May 2013 Masonry
A Joint Publication of NCSEA | CASE | SEI
STRUCTURE ®
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CONTENTS
FEATURES Rebuilding the Walls of Fort Jefferson
30
May 2013
By Craig M. Bennett, Jr., P.E.
A few hundred visitors a day take either a 45 minute seaplane ride or an almost three hour ferry ride from Key West to visit the Fort Jefferson National Monument. Unfortunately, settlement, loss of mortar and, most importantly, damage by embedded metals have nearly destroyed portions of the Fort. 160 years of moisture migration through masonry has left mortar so deeply eroded that bricks falling and touching bricks below is far from uncommon. Read how structural engineers faced these issues head-on.
DEPARTMENTS An Understated Entrance
By Rafael Sabelli, S.E. and Mark Waggoner, S.E.
44 Great Achievements
58 Structural Forum
William J. Mouton
By Richard G. Weingardt, P.E., S.E.
9 InFocus The Moral Virtues of Engineering By Jon A. Schmidt, P.E., SECB
14 Structural Practices Mechanical Anchor Strength in Stone Masonry By Kelly Streeter, P.E. and Keith Luscinski
18 Technology BIM for Masonry
By Tomas Amor, P.E.
22 Codes and Standards Harmonization of Allowable Stress Design and Strength Design of Masonry
By Edwin T. Huston, P.E., S.E.
51 Spotlight
By Greg Schindler, S.E.
By Joseph J. Luke, P.E., SECB
By W. Mark McGinley, Ph.D., P.E.
Visions of pyramids bring to mind the longevity of these amazing structures, but even the pyramids are in critical need of important structural renovations. Egypt’s Step Pyramid required careful planning and execution, due to the very dangerous condition of the burial chamber ceiling. A large portion had collapsed during the 1992 earthquake, and what remained was liable to collapse at any time. These renovations offer insight into the nature of the pyramids’ structural deterioration.
Filling the Void
Certifying the Practice of Structural Engineering
Design of Shelf Angles for Masonry Veneers
By Peter James
42 InSights
7 Editorial
10 Structural Design
New Theory on Egypt’s Collapsing Pyramids
34
COLUMNS
Changing Building Codes
By David Pierson, S.E., SECB
26 Historic Structures Modeling of Historic Structures By Sez Atamturktur, Ph.D. and Saurabh Prabhu
37 Just the FAQs Masonry Façade Condition Assessment Tool By Pamela Jergenson
A Joint Publication of NCSEA | CASE | SEI
STRUCTURE
®
38 Building Blocks
May 2013 Masonry
ON
THE
Shrinkage Forces on Concrete and Stone
COVER
Fort Jefferson National Monument is located in the Dry Tortugas, a group of sand bars 70 miles west of Key West, Florida. This great pile of 16 million bricks surrounding coral concrete cores was originally intended to defend a harbor for ships of the US Navy. The history of its construction and restoration are highlighted in this month’s feature on page 30.
Publication of any article, image, or advertisement in STRUCTURE® magazine does not constitute endorsement by NCSEA, CASE, SEI, C 3 Ink, or the Editorial Board. Authors, contributors, and advertisers retain sole responsibility for the content of their submissions.
STRUCTURE magazine
4
May 2013
By Garen B. Gregorian, P.E.
IN EVERY ISSUE 8 Advertiser Index 48 Resource Guide (Steel/CFS) 52 NCSEA News 54 SEI Structural Columns 56 CASE in Point
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Masonry has a New Edge. And it’s called HALFEN FK4. Introducing a new adjustable shelf angle with a thermal break.
H
ALFEN FK4 brickwork supports transfer the dead load of the outer brick veneer to the building’s load-bearing structure: an efficient construction principle developed with the experience of over 80 years of lasting technology.
Efficient Design As the demand for higher energy efficiency in commercial buildings continues to increase, the cavity between the brick veneer and the substrate is getting larger to allow for more insulation and air space. Along with this increased cavity size, the traditional masonry shelf angle, used to support the brick veneer at the slab edge, is also getting larger and subsequently heavier and more expensive to install. Architects & Engineer are looking for a more efficient support solution. The HALFEN FK4 brickwork supports use a thinner light weight shelf angle, eliminating brick notching while also providing a wider cavity.
Adjustability HALFEN FK4 brickwork supports provide continuous height adjustment of +/- 13/8” which compensates for existing tolerances of the structure as well as installation inaccuracies of wall anchors.
Structural Efficiency From load capacities starting at 785 lbs up to 2,360 lbs, the HALFEN FK4 brickwork supports allow efficient anchoring of brickwork facades in connection with HALFEN cast-in channels.
Reduced Thermal Bridging The HALFEN FK4 brickwork supports are off set from the edge of slab. Minimal contact with the building structure means reduced thermal bridging and lower energy loss.
Quality By using HALFEN FK4 brickwork supports, you profit from an approved anchoring system, excellent adjustment options and a complete product program covering all aspects of brickwork facing. Many advantages with one result: HALFEN provides safety, reliability and efficiency for you and your customers.
HALFEN USA Inc. · PO Box 547 Converse · TX 78109 Phone: +1 800.423.9140 · www.halfen.com · www.halfenusa.com · info@halfenusa.com
Editorial
Certifying the Practice of Structural Engineering new trends, new techniques and current industry issues By Joseph J. Luke, P.E., SECB
subjects do not need to be specific to the field of practice. The stricter standards are a benefit to the whole profession. For engineers practicing structural engineering in states that have SE licensing, the value of SECB certification can be less apparent. It would be very easy for an individual to believe that there is no advantage to applying for SECB certification. The value mainly comes in thinking long term, for the individual engineer as well as the profession as a whole. The success and implementation of SE certification across the US can be expected to result in the ability of structural engineers to easily practice in other states because of consistency in licensing requirements between states. At the NCSEA Leadership Forum held in Tucson, Arizona this past March, a Roundtable Session brought together structural engineers from all over the country, from firms of varying size and makeup, for a discussion of the current state of the practice of structural engineering and visions for the future. One interesting topic of discussion was practicing outside of the United States. Engineers that had been successful in that endeavor noted that there was utter disbelief, on the part of our overseas counterparts, at the dysfunctional state of the current practice of structural engineering in the US. A common thought was that structural engineers from the US should get their own house in order before they started moving offshore. In November 2012, SECB signed partnering agreements with both the Structural Engineering Institute (SEI) and NCSEA. These nearly identical agreements took effect on January 1, 2013. In accordance with the agreements, SEI and NCSEA have begun actively promoting SECB certification, especially in the states where SE licensure is not yet available. In return, SECB will actively promote SE licensure nationwide. SECB will also temporarily suspend its examination requirements* and reduce its application fee for members of SEI and the Member Organizations of NCSEA. The “Open Enrollment” period for licensed professional engineers to attain SECB certification began on January 1, 2013. An application for this open enrollment can be found at the SECB website at www.secertboard.org. Also on that web site one can find the history of SECB and information regarding its current activities. SECB firmly believes that its certification of structural engineers is an interim step towards SE licensure in all states, not an alternative, nor an end, in itself. In effect, SECB’s long term goal is its own disappearance.▪
Celebrating
years
1993-2013
Joseph J. Luke, P.E., SECB (jjluke@guerra.com), Senior Vice President and Director of Civil and Structural Groups at Jose I. Guerra, Inc. in Austin, Texas. He is Past President of the Structural Engineers Association of Texas (SEAoT) and is NCSEA Secretary and a Director of SECB.
a member benefit
structurE
®
The Mission Statement for the Structural Engineers Certification Board (SECB) is as follows: • To determine the level of unique and additional education, examination, and experience necessary to perform the science and art of Structural Engineering. • To provide a common national process for structural engineers to become certified. • To provide the public and stakeholders with an identification instrument that distinguishes an engineer with those unique and additional qualities necessary to perform structural engineering. The second statement implies that SECB is not intended to become the ultimate grantor of the certification, but is only to act as a vehicle toward the end of a broad based national process for certification. That certification process will require some consistency for licensing among the state Licensing Boards. The current condition of structural engineering (“SE”) licensing throughout the United States is that of a patchwork of disparate processes and requirements. Even the dozen or so states with SE licensure have considerably different requirements for that licensure. In an effort to promote public welfare and to aid stakeholders in selecting professional engineers qualified to do structural design, the National Council of Structural Engineers Associations (NCSEA) created SECB in 2003. SECB in turn created a certification process for engineers to demonstrate that they had the unique qualifications necessary to practice structural engineering. This certification process was not intended to supplant the licensing and regulatory rights of the States or other legal jurisdictions. For engineers practicing structural engineering in states that do not have an SE license, the value of SECB certification is fairly clear. SECB certification serves as a means of differentiating structural engineering from other engineering disciplines. SECB attempts to define standards for those wishing to practice structural engineering, including education and experience requirements. Having SECB following PE on a business card brings attention to the certification of structural engineers and helps start a dialogue between structural engineers and their clients and other professionals. It gives structural engineers an opportunity to explain to others how they are distinctly qualified, by education, examination, experience and continuing education, to call themselves structural engineers. An additional benefit of SECB certification is the much more meaningful continuing education standards compared to those required by most State Boards. The 15 PDH’s required annually by SECB must all be in subjects directly related to structural engiSTRUCTURAL neering. Many state engineering ENGINEERING INSTITUTE licensing boards have very loose requirements, if any, for continuing education subjects, and the STRUCTURE magazine
*As of January 1, 2013, the SECB examination requirement need not be met for applicants holding an active license or registration (as applicable) in any U.S. jurisdiction to act in responsible charge of structural engineering projects. The license and/or registration must have been awarded on or before July 1, 2005 and must remain valid continuously through the time of application.
7
May 2013
Advertiser index
PleAse suPPort these Advertisers
AZZ Galvanizing .................................. 39 Computers & Structures, Inc. ............... 60 CSC, Inc. .............................................. 50 CTP, Inc................................................ 33 Enercalc, Inc. .......................................... 3 Engineering International, Inc............... 16 Foundation Performance Association..... 24 Fyfe ....................................................... 23 Gerdau .................................................. 25
Halfen, Inc. ............................................. 6 Hilti, Inc. .............................................. 36 Integrated Engineering Software, Inc..... 45 ITW Red Head ..................................... 47 KPFF ...................................................... 8 NCEES ................................................. 43 Nucor Vulcraft Group ........................... 17 Polyguard Products, Inc......................... 27 Powers Fasteners, Inc. .............................. 2
Editorial Board Chair
Burns & McDonnell, Kansas City, MO chair@structuremag.org
Brian W. Miller
CBI Consulting, Inc., Boston, MA
Mark W. Holmberg, P.E.
Evans Mountzouris, P.E.
The DiSalvo Ericson Group, Ridgefield, CT
Dilip Khatri, Ph.D., S.E.
Greg Schindler, P.E., S.E.
Khatri International Inc., Pasadena, CA
KPFF Consulting Engineers, Seattle, WA
Roger A. LaBoube, Ph.D., P.E.
Stephen P. Schneider, Ph.D., P.E., S.E. BergerABAM, Vancouver, WA
Brian J. Leshko, P.E.
John “Buddy” Showalter, P.E.
John A. Mercer, P.E.
Amy Trygestad, P.E.
HDR Engineering, Inc., Pittsburgh, PA
Mercer Engineering, PC, Minot, ND
Chuck Minor
Dick Railton
Eastern Sales 847-854-1666
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sales@STRUCTUREmag.org
Davis, CA
Heath & Lineback Engineers, Inc., Marietta, GA
CCFSS, Rolla, MO
AdvErtising Account MAnAgEr Interactive Sales Associates
Jon A. Schmidt, P.E., SECB
Craig E. Barnes, P.E., SECB
QuakeWrap ........................................... 29 Quikrete ................................................ 13 RISA Technologies ................................ 59 SEAoSC ................................................ 40 SidePlate Systems, Inc. .......................... 41 Simpson Strong-Tie........................... 5, 21 Soilstructure.com .................................. 49 Struware, Inc. ........................................ 11 Williams Form Engineering .................. 19
American Wood Council, Leesburg, VA
Chase Engineering, LLC, New Prague, MN
COEUR D’ALENE CASINO RESORT HOTEL EXPANSION WORLEY, ID / PHOTO BY BENJAMIN BENSCHNEIDER
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STRUCTURE® (Volume 20, Number 5). ISSN 1536-4283. Publications Agreement No. 40675118. Owned by the National Council of Structural Engineers Associations and published in cooperation with CASE and SEI monthly by C3 Ink. The publication is distributed free of charge to members of NCSEA, CASE and SEI; the non-member subscription rate is $65/yr domestic; $35/yr student; $90/yr Canada; $125/yr foreign. For change of address or duplicate copies, contact your member organization(s). Any opinions expressed in STRUCTURE magazine are those of the author(s) and do not necessarily reflect the views of NCSEA, CASE, SEI, C3 Ink, or the STRUCTURE Editorial Board. STRUCTURE® is a registered trademark of National Council of Structural Engineers Associations (NCSEA). Articles may not be
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8
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inFocus
new trends, new techniques and current industry issues The Moral Virtues of Engineering By Jon A. Schmidt, P.E., SECB
A
ccording to Alasdair MacIntyre, applying virtue ethics to any practice is primarily a matter of recognizing the goods that are internal to it and the character traits that enable those who are involved in it to achieve them (“Rethinking Engineering Ethics,” November 2010; “Engineering Ethics as Virtue Ethics,” May 2011). David Miller argued that this is insufficient for practices that serve a wider social purpose (“The Proper Purpose of Engineering,” January 2013). When it comes to what constitutes a virtue in these cases, he wrote:
with others’ welfare in one’s product design … [is] as important to morality as any general principle.” How can following an approach along these lines facilitate deriving the moral virtues that are specific to engineering? Aristotle advocated locating most virtues at the mean between corresponding extremes of excess and deficiency that are deemed to be vices. With this in mind, adopting the standpoint of those put at risk by engineering endeavors in order to identify the types of behavior that engineers ought to avoid may lead to insights about those to which they should aspire. It seems logical to begin with the three virtues that MacIntyre identified as indispensable for every practice. Justice precludes both favoritism and indifference; every single person who will potentially be affected by what an engineer does deserves due consideration. Courage calls for being neither overconservative nor overconfident; in the words of Ross and Athanassoulis, engineers must “balance degrees of caution and (social) ambition that are appropriate to the circumstances and nature of [their] decisions.” Honesty means eschewing both deception and indiscretion; respect for confidentiality must be balanced with the public interest. Gene Moriarty, an electrical engineering professor at Illinois Institute of Technology, proposed a similar trio of virtues more closely tailored to engineers in his 2008 book, The Engineering Project: Its Nature, Ethics, and Promise, published by The Pennsylvania State University Press. Objectivity is a stance of impartiality or fairness that diligently examines all relevant factors and resolves each matter on the merits. Care entails assuming personal concern for another and then instinctively doing whatever the situation demands accordingly. Honesty encompasses cooperation and transparency as well as truthfulness. In light of the usual connotations of “objectivity” and “care,” it may appear at first glance that these two dispositions are incompatible. Moriarty is aware of this and explicitly downplays the association of objectivity with “coldness, lack of emotional involvement, bureaucracy … To be objective is not to be aloof, uninvolved, or uncommitted. It is to be disinterested rather than uninterested.” This notion of objectivity is perfectly consistent with the kind of empathy that genuine care demands, and that Busby and Coeckelburgh urge engineers to cultivate. Of course, these attributes are hardly exclusive to engineering; perhaps there is nothing more to being a virtuous engineer than being a virtuous person (in general) who happens to be an engineer. While this is accurate to an extent, Miller’s insight about purposive practices leads to the realization that Moriarty’s three moral virtues of engineering naturally align with the three components of its societal role. Engineers enhance the material well-being of all people by objectively assessing risk, carefully managing risk, and honestly communicating risk.▪
Very often a virtue is valuable both to its possessor and to those around him who benefit from its exercise. The possessor himself may value his attribute primarily because it enables him to achieve certain internal goods . . . But such an attribute only qualifies as a virtue because of other people’s valuations, and these will derive ultimately from the external purposes which the practice or practices in question serve. Therefore, if the proper purpose of engineering is the material wellbeing of all people, and if its basic societal role is the assessment, management, and communication of risk, then its moral virtues must be grounded accordingly. In the terminology of Allison Ross and Nafsikas Athanassoulis, the perspective of the potential harm-bearer carries greater ethical weight than that of the decision-maker (“The Internal Goods of Engineering,” March 2013). It is not simply up to engineers to define the limits of their own responsibility; instead, the public understandably assigns particular duties to them. Mechanical engineering professors J. S. Busby and Mark Coeckelburgh discuss the ramifications of this in a 2003 paper (“The Social Ascription of Obligations to Engineers,” Science and Engineering Ethics, Vol. 9, No. 3, pp. 363-376). They suggest that acknowledging their “ascribed obligations” would be beneficial to engineers, “not because they provide pre-formed rules that engineers can blindly follow, but because they can be used to help engineers develop a capacity for moral imagination.” The first step is to admit that “the picture of engineering as morally neutral is misleading.” It is true that engineers are not completely autonomous and rarely have the authority to establish the degree of risk that is acceptable for a given assignment (“The Social Captivity of Engineering,” May 2010). However, this does not absolve them from taking risk into account – especially its moral dimensions. Instead of only asking, “How can I justify the design that I want to develop?” engineers should also wonder, “How can I find the design that reasonably minimizes risk?” Busby and Coeckelburgh offer three motives for engineers to embrace what they call “ascribed ethics.” First, people generally behave in accordance with their expectations, so understanding common presuppositions about engineers and (especially) engineered systems may help better inform the design process. Second, non-engineers perceive risks that engineers are not in the habit of noticing, precisely because of their specialized expertise. Third, and most important, “The ability to imagine the implications of one’s actions, such as taking risks
STRUCTURE magazine
Jon A. Schmidt, P.E., SECB (chair@STRUCTUREmag.org), is an associate structural engineer at Burns & McDonnell in Kansas City, Missouri. He chairs the STRUCTURE magazine Editorial Board and the SEI Engineering Philosophy Committee, and shares occasional thoughts at twitter.com/JonAlanSchmidt.
9
May 2013
Structural DeSign design issues for structural engineers
A
nchored masonry veneer wall systems are commonly used throughout North America in residential, commercial and institutional construction. These exterior masonry veneers are non load-bearing and are usually assumed to be little more than an exterior finish of the building envelope. Using prescriptive design methods, masonry veneer can be supported vertically by foundations for heights less than 30 feet, or supported by the building frame for taller structures. The purpose of this article is to discuss the design of the vertical support of masonry veneers and how this design might be attempted under the current building code provisions. As shown in Figure 1, these exterior wall systems include an outer wythe (layer) of masonry veneer attached across an airspace to a backing wall by anchors. Typically, these backing wall systems can include sheathed wood, steel stud walls or concrete masonry walls. Masonry veneers can also be attached to poured concrete walls. The veneer wythe is most commonly constructed using units of clay or concrete masonry, bound together by mortar. These veneer masonry units vary from a nominal 25/8 to 4 inches in thickness. For the design of masonry veneer systems, model building codes in the United States reference Chapter 6 of the Masonry Standards Joint Committees’ Building Code Requirements for Masonry Structures, TMS402/ACI530/ASCE 5-08 (MSJC, 2008). The provisions in this standard describe two methods for veneer design, although the prescriptive method is used almost exclusively in North America. In these prescriptive design requirements, veneer backed by steel or wood stud wall systems over 30 feet in height must be supported at each floor level. Even though not required for other backing systems, masonry veneer wall systems are routinely also designed to be supported at each floor level to limit differential movement problems. As shown in Figure 1, the vertical support of the veneer is typically provided by a steel (shelf) angle that is attached to the building structural system. This connection often uses anchors embedded in the floor slab. The slab edge is then supported by a spandrel beam. In steel structural systems, the shelf angle can also be attached to the spandrel beams directly using shear plates as shown in Figure 2. The beams and supports are designed to resist the applied loads with beam deflections limited to L/600 under service level live and dead loads. The design of the shelf angle support is typically handled as part of the structural engineering design using a variety of design assumptions. These assumptions vary from assuming that the
Figure 1: Elevation of an anchored masonry veneer wall system.
Design of Shelf Angles for Masonry Veneers By W. Mark McGinley, Ph.D., P.E., FASTM
W. Mark McGinley, Ph.D., P.E., FASTM, Professor and Endowed Chair for Infrastructure Research, Civil and Environmental Engineering, J.B. Speed School of Engineering, University of Louisville. Dr. McGinley is an expert in masonry building systems, in particular, masonry building envelopes. He has been a primary author of all seven editions of the Masonry Designers Guide.
10 May 2013
Figure 2: Steel shear plate shelf angle support.
steel angles span in simple bending between anchor points while supporting a uniformly distributed dead load, to assuming the angle legs act as a bolted frame with a vertical dead load applied to the end of the horizontal leg (Figure 3). Unfortunately, none of these models accurately describe how the angle and veneer behave. The beam model ignores the significant torsion that is applied to the angle. Since angles have little torsional resistance they will rotate away from the slab, forcing greater loading on the angle sections near the anchors and may result in undersized shelf angle designs. Even though the frame model is more accurate and generally conservative, it also ignores the interaction of the brick and the shelf angle. In addition, the frame model requires assumptions be made relative to the effective width of the angle and tributary length wall. The thickness of the angle is highly dependent on these two assumptions, and designs often result in high angle thicknesses, especially as designers react
Shelf Angle Span Between Anchors
A) Uniform loaded Beam Shelf Angle Model
Shelf Angle Floor Slab
Veneer Weight
B) Anchored Frame Shelf Angle Model
Figure 4: Finite element model of masonry veneer and steel stud backing wall system.
Figure 3: Shelf angle design models.
To design the veneer as a beam, the allowable stress design procedures described in the masonry standards can be used. For example, if it is assumed that the dead load of a nominal 4-inch clay brick veneer produces a maximum vertical uniform load of 40 psf for a unit area of the wall face, then a uniform load (w) of 40 x height of brick above the angle must be resisted by the veneer acting as a beam. This load will produce a maximum moment and shear (Mmax and Vmax) of: Mmax = 40 h (Lspan)2/8 = 5 h (Lspan)2 Equation 1 Vmax = 40 h (Lspan) /2 = 20 h (Lspan) Equation 2 where h and Lspan are in feet and w is in psf. If it is assumed that the brick supports itself over the anchor spacing in simple elastic bending, then the critical maximum flexural stress (ft) produced by the moment (Mmax) must be limited to an allowable flexural tensile value (Ft). ft = Mmax / S ≤ Ft
Equation 3
where S (section modulus) = bd2/6, for a rectangular section S = t (h)2/6 Equation 4 The Masonry Standards Joint Committee [MSJC, 2008] code lists no limits for in-plane flexural tensile stress parallel to the bed joint. The flexural tensile stress limits (Ft) that it does present in Table 2.2.3.2 are for out-of-plane loading, and these vary from 15 to 80 psi for solid masonry units. Even though the allowable in-plane flexural tensile stresses parallel to the bed joints may be even higher than these values, the following analysis conservatively uses the lowest allowable out-of-plane strength value of 15 psi (solid masonry, Type N masonry cement Mortar). It should also be noted that the allowable flexural stress values in the 2011 version of the MSJC masonry code have been increased from the values described above, and even longer spans can be accommodated if these provisions are used.
STRUCTURE magazine
11
May 2013
For a solid veneer section, the design inequality is given by: ft = Mmax / S = 2.5 (Lspan)2/(t x h) ≤ 15 Equation 5 where h and Lspan are in feet and t is in inches and ft is in psi For a nominal 4-inch veneer, this reduces to: 2.5 (Lspan)2/(3.625 x h) ≤ 15 Equation 5 can be used to determine the maximum spans for various uninterrupted heights of veneer for a variety of veneer spans (anchor spacings) as shown in the Table (page 12). A similar analysis can be conducted with a 5 psi limit (1/3 of the previous limit) on the flexural stress and these opening heights are also shown in the Table. This lower Ft value is presented to illustrate that even if there is some question about the long term strength of the veneer, it can span a significant distance for even very low strength values. Furthermore, much longer spans can be realized if bed joint reinforcing is included in the veneer as these wires will allow the brick to act as a reinforced masonry beam. Shear stress is not critical, since calculations show brick sections will be able to support its own weight for spans over 50 feet without exceeding the MSJC code defined allowable shear stress of 37 psi. continued on next page
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to the higher insulation requirements of the new energy codes with longer angle lengths. Although not usual, more sophisticated finite element models of the veneer wall system can be constructed and, if the deformation of the ties and systems and supports are properly modeled, can be used to accurately analyze the behavior of the wall system. Figure 4 shows a typical model of a veneer wall system. The veneer can be modeled using plates elements, the ties modeled as axial elements and the stud backing modeled as beam elements. These models can also be extended to incorporate models of the spandrel beams as well. Although finite element analyses can provide accurate prediction of behavior of these systems, this level of analysis effort is not typically warranted in most designs. However, the author has conducted a number of finite element analyses on these systems during the course of failure investigations and retrofit studies, and the results of these analyses suggest a pattern of behavior that can be used to design shelf angles using more approximate analysis techniques. The finite element analyses indicate that the veneer and shelf angle supports interact, and the veneer is much stiffer than the angle, especially away from the anchor locations. Thus, the shelf angle provides much less support away from the anchors as the angle twists away from the slab edge or shear plate connectors. Away from the anchors, the steel angle provides much less support and the veneer is essentially acting as a beam, transferring the veneer dead load to the stiffer angle section near each anchor. Using this behavior as a guide suggests that a reasonable design approach would be to assume that the veneer will act as a beam spanning horizontally in-plane between anchors. If it is conservatively assumed that the brick veneer spans between anchor supports as a simply-supported, uniformly-loaded beam, then an appropriate anchor spacing and loading can be determined. Just how far a given height of veneer can span can be determined by applying the rational design methods described in the MSJC provisions.
Brick
Maximum spans for brick supporting its own weight.
Height of Veneer, h (ft)
Maximum Opening Span (ft) for Ft = 15 psi
Maximum Opening Span (ft) for Ft = 5 psi
1
4.66
2.69
3
8.08
4.66
5
10.4
6.02
10
14.7
8.52
20
20.9
12.04
30
25.5
14.75
Examination of the Table indicates that the brick is likely to support itself in simple bending if there is a sufficient height of brick. Even a 1-foot height of brick can span significant anchor spacings, although the lower heights of brick can easily be assumed to be supported by the angle even though the relative stiffnesses would suggest the brick will carry most of the load. The anchor spacing is thus likely to be determined by the anchor capacity for larger heights of veneer, although care must be exercised to account for any movement joints in the veneer to ensure they are not causing a break in the continuity of the veneer in locations that might over stress the veneer. This is not very likely, since the veneer can also cantilever over a significant distance. Once the anchor spacing is determined, the steel angle can be designed. In addition to acting as a form for the veneer and providing support to flashings and other wall systems, the angle must transfer the veneer loads to the structural support through the anchors. The tributary length of the veneer can be assumed equal to the span of the veneer used to determine the anchor spacing, and will include the additional weight of the shelf angle. The effective length of the angle resisting the veneer loading is a little more difficult to determine. Connection detailing, height of veneer, length of angle legs and angle thickness will all affect how much of the angle is effectively resisting these reactions. However, assuming that the effective length of the angle is 4 x (the nominal veneer unit thickness) has given reasonable results and appears to be supported by the results of the finite element analysis. This 4 x (the nominal veneer thickness) product was also used to determine the effective length of masonry wall under concentrated loads in older versions of the masonry standard, and thus has been used to describe effective width of steel and masonry systems in the past. Once the loads and the effective length of the angle are determined, it is a simple matter to determine the thickness of the shelf angle using the simple frame model as described in Figure 3.
As an example, a 10-foot height of 4-inch clay brick veneer and an anchor spacing of 6 feet would produce a loading of R = 40 psf x 10' x 6' = 2400 lb. Adding an additional 10 lb/ft for the angle weight results in R = 2460 lb. (note that the Table indicates that less than 3 feet of veneer could span this 6 foot anchor spacing, even if simple span supports and an allowable stress of 15 psi was assumed.) Using the 4t product, an effective length of 16 inches of steel angle can be used to resist this loading (4 x 4 = 16). If a 6-inch by 6-inch equal legged angle was used to support the veneer, the approximate angle loading shown in Figure 5 can be assumed. To determine the thickness of angle needed, the veneer weight is often assumed to be applied at the end of the horizontal angle leg. This condition will never happen since the angle will deform, forcing the effective loading point closer to the angle heal. In most cases, it would be reasonable and conservative to assume that the veneer weight is applied in the center of the veneer. If this assumption is used and it is assumed that a 2-inch cavity is present, then a moment on the lower leg would be Mmax = 4.19 in. x 2460 lb. = 10,307 lb. in. The 4.19-inch eccentricity was calculated to the center of the vertical angle leg assuming a ¾-inch thick angle, a 2-inch cavity, and one-half the brick thickness (e = 1.8125 + 2 + 3/8 = 4.19 inches). Assuming A 36 steel and using AISC LRFD Design procedures: Zx required = Mmax factored / 0.9 Fy = 1.2 x 10,307/0.9(36,000) = 0.424 in3 Since Zx = bt2/6 = and the effective length of the angle is 16 inches, a minimum thickness of angle should be 0.424 = 16 x t2/6 = 0.399 inches. A 7/16-inch, A36 steel angle would work to support this load. Note that that eccentricity used in the above calculation has a significant impact on the thickness of the angle and will vary depending on the angle and
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Shelf Angle 2400 pounds
Floor Slab
Eccentricity of veneer weight
Figure 5: Angle loading from 10 feet of veneer.
cavity configuration. This distance should be minimized as much as possible. The vertical leg of the angle would also have to be evaluated and sized. Equilibrium would suggest that the vertical leg would have to resist the same moment at the junction of the two legs, as well as the tension force produced by the shear force at the connection. For this case, the tension stress on the same effective length of the angle would be less than 500 psi (less than 2% of the capacity) and can safely be ignored. The vertical leg and horizontal leg would be designed for the same moment and thus be the same size. Note that this analysis ignores any shear lag effects. As can be seen from the analysis presented above, a reasonable angle thickness and support spacing can be determined. Deflections of the angles are not limited, since these deflections will occur as the wall is being constructed. The purpose of the deflection limits in masonry design standards is to preclude excessive cracking of the hardened unreinforced masonry. As these angle deflections occur primarily before the masonry sets up, they can usually be ignored. The previously described design method requires that the masonry veneer remains intact over the assumed anchor spacing. This is likely to be the case. Even if the brick does crack, however, there will likely be at most two sections of brick acting over a given span. This configuration puts much lower load on the steel shelf angle than a uniform loading and, in the extreme case, will create an arch whose thrust will be balanced by masonry on each side and/or friction on the steel angle.
Summary The previous discussion presents a design method that can be used to design the vertical supports of masonry veneer wall systems. It should be noted that this design methodology is based on a number of conservative assumptions and is likely to result in conservative designs in most typical design conditions. Designers are encouraged to analyze the veneer wall systems, and their supports using a finite element model in more unusual conditions.▪
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Structural PracticeS practical knowledge beyond the textbook
M
echanical anchor systems are commonly installed in historic masonry materials despite the lack of manufacturer-specified design values for this type of substrate. Scaffolding lateral supports, signage installations and telecommunication mounting systems all use these mechanical fasteners in natural stone materials. The current lack of codes, guidelines or recommendations for tensile and shear design criteria in historic masonry materials leaves structural engineers to improvise the design and specification of these anchors. Guidelines such as Appendix A, Guidelines for Seismic Retrofit of Existing Buildings in the International Existing Building Code (IBC), ASTM Standard E488-96: Standard Test Methods for Strength of Anchors in Concrete and Masonry Elements and Acceptance Criteria for Expansion Anchors in Concrete and Masonry Elements [ICC Evaluation Services 2005] are only relevant to concrete and brick masonry. Although field-testing is employed for some projects, more commonly an arbitrary reduction of the ultimate strength is used when designing these elements for use in natural stone. The creation of an empirical design equation for these values is arduous because, unlike concrete and concrete masonry units, historic building stone units are not manufactured materials, and their physical properties such as density and compressive strength vary from quarry to quarry and within quarry strata. The primary method for determining design values is a factor of safety approach. Factors of safety are divisors that are applied to the experimental average ultimate strength to allow for field conditions that invariably differ from a wellcontrolled laboratory environment. Currently, the factor of safety recommended for the design values in both shear and tension for both anchor types used in concrete is 4.0. A statistical COV (Coefficient of Variation) method is being considered as a change in approach, as methods in Strength Design of masonry becomes more widely used. The Coefficient of Variation for Mechanical Anchors is listed as between 10 – 15% [Powers Fasteners 2005].
Mechanical Anchor Strength in Stone Masonry By Kelly Streeter, P.E. and Keith Luscinski
Kelly Streeter, P.E., is a registered structural engineer and partner at Vertical Access LLC. She has continued to design and implement nondestructive evaluation projects at Vertical Access and received funding from the National Park Service to complete this research. Kelly can be reached at kelly@vertical-access.com. Keith Luscinski studied Operations Research and Industrial Engineering at Cornell University in Ithaca, New York. Keith can be reached at keith@vertical-access.com.
Methods and Materials The first task of this study included an online survey of the preservation engineering community, titled The Engineering Judgment Survey. A simple design problem was presented, asking for the selection of design values for a hypothetical installation. The laboratory portion of the study was designed as a screening experiment to evaluate a reasonably large number of variables (Figure 1) in order to determine
14 May 2013
Figure 1: Test specimens and variables.
which factors influence the response – in this case, the ultimate strength of the anchor installations. The primary (control) variables examined were: 1) type of stone (L=limestone; S=sandstone) 2) orientation of bedding planes 3) type of anchor 4) type of test: tension or shear The secondary (measured) variables were: 1) pulse velocity (all specimens) 2) compression tests (limited specimens) 3) failure mode (all specimens) Type of Stone The study utilized 10-inch cubes of both Ohio Sandstone (S) and Indiana Limestone (L), prepared and donated by Old World Stone in Burlington, Ontario. Each specimen was examined and marked with a unique specimen number, and each face was marked to control the bedding orientation during comparison of anchor strength as a function of stone “grain”. Bedding Orientation The orientation of the stone bedding plane relative to the axis of the bolt installation is a significant variable. Unlike concrete, limestone and sandstone are anisotropic materials and the anchors perform differently when installed in different orientations. For the tension tests, there are only two unique bedding orientations to study: perpendicular and parallel to the bolt installation. However, for the shear tests there are three different combinations of bedding orientation and pull direction to record. Type of Anchor One mechanical anchor was installed in the center of each face of each block following the manufacturer’s instructions for their installation in concrete. While a specific manufacturer’s anchors were used in this study, the methodology can be applied to other anchors. Powers Wedge-Bolt anchors (Figure 2) were installed
Figure 2: Wedge-Bolt (left) and Power-Stud (right).
cube splitting, face delamination and bolt pull out. The varied failure modes had a significant impact on the analysis of the data, especially for the tension specimens. Ultimate Tension Results
Figure 3: Tension failure modes: large cone (left) and bolt fracture (right).
with an embedment length of 2⅛ inches for the tension tests and 2¼ inches for the shear tests, following the minimum embedment recommendations listed in the specifications [Powers Fasteners 2005]. Powers Power-Stud anchors (Figure 2) were installed with an embedment length of 2 inches for both the tension and shear tests. Before any anchors were placed, pitch-catch Ultrasonic Pulse Velocity (UPV) measurements were taken through the stone cubes along all three perpendicular axes. UPV may be used in concrete to nondestructively determine compressive strength and help to determine anchor installation strength. Although there are no ASTM documents describing the relationship between UPV and compressive strength in natural stone, the goal was to determine the value of studying this variable in future research as a potential indicator of mechanical anchor installation strength.
the compressive strength of the samples in a controlled laboratory environment. This methodology could ultimately increase the confidence of structural engineers in the field, allowing them to use more realistic values and therefore fewer anchors when designing these installations.
Experimental Results Failure Modes: Tension The number of different tensile failure modes observed in the laboratory was unexpected, and presented another significant variable to track and analyze. In addition to the more classic failure modes of large cone failure and bolt fracture (Figure 3), four other failure modes were observed. Figure 4 displays four of the unexpected failure modes: small cone failure (a combination of partial pull out and then cone failure),
The results demonstrate, with just two exceptions, that the average ultimate tension strengths of Power-Studs and Wedge-Bolts in both stone types exceed the published design strength of these bolts in 4000 psi concrete. The predominant failure mode in sandstone varies by bedding orientation, with face delamination being the most common when the bolts were installed perpendicular to the bedding plane. Cube splitting and large cone failure were more common when the bolts were installed parallel to the bedding plane of the sandstone. The failure modes were more varied among the limestone blocks. In the Power-Stud limestone specimens, bolt failure was the most common, with the bolts breaking at the threads. The anchor-to-limestone bond exceeded the material strength of the anchor in 19 of the 24 samples. In contrast, only 2 of the 17 Power-Stud sandstone samples tested developed full strength of the anchor. In other words, the block failed first. It is not clear why this occurred; given sandstone’s greater compressive strength, we wouldn’t expect to see substrate-based failure in such a large number of specimens. Overall, the Power-Stud seems to be an excellent choice for limestone installations loaded
Engineering Judgment Survey Results The Engineering Judgment Survey showed that structural engineers tend to be extremely conservative when designing these anchors in tension and shear – probably due to the large variation of compressive values between and within types of natural stone. Even though the commonly accepted minimum Indiana Limestone compressive strength value is 4,000 psi [Indiana University], the designers were more likely to use the 2,000 psi concrete design value available from the anchor manufacturers. This is even more surprising in sandstone: with an accepted average compressive strength of approximately 10,250 psi [Richardson 1917], engineers were again more likely to use the 2,000 psi value. The only method to accurately determine compressive strength of stone is destructive, which is not an option in many cases involving historic structures. Therefore, the study investigated whether available nondestructive methods could be used to predict
Figure 4: Tension failure modes: small cone (upper left), cube splitting (upper right), face delamination (lower left), anchor pull-out (lower right).
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The overall performance of the anchors in limestone and sandstone was very promising. In tension, the Power-Stud proved to be an excellent choice for both Indiana Limestone and Ohio Sandstone installations, with capacities exceeding the 6,000 psi concrete designated values. For shear, the Wedge-Bolts exceeded the 6,000 psi concrete design values in all Indiana Limestone and Ohio Sandstone installations, regardless of bedding orientation and pull direction. Figure 5: Sandstone specimens that failed by cube splitting.
Experimental Design Drawbacks
in tension, regardless of bedding orientation. In sandstone, however, the Wedge-Bolts exhibited greater ultimate strength with lower variability between tests. Regardless of the observed failure mode, these results suggest that the published design values for installation in 4,000 psi concrete are appropriate, and in some cases conservative, for all variable combinations tested. These test results indicate that use of 2,000 psi concrete design values in Indiana Limestone and Ohio Sandstone is overly conservative.
The main flaw in the experimental design of the research was the use of such small stone cube samples. The decision to use the 10-inch samples size was driven primarily by a desire to maintain critical edge distances while maintaining manageable sample sizes. Although the minimum edge distances were maintained by using 10-inch cubes, the published ultimate strengths of the anchors in concrete do not mention the case where those critical edge distances are realized in all four directions – an unlikely condition in the field. As illustrated by the photos in Figure 5, edge distances did determine the failure mode observed on several samples. If edge distances had been greater, it is anticipated that a different failure mode and higher ultimate strength values would be realized in the samples where cube splitting was observed. This would have increased the sample size of meaningful data.▪
Ultimate Shear Results
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The Engineering Judgment Survey responses and the lab results for the shear tests showed an equally conservative tendency to underestimate the ultimate shear capacity of the anchors. With the exception of two specimen configurations, the laboratory data demonstrated that the ultimate shear strength of the bolt installations exceeds the published design values in 6,000 psi concrete, whereas the engineers surveyed chose to use the 2,000 psi concrete published. From the tests in both limestone and sandstone, the Wedge-Bolt is the superior choice over the Power-Stud for bolt installation in shear, with ultimate shear values exceeding the 6,000 psi concrete design level in every installation. As expected, the Wedge-Bolts
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provide much greater shear strength – in some cases two or three times greater than the Power-Stud. This trend is consistent with the higher published ultimate shear values of the Wedge-Bolts in concrete. Compressive Strength The destructively-determined compressive strength of the stone is a good predictor of bolt failure in tension in the limited number of tests performed. The selection of specimens to be tested destructively was based on the commonality of the variables, including failure mode, which greatly limited the sample size of compressively tested specimens. Destructive testing was not performed on any shear test samples due to budget constraints. Destructive testing of historic materials is obviously best avoided whenever possible; therefore, ultrasonic pulse velocity and Schmidt hammer tests were also employed, with the hope that they would have some predictive value in determining ultimate anchor strength. The pulse velocity data showed promise for further research in attempting to predict ultimate tension and shear installation strengths regardless of bolt and stone type. The next step for further research is to study limestone and sandstone samples from different sources with compressive strengths that vary over a larger range. The resulting data would likely increase confidence in the apparent linear and possibly predictive trends. The Schmidt hammer results did not have any predictive value for any documented variable. The surface hardness of natural stone, as tested using the Schmidt hammer test, does not appear to be an accurate measure of the compressive strength of the sample.
Conclusions The Engineering Judgment Survey confirms that engineers tend to be overly conservative when designing post-installed mechanical anchors in natural stone materials when a testing program is not feasible.
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Acknowledgments The authors would like to acknowledge the support of the National Center for Preservation Technology and Training (NCPTT), which supported the research through the PTT Grants Program. The complete grant report, including test results and additional information, is available on the NCPTT website (http://ncptt.nps.gov/wp-content /uploads/2008-05.pdf). Atkinson-Noland & Associates provided guidance and the compressive strength testing for the study. Michael Schuller, specifically, helped to edit the original report and provided valuable suggestions. Old World Stone prepared and donated the stone specimens for the research. Vertical Access LLC also supported the research by providing lab equipment, space and the time of Keith Luscinski to conduct the pull out tests and Kent Diebolt to direct the research. Readers may download the full-length article from www.STRUCTUREmag.org.
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Technology information and updates on the impact of technology on structural engineering
BIM for Masonry What’s Coming for Structural Engineers By Tomas Amor, P.E.
I
t’s been several years since the inception of Building Information Modeling (BIM) in our industry. Substantial progress has been made by the different software developers to provide us with the tools needed to tackle any type of project. As BIM use increases and progresses from design offices to construction trailers and fabrication shops, new needs emerge for the different professions and trades. When looking at structural building materials, the primary focus has been in the development of steel and reinforced concrete design and documentation tools. But what about masonry? Although it can be argued that many of the available features are interchangeable with concrete, masonry has very specific and unique needs that are not currently being met. There is great software available for masonry modeling and design, but BIM efficiencies come from effectively sharing information across the different platforms and avoiding duplication of data input. The primary challenge we face is how to transfer spatial and design information between the different programs. The engineering community has been relying on software developers to prioritize the creation of solutions to satisfy our industry needs. Unfortunately, there has not been a unified approach to develop those tools consistently for all building materials, see sidebar. The National Building Information Modeling for Masonry (BIM-M) Initiative was created to provide a roadmap to address that gap, and guide developers on masonry-specific needs.
BIM-M Initiative Tomas Amor, P.E., MBA, is a Senior Structural Engineer at Target Corporation in Minneapolis, MN. He chairs the National BIM for Masonry Structural Modeling Workgroup and is an Autodesk Revit Certified Professional. Tomas can be reached at tomas.amor@target.com.
The BIM-M Initiative was created to unify the masonry industry and all supporting industries through the development and implementation of BIM for masonry software. The initiative formed the following working groups to consider different perspectives in identifying challenges for masonry design and construction: Architectural Modeling, Structural Modeling, Construction Management, Construction Activities, and Material Suppliers Workgroups. These groups met last year to develop a vision for the development of BIM-M. The BIM-M Initiative was developed and funded by the International Union of Bricklayers and Allied Craftworkers (IUBAC), the Mason Contractors Association of America (MCAA), the International Masonry Institute (IMI), the National Concrete Masonry Association (NCMA), the Western States Clay Products Association (WSCPA) and The Masonry Society (TMS). The primary consultant has been the Digital Building Laboratory at Georgia Institute of Technology and the masonry coordination is being provided by Biggs Consulting Engineering.
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BIM and Other Materials Structural steel has had well-established analysis software with 3D-capabilities that contains a large amount of design information for decades. With such a robust technological foundation, software developers have been able to create bidirectional links that satisfy the immediate needs of engineers seeking interoperability efficiencies. Cast-in-place (CIP) concrete has similar welldeveloped finite element analysis tools that are very powerful and data-rich. Although certain design information, such as reinforcement, is not currently being transferred, there is specialized software that allows rebar and connections to be modeled. Additional modules are being developed to simplify those tasks on different platforms. Several BIM tools have also been created for wood and cold formed steel. There are plugins that recognize specific framing elements and use them to generate stick-frame layouts with analytical properties. This allows for the integration of design features and integrated generation of shop drawings. Like masonry, precast concrete has unique needs. The Precast Concrete Consortium/ Precast/Prestressed Concrete Institute (PCI) and the Charles Pankow Foundation sponsored an effort to develop BIM requirements specific to precast that is now well underway.
Structural Modeling Work Group (SMWG) The SMWG focused on the interoperability between architectural, structural, and structural analysis models. The group identified a need for advanced tools that support structural masonry analysis and design. The SMWG considered the functionality and interoperability of current structural tools for masonry, and provided a roadmap for improved software capabilities. The workgroup’s mission was to identify process and BIM software needs required for efficient structural engineering design and economical construction of masonry systems (Figure 1). The effort is focused on improving the competitiveness of masonry by providing more reliable tools and information-sharing that result in improved delivery and quality of masonry projects. As part of the roadmap, the workgroup created a list of must-haves in order to achieve the desired BIM tools for masonry. The list was prioritized in the following way: Software Interoperability for Structural Engineering Aside from everyone’s favorite spreadsheet, there are currently several standalone programs, like NCMA’s Structural Masonry Design System,
that help design masonry-specific elements. Information has to be transferred manually from one program to the next, which increases the likelihood of human error and inefficiencies. There are also sophisticated analysis and modeling programs that perform linear and finite element analysis of masonry systems. Although overall geometry can be transferred across some of these programs, they can be greatly improved for geometry updates and transfer of additional design information, such as rebar. There is also a need for modeling masonry-specific elements such as pilasters, bond beams, lintels, etc. in a way that they can be intelligent and transferred between platforms that perform different functions. The workgroup defined a need for analysis software interoperability for both “full building” and “individual element” design. There is a desire to have a reliable tool for “full building” design that shares information across platforms without having to duplicate data input. If reinforcing is assigned in the design software, it should be transferred to the documentation or fabrication software. A request was made to provide the necessary functionality to design building walls as systems, including foundations. Wall openings and overall layout coordination should be automated whether the system is resisting
Structural Engineer Wall Type Designation Gravity
Material Properties Cost competitive Regional availability
Lateral
Durability Sw & Mw
Wall Layout
Geographic location
Level of detail
Weight & dimensions
Bond pattern
Standardized properties
Multi-material type
F'm Mortar specification Grout specification Unit strength
Joint reinforcement Grouting (full/partial) Movement joint Control/expansion
psi color association?
Prism
Mortar bedding
R-value
Reinforcement layout Minimum reinforcement Seismic detailing
Fire rating
Connection design or graphical representation?
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Efficiency Arch model Assign walls Export to analysis software Roundtrips What information comes back? Bar layout 2D representation Critical areas
Incoming Outgoing
SDS2
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Project specifications & general notes
Design needs
Tekla
in-plane, out-of-plane, or gravity loads. Due to the iterative nature of masonry design, the software should be able to retain load distribution and load generation when changes to the design occur.
Link catalog of data
Analysis software full building Interoperability needs
Connections
Figure 1: BIM software needs identified during the SMWG sessions at Georgia Tech.
Model Spec Data
Temporary bracing Ability to apply control/construction joint Multi-materials same wall Want analysis & design package Foundation design Strength design or allowable Load generation Design shear walls Lateral load distribution Openings consideration Analytical property designation selection
Analysis individual elements Element design with plug-in
For the “individual element” design, the aspiration is to have at least one user-friendly plug-in that allows the engineer to design a specific masonry element within the documentation software. For example, design a
and established for each project. We’ve discussed ownership of information, but design teams also have to discuss what kind of information is being added to a model and why. Is there a need to know when a wall is fully or partially grouted? What’s the bond pattern? Where are the movement joints? Modeling this data can aid the design and field teams in many different ways. A strong tie between the project specifications and models will be required to manage that functionality. Global Interoperability Figure 2: Virtual mock-up of load-bearing masonry construction with structural elements highlighted. Courtesy of Lena Klein and Russell Gentry, Georgia Tech Digital Building Laboratory.
lintel inside of Revit using an NCMA plug-in and have that design accurately represented graphically. See Figure 2 for an example of a load-bearing masonry construction with structural elements highlighted. Efficiencies in Design Masonry design requires the input of many project specific material properties. In order to avoid duplication of work, we have to be careful about who has control over certain characteristics. Teams need to define ownership of information, not only ownership of model elements. If the architects model the walls that are also used for structural analysis, who specifies grout or mortar types? Is the color of block selected related to the unit’s strength? It’s important for each specialized party to have the ability to enter graphical and design data sequentially. That way, the model can evolve as data is layered onto different elements, instead of having to recreate them in different programs to serve one unique purpose. This will require software to allow properties of linked elements to be edited and structural components to be hosted to them. The programs will need to provide reliable change notification features and tolerate stable round trips without losing information. Alongside those tools, there is a need for a comprehensive masonry material catalog because there currently is large variability of product specification and availability across the country. The BIM-M initiative gave the SMWG the opportunity to engage with the Material Supplier Work Group, which helped clarify a common desire for standardization of materials. When achieved, this will simplify masonry design and modeling. When talking about BIM and transferring data across disciplines, the appropriate Level of Development (LOD) needs to be considered
The need for interoperability goes beyond architecture and structural. Other design teams, construction managers, general contractors, specialized sub-trades, and owners are interested in data output from BIM for different purposes. The ability to identify and communicate key parts of a structural masonry system during design would simplify and improve the coordination process. Once this type of intelligence is built into the model, rules can be established to identify no-fly zones for other disciplines to recognize. For example, ductwork or piping wall penetrations could be coordinated electronically to miss running bond beams or grouted cells. During construction, this added information would also benefit general contractors beyond clash detection. Construction Managers and General Contractors can use the increased functionality to plan construction sequencing, perform constructability reviews, overall scheduling, project management, and costing efforts. Ideally, these models will be passed onto fabricators and detailers, that can validate the data and extract 2D and 3D graphical representations to generate shop drawings effortlessly. In recent years, we are seeing that more sophisticated contractors and owners are benefiting from extracting information from the BIM at different stages of a project. Complete models can provide detailed bill of materials and quantity take-offs that can be utilized for cost estimation and analysis.
Next Steps The overall Roadmap for developing and deploying BIM for the masonry industry was released on January 31, 2013 (a link to the full document on the NCMA website is included in the online version of this article). This document captures the discussions and collaboration efforts from all five workgroups, and lays out the vision and groundwork for the next phases of the program. There are three phases of work being proposed: Development, Specification, and Implementation.
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During Development, the team will develop a data structure for the digital representation of masonry units and masonry systems. It will be an effort to bring consistency in design with the involvement of masonry suppliers. During this phase, the BIM-M initiative will formalize a relationship with major software developers. In the Specification phase, the focus will be on establishing a foundation for a software specification. With the involvement of software developers and contractors, there will be a focus on the development of structural engineering tools and construction workflows. In the final phase, Implementation, the BIM for Masonry initiative will become a reality. Efforts will be directed towards the implementation of BIM-M from virtual architectural massing studies to real construction.
Conclusion As our industry adapts to the new changes brought on by the latest technological advances, it’s exciting to see professionals and organizations come together to lead such an ambitious initiative to ensure equal progress is made for all building materials. This all-inclusive effort not only strengthens the ties between all the parties involved, but it also shows the commitment of the masonry industry to continuous improvement and adaptability. Software development for unique systems, such as masonry, requires a deep understanding of the material, its uses, and construction methods. It’s design professionals that need to help drive that development and aid software companies to understand what we need. BIM-M is intended to be representative of the overall masonry industry, and will be successful if it captures the actual needs and wants of our engineering community. We ask that you take ownership of this development with us and reach out to any of the groups or individuals mentioned in this article if you have feedback, input, or comments. Special thanks to the individuals that participated in the SMWG: Ross Shepherd, Ryan-Biggs Associates; Jeff Elder, Interstate Brick; David Biggs, Biggs Consulting Engineering; John Hochwalt, KPFF Structural Engineers; Amy Sellers, PES Structural Engineering; Jason Jones, PES Structural Engineering; Brian Johnson, Autodesk; Todd Dailey, Dailey Structural Engineers; Chad Boyea, PES Structural Engineering; Jamie Davis, Ryan Biggs Associates; and Craig McKee, Huckabee and Associates.▪ Others wishing to join this effort are requested to contact the author at tomas.amor@target.com.
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Codes and standards updates and discussions related to codes and standards
This article is provided by Ed Huston as the 2011 recipient of The Masonry Society’s Haller Award. Named for Professor Paul Haller, the Haller Award recognizes an individual engineer or engineering firm that has enhanced the knowledge of masonry in practice. Ed has an extensive background in masonry design, research, and teaching that has resulted in advancements in masonry design practice and code development. He continues to advance masonry knowledge through his ongoing design and investigation practice, as well as disseminate that knowledge through his teaching activities. The Haller Award committee congratulates him on his achievements and selection.
History of Masonry Harmonization Masonry structures have been built to endure; noteworthy masonry structures constructed hundreds, even thousands, of years ago, still exist. Masonry design and construction produces architecturally award-winning structures which are safe and durable. The main masonry materials in use today are brick, concrete masonry and glass block masonry. The primary masonry design methods in use today are allowable stress design (ASD), strength design (SD) and prestressed masonry design. The masonry standards referenced in the International Building Code (IBC) are the Building Code Requirements for Masonry Structures (Code) and the Specification for Masonry Structures (Specification). These two documents, along with their respective commentaries are jointly known as the Masonry Standards Joint Committee documents, or the MSJC documents. (Starting with the 2008 edition, The Masonry Society became the lead organization and the documents are often referred to as the TMS 402 and TMS 602, respectively.) They are jointly published by The Masonry Society (TMS), The American Concrete Institute (ACI), and the American Society of Civil Engineers (ASCE). These three organizations formed the Masonry Standards Joint Committee. The process of harmonization of design provisions in the MSJC documents dates back to the 1999 edition of the Code and Specification. The 1995 edition of the Code was comprised of twelve discrete chapters, which had almost no overlap. The 1999 edition of the Code and Specification reformatted the material into five chapters. Material common to different design methods, seismic design requirement, and quality assurance requirements were placed together in Chapter One. The 1999 edition of the Code and Specification had separate chapters for ASD and prestressed masonry design. There was a place holder for a future Limit States Design. The 2000 International Building Code included masonry provisions in Chapter Twenty-one. Section 2108 was for the Strength Design of Masonry. Many
Harmonization of Allowable Stress Design and Strength Design of Masonry By Edwin T. Huston, P.E., S.E.
Edwin T. Huston, P.E., S.E., is a Principal with the structural engineering firm of Smith & Huston Inc. in Seattle, WA. He has received numerous awards for his work and is a Past President of NCSEA.
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MSJC members had serious concerns about the requirements in this section, and set aside the work on a Limit States Design procedure to revise these strength provisions. This allowed them to include a SD chapter for masonry in the 2002 MSJC, so that they could begin shaping a workable set of strength design provisions. In keeping with seismic design changes in the Minimum Design Loads for Buildings and Other Structures (ASCE 7), the 2002 MSJC updated the seismic provisions for masonry and reorganized these provisions by Seismic Design Categories, rather than the Seismic Performance Categories used in previous editions of ASCE 7and the 1999 MSJC. The major harmonization effort in the 2005 MSJC was to change the design equations for anchor bolts, so that a designer would get approximately the same bolt spacing using either the ASD procedures or the SD procedures. The masonry modulus of rupture is used for the design of unreinforced masonry. It is also used in reinforced masonry to determine the cracking strength and the deflection of masonry elements just prior to cracking. The 2005 MSJC harmonized the modulus of rupture between the various design methods. Since ancient times, masonry corbels have been used to help translate a wall out-of-plane. The 2002 MSJC ASD procedures had requirements for the design of corbels, but the SD procedures did not. The 2005 MSJC harmonized the requirements for the design of corbels, applied these requirements to both ASD and SD and moved them to Chapter One.
Medieval tower with masonry corbels.
to mirror the design assumptions which previously existed at the beginning of the reinforced masonry portion of the SD chapter. With the availability of robust SD provisions, which have been substantiated by many laboratory test programs, the 2011 MSJC continued the harmonization process by reviewing these research results and by performing numerous trial designs comparing the results of the reinforcement requirements of elements designed by both the ASD and the SD procedures. We also reviewed historical code design provisions.
Harmonization in the 2011 MSJC – Allowable Stresses The 2011 MSJC added a set of design assumptions at the beginning of the reinforced masonry portion of the ASD chapter STRUCTURE magazine
The trial designs included both brick and concrete masonry; walls with a large variation of aspect ratio, walls with both very low and moderately high axial loads and a wide range of strengths. The goal was to encompass the gamut of designs that structural engineers encounter. We discovered that the allowable flexural stress limit of Fb = 0.33 f 'm was first codified in 1946, when normal masonry design strengths, f 'm, were about 800 psi. The trial designs indicated a very poor correlation between the flexural capacity of elements
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There is a saying that success begets success. Encouraged by the positive harmonization results of the 2002 and 2005 editions of the MSJC, the 2008 MSJC committee harmonized bearing requirements; effective compression width and provisions for concentrated loads, and moved all of these provisions to Chapter One. The 2008 committee also gathered most of the provisions for beams and columns and moved them to Chapter One. Many of the requirements for anchor bolts, such as placement, effective area and embedment length, were consolidated and moved to Chapter One. The anchor bolt provisions related to the design equations were left in the respective design chapters. The MSJC, and its industry partners also began the process of moving requirements out of the IBC and placing them in the masonry standard. Much effort had been spent in previous code cycles, trying to maintain provisions in the IBC and the MSJC which were exactly the same in the two documents. For example, both documents had hot and cold weather requirements. If the provisions were not exactly aligned, users were left to wonder why and whether one provision was deliberately different from the other. Removing these “transcribed” provisions from the IBC reduced the work load and the confusion factor. The seismic provisions for prestressed shear walls and for the maximum reinforcement provisions for Special Reinforced Masonry Shear Walls designed by ASD were moved from the 2009 IBC into the 2008 MSJC. For several decades, the IBC, and one of the predecessor legacy codes, required a 1.5 factor to increase the seismic shear forces on shear walls designed by ASD in areas of high seismicity, or, more recently, for Special Reinforced Masonry Shear Walls. The MSJC had a parallel design requirement in SD for capacity design for shear in Special Reinforced Masonry Shear Walls. The 2008 MSJC moved the ASD requirement from the IBC and the capacity design for shear requirement from the SD chapter of the MSJC and placed them side by side in the seismic requirements portion of Chapter One, so that designers could see that these requirements were focused on the same issue.
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Comparison of SD and ASD design with Fb = 0.45 f 'm.
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designed using SD and the same element designed using ASD and the 0.33 f 'm limit. The end result of this effort resulted in an increase of the flexural stress limit to Fb = 0.45 f 'm. The increase in the allowable flexural stress limit applies to all load combinations. Senior structural engineers will remember that the old working stress concrete design flexural limit was 0.45 f 'c. However, the 2011 MSJC was unwilling to increase the allowable flexural stress limit and to still allow a 1/3 stress increase. The trial designs confirmed that this could lead to unconservative designs. So masonry has now joined the other materials and eliminated this stress increase. Other stresses were also increased, although not all allowable stresses have been increased in the 2011 edition of the code. Based on research and testing, the bearing stress limit has been increased in both ASD and in SD. The composite action shear stress limit has been increased in ASD. The allowable steel reinforcement stress for flexure, direct tension and shear has been increased to 32 ksi. The allowable axial stress limit was not increased, because there was concern expressed
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between members of various subcommittees as to what effect this could have. For example, the prestress committee was unsure of the ramifications of this change on the design of prestress elements. This has been set aside as ongoing work for a future cycle. The trial design efforts did not result in perfect correlation between ASD and SD. This is because masonry is a non-homogeneous anisotropic material. In ASD, the safety factor for reinforcement is 2.5, but the safety factor for masonry can be four or more. Because of these inconsistencies, ASD will still be slightly more conservative in almost all cases.
Harmonization in the 2011 MSJC – Shear Design In the past, shear analysis in ASD of reinforced masonry has been treated very differently than in SD. In ASD, the shear stress on an element was calculated and compared to a lower level allowable stress which could be resisted by the masonry alone. If the shear stress on the element was less than the lower level allowable stress, the design was adequate, although reinforcement to meet prescriptive detailing requirements would still be required. If the shear stress on the element exceeded the lower level allowable stress, but was less than a higher level allowable (limit) stress, an amount of reinforcement was determined which could carry the entire shear. If the shear stress on the element exceeded the higher level allowable (limit) stress, the section was inadequate and had to be made stronger, thicker or longer. In SD, the shear demand on an element was compared to the shear capacity of the masonry and the reinforcement working together.
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In both design methods, the shear capacity was adjusted for the shear span of the element to promote ductility. This was analyzed by the M/Vd ratio of the element. Tall, narrow elements are flexurally dominated and have high M/Vd ratios. The shear capacity of such members was reduced to promote a ductile failure mode. Long squat elements are typically shear dominated, but generally have excess shear capacity. No reduction of the shear capacity of such members is required. Recent research compared eight different design methodologies, used, or recommended for use, around the world, with the results of laboratory testing. This research concluded that, of the eight methodologies, the TMS 402-08 SD for shear was the best predictor of shear capacity and that the TMS 402-08 ASD for shear in reinforced masonry was the worst predictor of shear capacity. Based on this research, the 2011 MSJC modified the SD methodology for shear capacity and converted it to an ASD methodology for shear stress, based on the stresses in both the masonry and the reinforcement. The modification kept the same terms in both methodologies, but modified (reduced) the constants which were applied to these terms. Trial designs were conducted to benchmark the ASD to the SD methodology. Particular attention was given to Special Reinforced Masonry Shear Walls, and these trial designs indicated that further reduction of the constants in the equations was required to achieve approximately the same results, regardless of the design method. In general, the amount of reinforcement required using ASD in the 2011 MSJC will be less than what was required using ASD in the 2008 MSJC. However, the amount of reinforcement required using ASD in the 2011 MSJC will typically be slightly more than the amount required using SD in the 2011 MSJC.
Future Harmonization Efforts While much progress has been made, there is still room for additional harmonization efforts. As noted above, some allowable stresses have not yet been harmonized and additional harmonization between ASD, SD and prestressed design has been held over as the work of future code cycles. The work that has occurred to date sends a strong message to the users of the TMS 402 (MSJC) that we are committed to this process.▪
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Historic structures significant structures of the past
W
ith the invention of mid-19th century naval weaponry, coastal fortifications in the United States were rendered obsolete as they lost their functional use of defense. More than 100 coastal forts, now over a century old, are considered national heritage structures to be preserved for future generations. Many of these brick masonry forts have incurred structural damage during bombardments, and further accumulated damage due to the harsh environments in which they were constructed. There exist no guidelines, however, to assist stewards in the safeguarding of these historically significant masonry fortifica- Figure 1: The current state of Fort Sumter National Monument: (a) aerial view, (b) barrel vaulted casemates, and (c) degradation tions. Therefore, a structural engineer must of brick and mortar piers. either rely on modern rules of material strength and structural behavior, or try to under- the masonry is not continuous at the interface, stand the failure mechanisms of the unreinforced thus forming a cold joint. This design is typical masonry on a more of Third System coastal fortifications in North fundamental level. America, as it served as a method of isolating An understanding the impact damage from enemy artillery on the of unreinforced outer walls. masonry can be In this article, we investigate the behavior of achieved through this structure under various foundation settlea combination of experimental and numerical ment scenarios. studies by gaining insights into macro-level strength-deformation behavior and micro-level Finite Element Model defects and crack growth of masonry structures. Development While uncertainties and errors inevitably arise in the development of such numerical models, experi- Finite element (FE) analysis is a widely accepted ments can ultimately reduce such uncertainties and method for analyzing historic masonry structures errors in predictions. due to its ability to model complex geometric 3-dimensional shapes and resolve nonlinear and material behavior. Fort Sumter National Monument anisotropic In the nonlinear analysis of masonry behavior, Fort Sumter, in Charleston harbor, SC, is a 19th to represent an appropriate failure criterion for century brick masonry coastal fortification. masonry, the elastic modulus, cracking strength Declared a national monument in 1948, it is and crushing strength of the homogenized best known as the site where the first shots of the assembly must be defined. This modeling step is American Civil War were fired in 1861. According typically when the majority of uncertainties are to archival documents, construction began in introduced, due in part to our lack of knowledge 1829 with the foundation, a man-made island (known as epistemic uncertainty) and in part to filled with nearly ten thousand tons of granite the natural variability of the material (known as and over sixty thousand tons of assorted rocks aleatory uncertainty). Any laboratory tests or on and aggregate. By 1860, the pentagonal-shaped site evaluations of material characteristics would structure (Figure 1a) rose nearly 50 feet high, help reduce the epistemic uncertainties, while the with three tiers built with locally made bricks aleatoric uncertainty (such as the spatial variabiland Rosendale mortar. After several devastat- ity of masonry) inevitably remains in the model ing bombardments between 1861 and 1865 and predictions. reconstruction efforts lasting into the early 20th During our studies on Fort Sumter, a prism century, only the lower first tier of the original sample along with 2.5-inch diameter cored fort stands with major portions reconstructed. samples were obtained on site (Figure 2, page The walls are made up of barrel vaulted casemates 28). The compressive strength and modulus of (Figures 1b and 1c) that once held guns and artil- elasticity were determined according to ASTM lery. Each casemate has a gun embrasure opening standard tests parallel and perpendicular to the in the exterior scarp wall allowing artillery to fire mortar bed joints. The tensile strengths of the from the fort in all directions. The scarp wall, brick and mortar samples are determined via is adjacent to the casemate piers and vault, but three-point flexural tests. The tensile capacity
Modeling of Historic Structures Simulation-Based Structural Analysis of Fort Sumter Considering Foundation Settlement By Sez Atamturktur, Ph.D. and Saurabh Prabhu
Dr. Sez Atamturktur serves as an assistant professor in the Glenn Department of Civil Engineering at Clemson University. Prior to joining Clemson University, she served as LTV technical staff member at Los Alamos National Laboratory. Dr. Atamturktur may be reached at sez@clemson.edu and further information about her work can be found at www.cuideas.org. Saurabh Prabhu is a graduate student in the Glenn Department of Civil Engineering at Clemson University. Mr. Prabhu may be reached at saurabp@clemson.edu.
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continued on page 28
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Figure 2: Coring of material samples revealed that although the construction drawings indicate a brick wall across the width, the construction of the fort is composite with tabby concrete infill.
Figure 3: The discontinuity between the scarp wall and barrel vaulted casemate isolates the structural damage to the scarp wall to prevent the casemate from collapsing in the event of an attack.
of the brick-mortar assembly is taken as the volumetric average of a representative cell. The properties of the tabby concrete infill are determined via diametral tests on the cored samples. Lastly, the densities of the materials are measured by taking a ratio of weight to volume of the specimen. Historic masonry monuments are typically a complex network of curved elements such as arches, vaults, domes, and buttresses with straight elements such as piers and walls. Some structural elements may also contain decorative moldings, surface texture or damage such as minor chipping, etc., making it difficult to reproduce the geometry in a numerical model. Such details can unnecessarily increase the model complexity and, thus, the computational demand. The fundamental principle in geometric modeling must be preserving the structurally important geometric features, such as cross sectional area, center of gravity, moment of inertia, etc. Over the past 150 years, Fort Sumter’s infrastructure has undergone significant and permanent deformations, material degradation, and discontinuities due to crack formations. In our study, terrestrial 3-D laser scanning with a Trimble CX scanner was implemented to digitally reproduce the fort’s geometry. A 3-D non-linear FE solid model of one of the casemates was developed using ANSYS 13.0. The model geometry was constructed from the wireframe models developed using the laser scan, and initial material properties were assigned according to the material tests. In analyzing masonry with the finite element approach, the geometric model was meshed with specialized elements designed for brittle materials accounting for cracking and crushing according to a predefined failure criterion. The size of the mesh was determined based on the trade-off between numerical accuracy and run times. A mesh
size of 0.2 meters typically yields a numerical uncertainty below the variability in the structure’s expected response due to environmental factors (5-6%). In our study, the discontinuous interface between the scarp wall and vaulted portion of the casemate (Figure 3) was treated as a contact surface and approximated by a friction coefficient. The coefficient was calibrated according to an experimentally measured ratio of displacements on the two sides of the interface when an instrumented hammer was used to impact one side. The casemate foundation was modeled assuming a linear relationship between the pressure on the foundation and the deflection, i.e. a Winkler type foundation. Thus, a series of vertical and horizontal linear springs was distributed throughout the base of the casemate. The stiffness of the springs represented the foundation stiffness, which required calibration to the experimental data. When developing finite element models of large masonry monuments, it is often necessary to isolate a portion of the structure. Such an approach, although necessary to keep the problem to a manageable size, results in unknown restraining forces between the structure of interest and components that are excluded from the model. To account for these unknown forces, the most computationally efficient approach is substructuring, which entails approximating the force-displacement relationship of the adjacent components through a small number of elements located at the interface. These elements are known as superelements, and they significantly reduce computational demands. While modeling one of the casemates of Fort Sumter, the adjacent casemates are represented with superelements, effectively reducing the problem to one third (Figure 4 ).
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Figure 4: The finite element model of the casemate is built recognizing the elastic constraints of the adjacent casemates as well as the unmodeled foundation.
Calibration of Material Properties Calibration refers to the systematic adjustment of model input parameters to match the solution with experimental observations. Non-destructive vibration tests were performed to extract the natural frequencies of the casemate, such that the model input parameters can be fine-tuned by comparing the predicted natural frequencies with actual measurements. Accelerometers were used to measure 30 minutes of ambient vibrations on 41 points on the casemate. The vibration response of the casemate, recorded in the time-domain, was post-processed to extract the first two natural frequencies at 27.48 Hertz and 45.2 Hertz. Natural frequencies are linear properties of the global behavior of the system and, thus, were used to calibrate material parameters that define linear behavior and boundary conditions. For the FE model of the casemate, three input parameters were selected for calibration: the elastic modulus of both the barrel vault and the walls and piers, and the stiffness of the foundation springs. The calibrated input parameters were obtained such that the FE model reproduced the measured natural frequencies with 10% accuracy.
Support Settlement Analysis Four settlement scenarios were considered with smoothly varying profiles including sagging settlements, pier settlements and tilting of the ground. Each settlement scenario is simulated with a maximum magnitude of 100 mm in increments of 2.5 mm. Scenario 1 (Figure 5) simulated an unsymmetrical sagging in the north-south direction, including both the scarp wall and piers. With this scenario, a significant through crack originating at the base of the scarp wall on the south side ran diagonally across the scarp wall. Also, severe cracking at the springing
vault. Cracking in the vault must be treated as an instability condition as the progression of cracks once initiated in these members was rather rapid (Figure 6).
Summary
Figure 5: The four settlement scenarios considered in the analysis.
of the arch on the south-side was observed. Scenario 2 simulated symmetrical sagging under the casemate in the north-south direction. This scenario resulted in a crack that began at the base of the scarp wall at both ends and converged in the center, forming a load bearing arch. Scenario 3 simulated the settlement of the north piers. Differential settlement of a pier caused an unsymmetrical cracking of the vault close to the pier that has settled, while the pier that has settled less experienced more damage. Scenario 4 constituted the tilting of the casemate in the north-south direction. This scenario resulted in heavy cracking of the south pier, diagonal cracks in the scarp wall and a rapidly developing through crack in the
The computer simulation indicated that unsymmetrical sagging types of settlements were characterized by diagonal cracking of the scarp wall, originating from the bottom on the less-settled side. Symmetric sagging under the casemate, however, formed cracks that originate from the bottom of the scarp wall from both sides and converge at the center forming an arch that spans the length of the casemate and bears the loads of the wall above. Cracks due to stress concentrations were seen for most settlement configurations at the intersection of structural members, such as the springing of the arches and vaults. Cracking of the vault was observed in configurations that involve differential settlements of the piers. Cracks, once formed in the vaults, progressed rapidly without warning as settlement increased. Thus, cracking of the actual vault should be taken as a structural stability concern. The formation and progression of cracks were observed to be unique to each settlement configuration. ADVERTISEMENT–For Advertiser Information, visit www.STRUCTUREmag.org
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Figure 6: Crack development under the settlement scenarios shown in Figure 5.
By utilizing visual investigations of these peculiar early warning signs in the form of cracks, the stewards of this historic monument can use these computer simulations to help draw conclusions whether settlement may be causing damage to the structure. This of course assumes that the cracks are not due to external loads, which too can be incorporated into the numerical model, making simulations a useful tool for historic structural assessment.▪
Rebuilding the Walls of Fort Jefferson By Craig M. Bennett, Jr., P.E.
F
Figure 1: Fort Jefferson, in the Dry Tortugas, is in a serenely beautiful setting, accessible primarily by ferry and seaplane.
Figure 3: Arches and vaults composed of 16 million bricks surround inner cores of coral concrete.
Figure 4: Settlements are as much as 24 inches per 200 feet. Here the water level in the moat makes settlement observation easy at Bastion 1.
ort Jefferson National Monument is located in the Dry Tortugas, a group of sand bars 70 miles west of Key West, Florida. This great pile of 16 million bricks surrounding coral concrete cores was originally intended to defend a harbor for ships of the US Navy, allowing the naval forces to control shipping through the Straits of Florida and, ultimately, to control trade through the Gulf of Mexico and into the Mississippi River. The fort occupies over seventy percent of Garden Key, one of the larger islands of what is now Dry Tortugas National Park (Figures 1, 2 and 3). The history of the construction of the fort is bewildering. Noted in 1829 by U.S. Navy Commodore John Rodgers as an ideal location for an advance post for the defense of the Gulf Coast, the study and design process occupied the next 17 years, culminating in 1846 with the start of construction. The fort was still not complete in 1865 at the end of the Civil War, by which time the invention of rifled cannon had made the fort itself obsolete. The fort was used, unfinished, as a Federal prison during and after the Civil War. It still remains unfinished today, serving as a marine research station and a National Park. Now accessible primarily by seaplane and ferry, the fort that once housed Dr. Samuel Mudd, imprisoned for tending to John Wilkes Booth’s broken leg, sees up to a few hundred visitors a day who take either a 45 minute seaplane ride or an almost three hour ferry ride
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from Key West to tour the casemates, snorkel among the sergeant majors and parrot fish, and occasionally camp at the edge of the beach. The fort has more than its share of structural issues, all of them issues that structural engineers face in working on many of our older masonry structures. Settlement, loss of mortar and, most importantly, damage by embedded metals have nearly destroyed portions of what the rifled cannon never had the opportunity to try to take out. Many of us see settlements in the neighborhood of an inch or two, and occasionally several inches, on existing structures. But between the middle of Front 2 and the tip of Bastion 1 at the fort, a distance of approximately 200 feet, the floor of the second level of the fort drops almost 24 inches, giving an average grade of one percent over that distance (Figure 4). The casemates show the movement with fracturing of the vaulting that makes modern structural analysis particularly challenging. Structural engineers are likewise accustomed, on many domestic buildings, to seeing masonry walls in need of repointing. But finding walls where bricks can be removed by hand is, fortunately, less
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Figure 2: The fort is reported to be the largest masonry structure in the western hemisphere.
Figure 5: Mortar loss is severe in many areas.
Figure 6: Corrosion of the embedded iron Totten shutters has severely damaged the masonry surrounding the embrasure openings.
common. Not here at Fort Jefferson, where 160 years of moisture migration through masonry has left mortar so deeply eroded that bricks falling and touching bricks below is far from uncommon (Figure 5 ). Fort Jefferson’s greatest challenge comes from embedded iron (Figure 6). The great military engineer, Joseph Totten, designed iron shutters to close and protect the embrasure openings from cannon fire. Unfortunately, the eight-inch thick, 1500-pound armor blocks were embedded between 18 and 24 inches into the masonry walls of Fort Jefferson. On Fronts 4 and 6, as well as on other portions of the fort, the damage to the scarp walls has been so severe that the iron has had to be removed and the walls rebuilt to a depth of as much as 24 inches (Figure 7). Reconstruction of these vaulted, loadbearing masonry walls was particularly challenging, as was any work at all 70 miles from the nearest building supply store. The logistical challenges of working in a particularly isolated environment were handled by a construction crew under the direction of Ken Uracius of Stone and Lime Imports and owner’s representative Kelly Clark of the National Park Service. The engineering design team, with input from the architecture team led by Susan Turner of Lord Aeck Sargent, was able to focus on the challenges of large-scale brick growth, tying new vaulting into existing, horizontal reconstruction of a structure originally built vertically, and movement and potential collapse of unresisted thrust in the vaulting. In modern construction, it is common to leave regularly spaced expansion joints in brick masonry, roughly every 24 feet in the southeastern Unites States. But the reconstruction of Fronts 4 and 6 required that roughly 400 feet of brick walls (Figure 8, page 32 ) be rebuilt without jointing and be done on a relatively tight schedule, with a non-hurricane working window of only six months a year. Fortunately, experience with similar issues at Fort Washington, Maryland had taught the team that they were able to force early permanent growth into the masonry with extended submersion of the bricks. Careful measurement work on extended brick soaks by Mike Schuller’s team at Atkinson-Noland of Boulder, Colorado had shown that seven to 28 day soaks could force enough growth in the bricks prior to
Figure 7: The damage to several fronts is severe but none as bad as that on Front 3.
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Figure 8: Front 4, now rebuilt, has over 400 feet of scarp without expansion joints.
their installation to avoid pushing the bastions apart. Without the pre-wetting, up to four inches of wall movement was anticipated. Tying new vaulting on the face of the scarp walls back into the existing vaulting behind the scarp was interesting. The Florida State Historic Preservation Officer had asked the team not to use metals to make a tension tie of the new construction back to the existing, although Series 316 stainless steel and bronze both have reasonably good track records of holding up well in a marine environment. By
judiciously cutting in the existing vaults and laying in tie bricks (Figure 9) then carefully laying out the brick coursing around the ties (Figure 10), the team was able to achieve a continuity similar to the original. The original 19th century construction of the fort had, of course, proceeded vertically from the ground upward, building centering (formwork) for the vaulting and removing the centering once the vaults had been built. Reconstruction was instead horizontal (Figure 11), starting in the moat and moving horizontally into the scarp walls, shoring as necessary and depending on arching action overhead wherever possible. Reconstruction did require that the brick coursing
Figure 9: Tie bricks let into the undamaged vaulting tie the new vaulting into the old. The coral concrete is seen above.
Figure 10: Layout of the vaulting incorporated the tie bricks.
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Figure 11: Even though the original construction was built vertically from the ground up, the reconstruction had to be horizontal working from the interior outward to the scarp.
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Figures 12, 13 and 14: Graphic statics calculations and two finite element models and confirmed that, without the scarp, the arch supporting the vaulting was on the edge of stability.
be laid out, course by course and wythe by wythe, in order to match the tie bricks and achieve the final bond pattern. Finally, the greatest concern was the possibility that deep cuts in the scarp wall could destabilize the casemate vaulting behind the scarp. Three different analytical models (two finite element and one based on graphic statics) had shown that the arch supporting the vaulting was close enough to being unstable (Figures 12, 13 and 14 ) that the depth of the reconstruction had to be tightly limited and that certain columns had to be restrained during the disassembly and reconstruction. While only one of the four engineering challenges of the construction was readily apparent before the design started, a careful focus on the short and long-term behavior of the materials and of the historic structural systems,
combined with input from the whole owner, design and construction teams led to a successful reconstruction of two of Fort Jefferson’s failing scarp walls.▪ This article is a condensation of presentations given at The Association for Preservation Technology, Victoria, British Columbia in September, 2011 and at The Masonry Society, San Antonio, Texas in October, 2011. Those presentations focused on different aspects of the same project. Craig Bennett, P.E., of Bennett Preservation Engineering PC, is a structural engineer focusing exclusively on existing, and primarily historic, structures. He can be reached at CBennett@BennettPE.com.
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STRUCTURE magazine
33
May 2013
New Theory on Egypt’s Collapsing Pyramids By Peter James
Figure 1: The Step Pyramid.
T
he author’s first introduction to working in Egypt was a project in Cairo’s historic old quarter following the 1992 earthquake that caused widespread and devastating damage. Cintec International began working on a contract to repair and reinforce a number of badly affected structures, including some 15 notable mosques and maqaads, which were strengthened using the firm’s patented anchoring systems. Following success in the old quarter, the focus moved to the internal reinforcement of the Temple of Hibis in the El-Kharga Oasis, 700 kilometres (434 miles) due south of Cairo. Construction on the Temple began in 672 BC, but unlike most other comparable structures, it had differential settlement problems due to poor soil conditions. Work on these buildings was completed with no damage to the splendor and history of the monuments. Soon afterwards, Cintec undertook its first pyramid restoration projects. These involved strengthening the connecting burial chamber corridors and ceilings of Egypt’s Red and Step Pyramids. The Red Pyramid is the third-largest of Egypt’s pyramids and was the first “true” pyramid built by Pharaoh Sneferu. Sneferu had built two previous pyramids, but these were not of a true triangular shape, and for structural reasons were not chosen by the Pharaoh as his final resting place. While work on the Red Pyramid was confined to strengthening the granite slabs immediately above the burial chamber’s corridor, Cintec’s next project, the Step Pyramid (Figure 1), required more careful planning and execution due to the very dangerous condition of the burial chamber ceiling. A large portion had collapsed during the 1992 earthquake, and what remained – a ragged, hanging, inverted group of large and small stones set in mud – was liable to collapse at any time. Cintec used its unique WaterWall airbags to support the ceiling temporarily without provoking further stone fall, before beginning work on final anchoring processes which are now halfway to completion. These ongoing projects offered insight into the nature of the pyramids’ structural deterioration.
any foundation movement. All of the missing cladding occurred at interfaces or changes of direction at the angles and between the ground and the cladding. A popular theory is that the missing cladding was removed by local opportunist thieves. At the lowest levels that could be the answer, but the same condition occurs at higher levels and in an apparently random manner, with no signs of indentations from temporary scaffolding or of any symmetrical cutting of the blocks to aid removal. It would have been extremely dangerous work. To dismantle a structure, you normally need as much scaffolding as you would to build it, and opportunist thieves would hardly have had sufficient resources. Indeed, if they merely wanted rough stones, they could have found them in the hills adjacent to the center of Cairo without the trouble of removing and transporting them 30 miles out of town. The damage here appears to be caused by a giant whose hand has swept across the face of the pyramid with enormous energy, sucking out the facing and leaving the ragged empty sockets. It is the author’s belief that in the case of the Bent Pyramid – in fact, in the case of all pyramids – the outer casing has been affected by thermal movement. The Bent Pyramid is the only one with any degree of stone casing still attached, making the mechanism of failure apparent. The distress at all of the perimeter edges suggests that the outer casing has expanded from the center outwards, and movement has taken place on all of the extremities.
The Bent Pyramid On one of the author’s visits to the Step Pyramid, he was asked for an opinion on securing the remaining outer cladding of the Bent Pyramid (Figure 2), another construction by Pharaoh Sneferu, located 40 kilometers south of Cairo. This pyramid’s top section sits at a slightly different angle to the main body, giving the structure its “bent” appearance. Before any structural restoration work could be considered, the exact nature of the pyramid’s defects had to be established so that the correct intervention could be carried out. From a visual inspection, the structure showed distress along all of its extremities (Figures 3 and 4). What were the clues? The pyramid did not appear to have STRUCTURE magazine
Figure 2: The Bent Pyramid.
34
May 2013
Figure 3: Major cladding damage at one corner of the Bent Pyramid.
It has been suggested that the original dimensions recorded by Flinders Petrie were inaccurate and that the dimensions taken in 2004 were larger by a small degree. This is to be expected of a structure that is still moving and increasing in size. Furthermore, the convex shape of the pyramid’s outer casing could be caused by the stones arching between fixed points. The transit of the sun across the region will vary over the seasons, heating one side more than another, giving rise to disproportionate movement, particularly at the extremities.
Figure 4: Limestone blocks cantilever out, where cladding below is no longer present, and eventually fail.
Additional Questions
Temperature Variation During the day, the temperature rises to 40⁰C (104⁰F) across the face of the outer casing, then at night cools to 3⁰C (37⁰F) because of the lack of cover and exposure to the prevailing winds. This gives an average daily temperature fluctuation of 37⁰C (67⁰F). The photographs of the Bent Pyramid show how thermal expansion has caused the blocks to move to the edges, where they have detached. It also shows how individual stones, unsupported, can cantilever and snap off and subsequently fall to the ground. Limestone has a coefficient of thermal expansion of 8x10-⁶, proportional to the change of temperature and to the original dimensions. Applying this yields (8x10-⁶) x (37⁰C) x (100m) = 30mm (1¼ inches) of movement per 100-meter (328-foot) run in all directions, although this is also dependent on the size of the gaps between adjacent stones. All movement from the thermal expansion of the casing would be taken up initially in the joints, but would also cause dust and stone particles to detach from the stones, filling the voids and gaps between them. This would reduce the amount of contraction possible at night, along with the stones’ natural propensity not to return to their original dimensions and position, and so the cycle would start again. Multiply this endless movement by the number of days that the pyramid has been erected and you have the reason why all the outer casing has moved to the extremities, where it has buckled or displaced against blocks moving in the opposite direction and then fallen off. It may then have been picked up by opportunists and removed from the site. STRUCTURE magazine
Another important question to consider is this: Why does the Bent Pyramid still have half of its outer casing attached, while the Red Pyramid and the Great Pyramids at Giza have virtually none? I believe that this is due to the increased skills of the craftsmen, who developed more knowledge and precision as the process of pyramid construction developed. They became able to provide better accuracy, build quality, and jointing of the slabs. The Bent Pyramid was probably built with less care, and with more voids between the stones that acted like expansion joints. The casing blocks being inclined inwards at the base of the pyramid may have limited the expansion. Finally, could the sight of the progressive damage to the outer edges of the pyramids, that would have taken place relatively soon after their construction, be the reason that – having spent so much time and energy constructing these wonderful monuments – the Egyptians changed their burial method to the Valley of the Kings? While the author is keen to stress that this is his opinion, rather than evidential fact, he suggests that thermal movement led to the crumbling of these magnificent structures, and eventually to their discontinued use.▪
35
Peter James (peterjames@cintec.co.uk), is the Managing Director at Cintec International in Newport, South Wales, United Kingdom. He has worked on projects around the globe, strengthening and restoring historically significant structures from Windsor Castle to the parliament buildings in Canada. May 2013
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Just the FAQs
Question For the critical examination or condition assessment of masonry façades, what is a good reference for practicing engineers to use?
Answer A good, comprehensive reference document that specifically addresses masonry façades is the recently published Guide for Condition Assessment of Masonry Façades (Guide) from The Masonry Society. It is different from the current stock of available documents that address a variety of systems that may be used in a building envelope. This Guide provides the methodology for assessing the condition of most any masonry façade, including terra cotta, and of any masonry detailing, including cornices, parapets, appurtenances, chimneys, and penthouse walls. It is intended to be a “living document” that will be amended periodically as the process is updated, techniques are changed, and the Appendix is expanded. For the practicing structural engineer, the Guide can be used for various purposes such as satisfying a local façade ordinance, preparing a due diligence report, formulating a maintenance program, or investigating masonry defects. Other audiences of the Guide may be building owners, agents representing the building owner, and building officials needing direction for, or an understanding of, masonry façade condition assessments. Early in the document, the Guide discusses factors that affect masonry performance, such as: gravity support, wind anchorage, seismic detailing, durability, and aesthetics. Each factor lists possible failure characteristics and the related defects that may be seen in a masonry façade. These portions of the Guide were written by practicing engineers experienced in the design and investigation of masonry. The main body of the Guide delineates the process for examining masonry façades. Starting with preparation before the site visit, the outline follows through the entire field investigation, testing, analysis, possible additional assessments beyond masonry façades, and the final reporting. The field investigation section is itemized to identify various masonry façade materials, components, systems, fenestrations, and hazardous materials possibly encountered. Visual observations are further discussed regarding documentation and defect types. The testing portion contains a comprehensive list of the current laboratory and field methods available, and it refers to the Appendix for more specific details on in-place test methods. Analysis of the assessment is discussed, including determining the causes of various masonry defects
questions we made up about ... Masonry
“Guide for Condition Assessment of Masonry Façades”, TMS-1700-12, from The Masonry Society, www.masonrysociety.org.
Masonry Façade Condition Assessment Tool
based upon the results of the field and laboratory work. Additional assessments that can occur are listed including those of other building envelope systems that are directly adjacent to, or impact on, the masonry façade. Lastly, where the final reporting is not dictated by local façade ordinances, a suggested written format is provided. The façade examination process is also graphically shown in a flowchart that contains checkpoints for “life safety issues” that may be revealed during the assessment. At any checkpoint, there are the “Yes” or “No” answers with arrows moving toward the next step. If there are any “life safety issues”, then the next step is to notify the building owner and recommend remedial action. After addressing the “life safety issues”, then the condition assessment process continues. Unique to the Guide is the Appendix that currently contains the special topics of: Distress Common to Terra Cotta Façades and Assessment Techniques of In-Place Masonry. Items covered in Distress Common to Terra Cotta Façades are a variety of defects that are typically seen only by close or intrusive examination. In Assessment Techniques of In-Place Masonry, both non-destructive and partially invasive methods are addressed. Currently, the Guide is available in printed form through The Masonry Society. In the future, it may also be available for on-line downloading. Watch for revisions to the Guide for Condition Assessment of Masonry Façades in future years.▪
STRUCTURE magazine
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By Pamela Jergenson, CCS, CCCA
Pam Jergenson, CCS, CCCA is the Secretary of the Existing Masonry Committee of The Masonry Society. As a part of that committee, she is the current Task Chair that has worked on the Guide for Condition Assessment of Masonry Façades since its inception. She can be reached at pjergenson@inspec.com.
Building Blocks updates and information on structural materials
D
urability, corrosion resistance and resistance to chemical attack is directly related to concrete’s permeability. If water cannot enter concrete, it cannot cause damage. Penetration of water into concrete is achieved by capillary action through actual physical passageways. Reducing these passageways will increase the resistance of concrete to water penetration. Certain concretes are watertight and water resistant, as well as strong and enduring in proportion to their absolute densities. Conversely, weak concretes are permeable and of low endurance in proportion to their porosities.
By Garen B. Gregorian, P.E.
Porous: Possessing or full of pores, permeable to liquids. Porosity: The ratio of the volume of interstices of a material to the volume of its mass. Volume of Permeable Voids: Volume of space where water or air can permeate. (From Webster’s 7th New Collegiate Dictionary, B4C Merriam Company, 1972.)
cement ratio of 0.4 or less, and then to consider using a supplementary cementing material or blended hydraulic cement”. (Nawy, 1977.)
Excess Water as a Cause of Porosity Aside from segregated pockets of stone which may also be caused by excess water, water voids are quantitatively more important than are air pockets as passageways for penetrating or percolating water. The larger the water to cement ratio, the greater the volume of capillary pores. Table 1 illustrates the relationship between water cement ratio, porosity, and permeability in concrete. “The first step in achieving a low permeability is to specify a water
Shrinkage Forces on Concrete and Stone
Definitions
Shrinkage Forces of Rocks Flow through porous rock, such as cemented sand or unfractured sandstone, is regular and reliable. Permeability of the rock mass is a reasonably well defined quantity which can be used in analyses. Porous rocks often have a large pore volume of about 10 to 30 percent or more. With reference to the Army Corps of Engineers Manual on Rock Foundations, Special Topics – in the “Free Swell Test,” a specimen of rock is ground and put into a tube where water is added and increase in volume is recorded. This
Table 1.
Garen B. Gregorian, P.E., MSCE, MSME, is a Consulting Structural Engineer in the offices of Gregorian Engineers (originally Zareh B. Gregorian & Associates, Inc.). Mr. Gregorian has collaborated in the design of numerous Commercial, Institutional, Healthcare, Recreational, Residential and Light industrial projects. Garen can be reached at garen@gregorianengineers.com.
Mix
W/C
Cure time
Permeability Hydraulic
Permeability Air
Porosity % Volume Permeable Voids %
1
0.26
1 day 7 days
Too small to measure
37 29
8.3 7.5
6.3 6.2
2
0.4
1 day 7 days
0.03 0.027
130 120
11.3 11.3
11.4 12.2
3
0.5
1 day 7 days
0.56 0.2
120 170
12.4 12.5
13 12.7
4
0.75
1 day 7 days
4.1 0.86
270 150
13 13
14.2 13.3
Table 2: From Masonry Design and Detailing for Architects, Engineers and Contractors (Beal, 1993).
Maximum water absorption requirements for building brick: Min. Water Absorption by 5hr boiling (%)
Max. Saturation Coeff.
Average of 5 brick
Individual
Average of 5 brick
Individual
Average of 5 brick
Individual
Severe Weathering
17
20
0.78
0.80
3000
2500
Moderate Weathering
22
25
0.88
0.9
2500
2200
No Limit
No Limit
No Limit
No Limit
1500
1250
Designation
No Weathering
Min. Compressive Strength
Structural clay load bearing tile wall Maximum water absorption by 1 hr boiling % average 16 to 28% Take 20%
38 May 2013
Table 3: From Rock Foundations Special Topics (Army Corps of Engineers EM 1110-1-2908).
Table 3 provides data on the Strength of Rocks.
Strength of Rocks Type of Rock
Tensile Strength (psi)
Tensile Stress (psi)
Compressive Strength (psi)
Youngs Modulus (psi)
Shale (Utah)
2494
831
31328
8441196
Shale (Pennsylvania)
203
67
14648
4525117
Granite (Georgia)
406.1
135
27992
5656472
Granite (Maryland)
3002.2
1000
36404
3683959
Granite (Colorado)
1726
575
32778
10239660
Marble (New York)
1696
565
18419
7832038
Marble (Tennessee)
942
314
15374
7005323
Sandstone (Alaska)
754
251
5656
1522896
Sandstone (Utah)
1595
531
15519
3103808
Slate (Michigan)
3698
1232
26106
11008360
Gypsum (Canada)
348
116
46267
—
Limestone (Indiana)
594
198
7687
7687000
Limestone (Germany)
580
193
9282
9253408
bulking is the increase in total volume of moist fine aggregate over the same weight dry, where surface tension in the moisture holds the particles apart causing an increase in volume. If alternate wetting and drying occurs, severe strain develops in some rocks causing an increase in volume and eventual
breakdown of the material. Porous concrete will crack similarly. Table 2 shows the relation between moisture added to dry aggregates versus percent increase in volume. The addition of 5% moisture by weight results in a 25% increase in volume, roughly.
Shrinkage Forces on Fresh Concrete As an example, the following concrete mixture is proportioned by absolute volume where total aggregate may range from 60 to 75%, cement from 7 to 15%, and water approximately 20%. Cement: 10% Air: 5% Fine Aggregate: 25% Coarse Aggregate: 45% Water: 15% Assuming 3000 psi concrete, the tensile strength will equal 6 √f 'c = 6 x54.6 = 328.6 psi. Although not allowed for major supporting structures, a factor of safety of 3 can be taken to obtain allowable stress in tension. Taking a factor of safety of 1/3, the allowable tensile stress (P) becomes approximately 100 psi (at design strength) = 14,400 lb/ft². If we take the allowable stress of freshly placed concrete to equal zero, then from the initial placement to final cure the average tensile stress is roughly 50 psi. continued on next page
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STRUCTURE magazine
COLORS
JOB#
39
FILE NAME
May 2013 OK as is
As the volume of the paste decreases by 20% (which is roughly twice the porosity), then this force on the area will be equal to 50 x 0.2 = 10 psi or 1,440 lb/ft2. To determine the shrinkage force per foot width of concrete, take a 1-foot by 6-inch slab section and determine the length at which it will crack.
For dry concrete, the porosity is 12%; hence, the force which will crack concrete exposed to dry and wet conditions will be about 12% of the allowable tensile stress. Other examples: • For rocks, a 25% increase in dry density can more than double the swell pressures developed in the material. The density of the stone is directly related to its tensile stress. Considering an allowable tensile stress of 199 psi for limestone, and taking a 25% increase in the dry density of a facade sample during swelling, then drying – shrinkage forces will approximately equal 25% of the tensile stress of 199 psi or 49.75 psi. • For unreinforced solid brick masonry with an allowable tensile stress of 7.5 psi, and porosity of 20% from Figure 1, the resultant in a drying shrinkage force will be 1.5 psi. • For Gypsum, taking a tensile stress of 116 psi will result in a swell pressure of 29 psi. The porosity of gypsum board must be considered in the calculation, as the increase in dry density of the material will be the deciding factor in the pressure. Hence, for a ½-inch thick gypsum board: ½ x (4 ft x12) x P = 116 and P = 5 psi (will crack if its porosity equals 5%.)
Percent increase in volume over dry rodded fine aggregate
A x P = L x A x D or L = P/D 1 ft x 0.5 feet x 14,400 = L x 1ft x 0.5 ft x 1,440 lbs/ft2/ft L = 14,400/1,440 = 10 feet.
40
Fine grading
Medium grading
30
20
Coarse grading
10
0
0
5
10
15
20
Percent of moisture added by weight to dry rodded fine aggregate Figure 1: Surface moisture on fine aggregate can cause considerable bulking, the amount of which varies with the amount of moisture and the aggregate grading. Reference PCA Major Series 172 and PCA ST20
The online version of this article contains detailed references. Please visit www.STRUCTUREmag.org.
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May 2013
InSIghtS
new trends, new techniques and current industry issues
Filling the Void By Greg Schindler, S.E.
A
s structural engineers, we typically don’t pay much attention to insulation issues. But modern energy codes are getting much stricter regarding the thermal integrity of the exterior building enclosure. You are likely seeing architects adding insulation in places that they didn’t worry about before, such as between slabs and foundation walls, and on the outside of the exterior sheathing on steel stud walls. These insulation planes can cause complications for structural engineers by placing a soft, non-structural element in the middle of our load path. The exterior insulation layer is one of those situations, especially where you need to support a heavy façade material such as brick or concrete. This insulation requirement is born out of the fact that steel studs are very thermally conductive and provide an easy path for heat to escape through the wall. For all the concern about insulation, there is a common place where the lack of insulation gets overlooked – hollow boxed steel stud framing elements. Architects often don’t realize that boxed framing elements often do not get insulated. Boxed elements such as bundled studs, box headers and outside wall corners typically are inaccessible to the insulation contractor after they are built, and thus cannot easily be insulated. Even so, you have probably seen architectural details that show the batt insulation pattern continuing right through these boxed framing members. To insulate hollow framing elements, either the framing contractor must insert batt insulation, such as rock wool, as they assemble the framing, or foam insulation must be injected later by
Brady ProX Header
the insulator. Both of these methods are time consuming and costly, and don’t actually get done unless the specifications are very clearly written to force the contractor L Header Trade Ready U Header to provide insulation Figure 1: Clark Dietrich headers. in hollow portions of the framing. Also, batt insulation material installed in the voids require a typical boxed header, EnviroBeam™ can get wet during construction and, once provides a pre-insulated version of box headers, enclosed with sheathing, can take a long time the E-Beam™ (Figure 3). to dry out. This has led to corrosion and mold As for jamb members at wall openings it is problems in some projects. preferable, where loads permit, to use enlarged Fortunately, cold formed framing (CFS) specialty stud shapes instead of bundled studs. manufacturers are coming up with new (Figure 4). They not only allow the insertion of products to allow engineers to avoid built-up insulation but they are also much easier and less hollow framing in many cases. These products costly to install than built-up bundled studs. fall into two categories; open shapes that are Such shapes are available from ClarkDietrich – engineered to be stronger than standard CFS HDS® Framing system, Scafco – Kwik-Jamb™ shapes, and pre-insulated framing members. system, or the Steel Network – JambStud® Several companies make open, insulatable and SigmaStud™ systems. EnviroBeam manuheaders. ClarkDietrich™ provides L-headers factures pre-insulated rectangular jamb stud and U-shaped Tradeready® headers (Figure shapes, the E-King™ system, also shown in 1). They also, along with Cemco, are manu- Figure 3. Since the insulation is already profacturers of the Brady ProX header system. vided, the E-King can be bundled to comprise Scafco® provides the Priceless Header™. These even stronger members. are both custom break shapes that are open Outside wall corners are also problematic if to accommodate insulation and also function one uses the traditional three-stud configuraas the track to attach the studs (Figure 2). tion of Figure 5a. After the sheathing is on, EnviroBeam has a preinsulated unit, the the corner is inaccessible for insulation. As is E-Header Sill™ that can be used for headers and often done in wood framing, a preferable consills, and consists of a pre-insulated hollow box figuration is shown in Figure 5b, sometimes shape that incorporates flanges to eliminate the called the “California corner”. A pre-insulated need for an added track. For situations that still
Scafco Priceless Header
Figure 2: Open headers.
E-Beam Header
Figure 3: Preinsulated headers.
STRUCTURE magazine
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May 2013
E-Header/Sill
Clark Dietrich HDS® Framing System
Scafco Kwik-Jamb™
The Steel Networks JambStud® SigmaStud®
Figure 4: Open jamb shapes.
A. Typical Corner
B. Calif. Corner
C. E-Corner
Figure 5: Corner options.
corner member is available from EnviroBeam, called the E-Corner™ (Figure 5c). While insulation isn’t part of our normal scope of work as structural engineers, we should be proactive wherever we can to provide the owner with a structure that does not compromise the energy efficiency that is ever more important these days. If hollow, un-insulated voids are built into the wall, the effective thermal opening sizes are significantly larger than typically used for heat loss calculations. On your CFS projects, consider using some of these or similar products. Avoid the use of box headers unless they are really necessary. In many cases they are not.
At the very least, discuss the issue of missing insulation in hollow boxed elements with the architect. Make them aware of the issue and suggest that they include language in the specifications that clearly calls for insulating hollow voids in CFS framing.▪ Greg Schindler, S.E., is an Associate in the Seattle office of KPFF Consulting Engineers. He is a past President of NCSEA and SEAW, and is a member of the STRUCTURE Editorial Board. Greg can be contacted at gregs@kpff.com.
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STRUCTURE magazine
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May 2013
More information can be found at these companies’ websites: Clarkdietrich.com Cemcosteel.com Scafco.com Steelnetwork.com Envirobeam.com These products are presented here as options to consider; however, the author and STRUCTURE magazine do not endorse or promote them.
Great achievements
notable structural engineers
William J. Mouton Tube Structure Pioneer and Foundation Innovator By Richard G. Weingardt, P.E., S.E., Dist.M.ASCE, F.ACEC
I
n addition to his many breakthrough structural innovations, noted New Orleans structural engineer and Tulane University professor of architecture, William J. (“Bill”) Mouton, Jr. (Figure 1) also stood out for an incredible list of other accomplishments. He held more than 20 patents, including erosion control concepts for Louisiana’s wetlands, regeneration systems for rebuilding sand beaches, a unique monotrack system for high-speed mass transit, and a counter-rotating combustion engine. Mouton designed an amphibian aircraft, a single engine short takeoff and landing aircraft with “swing wings,” and his patented “Coriolis” ocean turbine unit was featured on the cover of Popular Science (September 1980). That federally funded research project extracted energy from ocean currents deep below the surface. Mouton was internationally recognized as a pioneer in the design and erection of innovative long-span steel space-frame structures and tubular (“monocoque”)
Figure 2: Plaza Tower, New Orleans. Courtesy of Mary Mouton.
systems for high-rises, as well as modular prestressed concrete buildings. He was involved with more than 500 noteworthy projects, ranging from the American Sugar Dome in Boston, Massachusetts, the world’s largest parabolic dome at the time of its construction, to the Plaza Towers (Figure 2), for years the tallest building in New Orleans at 531 feet. Both projects, along with Studio Arms, a glass-roof space-frame covering more than an acre, were featured in a Museum of Modern Art exhibit on 20 th Century Engineering in New York City in the 1960s. On a visit shortly after it opened, Robert Disque, chief engineer with the American Institute of Steel Construction (AISC), recalled, “While I was visiting the Modern Museum, I was amazed to see an exhibit featuring a structural engineer. He was from New Orleans and someone I had never heard of before. There was a photo of a high-rise building, which the museum touted as unusual and beautiful. I saw immediately what the structure was. The frame was like a milk carton. Nothing liked it had ever been done. Instead of beams, girders and columns, it was designed as a tube.” Because he had never heard of such a thing at the time, Disque made an appointment to visit Mouton in New Orleans. When he got there, he said, “I had trouble finding him. His office was in the boondocks. His system came to be referred to as a monocoque or tubular structural system. I visited with him and put him on the next AISC program, which may have been 1965 or 1966.” At the AISC event, Mouton gave a comprehensive and in-depth presentation delineating the pros and cons of tube design for high-rise steel structures. In addition to using it for the Plaza Tower Building, Mouton also employed a unique deep pile foundation system that he had developed for supporting multi-story structures. (The pile testing for Plaza Towers is shown in Figure 3.) Prior to the Plaza project, constructed in the early 1960s, buildings in New Orleans were limited to fewer than 31 stories. Mouton’s deep pile system changed all that. Ron Flucker was with American Bridge, which was part of US Steel at the time, and was erecting Plaza
STRUCTURE magazine
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May 2013
Figure 1: William J. Mouton, Jr. Courtesy of Mary Mouton.
Tower. He got to know Mouton very well because of all the structural innovations that Mouton had incorporated into the building. Said Flucker, “He was the brightest structural engineer I ever met.” Mouton liked to tell his students and followers, “Always go beyond the book.” On a regular basis he told them, “The structural engineer should approach his problems as a creative visualist, not as a mere stress analyst. He should first think in three-dimensional terms, of transferring loads through space. Students of structure should first learn to visualize the action of structures, and then learn the mathematical intricacies of analysis. An overemphasis on analysis stunts the growth of the structural engineer’s imagination. Focusing his early professional gaze on the microscopic view of a twig (e.g., the analysis of an eccentrically loaded rivet connection), when he should first survey the whole tree (the basic structural system), should be avoided.” The Plaza’s steel superstructure was the first – or at least, one of the first – designed as a tube frame to resist lateral loads. The earliest known example of the use of a tube system for a skyscraper was the concrete-framed, 43-story DeWitt-Chestnut Apartment Building in Chicago designed by the legendary Fazlur Khan. However, Khan’s first structural steel tube skyscraper, the John Hancock Center in Chicago, was designed in 1965 and completed in 1969, a few years after Mouton’s Plaza Tower. In 1965, Mouton proposed a glass-paneled dome over Shea Stadium in New York. Its
silhouette looked like a flying saucer. The New Orleans Times-Picayune (June 6, 1965) reported, “Houston’s Astrodome could be fitted inside it.” The Shea project, alas, was never built. However, a few of Mouton’s other glass-roofed enclosures were, such as the Studio Arms structure, the patio roof of the Camillo Restaurant, and a large apartment-house courtyard. These came well before Buckminster Fuller’s famous 250-foot geodesic dome, featuring transparent acrylic panels and steel latticework, which served as an architectural centerpiece at the 1967 International and Universal Exposition in Montreal, Canada. Mouton’s largest completed dome, the Cajundome (Figures 4 and 5 ) in Lafayette, Louisiana, opened in 1984, had a clear span of 384 feet and was included, with many of his other works, as part of the Engineers of the Century exhibit at the Georges Pompidou Center in Paris in 1997. The exhibit also featured Mouton himself. In addition to innovative new construction, Mouton worked on structural renovations of numerous historic buildings, including the Federal Fibre Mills, Woodward Wright Apartments, and Henderson apartments, all in the New Orleans area. Bill’s younger brother Paul became an architect, and the
Figure 3: Pile testing for the Plaza Tower was pushed all the way to 462 tons without failure, well in excess of the required 180-ton design load. Courtesy of Mary Mouton. ADVERTISEMENT–For Advertiser Information, visit www.STRUCTUREmag.org
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two shared office space, often collaborating on projects. Individually and between them, they successfully completed a record number of low-cost and aesthetically appealing structures, including Mouton’s own home, a timber engineering masterpiece whose floors were raised above the New Orleans flood plain. It featured thick decking for its floors and roof, and 4-inch x 6-inch columns to allow for maximun clear spans. Bill was born in Lafayette, Louisiana in 1931, the oldest of four children. While he was growing up, he worked for his father, William Sr., who was a partner in the construction company, J.B. Mouton & Sons, founded in 1915 by his father (Bill’s grandfather). Bill’s clan of Moutons settled in southern Louisiana in the 18th century, after the British expelled Arcadia French colonists from Newfoundland. J.B. began the Mouton construction company with teams of mules digging excavations for houses, and then moved into increasingly larger commercial projects. The company still exists. As a boy, Bill was fascinated with scale model airplanes – designing, building and flying them. He even enlisted the help of his two younger sisters, Julie and Christine, to help him cut the balsa wood to make the models, some with 6-foot wingspans. Reported his oldest son, Dave, “Dad would work at odd jobs and, when he earned sufficient funds, he would rush down to the hobby store to purchase the latest balsa kit model. He would then lock the door to his bedroom and not come out until the plane was finished. This greatly concerned my grandmother as he wouldn’t eat, drink or go to the bathroom for hours on end.”
Figure 4: Cajundome, Lafayette. Courtesy of Wikimedia/Jcarriere.
Bill attended Tulane University on a ROTC scholarship and ended up assigned to Bermuda where he flew patrol seaplanes for the U.S. Navy in the 1950s. He carried his love of flying into civilian life when he became an enthusiast of soaring (or gliding) in non-motorized airplanes. He was one of the first active glider pilots in Louisiana and was the author of a number of seminal articles on the subject. Mouton earned a bachelor’s degree from Tulane in 1953 and a master’s degree in 1958, both in civil engineering. Shortly after marrying Elizabeth (“Libby”), who grew up around the corner from Bill, Mouton was offered a job with Martin Aircraft in Baltimore, Maryland, which would have required them to move there. According to their daughter, Mary, “He turned it down, saying, ‘What’s a Cajun going to do in Baltimore?’” Bill and Mary had five children: David, a petroleum engineer; Stephanie, a physician; Martin, an engineering draftsman; Mary, a public relations consultant; and Robert, a real estate attorney Said Mary, “After building the Plaza Tower, my father treated himself to a Porsche 911,
Richard G. Weingardt, P.E., S.E. (rweingardt@weingardt.com), is the Chairman of Richard Weingardt Consultants, Inc. in Denver, Colorado and the author of ten books. Two of his latest, Circles in the Sky: The Life and Times of George Ferris and Engineering Legends, both published by ASCE Press, feature the exploits of great American structural engineers who had significant influence on the progress of the nation. His 2012 book, Empire Man, is about Homer Balcom, structural engineer for the Empire State Building.
Figure 5: Interior of Cajundome. Courtesy of Mary Mouton.
STRUCTURE magazine
one of the very first in New Orleans. They were much, much cheaper then. In the early days, he always had little kids in tow, riding around in that Porsche with music blaring in his 8-track stereo. He came from a musical family, where mostly everyone played the piano. His favorite song that he played was ‘Night and Day’ by Cole Porter. That’s the only song I ever remember him playing. He also entertained us as little kids by playing the ukulele.” “On weekends, our father would take us to his office on Saturday mornings and then it was off to various airports where he flew his glider ‘NSOAR’ (actual tail number was N-8OAR). My father was one of the first glider pilots in the area and all of his recreation, and consequently much of our family’s, centered around gliding. We took vacations to California where the kids went to Disneyland and my father, ever the engineer, went to a glider port. NSOAR was a ‘self-launching’ sailplane. It would propel itself into the air, without the need of a tow from an airplane, which is more conventional.” At the time of his passing on June 30, 2001, at age 70, a victim of cancer, Mouton was no longer a full-time professor at Tulane. He was employed as chief engineer for Coastal Engineering and Environmental Consultants and was involved with the design of several B-2 stealth bomber hangars for the U.S. Air Force. Mouton was survived by his wife and their five children and their families.▪
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Steel/Cold-Formed Steel ProduCtS Guide a definitive listing of steel/cold-formed steel product manufacturers/distributors and their product lines Suppliers Cast Connex Corporation Phone: 416-806-3521 Email: info@castconnex.com Web: www.castconnex.com Product: Cast ConneX Universal Pin Connectors Description: Sleek, clevis-type fittings designed to connect seamlessly to round hollow structural section elements for use in architecturally exposed structural applications. The connectors have been carefully sculpted to provide smooth transitional geometry that is otherwise unachievable using standard fabrication practices.
CMC Steel Products Phone: 972-772-0769 Email: marketing@cmc.com Web: www.cmcsteelproducts.com Product: SMARTBEAM®, Castellated and Cellular Beams Description: CMC Steel Products manufactures the cellular and castellated SMARTBEAM – an innovative, economical and sustainable alternative for floor and roof framing systems. Manufactured from recycled materials, the beams are lightweight, have superior deflection properties, and can integrate MEP systems through the web openings. SMARTBEAM – The Intelligent Alternative.
Evolution1
steel stud heights. Custom heights are also available. Our Special Moment Frame is a structural steel product that uses SidePlate® moment connections.
type of structural member as a back mullion, SteelBuilt Curtainwall Systems can satisfy a nearly limitless range of design and performance requirements.
New Millennium Building Systems
USP Structural Connectors
Phone: 260-868-6000 Email: kevin.disinger@newmill.com Web: www.newmill.com Product: Steel joist, metal decking and castellated beams Description: New Millennium, a leader in BIM-based steel joist design, engineers and manufactures standard steel joist, architecturally unique steel joists, steel decking and FreeSpan castellated and cellular beams for wide-open bay designs. Has also introduced the Flex-Joist Gravity Overload Safety System for early warning of roof overloads.
Phone: 800-328-5934 Email: info@uspconnectors.com Web: www.uspconnectors.com Product: Cold Formed Steel Holdowns Description: USP Structural Connectors offers the LTS20B and HTT14S holdowns, designed for both new construction and retrofit applications for concrete to steel connections. For more information visit our website.
S-FRAME Software, Inc. Phone: 203-421-4800 Email: info@s-frame.com Web: www.s-frame.com Product: S-PAD Description: An entry-level, easy-to-use, stand-alone steel-member design and optimization application for small consulting engineering firms. Easy to use spreadsheet-type interface with advanced codechecking capabilities and auto-design to multiple design codes (AISC, CSA, EC, BS) for both strength and serviceability of columns, beams, and braces. Fully supports all of S-STEEL DESIGN checks.
Simpson Strong-Tie
Phone: 206-455-1978 Email: duane@envirobeam.com Web: www.envirobeam.com Product: Envirobeam Description: Green Pre-insulated CFS building components for exterior walls. Pre-insulated envirobeam perimeter roof curbs.
GT STRUDL Phone: 404-894-2260 Email: case@ce.gatech.edu Web: www.gtstrudl.gatech.edu Product: GTSTRUDL Description: GTSTRUDL for the analysis & design of civil works facilities, industrial, nuclear, offshore, transportation, and utilities. Key analysis features include linear/nonlinear static, dynamic, pushover, and buckling analysis. US and International Steel Design Codes plus ACI and British Concrete Design Codes. Optional modules for Base Plate Analysis and Multi-Processor Solvers are available.
Phone: 925-560-9000 Email: web@strongtie.com Web: www.strongtie.com Product: Simpson Strong-Tie SUBH Bridging Connector Description: The latest innovation from Simpson StrongTie for CFS framing. SUBH can reduce labor costs and increase installation productivity, with installation by a single installer and many applications requiring one screw. SUBH has been tested to include stud-web strength and stiffness in tabulated design values. Product: Simpson Strong-Tie DBC Drywall Bridging Connector Description: The DBC drywall bridging connector includes patent-pending design enabling one- or two-screw installation, significantly reducing labor and material costs. DBC is the first and only connector loadrated for smaller ¾-inch u-channel bridging. Extensively lab-tested as a system, ensuring that tabulated design values reflect stud web depth and thickness.
Technical Glass Products
Hardy Frames, Inc. Phone: 800-754-3030 Email: dlopp@mii.com Web: www.hardyframe.com Product: Hardy Frame® Panels, Brace Frames and Special Moment Frames Description: HFX-Series Panels and Brace Frames are fabricated with galvanized Cold Formed Steel to standard wood stud heights; HFX/S-Series to standard
Phone: 800-426-0279 Email: sales@fireglass.com Web: www.tgpamerica.com Product: SteelBuilt Curtainwall® Systems Description: Stronger and slimmer than traditional aluminum frames, SteelBuilt Curtainwall Systems enable openings with larger areas of glass, smaller frame profiles and greater free spans. Usable with almost any
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May 2013
Valmont Industries Phone: 402-359-2201 Email: kyle.debuse@valmont.com Web: www.hsssuperstruct.com Product: HSS SuperStruct Description: Custom HSS ranging from 12 – 60 inches in squares and rectangles.
Vulcraft/Verco Group Phone: 402-844-2570 Email: mike.klug@nucor.com Web: www.vulcraft.com Product: Steel Joists and Joist Girders Description: Open web-steel joists and joist girders are an engineered, truss-like construction component used to support loads over short and long spans. Steel joists and joist girders provide an economical system for supporting floors and roofs. Vulcraft joists and joist girders are designed/manufactured in accordance to the Steel Joist Institute. Product: Steel Decking Description: Used in many applications, but is particularly well suited to roofing and flooring. Vulcraft manufactures many different types of deck, including roof deck, floor deck, composite floor deck and cellular deck. A full line of deck accessories, such as end closure and pour stop, is also available.
Software AceCad Software Phone: 610-280-9840 Email: sales@acecadsoftware.us Web: www.acecadsoftware.com Product: StruM.I.S Description: Features and connectivity benefits designed specifically for larger steelwork fabrication companies. StruM.I.S Enterprise comes with additional training, implementation and allocated development specification time, for extended integration to ensure workflow with your current systems for maximum organizational success. StruM.I.S Enterprise is the complete software system for businesses.
Applied Science International, Inc.
Design Data
Nemetschek Scia
Phone: 919-645-4090 Email: sscoba@appliedscienceint.com Web: www.appliedscienceint.com Product: SteelSmart® System v7.0 (SSS) Description: Easily perform progressive collapse, blast, and seismic analysis for cold-formed steel structures with ELS’s easy to use cold-formed steel sections. Perform static or dynamic analysis with full nonlinear material and geometric behavior including buckling, post-buckling, P-Delta, contact, collision, and debris all automatically calculated by the solver.
Phone: 402-441-4000 Email: marnett@sds2.com Web: www.sds2connect.com Product: SDS/2 Connect Description: Enables structural engineers using Revit Structure for BIM to intelligently design steel connections and produce detailed documentation on those connections. The only product that enables structural engineers to design and communicate connections based on their Revit Structure design model as part of the fabrication process.
Phone: 877-808-7242 Email: info@scia-online.com Web: www.nemetschek-scia.com Product: Scia Engineer Description: Scia Engineer makes it easy to plug Cold Form Steel design into BIM. Go beyond simply member checks and produce full 3D structural models by integrated Cold Form Steel design with Structural Steel and Concrete. Design multi-story wall panels, trusses and even entire Cold Form Steel buildings in ONE program.
Devco Software, Inc.
POSTEN Engineering Systems
Phone: 541-426-5713 Email: rob@devcosoftware.com Web: www.devcosoftware.com Product: LGBEAMER V8 Pro Description: Analyze and design cold-formed cee, channel and zee sections. Uniform, concentrated, partial span and axial loads. Single and multi-member designs. 2007 NASPEC (2009 IBC) compliant. ProTools include shearwalls, framed openings, X-braces, joists and rafters.
Phone: 510-275-4750 Email: sales@postensoft.com Web: www.postensoft.com Product: TaperSTEEL Description: TaperSTEEL quickly and easily allows you to design (simple span, cantilevered or portal framed) I shaped steel plate girders and beams with varying web depth & varying flange width. The top and bottom of the beam or girder can slope to a ridge located anywhere across the span.
Product: Extreme Loading® for Structures v3.1 (ELS) Description: Easily perform progressive collapse, blast, and seismic analysis for cold-formed steel structures with ELS’s easy to use cold-formed steel sections. Perform static or dynamic analysis with full nonlinear material and geometric behavior including buckling, post-buckling, P-Delta, contact, collision, and debris all automatically calculated by the solver.
CSC, Inc. Phone: 877-710-2053 Email: sales@cscworld.com Web: www.cscworld.com Product: Tedds Description: A powerful software that will speed up your daily structural and civil calculations. Using Tedds you can access a large library of automated calculations or write your own, while creating high quality and transparent documentation. Product: Fastrak Description: An essential design and drafting software for steel buildings. Structural engineers use Fastrak to design simple or complex steel buildings. Produce clear and concise documentation including drawings and calculations, all to US codes.
Digital Canal Phone: 800-449-5033 Email: tblum@digitalcanal.com Web: www.digitalcanal.com Product: Digital Canal’s NEW Steel Design Description: Incredibly detailed “hand calculation” reports are a “must have” to learn the new AISC code! Steel Design is available in multiple span beams and column modules as well as full-featured frame/FEA programs. Try it free our website. (AISC 13th Edition, ASD 9th and LRFD 2nd Editions)
Phone: 800-376-6000 Email: sales@independencetube.com Web: www.independencetube.com Product: HSS A500 Description: Independence Tube produces HSS A500 Structural Steel Tubing at three locations. Squares (SQ) from 2 to 12 inches; Rectangles from 2½ by 1½ inches through 16 x 8 inches; Rounds from 1.66-inch OD through 16-inch OD; Walls from .109 inch through .688 inch.
News, Events, Book Reviews, Letters to the Editor and more!
www.STRUCTUREmag.org
Phone: 406-586-8988 Email: info@iesweb.com Web: www.iesweb.com Product: VisualAnalysis Description: Provides general-purpose analysis and design features in a friendly and easy-to-learn way. The tool provides design checks for cold-formed steel framing as well as that for hot-rolled steel, concrete, wood, and aluminum beams, columns, truss members and more.
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All Resource Guides and Updates for the 2013 Editorial Calendar are now available on the website, www.STRUCTUREmag.org. Listings are provided as a courtesy. STRUCTURE® magazine is not responsible for errors.
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Martin/Martin gains the competitive edge with Tedds Patrick McManus, Technical Director, explains how Martin/Martin saved time, improved consistency and enhanced quality control by standardizing on structural calculation software, Tedds.
“At Martin/Martin we work on a variety of commercial projects and specialize in arena and stadium work, defensive design and construction services. To meet the requirements of such demanding and differing projects we historically used software packages from multiple vendors. This was difficult to manage as each software package had its own interface and approached engineering problems differently. No single engineer knew every product in-depth, which created problems with quality control, consistency, and it impacted project scheduling. What we really needed was a single software package that could reliably and accurately do everything we needed.” “Tedds was our ideal solution because it provided an extensive library of calculations and created transparent output with detailed equations. It also reduced the need to perform calculations by hand, which had been very time consuming.
“Tedds has an extensive calculation library and produces transparent output with detailed equations.”
Tedds also offered us the capability to write our own calculations which has been invaluable. It works within the Microsoft Word interface, enabling us to develop custom tools that allow us to efficiently handle complicated problems that have not been well addressed by other software developers. This has given us a competitive advantage and we see great potential to take this further.”
“We have been able to write our own calculations in Tedds.” “Since standardizing on Tedds we have decreased our number of vendors, which has saved time for our information technology teams and our engineers speak to fewer technical support teams.
“Tedds is fast and intuitive and is used by all our engineers.” Tedds is really easy to use so it has become a staple tool for all our engineers, who now use the Tedds library daily for our quick component calculations. We have also standardized our output which immediately improved our consistency and quality control.
“Tedds has helped us to meet aggressive project demands and deliver a high quality service to our clients.” Without Tedds, calculations would have taken considerably longer to develop and verify, with less transparent output. Tedds is flexible, it’s regularly updated and the size of the library means we can quickly respond to the changing needs of our clients.” CSC thanks Martin/Martin for its contribution to this case study.
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award winners and outstanding projects
Spotlight
An Understated Entrance The New Atrium Roof at the California Academy of Sciences Plays a Supporting Role to a Landmark Building By Rafael Sabelli, S.E. and Mark Waggoner, S.E.
Courtesy of David Wakely.
Walter P Moore was an Award Winner for the Atrium Operable Roof – California Academy of Sciences project in the 2012 NCSEA Annual Excellence in Structural Engineering awards program (Category – Other Structures).
W
hen planning and designing the new operable roof at the California Academy of Sciences, the foremost concern was to allow the critically-acclaimed building to hold center stage while the new roof provided weather protection and ventilation quietly and elegantly in the background. Designed by world-renowned architect Renzo Piano, the California Academy of Sciences opened in 2008 to wide acclaim as the largest LEED® Platinum building in the world and an architectural landmark for the City of San Francisco. Built as part of an ongoing revitalization of Golden Gate Park, the 412,000-square-foot Academy is a single structure containing multiple venues including an aquarium, planetarium, natural history museum, and four-story rainforest. The Piazza – a glass-covered central atrium – serves as a multi-function gathering area for exhibit space, dining, and various community events. Furthering its function as a “living building” and taking full advantage of the pleasant northern California climate, the museum commissioned Walter P Moore to design a durable, high-performance operable roof over the Piazza that would allow open-air gatherings and maximize the amount of natural light entering the space. The original fixed Piazza glass roof that frames the opening is supported on a doublelayer stainless steel rod net system spanning the 66-foot by 90-foot opening. Working with such an iconic building required sensitivity as well as precise engineering to avoid altering the visitor’s experience of the space. Ari Harding, the Academy’s Director of Building Systems, described the process: We were focused primarily on functionality and the idea that the new roof should fit in so well with the original architecture that the installation would be transparent to our guest experience. Given the difficult access and location, we had a fairly extensive list of performance criteria which needed to be met for a new roof to be considered. Structural engineer Walter P Moore and mechanization contractor Uni-Systems
conducted a thorough feasibility study to explore options for retrofitting the atrium roof for operability. Several factors influenced the eventual solution and shaped the design of the new Piazza roof, including: • Existing Structure – The new roof had to be installed atop the existing double-layer stainless steel rod net structure without significant alterations to the existing structure and surrounding green roof. • Continued Operation – The installation of the new roof could not cause significant disruption to building operations or functions. • Aesthetics – The new roof had to integrate seamlessly with the existing architecture and visually reinforce the high-tech, progressive atmosphere. • Sustainability – The new roof had to adhere to LEED measures already in place and could not create any situation or use of materials that would contradict the museum’s LEED Platinum certification. • Operation and Maintenance – The new roof had to be designed for easy operation and maintenance while withstanding prevailing seismic and wind conditions. In response, Walter P Moore designed a series of eight stainless steel arches, each spanning 64 feet across the existing glass roof and weighing approximately 2,250 pounds. Thrusts from these arches resolve into an existing perimeter truss designed to withstand tension from the cable-net system, effectively counteracting those forces. Stainless steel cables stabilize the arches and provide the framework for the translucent panels to be opened and closed quickly and quietly. The roof closing/opening time is 2 minutes and 45 seconds. Two lightweight, stainless steel-framed panels, each 18 feet wide by 48 feet long, are carried
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by 24 high-strength, low-friction slide pad assemblies and driven by four belt drives which are fully concealed within the structural arches. The operable panels are clad with ½-inch thick, point-supported structural polycarbonate panels using a special formulation to provide for prolonged UV exposure. The unique use of point supports for the polycarbonate roofing sheets was supported through testing of prototype specimens by Uni-Systems. For ease of maintenance and to combat corrosion, engineered plastics were used for all exposed wearing components of the drive system, eliminating the need for lubrication. Each high-strength reinforced polyurethane belt is driven by a 1-hp electric motor (roof total 8 hp), controlled by a variable-frequency drive. Roof movement is controlled from a single Programmable Logic Controller, integrated within an operator control touch screen that is linked to the Academy’s building management system to provide position feedback and allow for the roof to be operated remotely via iPad. The final design is indeed transparent, integrating seamlessly with the existing fixed-glass roof and the surrounding 2.5-acre “living roof ” as though it was part of the original design. From below, visitors don’t notice the addition. Except when it rains.▪ Rafael Sabelli, S.E., was the structural engineer-of-record and principal-incharge for the design of the operable roof. He is a Principal and the Director of Seismic Design at Walter P Moore’s San Francisco office. Rafael can be reached at rsabelli@walterpmoore.com. Mark Waggoner, S.E., was the project manager of the operable roof. He is a Principal with the Research and Development Group in Walter P Moore’s Austin office. Mark can be reached at mwaggoner@walterpmoore.com.
GINEERS
AISC’s Code of Standard Practice for Steel Buildings & Bridges Mike West, S.E., Computerized Structural Design, S.C. The American Institute of Steel Construction’s 2010 Code of Standard Practice sets forth trade practices in the structural steel design community and the construction industry. The current Code is the result of deliberations and the establishment of consensus among design and industry representatives on the AISC Code Committee. Although the Code applies to both buildings and bridges, this webinar will focus on buildings. Michael West, principal at Computerized Structural Design, S.C. is the co-author of AISC Design Guide 3 Serviceability Design Considerations for Steel Buildings and AISC Design Guide 10 Erection Bracing of Low-Rise Structural Steel Frames. Mr. West is also the coauthor of AISC’s Lectures Intelligent Design and Effective Design. He has lectured extensively for AISC, both at NASCC and in AISC Seminars over the last 25 years, including over twenty presentations on the 2005 Specification and Manual. Michael West is a member of AISC’s Committees on Manuals and Textbooks and the Code of Standard Practice, as well as AISC’s Specification Technical Committee TC 13–Quality Control and Assurance, and he is Chair of AISC’s Certification Standards Committee. He also chairs the AISC-ACI Task Group on Tolerances for the Construction of Steel and Concrete Structures and the AISC-ACI Task Group on Coordination.
May 21, 2013
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Abrasion and Impact Resistant Slabs David Flax, Euclid Chemical Company There are many obvious projects where increasing the abrasion resistance of the slab is important, such as distribution centers/warehouses, maintenance facilities, and retail with heavy traffic. There are other projects that require increased impact resistance including manufacturing, loading docks, tracked vehicle maintenance bays, mining, and solid waste transfer stations. This webinar will cover how to select and how to specify the proper dry shake surface hardener or topping system. David Flax has a Civil Engineering Degree from RPI and over 35 years experience as a field engineer, a contractor, and a researcher with the Corps of Engineers. He has earned his CDT and CCPR from CSI, has specialized in concrete and has had dozens of articles published. He is on a number of national organization committees including the “Guide Specifications”, “Materials and Methods”, and “Repair of Construction Defects”, all with the International Concrete Repair Institute, and has spoken on these topics and others at The World of Concrete. EN
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News form the National Council of Structural Engineers Associations
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Nominations open for NCSEA Special Awards
May NCSEA Webinars
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Diamond Reviewed
These courses will award 1.5 hours of continuing education. Approved for CE credit in all 50 States through the NCSEA Diamond Review Program. Time: 10:00 AM Pacific, 11:00 AM Mountain, 12:00 PM Central, 1:00 PM Eastern. Register at www.ncsea.com. STRUCTURE magazine
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At the NCSEA Annual Conference each year, special awards are given to NCSEA members who have provided outstanding service and commitment to the association and to the structural engineering field. The NCSEA Service Award is presented to an individual who has worked for the betterment of NCSEA to a degree that is beyond the norm for volunteerism. The award is given to someone who has made a clear and indisputable contribution to the organization and to the profession. The Robert Cornforth Award is presented to an individual for exceptional dedication and exemplary service to the organization and to the profession. The award is named for Robert Cornforth, a founding member of NCSEA and treasurer on its first Board of Directors, a member of OSEA, and secretary of the Oklahoma State Board of Registration for Professional Engineers and Land Surveyors. Robert Cornforth Award nominees must be submitted by NCSEA Member Organizations The nomination form for these awards is available at www.ncsea.com, and the deadline date for nominations is July 1. Nominations are requested for both awards; however, awards are based on worthy recipients and may not be awarded each year. NCSEA Service Award honorees 1999 Gene Corley Rawn Nelson 2000 Tim Slider 2001 Norm Scheel 2002 Fred Cowen 2004 Craig Cartwright 2006 Stephanie Young 2007 Ronald O. Hamburger 2008 Jon Schmidt 2009 Timothy Mays 2010 Edwin T. Huston 2011 Marc S. Barter 2012 Emile Troup Michael Tylk
NCSEA Cornforth Award honorees 2001 Emile Troup 2003 Ben Baer Marc S. Barter 2004 Michael Tylk 2005 Craig Barnes 2009 David Bonneville 2010 William Holmes Robert Johnson 2011 Edwin T. Huston Wiliam L. Lavicka 2012 Ronald Milmed Nomination form available at www.ncsea.com, and deadline is July 1.
Is your MO a Diamond Review provider? NCSEA’s Diamond Review Program was developed to set a high standard for its Diamond Review-approved, continuing education courses for structural engineers, thereby assuring acceptance of resulting continuing education credits in all 50 states. Subsequently, NCSEA developed a Diamond Review Program for Member Organizations (MO’s) to help MO’s monitor their own continuing education courses for Diamond Review. This program is intended for in-state MO-administration only, allowing MO members to fulfill nationwide mandatory continuing education requirements by taking Diamond Reviewed courses offered and approved by their MO’s. If you would like additional information, contact Jan Diepstra at 312-649-4600, ext. 202 or jan@ncsea.com. May 2013
The NCSEA publications committee is proud to announce the release of its latest publication titled Guide to the Design of Building Systems for Serviceability In Accordance with the 2012 IBC and ASCE/SEI 7-10 by Kurt D. Swensson, Ph.D., P.E., LEED AP. This guide provides practical information and design examples related to the serviceability performance of buildings in accordance with the requirements of the 2012 IBC and referenced standards. Where current codes are silent or not specific, the example problems in the book include practical discussions of selected criteria that include detailed reviews of research publications, international references, and proprietary product information.
The book focuses on detailed explanations and examples of proper application of code provisions and standards for a broad scope or materials, building systems, and building components, covering the vast majority of limit states encountered in the structural design of buildings. It is arranged to provide the reader examples for review within a total building system containing 25 specific examples of serviceability design or evaluation arranged according to seven different building types. The performance of reinforced concrete, structural steel, masonry, and timber structural systems is evaluated with respect to the desired performance of various nonstructural components. Both static and dynamic loading is considered. Purchase this book at www.ncsea.com or www.iccsafe.org.
NCSEA News
NCSEA publishes new book on Design of Building Systems for Serviceability
News from the National Council of Structural Engineers Associations
Short Courses to be offered on Serviceability Book In conjunction with the release of the new book, NCSEA is also offering an associated course on the new guide. The course provides an overview of the book material while examining certain topics in more detail such as related use of structural freeware and practical applications of computer modeling to serviceability design. The 4.0 hour NCSEA Diamond approved course can be provided as a stand-alone course or as part of an arranged program such as a Member Organization (MO) annual meeting. The stand-alone course is being offered for $150 to $200 per attendee which includes a copy of the new publication, a binder of course notes, and food. MOs interested in holding the course should contact NCSEA Publication Committee Chairman Timothy W. Mays, Ph.D., P.E. at timothy.mays@citadel.edu.
The conference for practicing structural engineers Featuring technical and management sessions on structural engineering, including: • Keynote by Bill Baker, P.E., SECB, F. ASCE, FIStructE, Structural & Civil Engineering Partner, Skidmore, Owings & Merrill • Serviceability presentation based on NCSEA publication Guide to the Design for Serviceability: In Accordance with IBC 2012 and ASCE/SEI 7-10 by author Kurt Swensson, Ph.D., P.E., LEED®AP, Principal, KSI Engineers • ACI 550 session by Harry Gleisch, Vice President of Engineering, Metromont Corporation, and chairman of Joint ACI-ASCE 550, Precast Concrete Structures • ASCE 41 session • Practical Design of Complex Stability Bracing Configurations by Donald White, Ph.D., School of Civil and Environmental Engineering, Georgia Tech • And much more to come!
The Annual Conference will also include: • Social events that facilitate networking with fellow structural engineers; • [New] reception for Young Member attendees; • SECB reception and information on changes to application requirements; • A trade show featuring the best in structural engineering products and services. Check www.ncsea.com for continually updated information on Annual Conference educational sessions, events, and registration information.
Current NCSEA Annual Conference Sponsors: Bronze
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The Newsletter of the Structural Engineering Institute of ASCE
Structural Columns
ASCE Releases Report Card for America’s Infrastructure
SEI Team Visits New York to Study Effects of Hurricane Sandy
The new ASCE 2013 Report Card for America’s Infrastructure is now available at www.infrastructurereportcard.org and as a tablet and phone app on the iTunes and Google Play stores. Every four years, ASCE issues the Report Card which evaluates conditions and investment needs for major sectors of infrastructure – including roads, bridges, drinking water systems, ports, mass transit, and the electric grid. This year’s Report Card covers 16 infrastructure categories, and it’s being released as a digital application (or “app”) that includes videos, interactive maps, and other multimedia tools. Among the categories graded were bridges, dams, energy, rail, and roads.
In December and January, the Structural Engineering Institute (SEI) sent a reconnaissance team to New York City to investigate and document the performance of several buildings affected by the storm surge and flooding from Hurricane Sandy, which struck the city at the end of October. The team’s mission was to investigate the damage with a view to revising ASCE’s standard 24 (Flood Resistant Design and Construction) so as to reduce damage in urban environments. Once the members of the team have inspected all of the buildings on their list, their findings will be published by ASCE as a committee report. To read the entire story, see the ASCE News at www.asce.org/ascenews.
Students and Young Professionals
Local Activities
If you missed the ASCE Free eLearning webinar Careers in Structural Engineering March 13, it’s available online at www.asce.org/elearning under View Past Webinars. Listen to SEI Board officers and leaders discuss career paths and employment opportunities in structural engineering, and learn about opportunities for students and young professionals at SEI.
Structures Congress 2014 Call for Proposals Be part of the cutting-edge technical program of the Structures Congress 2014 in Boston, April 3-5, 2014. The Structural Engineering Institute is now accepting session and presentation proposals for the Structures Congress 2014.
Key Dates All Abstract and Session Proposals due June 12, 2013 Notification of Acceptance September 18, 2013 All Final Papers due December 18, 2013 (extensions not possible) Session proposals can take two forms: a traditional session with 4 papers presented, or a panel session with no papers and perhaps more audience interaction. In addition, you can submit individual abstracts that may be combined with others to form cohesive sessions. Topics will include but are not limited to: Bridges Buildings Seismic Wind and Flood Loads Sustainability Business and Professional Practice Blast and Impact Loading Nonbuilding and Special Structures Nonstructural Systems and Components Visit the Structures Congress 2014 website for more information and submission instructions http://tinyurl.com/dxlgyr9. STRUCTURE magazine
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The SEI Sacramento Chapter has been working hard with fellow ASCE members to develop a conference that will feature the latest international developments in orthotropic steel bridges, and would like to issue an invitation to bridge engineers to attend the conference. The conference dates will be June 26-28, 2013 in Sacramento CA. More information regarding registration and the conference agenda may be obtained from the conference web site at www.orthotropic-bridge.org/www/2013/2013_Home.html. This is the third in a series of conferences on this interesting topic and will discuss the latest additions to the ranks of orthotropic bridges that provide safe, efficient and seismically resistant transportation for vehicles on highways around the world. To get involved with the events and activities of your local SEI Chapter or Structural Technical Group (STG) http://content.seinstitute.org/committees/local.html. Local groups offer a variety of opportunities for professional development, student and community outreach, mentoring, scholarships, networking, and technical tours.
Journal of Structural Engineering Call for Papers Special Issue: Resilience-based Design of Structures and Infrastructures Designing resilient structural and infrastructural systems requires interdisciplinary collaborative efforts to formulate new approaches, and metrics that jointly consider performance and post event functionality goals that enhance disaster resilience. This special issue will deal with topics in technicalsocio-economic functionality of structures and infrastructures, probabilistic risk and resilience-based design principles for recommended practices and standards, optimal considerations in pre- and post-event retrofit and restoration, and resilience-based decision support systems for existing and new constructions and infrastructures. The abstract submission deadline is May 21, 2013. Full invited papers will be due by October 1, 2013. Visit the journal website at http://tinyurl.com/d9dtect for complete details. May 2013
Your New PDH Tracker and Personalized Hub for Continuing Education Manage your professional development and license renewal through ASCE’s new learning management system – myLearning. Track all your PDHs/CEUs, including those from other providers; obtain certificates of completion; take program-related exams; print or save transcripts of your professional development – all in one place! Make myLearning your personalized hub for continuing education and explore the comprehensive program catalogand track your PDHs. Visit the myLearning website at www.asce.org/mylearning/ and get started today.
Revision to Substation Structure Design Guide Underway The committee revising ASCE Manual of Practice 113 (Substation Structure Design Guide) will be having their first committee meeting April 29 – May 1, 2013 in Houston, Texas. This committee is no longer accepting new members, but asking for ASCE 113 users to contribute their ideas on which sections of the Substation Structure Design Guide need revision. If you have used ASCE 113 and have any input on the direction for the revision, please email SEI at SEI@asce.org using “ASCE 113” in the subject line of the email. A general overview of foundations typically used for substation structures will be added to the guide.
New Name And Chair for the Disproportionate Collapse Technical Committee standards, and technical advances. The new chairman is David Stevens of Protection Engineering Consultants.The DC Technical Committee is always looking for new members who can contribute to an interesting and challenging area of structural engineering.To join a SEI Technical committee, visit the SEI website at http://apps.seinstitute.org/committees/tadjoin.cfm.
Errata SEI posts up-to-date errata information for our publications at www.asce.org/SEI. Click on “Publications” on our menu, and select “Errata.” If you have any errata that you would like to submit, please email it to Paul Sgambati at psgambati@asce.org.
New ASCE Structural Webinars Available SEI partners with ASCE Continuing Education to present quality live interactive webinars on useful topics in structural engineering. Several new webinars are available: Wind Tunnel Testing for Wind Loads on Structures
May 1, 2013
Forrest J. Masters
Design of Bridges for Earthquakes
May 3, 2013
Mark Yashinsky
Evaluating Damage & Repairing Metal-Plate-Connected Wood Trusses
May 15, 2013
Jim Vogt
Seismic Assessment and Design of Pipelines
May 15, 2013
Donald Ballantyne
Designing for Flood Loads Using ASCE 7 and ASCE 24
June 3, 2013
William L. Coulbourne
Structural Thermal Bridging in the Building Envelope
June 5, 2013
James A. D’Aloisio
Damping and Motion Control in Buildings and Bridges
June 7, 2013
Brian Breukelman
Philosophy of Structural Building Codes
June 7, 2013
Dave K. Adams
Webinars are live interactive learning experiences. All you need is a computer with high-speed internet access and a phone. These events feature an expert speaker on practice-oriented technical and management topics relevant to civil engineers. Pay a single site fee and provide training for an unlimited number of engineers at that site for one low fee, and no cost or lost time for travel and lodging. ASCE’s experienced instructors STRUCTURE magazine
deliver the training to your location, with minimal disruption in workflow – ideal for brown-bag lunch training. ASCE Webinars are completed in a short amount of time – generally 60 to 90 minutes – and staff can earn one or more PDHs for each Webinar. Visit the ASCE Continuing Education website for more details and to register, www.asce.org/conted.
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The Newsletter of the Structural Engineering Institute of ASCE
The former Disproportionate Collapse Standards and Guidance committee has been renamed to the Disproportionate Collapse TECHNICAL committee. This was motivated by the recent creation of the Disproportionate Collapse (DC) STANDARDS Committee, which is chaired by Don Dusenberry and will create the ASCE standard for designing buildings to resist disproportionate collapse. The DC TECHNICAL Committee will return its focus to “stimulate research, prepare reports, develop conference sessions, review papers, and otherwise collect and transmit information concerning the analysis, design, and maintenance of structures”, i.e., the main activities of SEI technical committees. The DC Technical Committee would like to thank Bob Smilowitz of Weidlinger and Associates, Inc., for three years of fine leadership, as the committee simultaneously addressed design guidance,
Structural Columns
myLearning
Creating a Solid Risk Management Plan Tool 2-1: A Risk Evaluation Checklist
Tool 3-1: A Risk Management Program Planning Structure
CASE in Point
The Newsletter of the Council of American Structural Engineers
Don’t overlook anything! A sample itemized list of things you should look for when evaluating a prospective project.
This tool is designed to help a Firm Principal design a Risk Management Program for his or her firm. The tool consists of a grid template that will help focus one’s thoughts on where risk may arise in various aspects of their engineering practice and how to mitigate those risks. Once the risk factor is identified, then a policy and procedure for how to respond to that risk is developed. This tool contains 10 sample risk factors, with accompanying policies and procedures to illustrate how one might get started. The tool is designed to insert custom risks and policies to tailor it to individual firms.
Tool 2-4: Project Risk Management Plan This plan will walk you through the methodology for managing your project risks, along with a few common project risks and templates on how to record and track them.
You can follow ACEC Coalitions on Twitter – @ACECCoalitions.
You can purchase all CASE products at www.booksforengineers.com.
CASE Business Practice Corner If you would like more information on the items below, please contact Ed Bajer, ebajer@acec.org.
Legacy of the Recession There are several emergency trends affecting engineering firms that have been identified over the course of the current recession. One is engineering services being treated as a product rather than a service, or as some call it commoditization. Fee-based competition is becoming more common. Clients are expecting more for less with perfect results. The wealth of talent in baby boomers is diminishing with their retirement and leadership succession has become a problem. Recruiting new talent is also a problem. There are fewer qualified graduates and some professionals have left the industry. Some firms have met this by improving the work environment, including the culture and benefits for the workforce and trying to make themselves significantly distinguishable from competitors.
Teaming Agreements They are becoming more serious agreements with risk management issues and more indemnifications, warranties, confidentiality provisions and other significant legal terms. They can be so overloaded that they create more problems than they are worth. Some firms try to make them very simple and job specific. The Design Build Institute has a teaming agreement that has been used successfully by engineering firms. STRUCTURE magazine
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Recently Out – EJCDC Construction Related Documents The 23 documents in this new edition of EJCDC’s Construction series are specifically written for use on public and private engineered-facility projects, and include: • Core contract documents such as the Standard General Conditions, the Owner-Contractor Agreement form, and Supplementary Conditions • Forms for gathering information needed to draft bidding and contract documents • Instructions to bidders and a standard bid form • Bonds (bid, performance, and payment) • Administrative forms, such as change order forms and the certificate of substantial completion. What’s Changed • Revised instructions for Bidders, Bid Form, General Conditions and Supplementary Conditions • Improvements to the change order section in General Conditions • Revised language for bonds and insurance • Added new clause on differing site conditions Documents can be purchased as a set or individually. Go to www.acec.org/bookstore. May 2013
the meeting or have any suggested topics for the committees to pursue, please contact CASE Executive Director Heather Talbert at htalbert@acec.org.
Courtesy of Walter P Moore and Bliss Nyitray.
CASE Member Firm Wins Grand Award Congratulations go out to CASE Member firm Walter P Moore of Houston, TX for winning a Grand Award for its project, Marlins Park in Miami, FL. The project was also a finalist for the Grand Conceptor Award awarded at the recent Engineering Excellence Awards Gala held during the ACEC Annual Convention last month.
ACEC Business Insights EJCDC Unveils Updated Construction Series Documents The Engineers Joint Contract Documents Committee (EJCDC) released its revised Construction Series (C-Series) documents. The 24 C-Series documents can be used on both public and private projects in which the owner retains an engineering firm to prepare drawings and specifications and then engages a construction contractor to perform the work. The 2013 edition contains three new documents – Advertisement for Bids, Qualifications Statement, and Subcontract – and includes substantive changes to the Standard General Conditions document. For more information and to order these documents, go to www.acec.org/bookstore.
A/E Industry’s Premier Leadership-Building Institute Filling Fast for September Class Since its inception in 1995, the American Council of Engineering Companies’ prestigious Senior Executives Institute (SEI) has attracted public and private sector engineers and architects from firms of all sizes, locations and practice specialties. Executives STRUCTURE magazine
– and up-and-coming executives – continue to be attracted by the Institute’s intense, highly interactive, energetic, exploratory, and challenging learning opportunities. In the course of five separate five-day sessions over an 18-month timeframe, participants acquire new high-level skills and insights that facilitate adaptability and foster innovative systems thinking to meet the challenges of a changed A/E/C business environment. The next SEI Class 19 meets in Washington, D.C. in September 2013 for its first session. Registration for remaining slots is available. Executives with at least five years’ experience managing professional design programs, departments, or firms are invited to register for this unique leadership-building opportunity. As always, course size is limited, allowing faculty to give personal attention, feedback, and coaching to every participant about their skills in management, communications, and leadership. SEI graduates say that a major benefit of the SEI experience is the relationships they build with each other during the program. Participants learn that they are not alone in the challenges they face, both personally and professionally, and every SEI class has graduated to an ongoing alumni group that meets to continue the lifelong learning process and provide support. For more information, visit www.acec.org/education/sei/ or contact Deirdre McKenna, dmckenna@acec.org, or 202-682-4328.
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CASE is a part of the American Council of Engineering Companies
On March 5-6, the CASE Winter Planning Meeting took place in conjunction with the NCSEA Winter Leadership Forum in Tucson. CASE does two planning meetings a year to allow their committees to meet face to face and interact across all CASE activities. Over 30 CASE committee members and guests were in attendance, making this another well attended and productive meeting. During the meeting, break-out sessions were held by the CASE Contracts, Guidelines, Toolkit, and Programs & Communications Committees. The committees finalized the schedule of new products for release in 2013-2014, plus reviewed current documents for revisions and finalized speakers/sessions for the ASCE/ SEI Structures Congress this month, the 2013 ACEC Fall Conference October 27-30 in Scottsdale, and the 2014 ASCE/SEI Structures Congress scheduled for April, 2014 in Boston. New this year at the planning meeting was a roundtable discussion the night before with several topics, including specialty licensure, social media, BIM and insurance limits up for discussion. Prior to this roundtable discussion, the CASE Executive Committee met and finalized plans for the next year, including creating two new Ad Hoc Working Groups on Membership and Social Media and looking at ways to address issues that multi-discipline firms and firms that do primarily government. The CASE Summer Planning Meeting is scheduled for August 5-6th in Chicago. If you are interested in attending
CASE in Point
CASE Winter Planning Meeting Update
Structural Forum
opinions on topics of current importance to structural engineers
Changing Building Codes Are They Really That Bad? By David Pierson, S.E., SECB
I
n discussing my profession with a friend recently, I explained how we are bound (and protected) by building codes. I mentioned that it is a bit of a challenge keeping up with code changes, since a new code comes out every three years. I was a bit taken aback by his response. “Wow”, he said, “Can’t someone write a building code that lasts longer than three years?” His response prompted immediate reflection. That conversation has led me to re-think my stance on the necessity of such short code cycles. Considering that we measure the age of the Bible, the Torah and the Quran in centuries, it seems reasonable to question the need for a new building code every three years. Before I go any further, let me clarify something. I am not going to argue here against the complexity of the building codes. On this issue I sit silently on the sideline and applaud (with quiet golf applause) the fact that the code is too complex to be understood by someone without proper training and experience. In our profession, we have few barriers to entry better than a complex code. Strong barriers to entry are needed to keep demand for our services higher than the supply. That results in higher pay, and of that I am a proponent. But why do we need a new building code every three years? A popular answer to this question is that organizations promulgating codes need the revenue stream to stay in business. Surely it is necessary for those organizations to sell codes occasionally, and nobody begrudges them that. However, this motivation may cloud their ability to judge impartially the value of publishing a new code. Organizations should not exist for the sole purpose of selling codes and standards; they should be able to provide value to the design community (and be compensated for it) in other ways. Probably the most relevant response to the question of short code cycles relates to our increasing body of knowledge. For instance,
new technology enables advanced numerical methods to be utilized in design. Research, both academic and industrial, provides new options for structural systems. And natural disasters provide lessons regarding the performance of structural systems, thus presenting opportunities for improvement. Such advances should indeed be reflected in the building codes. But on what basis can we make the assumption that every three years we will have a sufficient increase in knowledge to justify changing the codes? Before a code is changed, there should be a requirement for a cost/benefit analysis. Too often the significant costs are ignored. Recently a person I know decided to estimate the cost of a complete building code. Starting with the IBC, she tallied the cost to acquire every referenced standard, plus the references in those standards. She stopped when she got to $100,000. Of course, nobody spends that much on these documents, but the point is still valid. Beyond that, time for learning a new code is a large cost to design firms, hidden somewhere deep in the overhead multiplier. Determining the benefits of a new code is a subjective endeavor, but the following question ought to be asked: If we do not adopt a new code and instead continue with the one currently in place, will the public still be adequately protected, and will the designs still result in economically feasible buildings? For example, I do not have any heartburn about the buildings that I designed using the 1997 UBC. Whatever improvement there has been in the codes since then, it has not been significant enough to cause concern about those previously designed buildings. I would ask anyone claiming to be concerned: Are you going back to the owners of the buildings that you designed under the 1997 UBC to tell them that they need to have their structures upgraded? Another issue is the academic research that creeps into the codes. While research
is certainly necessary and vital, many researchers seem to depend upon getting code changes incorporated in order to justify their work. It is not clear that they adequately consider whether such modifications are really improvements. Too few of those involved in the code development process ask the right questions. If a proposed provision indicates a 3% change in a calculated capacity, is that significant enough to justify a code change? How does it relate to the level of uncertainty still present on the demand side? Are the building codes supposed to ensure that the behavior of structures is accurately modeled with ultimate precision? Or are they intended to allow engineers to design safe, cost-effective structures within a reasonable time frame? How many different ways can we calculate 20 psf wind pressure on a building? There may be other reasons offered for the short code cycles, such as unintended consequences arising from previous changes. Upon serious reflection, however, I think we would find that most proposed changes can wait a few more years until the next code is published. For critical issues that cannot wait, addenda and supplements could be utilized. My questions to those involved in the development of new codes and design standards are as follows. If the code that you are now proposing to be adopted is so much better than the one that we are currently using, why will it be obsolete in just three years? Is the 2009 edition so problematic that we cannot wait until 2015 to replace it? If so, why did we adopt it? Are the codes to which we design really that bad? Five-year cycles would be better. What would be best? Do I hear six or eight?▪ David Pierson, S.E., SECB (davep@arwengineers.com), is a Vice President at ARW Engineers in Ogden, Utah.
Structural Forum is intended to stimulate thoughtful dialogue and debate among structural engineers and other participants in the design and construction process. Any opinions expressed in Structural Forum are those of the author(s) and do not necessarily reflect the views of NCSEA, CASE, SEI, C 3 Ink, or the STRUCTURE® magazine Editorial Board. STRUCTURE magazine
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