InSights

new trends, new techniques and current industry issues

UFC 4-010-01, DoD Minimum Antiterrorism Standards for Buildings, dated February 2012, outlines 21 standards that govern site planning and the design of structural, architectural, electrical and mechanical systems for Low and Very Low Levels of Protection. The current document was developed as an update to a previous version originally issued in October 2003 and modified by Change 1, in January 2007. Though some of the revisions were incremental and provided additional clarification to existing standards, others were significant and represent a major change in approach. Implementation of the updated criteria is likely to result in levels of hardening or analysis that vary from those required by earlier editions. The most obvious changes pertain to the standoff distances at which conventional construction may be used, the unobstructed space requirements, and the design of window and door systems. Each of the adjustments comes with opportunities, Updated Military Criteria but also potential pitfalls that could lead to uninfor Antiterrorism Design tended cost increases or criteria violations.

By Mark Gardner, P.E. and Spencer Quiel, Ph.D.

Mark Gardner, P.E. (mgardner@hce.com), is a managing engineer and Spencer Quiel, Ph.D. (squiel@hce.com), is a project engineer with Hinman Consulting Engineers, Inc. in Alexandria, Virginia.

Standoff Distances

Standard 1 outlines the conventional construction standoff distances (CCSDs) that permit the structure and façade, other than glazing systems and supporting elements, to be designed without specific analysis for blast effects. In the 2007 version, CCSDs and minimum standoffs were based solely on the building category and level of protection for a corresponding explosive threat. The 2012 version has overhauled this approach by specifying varying CCSDs for defined wall and roof construction types based on multiple construction parameters and limitations that were developed by a variety of dynamic calculations. As a result, this new version typically requires a larger CCSD for walls that are loadbearing versus non-load-bearing by allowing more damage to the latter. This approach allows the designer to tailor a conventional construction type to the available standoff. The CCSD for heavier materials, such as reinforced concrete and masonry, are smaller when compared to the generic values in the 2007 criteria, thereby permitting the use of such construction without blast analysis when less standoff is available. However, implementation of the reduced CCSDs is limited because they are only applicable for the specified range of element parameters, including spacing, span, supported weight, boundary conditions, and material strength. For example, two-way flat slab roofs do not qualify because such boundary conditions are not included in the recognized set of parameters. Other common roof types, such as steel-framed with wide flange shapes, are also not included. Per the criteria, “Any construction type that does not fall within the specified parameters needs to be analyzed for blast loads due to the explosive threats at the appropriate standoff.” It is reasonable to expect that structural systems that are similar to or stronger than those specified in the criteria can meet the intent of the CCSD criteria. However, the use of these systems without submitting dynamic analysis calculations may leave the design team in a position of having not met the criteria as written. The new version also states that all façade elements are assumed to conform to a pin-pin condition, which is not always the case. A cantilever condition, such as a wall below ribbon windows, is not considered. Therefore, if such elements are utilized, dynamic calculations must be performed to verify that they can resist the direct blast loads and the reaction from the window system, which can be represented as a point load at the end. This will generate the need for more analysis during design compared to the 2007 criteria, which had no restrictions based on the type of construction as long as the prescribed standoff distances were provided. In addition, the unobstructed space now extends out to the closest applicable standoff distance for Explosive Weight II, which applies to parking and roadways within a controlled perimeter and to trash containers, but not less than the minimum standoff distance for a qualifying construction type. The 2007 criteria required 33 feet of unobstructed space regardless of construction type. This change greatly increases the required interaction with the site and landscape design in coordination with the blast protection and construction type required in Standard 1.

Windows and Doors

Standard 10 outlines the design provisions for glazing systems, which are applicable even if the CCSD of the wall supporting or surrounding the window is met or exceeded, and also impose a tradeoff when the site design takes advantage of the reduced CCSDs for heavier construction types. Several significant changes were made to Standard 10 in the 2012 criteria (see Table). The structural elements supporting windows and skylights can now be designed statically by simply accounting for their increased tributary area relative to the rest of the wall. This factor is multiplied to the moment and shear capacity of the conventional wall or roof element to determine the required capacity of the supporting element and its connections to the structure, including any load-transferring elements such as kickers. Finally, the new version provides additional guidance for exterior doors, which must now

be tested to achieve the applicable level of protection in accordance with ASTM F2247. Previously, the doors merely had to swing outward. This requirement will present challenges for door manufacturers; they may be required to test their products for the smaller CCSDs that are now allowed for heavier construction types. Glazed doors must also meet the glazing and bite provisions of Standard 10.

Conclusion

In summary, the CCSDs have changed to allow threats closer to the building based on the exterior wall and roof types. The design team must carefully consider whether the chosen construction meets the parameters outlined in the new criteria, or if dynamic analysis will be required. Additionally, some of the parameters may have an adverse impact on project budgets. For windows and doors in particular, smaller standoffs allowed under the new criteria will typically increase the cost relative to previous versions. There will be a period of time before vendors adjust to these changes during which the products they offer may be severely limited and expensive. If these constraints are known in advance, then the design team can make informed decisions early in the process and avoid unanticipated expenses to the building.▪

Element 2007 UFC

Glazing Prescriptive Design w/ Table B-3

2012 UFC

Design w/ ASTM E1300 and ASTM F2248 (based on explosive weight, standoff distance, and glazing size)

Framing Design Loading per ASTM F 2248 Deflection < L /160 Design Loading per ASTM F2248 Deflection < L /160

AND

2X Glazing Resistance per ASTM E 1300 Deflection < L /60

Connections 2X Design Loading per ASTM F 2248 Design Loading per ASTM F 2248 Deflection < L /160

AND

2X Glazing Resistance per ASTM E 1300 Deflection < L /60

Supporting Structure 8X Glazing Resistance per ASTM E1300 Increase capacity of elements relative to typical wall by ratio of tributary areas

Skylights Same as Above Glazing requires dynamic analysis

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FOUNDATIONS

Slow and Steady Upswing for Foundation Business

By Larry Kahaner

Despite being whipsawed by the ‘ scal cli ’ negotiations, companies involved in the foundation sector report that business generally is up and growing, albeit at a measured pace. “Our customers tell us that there seems to be a slow but steady increase in work,” says Jim Hussin, Director, Hayward Baker, Inc. (www.haywardbaker.com), headquartered in Odenton, Maryland. “In our own company, we have seen a steady increase in business over the past two years and will perform a record volume of work in 2012.” Hayward Baker is a contractor specializing in foundations and geotechnical construction. eir services include grouting, ground improvement, structural support and earth retention, all of which are o ered as design-build services. One of their newer services is soil mixing which has gained popularity in the last few years, according to Hussin. He explains the process. “Soil mixing is a ground technique that improves the characteristics of weak soils by mechanically mixing them with a cementitious binder. e binder can be added as a slurry for dryer soft soils, or as a dry powder to very wet soft soils. To construct columns, a powerful drill advances drill steel with radial mixing paddles located near the bottom of the drill string. e binder slurry is pumped through the drill steel to the tool as it advances and additional soil mixing is achieved as the tool is withdrawn. To perform mass soil mixing, or mass stabilization, a horizontal axis rotary mixing tool is located at the end of a track hoe arm. is technique has been used to strengthen soft soils at sites of planned buildings, storage tanks and embankments.” Hussin adds that this soil mixing technique allows improvement of soft soils that were previously di cult to treat.

At Polyguard Products (www.polyguardproducts.com) of Ennis, Texas, CEO John Muncaster boasts of his company’s 20th straight year of sales growth. Polyguard does waterproo ng and corrosion protection, and they’re eager for SEs to learn about their Underseal Underslab waterproo ng membrane. “When you’re on a construction site and you’re about to pour a concrete slab, and you want to protect it from moisture or vapor or water, you want something that also will stand up to the abuse of construction. Traditionally, the industry has been using poly lms that become riddled with holes by the time the construction process is over. Our product not only waterproofs, which is vapor proo ng plus waterproo ng, but it has the ability to withstand the construction process better than anything out there,” says Muncaster. “People talk about protecting the whole envelope from primarily moisture, but what they’ve been using underneath the slab is like Swiss cheese by the time all the equipment has rolled on it, all the welding has taken place, and people have stomped around. And, literally, contractors using poly lm sometimes will punch holes in it to make the concrete slab dry faster… what I would like to emphasize is that our product is not just a vapor barrier but waterproof and damage resistant, too.” (See ad on page 32.)

Brendan FitzPatrick, Director-North America at Geopier Foundation Company, Inc. (www.geopier.com) based in Mooresville, North Carolina, says that their latest technology innovation, Geopier Densipact, provides further cost savings by densifying loose sand with on-site or local sand aggregate to develop allowable bearing pressures upwards of 12 to 14 ksf. “ e rapid densi cation of on-site soils combined with high bearing pressures a ords considerable cost and time savings to project teams,” he adds. Geopier prides itself on providing innovative technologies with a focus on reliable, cost-e ective foundation support solutions that deliver value to the project team, says FitzPatrick. “Many engineers have experience with a traditional Geopier ‘drill and ll’ technology that has been used for decades. Many customers have also experienced the bene ts of cost and time savings for their building foundations by using the displacement Rammed Aggregate Pier systems – Impact and Rampact–to reinforce loose saturated sands, or soft silt and clay, or contaminated soil where elimination of spoils generates additional cost savings to the owner.” He notes that these products, along with their new Denispact product, “provide an additional option to project teams for foundation and oor slab support, and expand our ability to serve our customers. For the right application, these additional tools provide further cost-e ectiveness than other Geopier options that the design team may have previously considered.” (See ad on page 33.) continued on page 34

Another company bringing ground improvement to customers is Subsurface Constructors, Inc. (www.subsurfaceconstructors.com), St. Louis, Missouri. “We are a full-service geotechnical contractor. We are one of the very few companies to o er both the full range of deep foundations and earth retention, in addition to serving as a design-build contractor for vibro ground improvement solutions nationwide,” says Lyle Simonton, Director of Business Development. “Our ground improvement division, although no longer new, continues to grow its ability to be competitive in all industries and geographic locations. We have designed and constructed vibro ground improvement for hundreds of structures of all sizes. We bring a signi cant amount of value to owners and developers who are seeking a lower-cost ground improvement alternative than the companies they’ve used previously. In the past year, we have completed several ground improvement projects in the east and northeast for developers of multi-family residential and commercial facilities. Engineers and contractors are starting to realize that ground improvement for their projects do not have to be high-cost solutions.” Simonton adds: “With some of the new equipment we’ve developed, we are becoming more mobile and even more competitive on projects that are a long way from our home o ce in St. Louis.”

Gina Beim, Senior Consulting Engineer, Marketing at Pile Dynamics (www.pile.com) in Cleveland, Ohio, says that the electrical utility sector has been a growth area for their products, which includes testing and monitoring systems for all types of deep foundations. “Two things have happened,” says Beim. “First, the sector is growing so there’s more construction. And second, the nature of the construction of these transmission lines is such that every so often a pole is supported by only one big foundation element: a monopile. It’s very important to test the quality and bearing capacity of this particular foundation element. In other cases, particularly in environmentally sensitive areas, this industry employs helical piles that up until recently had been a challenge to test (for capacity) by dynamic testing. Pile Dynamics has done some research and is now able to recommend

“Our customers tell us that there seems to be a slow but steady increase in work.”

how to undertake dynamic testing for this type of pile, and that is stirring up interest on the part of this industry.” She adds: “We have traditionally served the driven pile industry, the drilled shafts industry and the auger cast pile industry with instruments to assure quality of these types of piles. More recently, we have made certain recommendations in testing the capacity of helical piles so that they can be tested with the Pile Driving Analyzer. at’s a relatively new development that we are quite excited about, because consultants that provide these services are embracing this new way of testing.” Beim explains that in the past, the most often used method to evaluate the integrity of a drilled shaft was crosshole sonic logging, which is still by far the most widely-used method but it has some disadvantages. “ ermal integrity pro ling is also a method of examining the quality of these drilled shafts; this process is better because it looks at the entire cross-section of the shaft. Crosshole sonic logging does not. ermal integrity pro ling evaluates the alignment of the reinforcement cage and the shape of the shaft, which crosshole sonic logging cannot do, and it’s a test that can be performed much sooner than crosshole sonic logging. With these advantages, people are excited about it. We are seeing more and more interest in our ermal Integrity Pro ler, which ADVERTISING performs this new type of integrity test.”▪ OPPORTUNITIES

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