30 minute read
Structural Design
In New York City, there are over 100,000 masonry buildings that were erected prior to 1930. Typically these are three to six story unreinforced masonry (URM) bearing wall structures with wood floors and rubble foundations. As a consequence of the high number and density of these buildings, a high percentage of new developments occur adjoining an URM building. Excavating for deeper foundations of new buildings will require some type of underpinning. Although there are several methods available, New York City contractors seem to exclusively use strip (also called pin) underpinning. In recent years, investigations of several incidents showed that some contractors were paying scant attention to some critical activities (e.g. jacking). In some cases, strip underpinning might have been extended far beyond the range of where it had produced relatively safe results. About six years ago, the New York City (NYC) Buildings Department established a special unit to focus on excavation and underpinning. As a result, the number and gravity of incidents has declined substantially. While engineering input has increased, the question of what the most appropriate method of analysis should be has not yet been entirely solved by the engineering community. To motivate the profession towards a more in depth engineering approach to underpinning, the upcoming version of the NYC Building Code will require consideration of the effect of the soil lateral pressure on the structure of the building being underpinned. Given the high sensitivity of URM walls to out of plane loads, it is important to pay full consideration to the possibility that the soil lateral pressure acting on the underpinning might be partially transferred to the walls above. Although sometimes difficult, it is even more imperative to determine the effect of these pressures when the existing structure was erected based only on code prescribed empirical methods that did not fully include concepts like load path or wind design. Some of the engineering arguments that form the basis of this specific code requirement were presented in the May 2011 issue of this magazine (Cases of Failure of Unreinforced Brick Walls Due to Out-of-Plane Loads). This article presents a more detailed discussion of the lateral loads.
Empirical Methods and Lateral Loads
As long as underpinning is required only to transfer vertical forces to a deeper soil level, one only needs to verify that the transfer system does not introduce any eccentricity or local overstress and that the removal of overburden does not alter the soil bearing capacity. During the construction phase, strip underpinning functions as a soil retaining system as well, resisting pressures perpendicular to the wall. This dual function (support of vertical loads of the existing wall and support of lateral pressures developed as a consequence of the excavation) provides significant savings that probably explains the present ubiquity of the method.
design issues for structural engineers
The underpinning procedure requires jacking or shimming to ensure that the transfer of vertical forces occurs with minimal vertical displacement of the structure above. The jacking develops a high frictional resistance and, together with the subsequent grouting, produces a connection capable of transferring shear forces. Whether or not it is capable of transferring moments, the installation becomes continuous for shear loads. A load path is created. This continuity of foundation-underpin Effect of Lateral Soil Pressure makes impossible the prevention of transfer on Underpinningof lateral loads to the structure above. One can minimize the loads transferred by approaching the soil retention function of the By Dan Eschenasy, P.E. underpinning as a sheeting problem. This involves tying back the underpinning with anchors. Similar to anchored sheeting jobs, the engineer is required to select the tieback, its pre-stressing level and its timing in the construction sequence. Tie backs and horizontal shoring solutions are becoming common for deep underpinnings but are still rare for depths less than 6 feet. The analysis presented here concentrated on these lower ranges, since they are most common. Neither the technical literature nor design practice provide good examples of engineering calculations that take into account the effect of lateral loads on Dan Eschenasy, P.E. is the New York City Buildings Department Chief Structural Engineer. He is an Honorary Member of SEAoNY. existing structures. Many engineers consider such calculations unnecessary. They argue that the load or displacement transferred to the existing building is extremely small and gets dissipated in the system. For these engineers, the success of the operations requires merely conforming to some empirical principles of execution such as controlling the run of sandy soils, providing a carefully designed box for the approach pit, careful jacking of the pin, keeping water away from the pit, etc.
Sensitivity Analysis
Due to modeling uncertainties and the large variety of possible conditions, a sensitivity analysis was deemed the best approach. Sensitivity analysis is a methodology that evaluates how the uncertainty in the output of a model can be apportioned to
Figure 1: Phases of underpinning.
different sources of uncertainty in the model input. In this case the output is the magnitude of horizontal forces transferred to the existing structure. Some relevant results are presented here. Obviously, these results are for particular cases, and should not be used in calculations by others. It is also important to note that the lateral pressure on the underpinning results in unequal distribution of stresses on the underlying soil, concentrating stresses at the toe of the pin. The scope of the analysis was limited to underpinnings less than 6 feet in height and installed under walls of tenements less than 6 stories. The changes in pin toe pressure are described in terms of ratio of final toe pressure vs. pre-underpinning pressure. The large distance between the building’s shear walls restricting the out of plane movement of the bearing walls allowed us to neglect their direct effect on these bearing walls. In this simplified model, the lateral loads can be transferred to the underlying soil or horizontal diaphragms only. A rigid support at diaphragm level was assumed. The analysis used models that follow the steps of the installation process. The various static models are shown in Figure 2. Figures 3 to 6 depict the most relevant results, such as the theoretical effects on the underpin toe as well as the transfer of horizontal loads to
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the structure above. A typical underpinning procedure would be as follows (Figure 1): Figure 2: Structural models for underpinning Phase 3. Underpinning Phase 1 the masonry wall to the floor diaphragm were A sheeted approach pit is executed to allow digging under the existing foundation. The safe removal of earth from under the existing foundation is possible but also limited by the capacity of masonry (brick or rubble) to span several feet unsupported. After the pit under the foundation has reached the desired dimensions, a pin is poured. The top of the pin leaves a gap of several inches to the bottom of the existing foundation. The typical sequence of operation allows the simultaneous digging of several pits. analyzed (Figure 2). Case A: There is no positive connection to the floors. The resisting friction forces are small. The base of the underpinning prevents rotation. The case when the joist/wall friction is sufficient to transfer the horizontal forces is treated at Case B (see below). Case B: The base of the underpinning prevents rotation. Case B(1) corresponds to positive diaphragm (joist) anchorage to the wall occurs at the first floor. In some structures, the first wall diaphragm connection occurs only at the second floor since the first Underpinning Phase 2 floor joists were simply placed on the shoulder Jacking or shimming takes place to control the transfer of vertical forces from the foundation to the pin. The individual pin installation is finalized by packing grout in the gap between its top surface and the bottom of the existing foundation. offered by the rubble foundation. This situation is labeled as Case B(2). Case C: The base of the underpin cannot prevent rotation. For Case C(1) the first positive diaphragm-to-wall connection occurs at the first floor. Case C(2) corresponds to a first positive joist connection occurring at the Underpinning Phase 3 second floor. The underpinning is complete as pins cover the entire length of the foundation. No backDiscussion and Findings fill or additional supporting structure have During Phase 1, per the analysis, some pins been placed on the excavated side. Several could overturn under lateral soil pressure. possible conditions of the attachment of This analysis, also confirmed by the findings
Figure 4: Variation in toe stress – at rest pressure. Figure 5: Variation in toe stress – active pressure.
of several incident investigations, clearly justifies the need to provide some system to counteract lateral pressures. During Phase 2, jacking (or shimming) may result in an eccentric application of the load. The effects of various possible jack misalignments from the axis of the pin were evaluated. Misalignment from the center of the pin might increase the pin toe stresses by as much as 100%. Placing the jack perfectly at the middle of the pin avoids the application of a moment on the pin; but, since the position of the resultant of the existing vertical forces above is not exactly known, a misalignment might occur between the position of the jack and this resultant. As a consequence, some local stresses might double, but would probably be resolved within the masonry structure itself. Both Phase 1 and Phase 2 are temporary. Most accidents during these phases can be attributed to contractor errors and, as such, is out of the scope of this study. Stresses and displacements present during Phase 3 are usually not temporary. The presence of a new basement wall on the excavated side will only limit further rotation or horizontal displacement of the underpinning, but will not eliminate the stresses already present. For Case A, the analysis shows that under at rest soil pressure and depending upon the load of floors above and the pin height, the increase in stresses concentrating at the toe of the pin will reach 350% for a 6-foot underpinning (Figure 4 ). Case A occurs when no transfer is available (e.g. walls not anchored to diaphragm) or when the transfer path is damaged (e.g. end of wood floor diaphragm is rotten). The analysis also shows clearly that the lower the vertical load, the higher the possibility of overturning (Figure 3). In fact, in 2005 there were two collapses of one story buildings that were being underpinned. continued on next page
Th e modeling of Case B implies that as long as the soil bearing capacity is not exceeded, it will not infl uence the magnitude of horizontal forces transferred. Under at rest soil pressure, a 6-foot high underpinning will see a 200% increase in the stresses, concentrated at the toe. For this case, the horizontal load transferred to the building above seems to vary around 4 to 8% of the total lateral pressure. Th e load transferred to the fi rst fl oor might exceed 50 pounds per linear foot, a relatively small but not to be neglected load (see example in Figure 6). Such load might be suffi cient to break a deteriorating wall-to-diaphragm connection. Even though it would not drastically change the transfer of horizontal loads to the structure above, widening the pin towards the excavation side could signifi cantly reduce the pressure on the soil. However, in NYC it is rarely possible to provide such enlargement. In essence, Case C corresponds to a transfer of only vertical and shear forces to the soil (no moment restraint, rotation can occur). In Case C(1), the horizontal loads transferred to the fi rst fl oor might exceed 200 pounds per linear foot (see example in Figure 6). For this case, the horizontal load transferred to the building above seems to vary around 15-25% of the total lateral pressure. If the wall-to-diaphragm ties at the fi rst fl oor fail, the second fl oor diaphragm might be engaged. In this Case C(2), the horizontal load will diminish by about 40% compared with Case C(1). Case C requires verifi cation of the capacity of the wall/foundation/underpin as a column under combined vertical and lateral forces. For Case C(2), the column might become too slender. For some Case C conditions, some overstressing tensile
Figure 6: Horizontal force transferred at 1st fl oor diaphragm (example). stresses might develop and the compressive stresses might approach the allowable compressive capacity of rubble walls (as it was listed in old codes). Crushing and/or bowing of rubble walls were observed in several cases. When the grouted pin-foundation connection is not able to transfer applied moments, the column structure (bearing wall plus underpinning) might become unstable. Old masonry buildings were never explicitly designed to sustain horizontal forces and, as a result, even smaller loads might crack or rake the structure. In some cases when interior plaster walls participate in the transfer of lateral forces, they might develop cracks. Th e author has repeatedly seen such events. Raking of walls is likely to introduce additional moments as it shifts the position of the resultant of the masonry weight. Up to this point, the discussion of Case C and B involved only the at rest pressure that develops due to a stiff connection at the top of the wall. As noted, the size of the at rest lateral loads transferred to the structure might
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CAPWAP® reach levels enough to rake the wall or develop relatively signifi cant horizontal defl ections. For certain soils, these movements could change the nature of the lateral pressure from at rest to active. As the active lateral pressure is smaller than the at rest pressure, its eff ects might result in moments that the soil underlying the toe of the pin would be more likely to sustain. Similarly, displacements and consequent active pressure can occur when the diaphragm does not provide a rigid support (e.g. defl ects under load). Th e wall structure will be less stressed. But the reduction in lateral pressure can occur only subsequent to some larger defl ections or raking, that is, after some possible damage has occurred to the structure. A structural engineer might be summoned only at this late stage, when he/she will be able only to determine whether the damaged structure has the ability to sustain the smaller lateral loads.
Conclusions
Th e sensitivity analysis was able to verify that the magnitude of the lateral forces transferred to the structure is dependent on the location and rigidity of the diaphragm, and on the capacity of the base of the underpinning to prevent rotation. Th e percentage of the total lateral force transferred to the building increases as the stresses at the bottom of the underpinning concentrate toward its toe, making rotation likely. Th e horizontal forces that develop can reach levels suffi cient to damage the wall diaphragm connections and even rake some poorly built or deteriorating structures. Th ese modes of damage match some of the distressed unreinforced masonry walls that were investigated following underpinning incidents.
Th e New York City Building Code’s upcoming requirement of considering the eff ect of lateral forces on structures being underpinned seems fully justifi ed.
Underpinning might be more eff ectively designed by a collaboration of structural and geotechnical engineers.▪
ExcEllEncE in Structural EnginEEring
NCSEA 15 th ANNuAl AwArdS ProgrAm
2012 Panel of Judges
The judging was held Wednesday August 7, 2012 in Orlando, Florida at the offices of Cuhaci & Peterson. The 2012 awards jury included the following individuals from the Florida Structural Engineers Association: Roberto Archila, P.E. Cuhaci & Peterson Luis Bedoya, P.E. BBM Structural Kevin Casey, P.E. Paul J. Ford & Company Luis Correa, P.E. Integral Engineering, Inc. Justin Gatzemeyer, P.E. Amore Engineering Tom Grogan, P.E. The Haskell Company Roger Jeffery, P.E. AMEC Environmental & Infrastructure Amy Miller, P.E. National Ready Mixed Concrete Association Ron Milmed, P.E. FSEA Southern Florida Brett Rylands, P.E. Cuhaci & Peterson Mark Scala, P.E. A. Mark Scala, P.E., Inc. A t their annual meeting in St. Louis, Missouri on October 5, NCSEA announced the winners of the 2012 Excellence in Structural Engineering Awards. This awards program annually highlights some of the best examples of structural ingenuity throughout the world. Awards are divided into eight categories: four building categories which are separated based on construction cost, bridge or transportation structures, international structures, forensic-renovation-retrofit-rehabilitation structures and an “other” category which encompasses all types of non-building or bridge structures. In each category, three award winners were named with one project being named the Outstanding Project. All structures must have been completed, or substantially completed, within the past three calendar years. The 2012 Awards Committee was chaired by Carrie Johnson (Wallace Engineering, Tulsa OK). Ms. Johnson noted: “We had an outstanding group of projects this year and the variety of entries was fascinating. The quality of entries and the complexity of projects continues to grow. The judges had an enormous task to evaluate all of the projects and they did an outstanding job. The judging was really close in several of the categories, and the judges indicated that they had an interesting time reading about the various creative ways structural engineers resolve unique and challenging problems. It continues to make me proud to be a structural engineer.” Please join STRUCTURE® magazine and NCSEA in congratulating all of the winners. More in-depth articles on several of the 2012 winners will appear in the Spotlight Department of the magazine over the course of the 2013 editorial year.
Robert Wood Johnson University Hospital Proton Therapy Vault
New Brunswick, NJ O’Donnell & Naccarato, Inc.
Robert Wood Johnson University Hospital’s New Proton Therapy building is designed to house two MEVION S250 Proton Beam Therapy treatment machines which emit positively charged atomic particles that can be focused precisely on tumors. The 4,900 square-foot, three-story below grade concrete structure is located directly adjacent to a one-story medical office building, beneath its parking lot. Due to the confined site, deep excavation, radiation shielding requirements and coordination requirements with the medical equipment’s tight tolerances, the team had to incorporate many unconventional and innovative solutions into the design to produce a cost effective project on a tight schedule.
Outstanding Project
Salvador Dali Museum
St. Petersburg, FL Walter P Moore Located in downtown St. Petersburg, Florida, the new 66,400 squarefoot Salvador Dali Museum is an engineered work of surrealistic art that houses the largest collection of Salvador Dali’s work outside of his hometown in Spain. The challenge for the engineering team was to balance the aesthetic needs of a building of architectural significance while protecting the art and providing a structure capable of withstanding hurricane-force winds and flooding. Perhaps the most stunning feature – both architecturally and structurally – is the 60-inch-tall helical central stair that is supported by a singular stringer beam and was inspired by Dali’s life-long fascination with DNA structure.
Milstein Hall, Cornell University
Ithaca, NY Robert Silman Associates
Cornell University’s new building for the College of Architecture, Art and Planning befits its role as a place where design is taught. The two-story structure contains flexible studio space, a presentation area, gallery space, and a 250-seat auditorium. The challenging design incorporates a reinforced concrete dome at the basement level that rises from the concrete foundation wall up through the first floor, where it is crossed by a reinforced concrete bridge. On the second floor, a series of five steel hybrid trusses, that incorporate features of conventional and Vierendeel trusses, allows the building to cantilever dramatically fifty feet over University Avenue.
Outstanding Project
Crystal Bridges Museum of American Art
Bentonville, AR Buro Happold Consulting Engineers, PC The 201,000 square-foot Crystal Bridges Museum of Art is an iconic museum inspired by the local Arkansas landscape and the exotic suspension bridges of Bhutan. Its complex geometric forms were made possible because of creative design, cutting-edge engineering solutions, BIM technologies, and a thoroughly integrated design team. Unique among the eight-structure campus are two of the “water” buildings. Their foundation acts as a weir creating a pool and serves as the floor. The roof, constructed of glulam beams and clad in copper with skylights, is suspended from stainless steel cables.
Harbor Drive Pedestrian Bridge
San Diego, CA T.Y. Lin International The Harbor Drive Pedestrian Bridge is one of the longest self-anchored suspension bridges in the world. It was constructed to provide a safe, elevated footbridge over the busy Harbor Drive and existing train and trolley tracks. The main span of the bridge is 354 feet and the pylon is 131 feet tall. The pylon is inclined at a 60 degree angle from the horizontal and leans over the deck to support the single pair of suspension cables. For this bridge, the main cable is completely enclosed in a continuous stainless steel guide pipe. The Harbor Drive Pedestrian Bridge serves as a southern gateway to downtown San Diego and truly is a bridge fitting for America’s Finest City.
Photos courtesy of Brooke Duthie.
Outstanding Project
Regent Emirate Pearl Hotel
Abu Dhabi, UAE DeSimone Consulting Engineers Located amongst palaces and high profile skyscrapers on the Cornich Street of Abu Dhabi, the new mixed-use $287 million Regent Emirates Pearl development will rise and twist 840 feet above ground. The expansive plot area of 146,500 square feet provides some of the best views of the Capital City. The Pearl’s signature feature is the 45-story twisting elliptical floor plan and columns which contains 60 luxury serviced apartments occupying levels 1 thru 10 and a 5-star hotel occupying levels 11 thru the Roof. In plan, each floor rotates 0.56 degrees each level, with a total of 25 degrees of total rotation from Level 1 to the Roof. The expansive podium area includes five levels of restaurants, retail areas, spas, swimming pools, gym and more, with another 5 levels of underground parking. The total project build up area is 55 stories and 1.4 million square feet.
BC Place Revitalization
Vancouver, British Columbia Geiger Gossen Campbell Engineers, PC Decades after its opening, BC Place Stadium required updating. The stadium’s primary systems, the roof aside, remained sound and renovation was preferable to new construction. The original air-supported roof was deflated in May 2010 and renovations completed in September 2011. The new retractable roof and clerestory provide an open-air, or an enclosed but naturally-lit, configuration. The energy-intensive snowmelt required by the old air-supported roof is eliminated. Programmable, energy-efficient architectural lighting animates the venue. Revitalization was achieved with minimum intrusion and avoided costly and time-consuming structural alterations, resulting in a world-class, modern facility that is a new iconic Vancouver landmark.
Photo courtesy of David M. Campbell.
Photo courtesy of Michael Elkan.
Outstanding Project
Van Alen Books
New York, NY Robert Silman Associates
Van Alen Books was conceived of as an installation for the Van Alen Institute’s small storefront, intended to extend their mission of “promoting inquiry into the processes that shape the design of the public realm”. The store’s primary design feature is a seating area created from cantilevering recycled door stacks suspended
dramatically by steel rods from the existing wood joists above. Visitors to the store are encouraged to use the “steps” to peruse the design-related books that line the signature yellow walls. The effect is a sustainable, stunning, and interactive environment that both reflects and supports the Institute’s vision.
Photos courtesy of Danny Bright.
Killam Oil Company, Ltd.
Hollywood Park, TX Beicker Martinez Engineering
When the client requested a building with an architectural design that dictated concrete wall spandrels with horizontal ribbons of glass, the designers invented new technology. Using typical tilt up wall panel construction, the building design connected spandrels of all the floor levels together with steel columns embedded into panels with standard stud anchors. As a result, the panels could be lifted like typical tilt up panels. The outcome was the creation of a new patentpending product: ClearView Composite Wall System™. Unlike traditional tilt walls, the resulting panels provide true horizontal bands of unobstructed ribbon glass with load bearing tilt wall cost efficiency.
Centra
Iselin, NJ DeSimone Consulting Engineers
Located in Iselin, New Jersey, Centra at Metropark involves renovating and adding to an existing 4-story office building. The design utilizes existing basement space by adding light wells and a central oculus, creating 20,000 square feet of usable below grade office space. An additional 10,000 square feet were added above the fourth floor roofline, creating a new fifth floor. The floor plate of the addition is rectangular in shape while the existing floor plate is L-shaped. A signature tree-like column was created to support this addition and also create a dramatic entrance. The completed building epitomizes the concept of sustainability by updating the codes, standards of efficiency, and function of an existing yet outdated facility.
Centre for Interactive Research on Sustainability
Vancouver, British Columbia Fast + Epp
A cutting-edge facility for environmental development, The Centre for Interactive Research on Sustainability (CIRS) at the University of British Columbia was designed as a research and education facility focusing on the importance of sustainability. With a target of LEED Platinum and the Living Building Challenge status in mind, the designers and architects set out to maximize the use of wood in the project. The 59, 202 square-foot (5,500 square meters) building was constructed entirely of the local renewable resource, a rarity in multi-storey academic structures. CIRS encompasses offices, lecture halls, exhibition spaces, and laboratories for research and testing.
Buckner Companies Home Office
Graham, NC Stewart Engineering, Inc.
Buckner Companies, a North Carolina-based steel erector, envisioned a unique headquarters that highlighted past projects and exposed the building’s structure. The result is a structure composed of salvaged structural pieces of projects that have been recovered and stored by Buckner over the last 62 years. Examples include the conference room which was built from girders recovered from a renovation of Clemson University’s Littlejohn Coliseum and a connecting bridge originally erected by Buckner in 1972 on the campus of UNC Chapel Hill. Instead of components resulting from design decisions, on this project, the components drove the design. Structural engineers were challenged to reinvent the component’s intended purposes.
VanDusen Botanical Gardens Visitor Centre
Vancouver, British Columbia Fast + Epp
Courtesy of Nic Lehoux.
The new 19,000 square-foot visitor centre at VanDusen Botanical Gardens provides an iconic entrance point to the grounds. Engineers and architects collaborated to develop a dramatic free-form roof, made almost entirely of timber. Three-dimensional technology, a product of the use of three different modeling programs, ensured accuracy and precision in the design and construction of the multifaceted geometrical shape of the unique roof. The building targets LEED Platinum and Living Building Challenge status, as its 71 unique, prefabricated panels champion innovation, economy and sustainability.
Sanford Consortium for Regenerative Medicine (SCRM)
La Jolla, CA Hope Engineering, Inc.
Courtesy of Bill Robinson Photography.
SCRM unites San Diego’s top research scientists under the “Collaboratory”, a 150,000 square foot facility with a total construction cost of approximately $85 million. Construction of the main building consists of two-way concrete flat-slabs, concrete columns, and shear walls on a conventional foundation system. “Punched” exterior shear walls serve to maximize interior natural light and provide seismic resistance. Exterior steel framed office “Pods” rest on cantilevered tapered concrete slabs extending over 16 feet from the supporting exterior wall. The auditorium building consists of elevated floors of composite steel beams with metal deck and concrete fill. The cantilevered perimeter framing creates an illusion of a floating structure.
Category 4 Award Winners
8 Spruce Street–Beekman Tower New York, NY WSP Cantor Seinuk
8 Spruce Street – Beekman Tower is Manhattan’s tallest residential building. Designed by Frank Gehry, for client Forest City Ratner, the 870-foot tower rises to 76 stories. The tower boasts a striking rippled facade, made possible by a ground-breaking structural design. Floor plates and slab edges are unique to each level, and innovative column, beam, and shear wall configurations accommodate the steel curtain walls, providing flexibility for a wide range of interior space configurations while also ensuring structural stability in high winds. At the foot of the building, a six-story podium contains a public school and ambulatory care center.
Category 5 Award Winners
Lake Champlain Bridge Replacement – Crown Point, NY
Chimney Point, VT HNTB Corporation
The Lake Champlain Pedestrian Bridge is a lifeline connecting two communities, Crown Point (NY) and Chimney Point (VT). The replacement bridge maintained the signature aesthetics that fit the landscape and provided a stronger, longer lasting bridge from start to finish in just over two years. Several team innovations helped to fast-track construction activities, including the decision to construct the arch span offsite, concurrently with the approach spans, and then float it into place. The team’s accelerated project delivery resulted in the bridge opening to traffic in just 20 months – a dramatically shorter time frame than normally associated with traditional methods.
Category 6 Award Winners
Marina Bay Sands
Singapore Arup
The Marina Bay Sands Integrated Resort was unrivaled in scale, complexity and speed of execution. The engineering design included a number of firsts for the construction industry as a whole. Project highlights include three curving, uniquely shaped high-rise hotel towers topped by a SkyPark (the world’s largest public cantilever, lifted into place with one of the highest strand-jacking operations ever undertaken), a lotus-shaped museum, and extensive casino, convention, retail and entertainment spaces. The development necessitated deep excavations across the site. Four cofferdams, among the largest ever used, were deployed at the unusual depth of 62 feet.
Photo courtesy of Timothy Hursley.
Kauffman Center for the Performing Arts
Kansas City, MO Arup
The 356,000 square-foot Kauffman Center for the Performing Arts serves as the focal point of Kansas City’s burgeoning arts district. To achieve the architect’s ambitious vision while providing excellent sound quality within the performance spaces, engineers helped design a box-in-box facility that is actually three separate buildings: two dense concrete performance halls covered by a lightweight steel structure with a glass wall and roof supported by a cable net. Structural engineers worked closely with other disciplines to achieve elegant solutions for fire protection, climate control and other issues. The thoughtful use of steel and concrete for different areas of the facility saved the client time and money.
Tempe Town Lake Pedestrian Bridge
Tempe, AZ T.Y. Lin International
The Tempe Town Lake Bridge connects bike and pedestrian paths from the north and south sides of the lake, allowing runners, walkers and cyclists to cross without having to compete with vehicular traffic at major intersections. The structure is a 4 span simple tied arch bridge, each comprised of tubular parabolic arches that lean into each other and cross at the quarter points, lending to its distinctive shape. Exposed “up-close” connections and simple railing details invite users to observe elegant, yet simple “engineering”.
Al Hamra Tower
Kuwait City, Kuwait Skidmore, Owings & Merrill LLP
At 413 meters (1,355 feet), Al Hamra Tower is among the tallest buildings in the world. Its unique sculpted form sets it apart from other towers. The structural system and exterior were developed symbiotically through digital design. A spiraling slice subtracted from a prismatic volume generates the building geometry and results in a cantilevered office wing that wraps around a courtyard. The two resultant cut surfaces are hyperbolic paraboloid reinforced-concrete walls that extend the full height of the tower and participate in the lateral and gravity force resisting systems.
J. Paul Leonard and Sutro Library
San Francisco, CA Simpson Gumpertz and Heger
In 2007, San Francisco State University undertook an extensive program to remodel, retrofit and expand their existing library. The expansion included a complete seismic upgrade, architecturally significant study spaces, and a high-density vault to house the majority of the library’s collection. This was a design-build project, which allowed Structural Engineers to work closely with the architect, owner and contractors while developing design solutions. A performancebased approach to the retrofit resulted in an economical and effective design. For example, engineers designed concrete shear walls to rock on new shallow foundations, eliminating a need for new micropiles.
Photo courtesy of David Wakely.
Atrium Operable Roof – California Academy of Sciences
San Francisco, CA Walter P Moore
The California Academy of Sciences commissioned the design of a replacement operable roof over the Piazza, a glass-covered central atrium. Key factors in the design were visual integration with the Renzo Piano-designed Academy, a high-tech aesthetic, durability, and ease of operation. Eight stainless steel arches span 64 feet across the existing glass roof, blending seamlessly with the existing structure. Lightweight, translucent polycarbonate panels slide along the top of the arches via highstrength, low-friction slide pad assemblies. To ease operation and maintenance requirements, engineered plastics requiring no lubrication were used for exposed components, and roof movement is operated remotely via iPad.
Photo courtesy of David Wakely.
Chelsea Piers, Connecticut
Stamford, CT WSP Cantor Seinuk
Chelsea Piers, Connecticut is a 400,000 square-foot sports, entertainment and educational facility. Formerly a Clairol manufacturing plant, the adaptive reuse design saved the old building from being demolished and ending up in a landfill. An economic solution to remove 23 existing columns was necessary in order to achieve 100-foot column free zones. Leaving the entire roof structure in place, king post trusses constructed out of the in-place existing roof structure allowed for the columns to be cut away. The structural engineering solutions were creative, economical and sustainable, resulting in limited demolition and limited use of new materials.
Miami Beach Soundscape
Miami Beach, FL Douglas Wood Associates, Inc.
Soundscape is a small park in lively South Beach. It’s also a state-of-the-art theater for live projection of concerts from the adjacent symphony and for public movie and art presentations. The park’s primary structures are three large aluminum pergolas (shaped to resemble cumulus clouds) and numerous outdoor theater elements, including giant “Ballet Bars” and a project tower (enshrouded in its own “cloud”). Undulating concrete seating walls and a maze of concrete walkways complete the project. This project stands out for accommodation of its complex audio-visual systems within a sculptural expression of its structural systems.
Photo courtesy of Robin Hill.
