exposed structural steel

Connecting

on Campus By James Pospisil, P.E.

A new exposed steel pedestrian skyway links two parts of the Duquesne University campus.

Duquesne University is situated for the skyway, the idea of using an exposed would be prefabricated near the site in one on an urban campus in the Hill District of structural steel frame was conceived. The lane of Forbes Avenue while the towers and Pittsburgh. Bound on the south and west ability to prefabricate most of the structure 14 ft of cantilevered bridge span were being by a formidable cliff and the north and either in the shop or near the site allowed erected on each side of the street. Erection east by city streets, the university has faced the design team to resolve construction dif- of the center portion of bridge would occur challenges in meeting the growing need for ficulties related to the small site and accel- in a single lift on a single weekend. Dnew facilities. erated the construction schedule. Architec- During the tower assembly process, This January, a new multi-purpose turally exposing the steel simplified many plumbness was maintained to within ¼ in. facility with a ballroom, fitness center, and details while mirroring existing architec- to ensure that the prefabricated center span retail space for the university was com- ture on campus and highlighting the steel would fit. Erection of the center span was pleted north of campus across Forbes Ave- heritage of Pittsburgh. planned for early morning so it could be nue. During the initial planning phase for installed “cold,” reducing thermal growth the new building, a pedestrian skyway was Constructability Challenges and minimizing the overall length. To keep envisioned to connect the parking garage A key parameter affecting many aspects the visible connections clean, only ½ in. of and main campus to the new building. The of the design was the city’s desire to limit shim space was provided at each end of the skyway would improve student access to the ongoing closures to Forbes Avenue to one center span, allowing for tower plumbness new facility by creating a safe way to cross lane and total closure of both lanes to one and center span length tolerances of 1 in. the street, and would also function as the weekend. In addition, the close proxim- maximum. The total time needed to erect primary conduit between the new build- ity of the parking garage to the south and the steel frame was approximately four ing and the central campus steam, chilled ongoing construction of the new building weeks. water, and telecommunications infrastruc- to the north added to the complexity. With ture. In addition, the skyway would serve a only one lane of Forbes Avenue accessible Trussed Towers symbolic function as well, as a gateway to for construction, and a minimal lay-down The primary structural components the campus and Hill District. Construction area between Forbes Avenue and the park- of the skyway include trussed towers on of the skyway would take place concurrent ing garage, careful planning and extensive each side of Forbes Avenue, with a central with that of the new building, due to the prefabrication were necessary. Vierendeel truss span partially supported by critical nature of the infrastructure connec- Critical splice locations were identified two tapered plate girder arches. The trussed tions, and a total of nine months was allot- to maximize shop prefabrication. To elimi- towers measure approximately 16 ft by 16 ted for building the skyway. nate the need for long-term street closures, ft and have a network of rod and clevis X- During the schematic design process the center 74-ft portion of the bridge span brace assemblies providing stiffness and sta-

MODERN STEEL CONSTRUCTION may 2008 Above: One inch was allowed for tolerances when erecting the prefabricated center span. Right: The tower columns support the pedestrian bridge; access to the bridge occurs through the adjacent structures. bility. The south tower is more than 140 ft ported by a walkthrough Vierendeel truss and overall stability. After evaluating the tall, while the north tower is 100 ft tall. with intermediate ties to the tapered plate subassemblies, the structure was evaluated In each tower, horizontal X-bracing lev- girder arches. The chords of the Vierendeel as a whole. A three-dimensional finite ele- els occur every 19 ft and are comprised of truss are made up of continuous W14 ment model considering wind, seismic, live, 2-in.-diameter 36 ksi rods with a #5 clevis members. Vertical truss elements are HSS and temperature loads was used to verify and 2-in.-diameter pin at each end. Three 10×6×1 with bolted end-plate moment con- member stresses and structure deflections. horizontal bracing levels occur between nections to the truss chords. The tapered Due to the height-to-width ratio of the ground and the bridge deck. Verti- arches, while aesthetically important to the the trussed towers and the concentration cal X-bracing consisting of 2-in.-diameter architecture of the bridge, are structural in of lateral load more than 75 ft above the 50-ksi rods with a #8 clevis and 3-in.-diam- nature. They provide intermediate support base, very large overturning moments are eter pins occurs at every level except the to the Vierendeel truss and add significant present at the foundations. Further com- street. To fully develop the higher strength lateral stiffness to the structure by function- plicating the foundation design were the vertical rod bracing, up-set threaded ends ing like a knee brace between the bridge physical constraints imposed by the exist- were specified at the clevis connections. ing parking garage at the south tower and Tower columns are made up of multiple new building at the north tower. Different 3 3 HSS 10×10× ⁄8 members with HSS 16×8× ⁄8 Architecturally exposing foundation systems were chosen for each horizontal members passing between at tower to accommodate the specific design the bracing levels. To allow for pedestrian the steel simplified requirements. access to the garage and new facility, verti- The south tower used a spread footing cal X-bracing was omitted on the first level. many details. bearing on shallow bedrock. Because the Here, at the weakened story, each tower col- existing parking garage foundations were umn was made up of four HSS 10×10 mem- within inches of the new tower, overturn- bers acting as a moment frame. Between the and the towers. The total span of the arch ing resistance of the foundation could not first braced level and the bridge deck, the and truss system between the towers is 102 be achieved by using a large footing. Tak- tower columns were reduced to three HSS ft, and approximately 240 tons of structural ing advantage of shallow competent bed- 10×10s, and above the deck they were fur- steel were used in all. rock, a permanent post-tensioned rock ther reduced to two HSS 10×10s. The col- anchor system was chosen to achieve over- umn reductions could occur above the first Foundation Stability turning stability. A total of six 150-kip rock braced level because the towers behave more With the structural concept and basic anchors were used to anchor the founda- like a truss. The HSS 16×8 horizontal mem- construction sequence in mind, detailed tion to the bedrock. The post-tensioning bers passing between the column groups analysis and design of the individual fram- force and anchor layout were chosen so at the tower corners are spliced with end ing components and subassemblies was the footing would be in constant contact plates at the center. This detailing approach performed. The structure was analyzed as with the bedrock, even under the extreme allowed for the complicated welded connec- three individual subassemblies: the north overturning load. tions at the columns to be performed in the tower and cantilever, the south tower and At the north tower, lower bedrock and shop, while simpler bolted end-plate con- cantilever, and the center bridge span. Con- the concurrent construction of the new nections were used in the field. struction and environmental loads were building resulted in a much different foun- The concrete bridge deck is approxi- considered, and the three primary subas- dation design. To resolve the overturning mately 75 ft above the street and is sup- semblies were evaluated for member stress forces, an aggregate-filled concrete ballast

may 2008 MODERN STEEL CONSTRUCTION forces; therefore, rock anchorage was not problems were identified by the design required. team and integrated into the structural and The superstructure-to-foundation con- architectural concept for the skyway. And nection involved transferring moment and of course, success was also the result of the shear in both directions in combination design flexibility afforded by exposed struc- with vertical load either up or down. A non- tural steel. bonded post-tensioned anchor bolt system was chosen to simultaneously resolve all James Pospisil is an associate with Barber & loads. Post-tension forces were chosen to Hoffman, Inc. achieve a constant minimum contact pres- sure between the tower base plates and the Owner concrete piers, eliminating cyclical loading Duquesne University, Pittsburgh on the anchors while maintaining enough Architect contact pressure to resist shear through WTW Architects, Pittsburgh friction. A total of eight anchors were post- Structural Engineer tensioned to 50 kips at each tower column. Barber & Hoffman, Inc., Cranberry After tensioning and testing, the anchors Township, Pa. were grouted, and custom steel anchor caps were filled with grout and welded into General Contractor place to protect the anchor heads. Tedco Construction, Carnegie, Pa. Steel Fabricator Steel rods and clevises provide visually unob- trusive lateral bracing for the supporting Successful Skyway Mazzella Welding & Fabrication, Inc., towers. The successful completion of Duquesne Wellsburg, W.Va. (AISC Member) University’s skyway was due in large part to Steel Erector box was designed. The ballast box is sup- early interaction between the design team Century Steel Erectors Company, ported by caissons founded on bedrock and the steel erector and contractor. With Dravosburg, Pa. (TAUC Member) and was configured so that the new build- input from each entity, critical construc- Structural Engineering Software ing could be constructed without affect- tion issues were raised and addressed while RAM Advanse ing the stability of the tower. The weight the design was in the early stages. Practi- of the box was used to resist overturning cal solutions to anticipated construction

MODERN STEEL CONSTRUCTION may 2008