Built in 1972, the building’s existing façade resembled a tiered wedding cake, which posed a myriad design challenges. The building presented a number of engineering challenges, not the least of which was an aggressive construction schedule. The design included connecting the new space directly to the adjacent hospital by filling in the wedding cake tiers of the Tower. But early analysis proved the existing columns and transfer girders were inadequate to support the infill framing. The existing Tower’s structure didn’t conform to the current seismic standards required by Philadelphia building code. An upgraded lateral system would need to be inserted into the existing building or the new construction would need to supply the added capacity.
Additionally, columns in the new South Tower were required to be transferred out at the third level to accommodate a new ambulance entrance beneath the Tower; steel transfer trusses were depth-restricted to 9’ 0” to provide adequate clearance for the ambulances while aligning the floors above with the existing hospital. And to meet schedule demands, the fit-out of the third floor had to begin as building erection continued above. These construction sequencing and fit-out issues required extremely stringent deflection limitations. Site restrictions also meant that each 35-ton truss had to be field-assembled.
The Phase II Northwest Tower was to be constructed above the operational loading dock area servicing not only CHOP but the adjacent Hospital of the University of Pennsylvania. This area was critical to the operations of both hospitals and therefore must remain open at all times during construction. At the third floor of the new tower, long span structure was required to provide an open space for the employee dining facility at ground floor. The new construction also required the relocation of the existing helipads.
Because the existing structure was inadequate to support infill framing, the tiered façade appeared to limit available space. But by threading four new columns into CHOP’s famous atrium and suspending floor framing above the existing hospital–from new long span trusses supported by the four columns—much-needed space was created in the South Tower.
The resulting design scheme featured new floor framing supported by hangers from roof-level trusses to fill the “void” above the wedding cake shaped existing building. Each truss was supported on two columns, one outside and one inside the building that threaded through the existing atrium, emergency department and other critical care units. A temporary erection column was used to allow portions of the infill framing to be built from the ground up with support from the existing hospital. After the roof-level truss was field-assembled, the temporary erection column was removed and infill framing was hung from the truss. Additional infill framing was added from the top down; floors were hung from the roof-level truss. Through it all, the hospital remained fully operational.
The solution to the loading dock area incorporated transfer trusses in the floor of the structure above. To achieve the open space in the employee areas, floors above were again hung from transfer trusses, this time located in the top floor which was occupied by a major mechanical level.
A careful sequencing of construction was required to achieve these multiple goals. The transfer trusses above the loading dock were design not only to support the floors up to the employee level in the final configuration but, also to support all of the structure during construction up to the roof. A temporary column placed on each truss at the employee level, served as support as the floors above were erected. The trusses were constructed and roof steel was completed while supported on this temporary column. Once the structure was complete, loads were jacked off of the temporary columns and they were removed creating, the open employee space. Using detailed structural analysis of the required procedures, the floors above were released to be hung from the trusses. The floors above were erected high to compensate for the truss deflections and the floors above settled into the final position.
Using structural steel enabled the design team to incorporate multiple transfers and trusses in a complex design.. The strength, relatively light weight, and the ability to assemble and dismantle steel trusses and temporary supports proved critical to meeting the many transfer requirements. And the steel trusses provided the stiffness needed to limit deflection. Choosing steel also expedited the tight construction schedule and met the complicated demands of construction sequencing.
The Harman Group not only provided Structural Engineering for the design team but also Construction Engineering for the construction team to assist in implementing the complex procedures and temporary works for this project.
Outstanding Project/First Place Award, Buildings Over $300 Million, Delaware Valley Association of Structural Engineers (DVASE)
Outstanding Project Award, Delaware Valley Association of Structural Engineers (DVASE)