Residence with Dramatic Roots
The Harper, a 25-story, 260,000 square foot mixed use residential building in Philadelphia, which features 183 apartments and two floors of retail. Located at the site of the former Boyd Theater, The Harper includes premier retail space, creative office space, an outdoor park with a pool and an underground garage. The building tops out at 314 feet above grade and is the tallest all-steel and hollow core concrete plank structure in the city with no concrete core.
The Harper features several creative solutions to structural problems posed by the owner and the architect. Of paramount importance to the retail plan was the elimination of every other column on the standard 24’x26’ column grid that was used in the parking area and the apartment tower. As a result, outside of the tower footprint we have long-span steel girders which enable large column free spaces in the retail. Below the apartment tower, we transferred out nine columns using five stick-framed trusses and two plate girders.
The owner was sensitive to the costs incurred by transferring out columns to free up more space in the retail areas and asked how we could reduce structural costs. The Harman Group, now IMEG decided to use grade 65 A572 steel for all columns and truss members 90 pounds per foot and heavier. As a result we saved approximately 200 tons of material, saving the client hundreds of thousands of dollars. According to Arcelor Mittal steel, The Harper is the first residential tower, and only the second building in Philadelphia, to use grade 65 steel.
In addition to the column transfer and use of grade 65 steel, part of the challenge of building on this site was the limited amount of laydown area for the steel and limited crane capacity. This lack of space and crane capacity, typical for an urban environment, meant that the transfer trusses could not be built on the ground and lifted in one piece to the transfer level. Zero camber can be difficult to achieve when stick-building trusses. The top or bottom chord of the truss is erected first and will deflect significantly under its own weight. If not corrected, the transfer columns will start off too low, causing the entire building to be too low at those column locations.
The trusses were stick built and the deflection in the top chords, just from self-weight, was between 5/8 of an inch and 1 ¼ inches. To correct for this deflection, first the trusses were erected, all of the bolt holes were stuffed, but not tightened. Then the steel erector jacked up the trusses to set zero camber. Shoring and spreader beams were added to distribute the load from the jacking operations. Then, the tower crane was used to pull up on the truss until the crane hit its load limit, and then jack pressure was applied. As the jacks took load off the crane, the crane applied more force, and so on until the trusses were level.
After the surveyor called the truss at the proper elevation, all of the bolts were torqued to spec (1 1/8” A490-SC TC bolts). When the bolts were tightened, the jacks and the crane were released and the surveyor did a final check of elevation. After torqueing all the bolts, deflection due to self-weight of the truss was minimal, 1/16 of an inch or less (almost exactly zero camber).
Through creative engineering, the Harman Group, now IMEG was able to bring the vision of the architect and the owner to life while staying on budget.
This project was awarded a Merit Award from the Delaware Valley Association of Structural Engineers.View the Gallery Print PDF
Structural Systems Used
Structural Steel Frame