Rising to the Challenge: 1200 Intrepid
The east facade of the four-story 1200 Intrepid office building in the Philadelphia Navy Yard resembles a shockwave. The striking architectural precast design on the $19 million structure is possible thanks in large part to innovative engineering from the team at High Concrete Group LLC.
Architect BIG created the multi-radius shape to visually connect the building to a circular track and a park in front of the building. The radii vary from just over 320’ at the ground floor to just over 8,900’ at the parapet. One of the key engineering challenges was to create the curvature from precast panels.
“I was itching for a challenge, something new and unique,” said High Senior Engineer George Burnley. “When I saw the initial sketch I said, ‘Let’s try it.’”
The 4”-thick panels have 16” returns and a variable chamfer ranging from 3-⅝” to 10-⅝”. The tops of the panels are not as wide as bottom due to arc length decreases as the ever-larger radius moves up the facade. High made the varying chamfers by exporting the complex geometry from a BIM model to a CNC foam cutting machine to produce the negative image of each unique panel. High coated the foam with rosin for a smooth finish in the precast concrete forms.
Each panel is supported laterally with four pivoting tie-back connections custom-designed by High. The connections are unique in that they allow the panels to be connected to the structural framing and accommodate all the panel variations between plumb and 22-⅓ degrees at the center of the facade. The entire curved facade was done with only one lateral connection detail. The pivot action minimizes the lateral movement of the facade due to the vertical deflection of the steel framing. At the request of the owner, a secondary connection was added to the middle of each panel to serve as backup support.
The building’s steel frame was designed to support lateral loads, but not gravity loads. So the team designed the 8,000 to 13,000 lb. panels to support each other, taking the weight to the foundation. The resulting facade creates an aesthetic versatility that is unique to the project.
“We had to figure out the mechanics of the load support,” said Burnley. “The panel support travels at the facade angle, but gravity is vertical.” The tie-backs brace the panel to allow for the inclined load support.
Pockets formed into the sides of the panels allow them to interlock, supported by two 2-½”-thick shear bars cast into each panel. Panels above the windows support the panels on either side of the windows, which in turn are supported by the panels below the windows, and on down the facade. At first, the connections were planned to be above the floors, but because of window and sealant details, they were located under the floor near the bottom of the steel beam.
Also unique to the project are bearing devices designed to be flush to the panel backs to allow them to stack for load support. The solution simplified caulking and finishing. “This was courageous,” said Burnley, who doubts that it has been done before. “It solved a lot of problems.”
One of the largest fabricators of precast concrete buildings in the United States, High Concrete Group has earned a reputation for taking on the large and challenging projects that others can’t or won’t do.
“It was a complicated project,” said Burnley. “Early during the conceptual design phase I could have turned the job down and no one would have questioned me.”
After a pause, he smiled and said, “But this is what we do.”
George Burnley retired in September 2020 after more than 35 years at High Concrete Group.
High Concrete Group LLC