Cantilevered Cube Complicates L.A. Courthouse
Thanks to a structural-steel hat truss, the 10-story tower of the 240-ft-tall New United States Courthouse, Los Angeles, appears to balance on its core, hovering over the ground as if it were a sculpture on a pedestal.
The design, which elevates the bulk of the cube-shaped building off the ground and hangs it from the penthouse truss, improves defenses against vehicular assault at the street level. But minus perimeter columns at grade, the steel structure of the boxy tower was not self-supporting during construction. That meant some extra legwork—including an elaborate falsework system complete with hydraulic jacks—to build the $400-million courthouse, which is 70% complete.
“In a typical building, you have perimeter columns that meet the ground,” says Mark Sarkisian, structural and seismic engineering partner in the San Francisco office of the project’s architect-engineer Skidmore, Owings & Merrill LLP (SOM). “In this case, those perimeter elements don’t reach the ground,” he adds. They stop at the second floor.
“Since we had a building with a cantilevered perimeter that wouldn’t support itself until we completed the hat truss, it was our job to design and deliver falsework,” says Bob Hazleton, vice president of the Herrick Corp., the job’s Stockton, Calif.-based steel fabricator-erector.
The falsework was a system of 24 stilts—42-in.-dia pipe columns—around the perimeter of the 220-ft square building. Each 50-ft-tall stilt was installed below a permanent column location so that each perimeter column could be erected directly on top of it.
In essence, the stilts served as perimeter columns up to the second level. That strategy allowed a conventional construction sequence of the steel frame, says Sarkisian.
Clark Construction Group of California started work on the courthouse in August 2013 under a design-build contract with the U.S. General Services Administration. The courthouse, which sits on a sloping 3.6-acre site, is on time and on budget, says Duane Allen, GSA’s project manager. He credits the design-build process for keeping the courthouse on track for its opening next July.
The building, which contains 24 courtrooms and 32 judges’ chambers, will house the U.S. district court and central district court of California.
The 600-ton hat truss consists of an orthogonal grid of twelve 220-ft-long structural-steel trusses that span opposing perimeter columns across the top of the building’s reinforced-concrete shear-wall core. Each 17-ft-deep truss is composed of welded-steel, wide-flange top and bottom chords. Trusses are embedded in the core walls.
Buckling-restrained braces in the trusses link the core wall elements. “The braces significantly reduce drift and increase the performance of the system,” says Sarkisian.
The hat truss supports approximately 6,000 tons, or the load of 10 floors. Each floor cantilevers 35 ft from the core, which forms a 160-ft square in plan. Truss fabrication lasted nearly four and a half months, due to the requirements for the welded connections, says Hazleton.
Crews first cast the reinforced-concrete foundation. Then, in early 2014, starting in the basement, workers installed the silts. Each stilt sat on 2 in. of four stacked shims. The stilts—each with a steel jacking collar around its base that, ultimately, would help to remove them—extended up 50 ft to the underside of the building’s second level, according to Patrick M. Hassett, the owner of Hassett Engineering, which developed the stilt’s connection to the permanent column under the second floor and engineered the jacking system.
The collars were composed of two parallel horizontal rings separated by four vertical stiffeners to “spread the eccentric jack forces [evenly] around the pipe’s wall,” says Hassett.
Beginning at the second floor, construction continued in a conventional manner. At the top of the building, Herrick took one month to erect the trusses and another two months to complete the field welding of all the connections, says Marshall Singh, Clark’s project executive.
After workers completed the truss installation, crews removed the stilts in a carefully sequenced order, using hydraulic jacks. Four jacks were placed under each collar of each of the 24 stilts.
The shims were removed in a clockwise sequence, cycling twice around the base. Two shims were removed in each cycle.
The jacks lifted the stilt just enough to slide a piece of paper between the stilt and its four shims. That released the pressure and allowed workers to slide the shims out, says Andrew Kregs, SOM’s project engineer.
For each individual jacking operation, crews removed the shims from three pairs of opposing stilts. “We didn’t want differential deflection of adjacent trusses,” says Krebs.
The synchronized operation began in May and took three days. Workers then spent a week cutting out the stilts, first removing the upper half.
During the tower’s steel erection, permanent columns were in compression as loads traveled down to the stilts. When crews removed the shims, the permanent columns engaged and began hanging from the hat truss. The trusses deflected as calculated, causing about 1.5 in. of downward movement, says Kregs.
To accommodate the anticipated movement, Clark crews built the floors around the perimeter of the tower at a slightly higher elevation than the final elevation.
To anticipate all final elevations, the team developed a virtual computer model of the building. “A full 3D model of all foundations, walls, beams, columns, slabs and braces was created and used to coordinate structural elements” with the architecture and building mechanical systems, as well as to produce construction documentation, says SOM’s Sarkisian.
In addition to the building information model, Herrick created a detailed 3D connection model of the hat truss. Also, SOM modeled the reinforcing steel in the shear walls at key coordination locations.
“These models were all critical for proper [preconstruction] coordination between multiple subcontractors,” says Sarkisian.
The building is going for the highest rating—Platinum—of the U.S. Green Building Council’s LEED certification system. A combined mechanical plant for cooling, heating and power has two 5,000-amp substations and a 2500-kW diesel generator for emergency power. The system can be converted to biomass as a fuel. A 400-kW, roof-mounted photovoltaic array is designed to produce up to 525,000-kWh annually.
The project’s 220,000-sq-ft pleated and glazed curtain wall gives the courthouse a variable appearance that changes as the sun moves while reducing solar heat gain by nearly 50%. The glass cladding makes the 627,000-sq-ft courthouse appear lighter and more transparent, says José Luis Palacios, SOM’s design director.
The pleated facade is a response to the downtown street grid, which is about 38° off of a true north-south axis. Because the courthouse aligns with the street grid, transparent panels on the north and south sides maximize daylighting, while opaque panels on the east and west sides minimize solar gain, says the architect.
GSA’s Allen says that while the design-build process has saved time and money, it does require the team to manage design work. “The design has been ongoing for the past two and half years, even though construction is underway. It gives everybody a sense that they can continue to design, and at some point we’ve had to say, ‘No, the design is done. We need to get the building built,’ ” he says.