By Kevin Wilcox
Engineers develop an innovative structural system to meet the challenges of a steeply sloped site in Los Angeles.
The 34 ft cantilevers in all directions create the impression that the building is floating, especially at night. Photo by Bruce Damonte, courtesy of SOM
March 15, 2017—The new federal courthouse in downtown Los Angeles casts a modern, elegant silhouette against the city's skyline. The 10-story building is a cube of pleated, tempered glass that gives the impression that it is floating above the site when viewed from the street. Beneath this stylish exterior, however, is a complex structural engineering system.
The 633,000 sq ft building was designed and engineered by the Los Angeles office of Skidmore, Owings & Merrill LLC (SOM). The design and engineering direction was led by Craig Hartman, FAIA, the senior consulting design partner in SOM's San Francisco office, and Mark Sarkisian, P.E., S.E., LEED AP, M.ASCE, a structural and seismic engineering partner of the firm who also works in the San Francisco office. SOM worked with Clark Construction Group, LLC, in a design/build partnership that delivered the project in just three and a half years from design competition to completion.
The U.S. General Services Administration owns the building, which houses many of the courtrooms and judicial offices of the U.S. District Court for the Central District of California. The building occupies a steeply sloped site bounded by First Street, Hill Street, and Broadway, which presented design and engineering challenges.
The General Services Administration asked the teams that entered the competition to provide the best possible value while accommodating the programmatic needs of a busy federal courthouse and to include 24 courtrooms in their designs. A committee led by Margaret Morrow, then a federal judge, also charged the design team with creating a great work of civic architecture that would make room for art and embody a high level of environmental stewardship, recalls Hartman.
"They were very high-level directions, but they really demonstrated a . . . concern for making a great work of architecture," Hartman says. "Our vision [was to] make a building that really honored the civic agenda and mission as a courthouse and [that would] clearly be a strong civic addition to the city of Los Angeles."
The building is elevated 30 ft above a steeply sloped lot in a prime location. Photo by Bruce Damonte, courtesy of SOM
In pursuit of simplicity, clarity, and beauty, the architectural team arrived at the concept of a cube elevated approximately 30 ft. The building is set askew of the Cartesian grid of the city, as it faces First Street. Because of this, SOM developed the concept of a tempered glass facade with panels that would fold outward at an angle of 38 degrees, placing the glass in line with the grid.
"The folded glass system, if you look at it in plan, is essentially a triangle," Hartman says. One side of that triangle, which is exposed to the sun, is opaque to minimize solar gain. The other side is transparent glass, affording "an enormous amount of transparency [from] inside—the judges' chambers, the jury assembly areas . . . have tremendous views."
Although this facade system carried a higher price than conventional flat glass, it not only creates visual richness for the building's exterior but also reduces solar heat gain by an impressive 48 percent, Hartman says.
The interior of the building is sleek and modern, with floating, cantilevered staircases, glass railings, and balconies that surround a dramatic central atrium that extends the height of the building, flooding the interior with natural light. Perimeter corridors that serve the judges' chambers and jury areas are naturally lit and feature an undulating wall created by the angled facade. Such materials as natural stone, stainless steel, and glass were chosen both for their durability and their ease of maintenance.
The sloping site, which is 28 ft higher at Hill Street than at Broadway, was already cleared as part of a previous effort to develop a courthouse at the site. The site's slope was a key element in the decision to raise the building above the topography and essentially suspend it via trusses from four large concrete cores.
A dramatic central atrium is topped by skylights. Photo by Bruce Damonte, courtesy of SOM
"The sloping topography is very visual and evident, but the simplicity of the civic architecture is also very clear," Hartman says.
Because the building is cantilevered approximately 34 ft in all directions from the four cores, the site has much more public space than if it were occupied by a more conventional tower. Meandering trails connect Hill Street to Broadway in a manner that meets the requirements of the Americans with Disabilities Act.
Hanging the building from the four robust towers of ductile reinforced concrete was one of the key engineering challenges of the project, says Sarkisian. "Typically, with a federal building we are very concerned about columns that meet the street and any potential threats," he explains. "The conceptual design was to remove all of . . . the columns at the bottom of the building. So, all of the load—whether it is lateral load or gravity loads—is essentially resisted by the four legs. These walls, more or less, do everything."
The soil conditions at the site were good, enabling the engineers to employ a mat foundation. In the areas that would receive the heaviest loads, the soils were strengthened by the addition of soil-cement piles. The thick concrete cores are enclosed in stone to form a dramatic entry point for the building, with spectacular views of the expansive central atrium.
"The thing I believe makes this a really good idea is it creates these resilient capsules, or pathways, within a service area of the building—elevators, stairs—where concrete is there to create an enclosure but also to resist the structural loads," Sarkisian says.
The cores are connected at the top with unbonded steel members on bonded braces. These braces are designed to add ductility to resist seismic forces during an earthquake. The braces would concentrate any damage in one area and were designed so that they can be replaced if needed.
The building was designed to maximize natural light while minimizing solar gain. Photo by Bruce Damonte, courtesy of SOM
The rest of the structure employs a lightweight steel system, which also offers an advantage during an earthquake. The team employed a three-dimensional steel truss system proffered more than 100 years ago by the mathematician Anthony George Maldon Michell. The trusses are arched; Sarkisian likens them to half of a bicycle wheel when viewed in cross section. The truss design is so efficient that the team could reduce the amount of steel used for the cantilevers by 25 percent.
"So here was a mathematician who described the shape of these perfect cantilevers, and we use the same principles to come up with the design of these cantilevers," Sarkisian says. "It's a component in the building that is based on mathematics and fundamental engineering concepts which may be used in other ways and in other buildings."
These cantilever trusses support the perimeter columns of the building, which are inset approximately 6 ft to enhance views from the exterior hallways and corner office suites. Because these columns are in tension, rather than compression, the members can be smaller, Sarkisian notes, further reducing weight.
"Most importantly, we addressed the issue of redundancy of the perimeter frame," he explains. "The frame is fully moment connected so that if, for whatever reason, there was a loss of an element within the frame, loads could be redistributed to neighboring columns and beams."
This structural system created a challenge for the construction team. As Sarkisian explains, even though the perimeter system doesn't meet the ground, "you need to build it from the ground up. Clark Construction did an excellent job with this."
The perimeter of the building was built on a series of temporary columns. Because the perimeter columns are in tension, the building would eventually have to be lowered into place when it was complete by using jacks to lift temporary columns and then remove shims placed below these columns. This meant the engineering team had to precisely calculate exactly how much the building would move at multiple points along those columns.
"We built the perimeter frame high relative to the core," Sarkisian says. But the top floors were not built as high as the bottom floors because when the tension loads were placed on the columns, the overall column displacements would be greater at the bottom than at the top.
"So, we had to come up with an engineered idea of where the building needed to be during construction and then where we expected it to be in the final case," he explains. "And we were, I have to say with great appreciation, pretty much right on after the loads were transferred from the temporary columns."
The building opened in late 2016 to positive reviews from the public and the judicial community. "It's a unique building," Sarkisian says. "But we found it to be, given the site and the concerns, . . . a very appropriate idea."