New United States Courthouse – Los Angeles – Structural Engineering

SOM’s design concept for the New United States Courthouse in Los Angeles began with the idea to “float” the building above its surrounding plaza. To achieve this, the firm engineered a three-dimensional steel roof hat truss system that cantilevers symmetrically beyond the robust cores of the building and over to its sides. In doing so, the system supports the outer bay of the courthouse, allowing the cubic volume to appear as a singular, hovering form. This creates standoff distance between perimeter building columns and the curb line, which helps protect the facility from blast threats.

Due to the location of the building, seismic design was also a critical consideration for its structural engineering. To control lateral drifts, the roof truss is utilized as a mega link beam that connects the reinforced concrete shear walls at the courthouse’s top story, which reduces ductility demands during seismic events. In testing, this resulted in a change in the lateral mode of deformation from single curvature to double curvature — more typical of an outrigger system — reducing peak seismic story drift ratio by 30 percent. Buckling restrained braces were found to be the most efficient linking brace member, using 2000 KIP maximum capacity elements.

The design of the truss system was inspired by the results of topology optimization iterations, evaluated for efficiency using Maxwell’s theorem of load paths. This optimization is similar to the optimal layout proposed by Michell’s work on frames of least weight, or Michell trusses, which represent the stiffest layouts for the least amount of material in a continuum. SOM’s final truss geometry, which resembles a bicycle wheel, resulted in a 13 percent reduction in total steel weight for the trusses compared to traditional configurations.