A workhorse with showhorse trappings as well, the $289-million Sixth Street Viaduct in Los Angeles features a series of technical innovations that sets a new threshold for seismic safety. The 3,060-ft-long, six-lane concrete-tied arch bridge carries 20,000 vehicles per day on average. The bridge is supported by 10-ft-dia cast-in-drilled-hole piles that are up to 165 ft deep. A helical pedestrian ramp connects to a 12-acre public park.

The constrained site included U.S. Highway 101, four surface streets, an industrial area, the Los Angeles River, electrical power lines and 18 active railroad tracks operated by six railroad agencies. Property-use rights and groundwater issues presented additional challenges. Despite the ten spans having seemingly similar arches, their geometry and detailing varied enough so that the team used 3D BIM models of each knuckle joint to identify clashes and resolve constructibility issues.

The project advanced bridge engineering through a novel approach to seismic isolation by integrating the arch ribs with the Y-bent arms, which required placement of the seismic isolation bearings within the vertical height of the Y-bent columns. “This led to a new seismic isolation design methodology that requires the isolation system to protect the structure while the structure protects the isolation system,” says Michael H. Jones, senior civil engineer at HNTB. “Should the one-in-a-thousand-year seismic design event ever be exceeded by about a factor of two, a secondary seismic isolation system is activated that protects the isolation system.”

It is the first U.S. structure to be designed with post-tensioning multistrand couplers, according to Jerry Iniguez, vice president of operations at Skanska USA Civil.