Replacement of the sixth street viaduct
Los Angeles
Better road/bridge
Presented by: HNTB Corp. and Skanska/Stacy and Witbeck JV
Region: ENR California
Owner: City of Los Angeles Engineering Office
Lead Designer/Structural Engineer: HNTB Corp.
Principal Architect: Michael Maltzan Architecture
CM/GC: Skanska-Stacy and Witbeck
Assembly Engineer: COWI
Bearings: Earthquake protection systems
A workhorse with showhorse trappings as well, Los Angeles’ $289 million Sixth Street Viaduct features a number of technical innovations that set a new threshold for seismic safety. The 3,060-foot-long, six-lane concrete tied-arch bridge carries an average of 20,000 vehicles a day. The bridge is supported by 10-foot-diameter bored piles that are up to 165 feet deep. A spiral pedestrian ramp connects to a 12-hectare public park.
The restricted site included US Highway 101, four surface streets, an industrial area, the Los Angeles River, power lines, and 18 active railroad tracks operated by six railroad agencies. Property rights and groundwater issues presented additional challenges. Although all ten spans had seemingly similar arches, their geometry and details varied enough that the team used 3D BIM models of each hinge joint to identify clashes and resolve constructability issues.
The project advanced bridge engineering by taking a new approach to seismic isolation by integrating the arch ribs with the Y-bent arms, which required the placement of the seismic isolation pads within the vertical height of Y-inclined 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 be exceeded by a factor of two, a secondary seismic isolation system is activated that protects the isolation system.”
According to Jerry Iniguez, vice president of operations for Skanska USA Civil, it is the first structure in the United States to be designed with post-tensioned multi-strand couplers.
