New L.A. Bridge Built with Seismic Resilience in Mind

Brief 

The iconic, 3,500-foot-long Sixth Street Viaduct in Los Angeles was deteriorating and vulnerable to seismic shocks when it was demolished in 2016. The bridge that will replace it will feature a low-shrinkage, 6,000 psi concrete mix and triple-pendulum isolation bearings at the stem of the Y bent columns.

Insight

Like any good story from Hollywood’s hometown, the Sixth Street Viaduct in Los Angeles is about to get a sequel.The original bridge, made mostly of concrete but with two pairs of braced steel arches, was constructed in 1932 and extended across the Los Angeles River, several railroad tracks, the U.S. 101 freeway, and various local streets.

At 3,500 ft long and 46 ft wide, it was the largest of more than a dozen historical structures erected during the first four decades of the 20th century to span the Los Angeles River.

But the iconic Sixth Street bridge — which appeared in numerous movies, television shows, music videos, and commercials — suffered from a fatal condition that was not understood at the time the viaduct was constructed. Its concrete included an aggregate that caused a chemical reaction known as alkali-silica reaction, which severely damaged the structure over time.

Although “various costly restorative methods were tried in an ongoing effort to save the viaduct … all of them failed,” explains the Los Angeles Bureau of Engineering’s website about the Sixth Street Viaduct. In addition, “seismic vulnerability studies concluded that the viaduct had a high vulnerability to failure in the event of a major earthquake.”

Consequently, the original bridge was demolished in 2016. The Bureau of Engineering, together with the city’s Bureau of Contract Administration, is leading a $588 million project to replace the Sixth Street Viaduct. The new crossing, expected to be completed this summer, represents the most expensive bridge project ever in the city of Los Angeles.

Project partners

HNTB, an infrastructure design firm with offices across the nation, won a 2012 international competition to design the replacement bridge. HNTB is serving as the architect of record and engineer of record for the project.