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After Much Study, 6th Street Viaduct's Saving Highly Doubtful

By Eric Richardson
Published: Monday, September 29, 2008, at 04:26PM
Sixth Street Viaduct Eric Richardson [Flickr]

Looking east along the 6th Street Viaduct, in a file photo from February, 2007.

The 6th Street Viaduct, opened in 1933, has served as the setting for countless movie and television shoots over the years. Time appears to be running out for the iconic span, however, as studies continue to conclude that a replacement is necessary to make the structure safe for future generations.

The viaduct has concrete "cancer," specifically a condition known as Alkali-Silica Reaction (ASR) that causes the concrete to crumble from the inside, steadily reducing the chances that the structure would survive a major earthquake.

Studies done on the bridge show that it has a 70% chance of collapse in the next 50 years, far higher than the 10% chance that standards allow. ASR occurs when concrete aggregate that contains reactive elements combines with silica and moisture. Unlike any of the other river spans, the 6th Street Viaduct used an on-site plant for its aggregate. The materials used happened to contain the perfect mix of ingredients for ASR.

Last January, the Bureau of Engineering held a series of meetings kicking off a seismic improvement project for the span. Their message was that ASR has so damaged the structure's concrete that a full replacement is likely the only solution.

Since then, additional studies have been done on ways to mitigate the dangers of the ASR and save the historic bridge, one of a dozen to span the Los Angeles River near Downtown. A panel on the future of the bridges was held at the Getty Center in April, audio of which is available on the Getty's website.

Despite extensive research, the retrofit alternatives continue to come up short.

Before the end of the year, the Bureau will be releasing a Draft Environmental Impact Report that will contain two retrofit alternatives and three replacement options. Whereas replacement options are designed to last 75 years, retrofit options would max out at 30. They would also involve significant visual changes to the structure, filling in the space between support piers and replacing significant amounts of substructure.

The three replacement options provide different takes on what a new bridge would look like. All would soften the span's current kink, replacing it with a more even curve. They would also widen the roadway, adding shoulder space, and bring the sidewalks up to ten feet. Overall, the bridge would widen from 77 feet to 94 feet.

$245 million dollars have currently been signed to the project, though total cost is estimated at $300 to $400 million. Funding will come from the state and federal government. The project's current timeline puts the start of construction in the first half of 2011.

At the June CCA Downtown Housing Forum, developer Paul Solomon of Linear City expressed his position that the bridge should be saved by removing auto traffic and making it a pedestrian span. Asked about the idea, engineers from the Bureau confirmed last week that such a plan would do nothing to make the bridge less likely to fall. Load doesn't play into the rate at which ASR eats away at the bridge's support.

BELOW: These three diagrams represent the replacement designs that will be found in the project's Draft EIR, to be released this winter.

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