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Sixth Street Viaduct has Cancer; Suggested Treatment: New Bridge

By Eric Richardson
Published: Wednesday, January 24, 2007, at 10:50AM
6th Street Bridge Eric Richardson [Flickr]

The Sixth Street Viaduct, opened in 1933, is perhaps the gem of Los Angeles' historic spans over the LA River. It's slowly dying, though, thanks to a chemical reaction often referred to as a concrete "cancer." Yesterday marked the first public meeting in a seismic improvement project that may well see the bridge torn down and replaced.

Obviously this is a big deal for Downtown on several different levels. First, it's a historic structure facing possible demolition. Second, it's a massive construction project that would have the bridge out of service for several years and cost over $200 million.

It's also an acute reminder that not everything can be saved. The desire to save and celebrate the past has to be weighed against the need for safety and new technologies.

Now's the time to start thinking: If you were to build a new Sixth Street viaduct, what would it look like? Would it be a recreation of the existing bridge, or would it be something new and modern?

Much more after the jump...

ASR -- Alkali-Silica Reaction

When the viaduct was built in 1932 it used an specially constructed on-site plant to provide its concrete. This was a revolutionary technique, but the product that came out of this particular plant turned out to have a high alkali content and certain reactive elements in its aggregate. As water seeps into the concrete over time these these conditions combine to cause an Alkali-Silica Reaction.

The ASR reaction creates a gel by-product. Since this by-product is larger than material it consumes, pressure is created on the surrounding structure. This can cause cracks in the concrete, thereby allowing in more moisture and accelerating the situation. The undermining of the concrete lessens the strength of the structure.

How Bad is It?

The City's Bureau of Engineering has been involved in numerous projects to try and slow the weakening of the viaduct. Most attempts focus on sealing the structure to keep water out. These efforts may have slowed the bridge's disease, but can't do anything to cure it.

The current estimates are that the viaduct has a 70% probability of collapse due to a major earthquake within 50 years. That's just slightly greater than the 2% standard that bridges are designed for today.

Samples have been taken from 88 points on the bridge. They show a generally bad situation, but one that's particularly dire in the underground footings that support the columns. Testing shows that all of the footings are suffering from severe degradation. This makes sense, since underground the footings are sitting in moisture.

Interestingly, problems with the bridge started soon after it was completed. In the 1940s a set of towers that originally rose in the center of the span were removed after being shown to be unstable. The problem has continued to worsen in the years since.

Curiously, this is the only one of the historic LA River bridges to suffer from ASR.

And So They Want to Replace It?

The study that's now kicking off will examine all options and present them to the city, but the preliminary indication is that replacement is likely to be the recommended course of action. This is the option supported by Caltrans.

A large chunk of funding for the project is already earmarked. $200 million was set aside for the Sixth street project in the recent Prop 1B bond measure. This would cover a basic new bridge, but a basic bridge is unlikely to be welcomed here. It's yet to be determined what it might cost to replace the existing design or to create a new landmark bridge.

Whatever design a replacement might have, it would likely expand the existing roadway to meet current standards for lane width and median separation. Preliminary plans also include ten foot sidewalks linking Downtown to Boyle Heights. Design would also be coordinated with the project to redesign and restore the river.

Public Input

6th Street Bridge MeetingThe project team hopes that the study and environmental process for this project can be completed in three years, an aggressive duration given that both a state EIR and federal EIS are needed. Last night's meeting at Art Share, previously mentioned here, was the first in what will surely be many public meetings.

In mentioning the meeting last week I criticized the lack of notice that this meeting was going on. It turns out that fliers were sent to those living or owning property within 2500 feet of the viaduct, as well as to various other offices and groups considered to be stakeholders in the project. While that's a start, I wish there had been some better way to explain to Downtown as a whole what was being discussed.

Public Comments

The meeting was well attended by those from the Arts District community as well as those from Boyle Heights. The audience was perhaps thirty or so.

There was a wide range of comment given, but many comments focused on one central issue: trying to save the existing structure instead of tearing it down. Those commenting simply did not want to be convinced that the bridge needed to be torn down and replaced.

Many keyed on the consultant saying that one option under study was a $40 million project to retrofit the bridge by wrapping columns with steel and filling in some interior arches. The guess is that this could extend the life of the bridge thirty years.

It's a valid argument, and one that the study will look at, but in the end I think many people imagine the outcome of this as different than it really would be. Wrapping and filling columns would permanently and negatively alter the bridge and its aesthetics. Yes, the structure would have its life lengthened but it would no longer be the same.

Moving Forward

There's another meeting giving the same presentation tomorrow night at 6:30pm, at Saint Isabel Church (918 S. Soto St). There will certainly be more meetings to come on both sides of the bridge.

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