Sixth Street Bridge in bad shape

How bad? An engineering consultant told the Downtown News that it's "seismically sick." Others warn that an earthquake could bring it down. Repairs to the 1932 structure, which carries 11,000 cars a day, aren't expected to begin until 2011, which is a long time to hope that something bad doesn’t happen. During a public meeting last Tuesday, a fellow from Parsons, the big engineering and design company in Pasadena, said “the Sixth Street Viaduct has a form of cancer."

The root of the Sixth Street Viaduct's deterioration lies, essentially, in its genetic makeup. For 75 years, a phenomenon called Alkali-Silica Reaction has been eating away at it from the inside out. ASR occurs when the right "freak mixture" of chemicals come together while mixing concrete, said Bingham. In certain proportions, the silica in the sand and gravel aggregate used to make concrete react with the alkali in cement to produce a gel-like substance. That gel swells when exposed to water, resulting in expansion and cracks. The Sixth Street Viaduct, said Wally Stokes, environmental coordinator for the city's engineering bureau, is the only city-owned bridge afflicted with ASR. He said the condition has greatly reduced the bridge's capacity to withstand an earthquake.

According to a report released by the Bureau of Engineering last month, there is a 70% chance that the bridge will collapse due to a major earthquake in the next 50 years. The normally accepted collapse probability for bridges is 2% or less over 50 years. That 70% risk, Stokes says, is not as bad as it sounds. The percentage, he explained, is based on the assumption that an earthquake of at least 7.5 magnitude will hit a nearby fault line in the next 50 years. Furthermore, he added, the term "collapse" means that "the bridge would be unusable," not "that it would fall down like London Bridge."