California I-10 Bridge Collapse

On Sunday, July 19 an end span of California bridge 56-0576R on I-10 over Tex Wash collapsed in a flash flood.

Built in 1967, the concrete slab bridge is 90 ft. long with three 30 ft. spans at a 10 degree skew. The bridge was identified as functionally obsolete (most likely indicating vehicle capacity was inadequate or the geometrics substandard) but was not listed as structurally deficient. In fact, according to uglybridges.com, the 48-year-old bridge had received a good rating in a 2013 inspection. The overall sufficiency rating was 91.5/100, deck 6/9, superstructure 7/9, and substructure 7/9. Furthermore, the channel was described as stable.

So how did the bridge come tumbling down? Flash flood is likely the key word. The storm dumped 6.7 inches of rain on a county where the average annual rainfall is roughly 3 inches. The channel may have migrated toward the abutment during the flash flood, causing significant undermining and leaving the bridge foundation unstable.

Heading in to an El Niño year, this raises questions about the ability of California infrastructure to withstand increasingly extreme weather. According to the 2014 National Bridge Inventory, California has 25,406 bridges, with 2,501 identified as structurally deficient and 4,306 classified as functionally obsolete.

Interstate 10 serves as an important route between California and Arizona, and the economic impacts of this bridge failure are likely to be staggering. The collapsed eastbound structure typically carries in excess of 11,000 vehicles per day, roughly 40% of which are trucks. Granite Construction was hired to enact emergency repairs to the westbound bridge (namely, filling in the abutment undermining and placing tons of riprap). Granite also constructed crossovers that allow eastbound traffic to be diverted on to the westbound structure, with traffic resuming on Friday, July 24.

The cost to "rebuild" the eastbound bridge is estimated at $5 million, with $2 million of that coming from the Federal Highway Administration. I'm not entirely clear whether "rebuild" means "repair" or "replace". Caltrans' stated goal of September completion makes me think "repair", but then again, design of a workhorse bridge is fairly quick and history has shown that with financial incentives contractors can just about move mountains.