Dramatic Bridge Collapse Snarls Washington Traffic

While some have tried to connect the bridge collapse to America's chronically underfunded infrastructure, that may be a bit of a stretch in this case. The Seattle PI described a "truck with a too-high load" as the likely cause of the collapse. 

As for the condition of the truss bridge itself, Stephen Lee Davis of Transportation For America explains the difference between functionally obsolete and structurally deficient:

 (T)he bridge was considered “functionally obsolete” by state and federal inspectors, which is a designation that could mean any number of things, none of which have anything to do with structural safety. The lanes could be narrower than today’s standards, the weights allowed could be less than an interstate bridge built today, or built using materials that would be considered obsolete today.

However, the bridge was not considered “structurally deficient” at the time of collapse, which means that a bridge requires repair, rehabilitation or replacement, along with much more regular inspections. To be considered structurally deficient, one of the three major components of a bridge (deck, superstructure, substructure) has to score a 4 or below on a scale of 1-10.

The bridge collapse is a bit reminiscent of the August 1, 2007 I-35W bridge collapse over the Mississippi River in Minneapolis, killing 13 people and injuring 145. However, the two collapses are quite different. In Minnesota, the National Transportation Safety Board pointed to bends in gusset plates as the likely cause, unlike the specific incident described by State Patrol Chief John Batiste in the Chicago Tribune:  

(Batiste) said a semi-trailer truck heading southbound struck the bridge just before part of it collapsed. The bridge has metal overhead beams.

"The size of the load he was carrying appeared to create a problem, causing him to strike the bridge," Batiste said. He said investigators were talking to the driver and inspecting the truck.

The effect on traffic of the bridge collapse will be far-reaching, writes Davis.

It’s hard to accurately describe how crucial this interstate connection is. I-5 runs from Canada to Mexico within the U.S. and touches almost every single major city on the west coast. It’s a vital corridor not only commuters but also for freight traffic — 12 percent of the daily traffic on this bridge was truck traffic.