Why Did The Washington I-5 Bridge Collapse And How Did Everyone Survive?

A bridge over the Skagit River along an interstate highway partially collapsed in the US state of Washington, leaving cars with people in the water, according to several media reports.

Three vehicles were in the water after the collapse occurred at 7 p.m. local time (10 p.m. EST May 23) on Interstate 5, the main freeway running along the West Coast, NBC News quoted Washington State Patrol as saying.

Skagit Valley Herald reported a sheriff's office rescue boat arrived at the scene, adding that rescue crews were looking for the surviving people in the water, according to Fox News. Several people were reported to be seen who already escaped their swimming cars and thus survived the bridge collapse unscathed. The reason could be that the bridge is constructed as a pure steel structure which can collapse in a smoother way, as the steel stretches and tears over a period of time - as opposed to a concrete structure that breaks much more suddenly and thus also free-falls for a longer distance. Combined with the rather low height of about 10 meters and water underneath, a fall inside of a shock absorbing car is survivable.

The bridge was built in 1955, according to the website for the privately run National Bridge Inventory Database -- concrete and steel structures can reach the end of their life after 60 years and often need to be renewed thoroughly to live on after such a period. And indeed, the National Bridge Inventory Database gave the bridge just a 57.4 rating, barely above the critical level of 50 or less where immediate replacement is mandated, according to the American Society of Civil Engineers. The final straw came apparently a bit early for the bridge, also already classified as "fracture critical," meaning it lacked redundancy and a major structural failure could cause its collapse, as a truck hauling an over-sized load was the likely trigger according to a comment by an early investigator on site.

The general bridge situation looks pretty dire as well, getting an overall C- grade in the Engineers 2013 infrastructure report card, which will surprise only few. The statewide average rating for Washington is 80, according to an Associated Press analysis of federal data, but 759 bridges in the state have an even lower sufficiency score than the collapsed bridge! The highway and bridge are the main corridor for car traffic between Seattle and Vancouver, Canada, crossed by around 71,000 cars per day.