Bridge that collapsed in Washington is common type

The Skagit River Bridge that collapsed after being struck by an over-height truck on May 23 in Washington State was opened in 1955 and is a through-truss steel truss type structure.

The 58-year old bridge is one of thousands of through-truss type bridges in the U.S. The design is now considered obsolete and designated “fracture critical” because if just one component is compromised, the entire structure can collapse. This bridge measures 339 metres total length and is 22 metres wide. It had four spans in total and a curved overhead arch.

While the U.S. National Transportation Safety Board are still investigating, it is believed that the truck struck and damaged the compression chord in the overhead truss, causing a major section of the bridge to collapse into the Skagit River. No-one died in the incident, but two cars fell into the water and their occupants had to be rescued.

The bridge is near Mount Vernon on Interstate 5 between Vancouver and Seattle, on the route to one of the busiest transborder crossings between Canada and the U.S. The bridge carried around 71,000 vehicles a day, and officials are now scrambling to build a temporary structure to replace the 48-metre section that was damaged.

The truck that hit the bridge was carrying a large drilling casing as its load and was accompanied by a pilot car. The truck measured 4.8 metres high and could have cleared the centre southbound lane where the clearance is 5.2 m, but it was travelling on the outside lane which only had 4.45 metre clearance. According to the NTSB, the bridge had higher than the standard clearance and did not require signage.