Friday - May 24, 2013

Trestle Down

I-5 - Skagit River Bridge in Washington Suffers partial collapse after truck impact
Three people rescued from river, no fatalities or serious injuries



photo credit: Martha T, on Flickr

A truck carrying an oversize load struck a bridge on the major thoroughfare between Seattle and Canada, sending a section of the span and two vehicles into the Skagit River below, though all three occupants suffered only minor injuries.

It happened about 7 p.m. Thursday on the four-lane Interstate 5 bridge near Mount Vernon, about 60 miles north of Seattle, and disrupted travel in both directions.

Initially, it wasn’t clear if the bridge just gave way on its own. But at an overnight news conference, Washington State Patrol Chief John Batiste blamed it on a tractor-trailer carrying a tall load that hit an upper part of the span.

“For reasons unknown at this point in time, the semi struck the overhead of the bridge causing the collapse,” he said.

The truck made it off the bridge and the driver remained at the scene and cooperated with investigators.

Two other vehicles went into the water about 50 feet below as the structure crumbled. Three people were rescued and were recovering Friday.

Drivers were told to expect delays. Detours have been set up to try to ease the congestion. Batiste urged drivers to avoid the area if possible, especially over the Memorial Day weekend.

Traffic along the heavily travelled route could be affected for some time. The bridge is used by an average of 71,000 vehicles a day.

The bridge is a four section simple span Warren through truss with alternating verticals, a robust design for a steel bridge. Overhead clearance is 15.3 feet, 18” more than maximum legal truck height. Details are still emerging, but it looks to me like the truck went off the road, through the thin concrete guard rail, and took out one of the verticals, which in turn yanked out one of the central sway braces and tore the top chord along one side. With the top chord broken the truss folded inwards at that point, while dropping. And that’s exactly what you see in the picture: two triangular sections folded together because the top chord blew out. The dropping pulled both ends off their footings, and the whole section went down. Because this bridge is a simple span design - each section is separate and shares a footing on each pier - the damage was limited to only that one section. If you look closely at the large version of the picture on Martha T’s Flickr page, you can also see some damage to the third vertical from the left on the next section. I don’t know if that is new or old. Must have been one helluva impact.

This 1112 foot long bridge was built in 1955, and was recently (in bridge terms) rated as sufficient. More bridge info.

Posted by Drew458     on 05/24/2013 at 11:21 AM   
Filed Under: • Bridges •  
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