The science behind the I-5 bridge collapse

June 8, 2013 in News, Tension and Compression by Chris Singer

A few weeks ago, a large section of the I-5 bridge over the Skagit River in Washington State crumbled and crashed into the river below.

Here’s video of the collapse:

Luckily, there were no fatalities but this event could have been far worse and is another sign of our country’s infrastructure is in desperate need of more than simple facelifts.

After some research, I found some interesting facts about this particular bridge and the state of America’s bridges as a whole and I thought I would share:

*  This particular bridge over the Skagit river in Washington state was rated “functionally obsolete” by the Federal Highway administration.

* What does “functionally obsolete mean?” From the National Bridge Inventory:

Functionally Obsolete (FO) Status – Highway bridges classified as functionally obsolete are NOT structurally deficient, but their design is outdated. They may have lower load carrying capacity, narrower shoulders or less clearance underneath than bridges built to the current standard.

* A November 2012 inspection of this bridge resulted in a score of 47 out of 100 on a sufficiency rating. Not good to say the least. According to the American Society of Civil Engineer’s Infrastructure Report Card, bridges in the U.S get a C+ rating with 1 in 9 of the nation’s bridges being structurally deficient (see more here:


Parts of a truss bridge

* This particular bridge is a truss bridge. Here’s a good article from How Stuff Works about truss bridges. Basically though, truss bridges are composed of steel beams in the shape of triangles. A triangle is the strongest geometric shape so with three beams arranged in a triangle, meeting at three corners, the beans cannot shift position without a corner coming apart. You can test this by taking toothpicks and making different shapes. You’ll notice that a square or pentagon can change it’s shape without the joints breaking. A truss bridge will hold together as long as the beams themselves don’t break and the joints stay together.

* So why did this bridge collapse? Again from

When an enormous truck hit the Skagit River bridge, it likely bent one of the triangle beams near the top of the truss structure. Once this beam bent or broke, the corners were no longer held in place. An educated guess as to the cause of the disaster would be a chain reaction of collapsing truss triangles triggered by the single broken beam. A single beam eliciting a complete failure, however, may suggest that further weaknesses existed in the bridge.

* Another good video explanation of why this bridge collapsed:


For further reading and learning:
* The Tacoma Narrows Bridge
* Ugly Bridges
* The Physics Behind Famous Bridge Collapses
* Let’s Build A Bridge
* Building bridges from
* Bridge Building Video
* Engineering a Bridge