The I-5 Bridge Collapse

by briancpotter

The I-5 Bridge in Washington

Right on the heels of my last post about structural collapse, we get news that the I-5 bridge in Washington has collapsed.

Information is still limited, but based on the facts available, it looks as if it was caused by a truck carrying a heavy piece of mining machinery striking a cross member. The 50 year old bridge wasn’t classified as “structurally deficient“, but it was “functionally obsolete”, indicating it probably had less clearance height than a modern bridge.

First off, despite what some news outlets are reporting, there are no indications that the bridge was “unsafe”. Functionally obsolete means things like lane width, approach curvature, clearance, and other nonstructural aspects aren’t up to modern bridge code standards. However, none of this has anything to do with the load carrying capacity of the bridge, or the status of the structural members.

So is this just another example of why we need more infrastructure spending? I’m not so sure.

Based on the facts available, there are several possibilities for assigning blame:

1) Faulty Inspection – it’s possible that the bridge was in fact structurally deficient, and it simply went unnoticed by inspectors.

2) Faulty Design – Some engineers have stated that the damage of a single member like this should not have caused a full-scale collapse. I tend to agree, to a point – it’s very hard to completely remove single points of failure, and the impact load of a truck is astonishingly high. Still, for a bridge that has probably seen a million trucks pass over it, it should have been considered.

3) Faulty Code – This bridge was built over 50 years ago. However, even now the code only requires substructure elements (piers, columns, supports etc.) to be designed for impact load, while completely ignoring superstructure members like the one that was damaged.

Right now it looks like 2) is a good culprit. It seems unlikely an inspection would have caught functional issues and missed structural ones, and it is simply unreasonable to require all members to be designed for  400,000 pounds of truck hitting it. However, though CURRENTLY the danger of progressive collapse is well understood and taken into account, it’s not clear if that was standard practice, or even well understood, when this bridge was designed. So this may be a case of something being bad design today, yet within the accepted practice of the time. And since inspectors aren’t going to re-design from scratch every bridge they inspect, this probably isn’t something an inspection would have caught.

Regardless, none of those things are a problem you fix by throwing more money at them. Another billion dollars won’t stop engineers from making mistakes, won’t result in stricter code provisions, and won’t dramatically revise the duties of a bridge inspector – at least not in the current narrative of what our infrastructure problems are. To address those things, to the extent that they need addressing, requires institutional changes – a stricter design review process, different inspector training programs, etc. You don’t just write a bigger check to the ASCE.

We’ll see how this develops as more information becomes available. But right now it doesn’t look like this is an Our Crumbling Infrastructure problem.