If bridges are ‘safe,’ why do they collapse?

Because safety doesn’t mean what we think it does.

At Keystone Crossroads, we’ve been learning a lot about Pennsylvania’s crumbling infrastructure, including its bridges.

Twenty-two percent of Pennsylvania’s bridges were structurally deficient as of December 2014. That’s the second-highest percentage in the country, behind Rhode Island. (Up until last year, Pennsylvania had the top spot). 

What does that mean? Not what you might think. “Structurally deficient” is a technical term engineers use to label a bridge for repair or replacement. Bridges with this label aren’t necessarily unsafe, engineers say. If a bridge is unsafe, it’ll be closed by inspectors, according to the Pennsylvania Department of Transportation. That’s what we reported in our recent series on the state’s bridges. 

After hearing that explanation, we still had some questions. The states all follow federal safety standards. Yet more than 180 bridges collapsed in the United States between 2000 and 2012, according to data from an Ohio State University research report. If unsafe bridges are closed by each state, why do bridges collapse? Is there something wrong with the safety standards? How do bridges slip through the cracks?

Engineers say there are a few reasons. Sometimes a bridge receives a physical blow that it wasn’t built to withstand, like an earthquake, boat collision or a tractor-trailer hitting one of its components.

Sometimes bridge collapses are due to human error. Maybe a bridge inspection wasn’t done as thoroughly or carefully as it should have been.

And other times a bridge has a structural weakness that engineers don’t recognize as a problem. For instance, the I-35 bridge in Minnesota collapsed in 2007 because of a design flaw: its steel plates were too thin, according to the National Transportation Safety Board. At the time, the report said, engineers didn’t understand the importance of the steel plates.  

As a result, many similar bridges were repaired, and inspectors look for this design flaw in their monitoring. “Now, gusset plates aren’t taken for granted,” says Andy Hermann, former president of the American Society of Civil Engineers. “They’re looked at very closely and analyzed. You get that with a lot of bridge failures.”

Engineers are always learning from bridges failures, and incorporating that knowledge into safety standards. So theoretically, our bridges keep getting safer.

Many of us see things as either “unsafe” or “safe. But some say safety isn’t a binary term, and it doesn’t mean eliminating all risk. It just means that we’ve accepted a certain (negligible) chance that a disaster will happen. In other words, in a “safe” bridge system, it’s unlikely that a bridge will collapse, but it’s not entirely impossible.

Hermann says that even if it were possible to eliminate risk, that “would be a price tag way higher than we can afford. Right now we don’t even have enough money to do the maintenance we need to do on bridges,” he says. “We have to live with the safety factors we build into the bridges.”