Thousands of bridges across the United States are crumbling. Thousands more are on their way there. According to the most recent analysis by the American Road and Transportation Builders Association, more than 47,000 bridges in the U.S. are structurally deficient.
And it's likely to get worse, not simply as a result of time and traffic, but due to climate change.
In a new study, a pair of engineers at Colorado State University outlined the potential effects of climate change on the structural integrity of bridges in the U.S.
The duo's research focused on the country's 80,000 "simply supported steel girder bridges," a design common since World War II that consists of longitudinal beams spanning two piers.
Girder bridges also feature expansion joints, which link bridge spans and allow the bridge to expand and contract with seasonal temperature fluctuations. Because these joints regularly clog with debris and must be cleaned, girder bridges require routine maintenance.
As a result of poor oversight and limited transportation budgets, maintenance has been deferred on thousands of bridges around the country. Clogged joints prevent bridges from properly expanding.
Engineer Hussam Mahmoud and his research partner Susan Palu, who recently graduated from Colorado State with a master's degree in civil engineering, reasoned that as global warming continues and temperatures rise, bridges will be subjected to greater heat stress.
Heat stress can cause bridges to buckle and crack -- structural defects that can become exacerbated by the stress of passing cars and trucks. All this stress is made worse by clogged joints.
Using their understanding of thermal heat stress, Mahmoud and Palu built a model to predict the effects of rising temperatures on the structural integrity of simple girder bridges. The researchers suggest bridges built in cooler climates will be most vulnerable to climate change, as they weren't designed to withstand as much summertime expansion.
According to the study, published this week in the journal PLOS One, bridges in the Northern Rockies and Plains, Northwest and Upper Midwest are likely to experience the greatest increase in thermal stress. Bridges in the Northeast and Southeast will be less susceptible.
"We surprisingly find that potentially most of the main load carrying girders, in the analyzed bridges, could reach their ultimate capacity when subjected to service load and future climate changes," the researchers wrote in their paper.
The study's authors hope their findings will move transportation and infrastructure policymakers to account for the effects of climate change when prioritizing repairs.
"We as engineers must start to look beyond what we have initially been taught on how to analyze systems and start to think about what climate change is going to do to our understanding of component-level behavior and system-level performance," Mahmoud said.
