A new report by a Washington, D.C.-based group says over 47,000 bridges across the country need repairs, according to an article by NPR. The group, The American Road and Transportation Builders Association (ARTBA), estimates an 80-year timeline to fix all the bridges. But the construction and deterioration of bridges are complex matters.
The ARTBA bridge report seems especially concerning in light of a bridge collapse in Tennessee on April 1. According to the NPR article, the number of structurally deficient bridges is actually down by 7,000 since 2017—but that decrease is due to the Federal Highway Association’s newly relaxed standards of bridge quality, not because the 7,000 bridges were repaired. Road maintenance is a longstanding and common issue at city, state, and federal levels. So why haven’t we figured it out yet?
Girders and Complications of Bridge Structure
Countless bridges in America are “girder bridges,” which are exactly what they sound like—bridges whose roadways are supported by several long girders made of steel or concrete. These girders usually resemble a capital letter “I” when seen from either end. The parts of the I-shaped girder that lie horizontally are called flanges, while the vertical stick of the “I” is called the web. “Most bridge girders are custom-fabricated by welding individual steel plates together to form the flanges and the web,” said Dr. Stephen Ressler, Professor Emeritus from the United States Military Academy at West Point. This type of girder is called a plate girder. Plate girders can be built up to about 300 feet in length, which is triple the length of the types of girders manufactured for residential construction, but there are drawbacks.
The first problem is called lateral torsional buckling. “In general terms, buckling is an instability failure that occurs when a compressed structural element kicks out sideways,” Dr. Ressler said. “In bending, an I-shaped girder has its top flange in compression as well, so the top flange kicks out sideways, resulting in a twisting or torsional failure.” In simpler terms, looking at the I-shaped girder straight on, the near end of the “I” may look as though it’s falling over, but the far end still stands straight up. The entire girder twists dangerously from one end to the other.
Another problem is related to the thinness of the web—the vertical stick portion of the I-shaped girder—and it’s called web buckling. When the web of a girder buckles, it fails to stand vertically, looking instead like an arc or parenthesis bending outwards. This problem can be solved either by building thicker, pricier webs or by using web stiffeners, steel struts to support thinner webs and keep them standing up straight. So why aren’t they used more often? “The problem with web stiffeners is that they’re expensive to fabricate and install—often offsetting the cost-savings that might have been gained from using a thinner web,” Dr. Ressler said.
Why Bridges Collapse
“Bridge engineers take every reasonable precaution to avoid failure, but the ugly truth is that a zero probability rate of failure is impossible to achieve,” Dr. Ressler said. “The most important reason for structural redundancy is fatigue—the progressive accumulation of damage in metals subjected to repetitive loading.”
Dr. Ressler explains that in steel bridge girders, microscopic cracks or defects occur most often at welding points. When subjected to bearing loads over and over, the minuscule defect spreads and worsens. “Over time, as this process continues, the extension of the crack gets progressively larger as the concentration of stress at the tip of the crack gets higher,” Der. Ressler said. “At some point—typically after several hundred thousand load cycles—the crack can reach a critical length at which point a sudden, catastrophic fracture of the entire girder occurs.”
As the ARTBA bridge report shows, solutions to girder bridge failures are either imperfect or need implementation, depending on each bridge. These repairs are costly and currently in debate in Congress, but large-scale infrastructure spending could prevent further incidents like the Tennessee bridge collapse.
Dr. Stephen Ressler contributed to this article.
Dr. Ressler is Professor Emeritus from the United States Military Academy at West Point. He earned a B.S. from West Point and an M.S. and a Ph.D. in Civil Engineering from Lehigh University, as well as a Master of Strategic Studies from the U.S. Army War College.