A Reuters article reported that a 100-year-old bridge in North Dakota collapsed from the weight of a 42-ton truckload of beans. The tractor trailer was overloaded, leading to the collapse of the 56-foot wooden bridge. Other bridges may face similar dangers.
According to Reuters, the Grand Forks County Sheriff’s Office called in the incident and said that while the truck had made it across the Goose River bridge, it had become stuck due to the bridge collapse. The driver was uninjured, though police issued him an $11,400 citation for exceeding his load limit, which was reportedly triple the amount the bridge could have supported. Unfortunately, many historic bridges across the United States could suffer similar fates due to their tonnage limits and less advanced designs.
The “I” of the Beholder
Most bridges are supported by girders that help them bear loads, and many of those girders are made of steel. “Steel is an ideal material for girders—first because it has equal strength in both tension and compression, and second because it can be formed into these highly-efficient I-shaped cross sections,” said Dr. Stephen Ressler, Professor Emeritus from the United States Military Academy at West Point. “So why are steel girders usually I-shaped? The answer is that the I is an inherently efficient shape for carrying loads in flexure.” In other words, I-shaped girders—which look like a capital I when viewed from one end—flex and bend far less when weight is applied on top of them than, say, a square shape.
“Maximum compression occurs at the very top of the beam and it decreases linearly to zero at the neutral axis; maximum tension occurs at the bottom and also decreases linearly to zero at the neutral axis,” Dr. Ressler said. “Clearly, the material closest to the top and bottom of the beam are doing most of the heavy lifting in flexure. Thus, if we concentrate most of the beam’s material away from the neutral axis, we can greatly enhance the beam’s load-carrying capacity.” This means that the horizontal bars on the I-shaped girder do far more of the load bearing and support than it may appear. Unfortunately, no bridge is guaranteed to never break. In April 2019, the American Road and Transportation Builders Association declared 47,000 bridges in America structurally unsound.
Web Shearing in Older Bridges
I-shaped girders have three components to them: the horizontal bars are called flanges, while the long, vertical bar is called a web. However, over time, as repeated stress loads are placed on the I-shaped girders on bridges—in other words, as vehicles drive over them—the flanges and web can become distorted. A flange can kick out to the side instead of standing straight up, for example. However, older bridges often face a problem called shearing.
“This phenomenon is quite complex, but at the most fundamental level, it’s the tendency of the girder-web to distort from a rectangular shape [when viewed from the side] to a parallelogram shape on either side of the applied load,” Dr. Ressler said. In older bridges, the most common way to address shearing is by adding web stiffeners along its length. These additional supports run from the bottom flange straight up to the top.
So why aren’t more bridges fixed this way? “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.
For now, unless engineers address problems like faulty I-shaped girders and thin webs, thousands of other bridges like the one over Goose River in North Dakota could collapse, causing more trucks to spill the beans.
Dr. Stephen Ressler contributed to this article. Dr. Ressler is Professor Emeritus from the United States Military Academy at West Point and a Distinguished Member of the American Society of Civil Engineers (ASCE). 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.