Determining whether a bridge is reaching the end of its useful life is part science, part sense, part adventure.
There are 600,000 bridges in the United States keeping people and goods moving over water and land, through city and country. But, as traffic increases and infrastructure continues to be underfunded, many aging bridges are approaching the end of their useful life. In fact, reports estimate the number of structurally deficient bridges to be 64,000.
Assessing what bridges need to be repaired, and when, is left to an unseen fleet of bridge inspectors — men and women who climb in and around structures to asses their condition. To find out how they do it, we went out on the road with veteran SEH bridge inspectors Jason Triplett and Paul Jordan as they look, touch and listen to bridges on several fish and wildlife refuges in North Dakota and Washington.
While 21st century bridge inspection procedures include everything from snooper trucks to aerial drones to ground-penetrating radar and ultrasonic testing equipment, simple visual, tactile and auditory techniques remain the most reliable and cost-effective methods for detecting deficiencies.
According to Triplett, more than 99% of inspection techniques are visual, auditory or tactile.
It’s called “sounding” and Triplett uses it and other tactics to determine how safe our bridges are. Problems can often be found by tapping the surface of a bridge and listening to the difference in the way the tapping sounds.
“We run a rotary impact tool across the bridge surface,” he says. “The different sounds you get are very apparent.”
When the sound changes from a high-pitched clacking to a low rumbling noise, there is evidence the bridge deck may be delaminating underneath. Delamination occurs when the top layer of concrete is separating from the layer below it. This separation is a pathway for moisture to attack the interior of the structure, destroying it from the inside out.
A geologist hammer can also be used to sound different areas of the bridge. Inspectors listen for delamination that they might have missed just by looking at it.
Here, Triplett uses a hammer to tap along the surface of a timber bridge, looking for spots of possible rot.
Concrete structures also relay different pitches when tapped with a hammer. Here, Triplett is able to identify a specific area of a bridge wingwall that is delaminating from the inside.
Sometimes, inspectors let technology take over. High-tech vehicles and equipment make the job easier in many circumstances.
“With steel bridges, it’s much easier to see how they’re aging,” says Triplett. “As anyone with a car in the Northeast can tell you, rust is easy to spot with the naked eye and once it gets going, it’s really hard to stop.”
Gauging how much of the steel is gone due to rust is a challenge. Over time, rust will create layers, so you can’t simply measure the thickness of the layers to tell how much steel is gone.
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One method, used by SEH crews, involves removing the rust layers with a wire brush to expose the bare metal. They then use a “D-Meter” to measure how much of the steel remains intact.
Snooper trucks allow bridge inspections from a bucket safely underneath the bridge. The articulating arm is operated by the inspector as they position it in the right location.
“A lot of time you have rely on good old-fashioned technology to get the job done,” says Jordan. “To perform the necessary inspection underneath a bridge, it’s just a lot easier and safer to use a snooper.”
Bridge inspections often require watercraft. Triplett likes to use a jon boat because of its stability and portability.
Even in a boat on the water, an inspector never knows what they are going to encounter.
“Luckily, we were in North Dakota on these inspections, so we didn’t come across any reptiles or venomous insects during our boating excursion,” says Triplett.
Though the job is not without hazards, Triplett assures us that identifying possible concerns with a bridge far outweigh the potential dangers encountered out in the field.
“It’s all about connecting people and getting them from one place to another,” Triplett says. “These bridges are what connect our society, and it’s important to make sure people can cross them safely.”
“It’s up to us to gather the information that tells whether a bridge needs to be repaired or replaced,” adds Jordan. “It’s important that information is right, every time.”
According to Triplett, gathering that information is the first step toward safety.
"Our infrastructure is crucial to our way of life, and many of us take for granted that our roads and bridges will always be there,” says Triplett. “But the fact is that they won’t, unless we take action—I like to think that as bridge inspectors, we provide the information that is the critical first step to providing safe bridges for everyone."
Jason Triplett, PE, is a senior structural engineer, bridge inspector and proponent of safe passage for bridge users everywhere. Contact Jason
Paul Jordan, PE, is a senior professional engineer and bridge inspector dedicated to keeping bridges safe for everyone. Contact Paul