Ma and Saykat’s safety-conscious drop tower improves TN bridges

If you are inside the John D. Tickle Engineering Building at the University of Tennessee, Knoxville and think you hear a semi truck slamming into a highway overpass, your ears aren’t deceiving you. You’ve just stopped by during an experiment that promises to improve Tennessee’s bridges.

“The majority of our bridges in Tennessee—and nationally—were designed according to old specifications, with a (lower) clear height,” said Department of Civil and Environmental Engineering (CEE) Professor John Ma. “So if you have a (tall, modern) vehicle driving underneath, then they’re going to hit the bottom of the support beams, (which) can cause tremendous damage.”

High-energy vehicle strikes strip away concrete protection, exposing the structural steel within overpass support beams. That changes stress distribution across the bridge and makes the steel susceptible to corrosion. Besides, Ma said, even if road engineers have confirmed that an overpass is safe to use after a collision, Tennesseans feel anxious driving under a bridge with a large chunk missing.

“Tennessee itself has 20,000 bridges,” said Sazzadul S. Saykat, a PhD student in Ma’s lab. “We reviewed a bridge near Memphis that has been hit (and repaired) 13 times. That means that 13 times, they had to close the bridge and rebuild—which causes not only expense, (but) also a lot of effort and a lot of time of the (TDOT) personnel.”

That’s why the Tennessee Department of Transportation (TDOT) and the Federal Highway Administration tasked Ma and Saykat with finding a cost-effective, labor-efficient way to make the state’s bridges more robust.

Ma and Saykat have developed two new beam designs that should be an improvement, but theoretical simulations aren’t enough to change civil engineering guidelines. Generating the loads necessary for their experiments requires dropping a 1,500-pound weight from up to 12 feet in the air. To achieve this safely, the team worked closely with both the CEE and University of Tennessee university-wide safety committees to build a large drop tower, which is a sturdy steel construction that lets researchers very precisely drop a weight onto a material sample.

“The research team needed to ensure the load would land exactly where it needed to, every time,” said CEE Department Head Chris Cox. “It took more than a year of design reviews and revisions with Sazzadul, Dr. Ma, our safety committee, and the machine shop crew who built the tower to get there, and I’m proud of how seriously everyone treated it.”

Drop Tower Safety and Long-Term Use

When Ma and Saykat accepted TDOT’s challenge, they needed to create an extremely detailed and accurate simulation of how bridges fare after collisions from overheight vehicles. They evaluated 40 bridges across Tennessee, working through police and TDOT reports of vehicle strikes to help ground the simulation in real physics.

To test their new designs, they rotated the crash scenario 90 degrees, placing the different bridge beams on the ground beneath the drop tower.

“(Everyone) was very conscious about the safety of the whole thing,” Saykat said. “Imagine if the magnet was the only thing holding the weight up, and the electricity went out for a millisecond. This can cause death if we are not safe. That is why we have chains holding it up and iron bars supporting it from below when it’s not in use.”

In addition to the chains and iron bars supporting the weight itself, the tower’s impact site is surrounded by thick metal grating to prevent debris from escaping. Preparing for a drop involves about half an hour of careful positioning and safety checks, Saykat said.

“The safeguards on the tower, including the containment grating, the release mechanism, crane operations, and the freefall pathway, went through multiple rounds of review before we’d sign off on it,” Cox said. “We didn’t approve a single drop until we were confident that a power blip, an unexpected test outcome, or a simple mistake couldn’t put anyone in harm’s way.”

Building a Better Beam

Saykat collaborated with Ross Prestressed Concrete, Inc., a local company, to build three beams for testing: one built to the current TDOT standard and two new designs that he and Ma believe will better absorb high-energy impacts.

Ma will use data gathered during Saykat’s drop-tower experiments to calibrate the full-scale 3D fundamental model of a real Tennessee bridge.

The pair hopes that their tests and simulations convince TDOT to make the switch to more impact-resistant bridges, reducing material and labor costs. If the improvement is as dramatic as they expect, Saykat foresees the new beam design spreading across the nation.

“If the research goes according to the plan and (TDOT) chooses to use the best-performing design, we are expecting that (you can have) the same vehicle impacts (but) the damage will be less,” he said. “That means that the repair cost will be much lower and (TDOT) can invest their time on other important things. And if this work benefits Tennessee, (then) all other states, they will see that and they will also (implement and) benefit from the new designs.”