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Beam breaking leads to better bridges

"I'm one of the few engineers who gets to design something, see how it behaves and break it. I keep learning each time I break something."

By Mark Berry

 

 

Bridge beam breaking close upBeam breakingTest frame design graphic
Images provided by Bob Peterman.

Top left: Failure of this rectangular test beam was due to strand rupture. Top right: The large t-beams broke at about 120,000 pounds. Above: This revised test frame has a galvanized metal roof, which provides protection for test equipment and allows work to proceed during light rain.

 

Kansas State University civil engineering assistant professor Bob Peterman keeps video clips of destruction.

In one video, a metal block called a spreader beam pushes down in the middle of a long concrete beam that is held down at both ends. Cracks sprout on the beam and spread out like spider webs, before the concrete breaks in a groan and a cloud of dust. A layer of concrete on top of the beam ripples like a wave.

The demolition isn't just for amusement. It's leading toward improvements in the way engineers build new bridges and reinforce old ones.

Peterman was the driving force behind the acquisition and design of three devices that test the strength of structural beams. The devices press on the beams -- much like a truck would as it passes over a bridge -- until they find the breaking point.

The biggest beam-breaker is 52 feet long and can exert 500,000 pounds of pressure. It is located at 925 Carlson Street in Manhattan, behind K-State's Civil Infrastructure Systems Laboratory. The $120,000 device was donated by Havens Steel of Kansas City, Mo. A second smaller frame is at the same site. A third sits in the basement of Fiedler Hall on K-State's campus. Twenty-two industrial sponsors helped fund the equipment.

The first test was performed in April 2001. Peterman brings his students to the tests to give them hands-on experience. The engineers average two tests per month, as some tests take a couple weeks.

"Usually there is always something being tested," Peterman said.

Peterman is working with the Kansas Department of Transportation on developing fiberglass beams. Many old concrete bridges in Kansas were built in the 1940s and 1950s. Trucks have gotten heavier and wider since then, but the bridges have only deteriorated. Counties are required to bring the bridges up to federal standards if they want government money to fix them.

One way to do that is to widen the bridge and make the surface on top lighter. That can be done with a layer of reinforced, honey-combed fiberglass, which is much lighter than concrete and just as strong, Peterman said. Engineers can tear off the aging concrete deck and replace it with the fiberglass structure. They can also use the fiberglass to widen the bridges, which often can't accommodate two passing cars. Three bridges in Kansas use fiberglass structures, Peterman said.

The problem is, Peterman found that fiberglass structures don't break slowly when too much force is placed on them. They explode. Because of this, engineers need to use a larger safety margin than usual, which makes them more expensive. Peterman said beams should instead fail gradually and deform, so drivers will be able to see that the bridge is broken.

"We're working with the manufacturers right now to change how this fiberglass member is constructed so that it isn't explosive," Peterman said. "What we don't want is a structure that appears perfectly fine and then all of a sudden fails catastrophically."

The machines are also equipped to test columns and joint connections, but most of Peterman's work so far has been testing beams that would be used in bridges. The beam breakers can simulate the load of 100,000 trucks going over a beam per day.

"It's one thing to stand up against a one-time load, but we want to see how it's going to perform long-term," Peterman said.

Peterman is also testing a reinforcing mat made of carbon fibers woven together by a cotton thread. The thin mat can withstand 3,000 to 4,000 pounds of force per inch width. When it is glued to the side of a bridge's beam, the beam becomes 50 percent stronger, he said.

"I love what I do. It's fun. I get to investigate new technologies long before most design engineers find out about it. And then I get to take it into the classroom," Peterman said. "I'm one of the few engineers who gets to design something, see how it behaves and break it. I keep learning each time I break something."

Summer 2002