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Getting the hang of bridge-building

Engineering professor Clay Naito straps himself up for the stress test.

Transforming a two-dimensional drawing of a bridge into a 3-D model of an actual span is the first step into the real world for first-year civil engineering students at Lehigh.

Students in one Engineering 5 class who could make the leap successfully recently got the chance to “hang” their teacher—Clay Naito, assistant professor of civil and environmental engineering.

Naito assigned the 20-plus first-year students in his class this fall to design and build model truss bridges that weighed 3 ounces or less, were made of lightweight balsa wood, and could sustain a load of 180 pounds.

The finished models were tested on a dynamic load-testing machine in Fritz Lab, where bridge decks and other samples are typically subjected to thousands of cycles of fatigue (repeated stresses) to simulate the constant punishment that bridges take from passing cars and trucks.

But instead of using the machine itself to impose stress on the bridges, Naito decided on a novel, more personal approach. Donning a hard hat and stepping onto a lift, he crisscrossed his chest with straps and attached the straps to a movable cross brace several feet above him.

His total weight—body plus hard hat plus loading attachments—now amounted to 180 pounds and rested on the lift.

One by one, the teams of students then fixed their model bridges just beneath the cross brace to which Naito was attached. The students then began lowering the lift on which Naito was standing, gradually transferring his weight from the lift to the cross brace.

As the brace slowly imposed more and more of Naito’s “load” to the model bridge, the students listened expectantly for the first hint of a crack and sign of failure.

“That doesn’t sound good at all,” said one student as the first bridge began cracking long before it was subjected to Naito’s full weight. A split second later the span snapped in the middle, pulling apart the truss connections at one end of the bridge.

Indeed, all the model bridges would fail; none left Naito hanging in the balance. The weaker models did so in dramatic fashion, sending pieces flying in several directions, while the stronger bridges buckled more gracefully, bending before breaking, as bridges are designed to do in the real world. The most successful team attached a diagonal cross beam to prevent its truss from twisting, but still gained only an additional second or two of longevity.

Right design, wrong material

A balsa wood bridge designed by students snaps under the load.

“Engineering 5: Freshman Design Experience,” a requirement for first-year engineering students, gives students a look at all the engineering disciplines and allows them to do hands-on design projects. Students complete two, five-week sections in a semester and gain exposure to two different engineering fields.

Naito praised his students for their designs and said they had done a commendable job of learning the basics of structural design—hand calculations, analysis, stress, strain, materials, bridge components and testing—in just five weeks.

But he said he might have students use a more uniform, less variable material when he teaches the course in the spring.

“You did the design process right,” he told the students, “but you failed because of the material. Balsa wood is a variable material, so one flaw in the wood can cause failure.”

Students said they enjoyed using Visual Analysis, a software construction- design program selected by Naito. But they noted there was little margin for error in the actual assembly of their bridge models.

“On the computer you can test different designs and see how well each one will do,” said Stacy Sommerfield ‘08. “The hard part is getting the actual bridge to behave the way the computer says it will.”

“Our bridge should have held 280 pounds, on paper anyway,” said Judd Vear ‘08, whose team came closest to “hanging” the professor. “The computer software enables you to cut everything really close, but your assembly has to be almost perfect.”

--Kurt Pfitzer

Posted on Tuesday, November 16, 2004

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