A lesson in bridges ends up in splinters

April 18, 1994|By Anne Haddad | Anne Haddad,Sun Staff Writer

For about a month now, students in the science research classes at South Carroll High School have hounded teacher Robert Foor-Hogue with one question: "When are we going to break our bridges?"

A strange request, and a strange goal: building something just to break it.

But the only way to judge a bridge is to see how it takes weight, Mr. Foor-Hogue said.

Friday, in the school auditorium, the students took turns connecting their balsa wood bridges to a box-like instrument called a stress analyzer. A flat, square weight is attached to the middle top of the bridge and pulled down in increments by the machine. A liquid crystal dial shows how many pounds of pressure per square inch are bearing down at each step of the way.

"An ideal bridge shows stress in multiple places and has multiple failures," Mr. Foor-Hogue said. "A good bridge basically will explode."

Not the kind of thing a person wants to think about when driving across the Chesapeake on the Bay Bridge, but further explanation makes sense:

"What a bridge must do is take the weight coming down and send it everywhere," he said. "Then you have something that, if it was real, would do quite well."

The steady, static weight of the stress analyzer is a greater force than the moving load of traffic, Mr. Foor-Hogue said.

Remarkably, these balsa wood bridges could weigh no more than 175 grams, a fraction of a pound. Students connected the pieces with standard wood glue. No nails.

Junior Jason Mertz's bridge at first was 110 grams and too light, not meeting the minimum 125 grams. He had to add a few more pieces, but Mr. Foor-Hogue said the first version still would have held a lot of weight. Students also figured the ratio of the bridges' weight and strength, with the goal being to have as light a bridge as possible that would still hold a lot of weight. In addition to building the bridges, students had to submit a report on where they thought the models would break first.

"Designing it and putting it together were totally different," said senior Molly Cage of Mount Airy.

"When you're designing things, you're thinking about how it looks. A lot of things that you thought would work, don't.

"I didn't know anything about bridges, but we read a lot of material," she said.

Molly put together an arch bridge that withstood 144 pounds of pressure per square inch.

Many of the bridges took a lot more abuse.

It took 359 pounds of pressure per square inch to break junior Tom Schaeffer's truss bridge, the strongest.

It may not have looked as elegant as some of the other bridges, especially the elaborate arches some students built. But Tom's bridge was "extremely well-designed" and sturdy, Mr. Foor-Hogue said.

Senior John Macinnis came in third in the weight his bridge held, 308 pounds, but he won for the best-looking truss bridge.

Arch bridges were in a separate category, and junior Tom Clark came in first for sturdiness, with his model holding 279 pounds per square inch. Junior Jason Mertz came in third for sturdiness, his bridge holding 167 pounds, but first in aesthetics.

Second-place winner for the arch category was Melina Delahoz, a senior whose bridge also held 167 pounds.

Second place in truss went to Justin Barnhouser, a senior whose bridge held 339 pounds.

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