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Design flaws tell no lies

Mountain Middle students test their engineering skills

As their most recent project, sixth-grade science students at Mountain Middle School built a bridge to somewhere – arriving very likely at a solid grasp of structural engineering, ratio, force and gravity.

The students’ classroom calculations that went into building truss bridges of balsa wood were tested in public view Wednesday when the structures were subjected to increasing weight until they collapsed.

The objective was to build a bridge with the highest strength-to-weight ratio at the lowest design costs.

Testing took place before a live audience of 200 family members, friends and teachers in the Fort Lewis College ballroom.

Student projects are exhibited in public for a reason, Shane Voss, head of school, said. It brings out more effort and improves the quality of work.

“Public exhibits drive our projects,” Voss said. “Students have an authentic audience.”

Students prepared themselves by consulting professional bridge designers and visiting bridges in the area, sixth-grade math and science teacher Josh Dalley said.

Truss bridges incorporate interlocking triangular units, which differentiate them from suspension bridges – the Golden Gate – or arch bridges – the Ponte Vecchio in Florence, Italy.

Working in two-member teams, students drew sketches and followed up with blueprints. They next ran their designs through the West Point Bridge Designer, a computer program that analyzed them and revealed potential places for improvement.

“As a result, they were able to change their design,” Dalley said.

The end result was 29 balsa-wood truss bridges.

“No two look alike,” Dalley said.

In the stress test, two tables were set a few inches apart, with the bridge being tested spanning the gap. River pebbles were emptied, a cup at a time, into a bucket that hung from a scale attached to the bottom of the bridge. The scale recorded the weight of the rocks needed to bring the bridge to the point of failure.

The winning team was Leit Schafer and Molly Poitras. It took 14 pounds 13.7 ounces of pebbles to collapse their bridge, which weighed 0.96 pounds.

Stated another way, the bridge had a strength-to-weight ratio of 245, meaning it sustained 245 times its own weight before giving out.

daler@durangoherald.com



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