BUILDING BETTER BRIDGES: STUDENTS USE TOOTHPICKS FOR CONSTRUCTION

Linda Tansil's Central High School physics classes had three weeks and 60,000 toothpicks donated by Wal-Mart. Randy Hitt has a few years and many tons of concrete and steel.

But their goal is the same: to build a better bridge.

Hitt, area engineer for the Missouri Department of Transportation, was at the school Monday to evaluate the model bridges built by the classes. Hitt is in charge of supervising the contracts for the new Bill Emerson Memorial Bridge under construction at Cape Girardeau.

Tansil's students built tiny solid bridges, flimsy bridges, sweeping bridges and bridges unsafe at any speed.

Most were constructed of only toothpicks and glue, though pasta was the primary ingredient in one case. "Most of our bridges aren't edible ...," Hitt said.

They were judged on the basis of their plans, the quality of construction and weight-bearing ability.

Aside from the materials used, little new has been developed in the design of bridges since the Romans, Hitt said.

The students' bridges were based on 11 different types of truss designs, the type employed in the current Mississippi River Bridge. The Emerson Bridge under construction is a cable stay design, which has more flexibility than a truss bridge and thus greater seismic safety.

Unlike the toothpick bridges, the Bill Emerson Memorial Bridge is designed to withstand 100 mph winds, earthquakes and the possibility of being rammed by fully-loaded barges.

"If two of the three cables failed, the bridge would still be standing," Hitt told one of the classes.

The goal, Hitt said, it to design for safety without overbuilding and adding cost.

Making toothpick bridges gives her physics students a chance to be creative, Tansil said. They're learning about torque and distribution of loads. And they had to compute the mass of the bridge and the weight of the truck that would cross it.

Their goal was to understand "the structurally strongest way to build a bridge," she said.

Hitt invariably found something good to say about each design while also pointing out flaws that could cause the bridges to fail under stress.

"It's like a chain if the weakest link fails," he said.

He focused on the critical points -- the piers and the cross bracing at the top that may look superfluous but shares stress when the bridge is put in motion.

The critical points of the Mississippi River Bridge will be exploded when the time comes to tear it down, he said.

The students' bridges had to come in under a certain weight, had to be wide enough to accommodate a miniature truck and high enough to allow a boat to go underneath.

The final test was each bridge's ability to bear a weight of 5 kilogram for 10 seconds. Few could. The builders donned safety glasses as Tansil suspended increasing amounts of weight from each bridge.

Indeed, some splintered, and one bridge seemed to waver before any weight was added at all. But the sturdy model built by Trish Zimmer and Katie Barbour didn't flinch as each kilogram of weight was added. It still stood firm at 5.

Building a toothpick bridge may seem an academic exercise but Hitt pointed out that the crane capable of lifting a backhoe at the bridge construction site is made of thin bars woven into a design that makes the crane incredibly strong.

Hitt, a Cape Central graduate, made a toothpick bridge himself when he was in high school.

"It gives you a basic appreciation of what goes into design and what the critical factors are," he said.

"Some things in physics are hard to apply in the real world."