Kristin Smith knows how to build a bridge. Her balsa wood structure weighed only 1.2 ounces, but it held out until 66.6 pounds of weight finally proved its undoing. That's 55.5 pounds per ounce.

And it made Kristin the winner in the sixth grade bridge-building competition at Silver Mesa Elementary School.Of the school's 140 sixth-graders, 46 built bridges from the balsa wood kits provided by teacher Darlene Becknell.

Besides the kit, which included a base 4 by 12 by three/- sixteenths inch, and 9 feet of three/sixteenths square sticks of balsa, the children were armed with some information.

Such as: the triangle is the strongest geometric figure. It distributes a load more evenly. The stronger joints are, the better a load will carry.

Then they were turned loose to design their bridges, Becknell said. "They didn't have to stick with triangles and they could make their bridge any height they wanted."

The students worked at home, enlisting help from parents (and neighborhood engineers, if they were so lucky as to have one) and the contest was conducted over a several weeks so those on all four tracks of the year-round school could participate.

On competition days, a rope was draped across the center of each bridge and a variety of weights added, including weights from a bench press set, ankle weights and the kids' science books, which were found to weigh one pound each.

There was intense interest as the weight accumulated and the students waited for the first warning "crack" that mean the bridge was about to give way.

"We tended to get a little bit loud at those points," Becknell said.

First-, second- and third-place winners all were girls, a fact that is encouraging in a Utah school, where females tend to take fewer math and science classes.

Elizabeth McPartland's bridge actually held the greatest load at 143.4 pounds, but since it was heavier - 2.9 ounces - it came in second with 49.4 pounds per ounce. Third place went to Cindy Pryzbyla whose 1.7-ounce bridge didn't cave in until it was holding 64.2 pounds - 37.8 pounds per ounce.

In the end, the participants had nothing left but splintered balsa wood, some certificates - and a better understanding of how to make a few ounces support a great many pounds.