Montshire Minute: Bridges and Structures

Originally aired during the week of August 14, 2006

Monday

Our early ancestors knew how to apply physical laws when they created dwellings that could be moved from place to place. Imagine a peaked tent, with two long ropes staked to the ground at each end. The middle of each rope is looped over a notch at the top of the pole, which stands in the middle. So you've basically got a teepee with a square foundation. Pulling on a rope will not only tighten the walls of the tent, but will also push down on the pole. The forces push and pull against each other to keep the structure sturdy. When you grip the doorknob of a closed door and pull as hard as you can, your arm is straight and under tension, right? Now, push against the doorknob. You are now experiencing "compression." From a small footbridge to the Empire State Building, every structure we build relies on tension and compression. Sounds kind of stressful, doesn't it?

Tuesday

It's fun to watch the frame of a new building go up. The beams, those weight-bearing horizontal lengths of steel or wood or other material, and the vertical columns that support the beams, will hold the building up. It's also safe to say that we're watching tension and compression at work. Mario Salvadori illustrates this idea with a simple experiment in his book The Art of Construction. Place a book under each end of a plastic ruler, and push down gently on the middle of the ruler until it bends. The underside of the ruler is under tension - and the top of the ruler is being compressed. In a framed building, the beams are exposed to the weight of the floor, much like the ruler spanning the space between the two books. And the beams are compressing the supporting columns. So the push-me pull-me effect works together to keep the framework strong.

Wednesday

Until the late nineteenth century, stone arch bridges were the only kind of bridges that could span great distances. Eventually a "truss" system was developed for long bridges that required a very large or heavy beam to support loads. Since a truss is composed of triangles (the strongest of polygons because their shapes cannot be distorted), a truss bridge can support heavy loads with its relatively small weight. A square, on the other hand, can twist into a variety of shapes, with the angles allowed to change even when the lengths of the side stay the same. Look for the triangle shape in radio towers, steel bridges, or even in the beams holding up the Museum's ceiling during your next visit to Montshire!

Thursday

Although European countries including Austria, Germany and Switzerland built timber-framed covered bridges, America's versions were remarkable for their length as well as their elegant and efficient engineering. By the 1860s and '70s, the timber truss was eclipsed by spans of iron, which in turn were replaced by concrete and steel bridges in the 1920s and '30s. Covered wooden bridges quickly fell out of favor. Of the 10,000 or so covered bridges that once graced the American landscape, fewer than 800 remain. Vermont and New Hampshire are lucky enough to have about 150 bridges still standing. To learn more about the history and engineering of these structures, visit Covered Bridges: Spanning the American Landscape, a new exhibition from the Smithsonian Institution Traveling Exhibition Service (SITES) and the National Park Service's Historic American Engineering Record (HAER) at the Montshire Museum through October 15.

Friday

In the late nineteenth century, suspension bridges began to appear. Primitive forms of this design, made of wood and suspended from ropes, had long been used to span small distances, but it was impractical to build large suspension bridges until steel became cheap and easy to produce. A relatively light metal, steel can withstand tremendous amounts of stretching and compressing. But engineers also need to allow for a certain amount of "torsion," or twisting, in their bridge designs. Is a good example of how wind can cause torsion to damage or, in this case, destroy a suspension bridge. In 1940, the Tacoma Narrows Bridge in Washington state, only a few months after it opened to traffic, twisted so violently in the wind that its center span collapsed. Since then, engineers test bridge models in wind tunnels prior to construction and build their decks more stiffly.