The Living Bridges
While the first bridges were likely nothing short of logs toppled over creeks, most of humanity’s bridge-building legacy is a story of artificial structures crafted out of the elements. We can find, however, one of the most striking exceptions to this rule in the Meghalaya region of northern India.
During monsoon season, locals here endure some of the wettest conditions on Earth, and rising floodwater’s cut the land into isolated fragments. Build a bridge out of woven vines or hewn boards and the rain forest moisture will inevitably turn it into compost. As you can see from the photo, the local people developed a rather elegant solution to the problem: They grow their bridges out of natural vegetation. In doing so, they turn a large portion of the bridge maintenance duties over to the bridge itself.
Building a living bridge takes patience, of course. The local villagers plan their constructions a decade or more in advance. The War-Khakis people, for instance, create root-guidance systems from the hollowed halves of old betel nut tree trunks to direct strangler fig roots in the desired direction. They simply direct the roots out over a creek or river, spanning it, and only allow the roots to dive into the earth on the opposite bank. The larger living bridges boast lengths of up to 100 feet (30 meters), can bear the weight of 50 people and can last upward of 500 years
But the weight of car or foot traffic is far from the only force affecting a bridge.
Additional Bridge Forces: Torsion and Shear
So far, we’ve touched on the two most important forces in bridge design: compression and tension. Yet dozens of additional forces also affect the way bridges work. These forces are usually specific to a particular location or design.
Torsion, for instance, is a particular concern for engineers designing suspension bridges. It occurs when high wind causes the suspended roadway to rotate and twist like a rolling wave. As we’ll explore on the next page, Washington’s Tacoma Narrows Bridge sustained damage from torsion, which was, in turn, caused by another powerful physical force
The natural shape of arch bridges and the truss structure on beam bridges protects them from this force. Suspension bridge engineers, on the other hand, have turned to deck-stiffening trusses that, as in the case of beam bridges, effectively eliminate the effects of torsion.
In suspension bridges of extreme length, however, the deck truss alone isn’t enough protection. Engineers conduct wind tunnel tests on models to determine the bridge’s resistance to torsional movements. Armed with this data, they employ aerodynamic truss structures and diagonal suspender cables to mitigate the effects of torsion.
Shear: Shear stress occurs when two fastened structures (or two parts of a single structure) are forced in opposite directions. If left unchecked, the shear force can literally rip bridge materials in half. A simple example of shear force would be to drive a long stake halfway into the ground and then apply lateral force against the side of the upper portion of the stake. With sufficient pressure, you’d be able to snap the stake in half. This is shear force in action.