Destroyer of Bridges

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Weather, Destroyer of Bridges

While wind can certainly induce destructive resonant waves, weather as a whole unleashes a host of destructive assaults on the bridges we build. In fact, the relentless work of rain, ice, wind and salt will inevitably bring down any bridge that humans can erect.

Bridge designers have learned their craft by studying the failures of the past. Iron has replaced wood, and steel has replaced iron. Prestressed concrete now plays a vital role in the construction of highway bridges. Each new material or design technique builds off the lessons of the past. Torsion, resonance and poor aerodynamic designs have all led to bridge failures, but engineers continually bounce back with innovations to solve design problems.

Weather, however, is a patient and unpredictable adversary. Cases of weather-related bridge failure tend to outnumber those of design-related failures. This trend can only suggest that we have yet to come up with an effective solution. To this day, no specific construction material or bridge design can eliminate or even mitigate these forces. After all, we’re talking about the same forces that degrade whole mountain ranges and forge deep chasms in the earth. By comparison, a man-made bridge is nothing.

As with the ancient Incan suspension bridges, the only deterrent is continual preventive maintenance.

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The Living Bridges

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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

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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.

Truss Bridge

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Truss Bridges: Beam Bridges With Braces

Travel around the world, and you’ll encounter dozens of variations on your standard beam bridge. The key differences, however, all come down to the design, location and composition of the truss.

During the early Industrial Revolution, beam bridge construction in the United States was rapidly developing. Engineers gave many different truss designs a whirl in an attempt to perfect it. Their efforts weren’t for naught. Wooden bridges were soon replaced by iron models or wood-and-iron combinations.

 

 

All these different truss patterns also factored into how beam bridges were being built. Some takes featured a through truss above the bridge, while others boasted a deck truss beneath the bridge.

A single beam spanning any distance undergoes compression and tension. The very top of the beam gets the most compression, and the very bottom of the beam experiences the most tension. The middle of the beam experiences very little compression or tension. This is why we have I-beams, which provide more material on the tops and bottoms of beams to better handle the forces of compression and tension.

And there’s another reason why a truss is more rigid than a single beam: A truss has the ability to dissipate a load through the truss work. The design of a truss, which is usually a variant of a triangle, creates both a very rigid structure and one that transfers the load from a single point to a considerably wider area.

While truss bridges are largely a product of the Industrial Revolution, our next example, the arch, dates back much further in time.