InTrans / Aug 06, 2014

Bridges: May the force be with you

Go! Magazine


posted on August 6, 2014

Have you ever looked at a bridge, no, I mean really looked at it?

Bridges can range from logs used to cross streams to the massive steel and concrete suspension bridges that span entire bays. Regardless of their complexity, there are two forces that all bridges must overcome: “compression” that pushes in towards the center of the bridge (or span) and “tension” that pushes down and towards the outside of the bridge. Both forces are active at the same time (as the top undergoes compression, the bottom experiences tension). If either becomes too great, the bridge collapses. Designers have to account for both, but they also have to allow the bridge to be somewhat flexible.

Two additional forces that affect larger bridges, notably suspension bridges, are “torsion,” or the force of wind, and “shear” the force of the bridge span being pulled in two directions. Torsion occurs when high wind causes the suspended roadway to rotate and twist like a rolling wave. On the other hand, shear force can literally rip bridge materials in half by forcing two parts of single structure in opposite directions. Let’s look at four very different types of bridges:

Suspension bridges

Suspension bridges are the most expensive, most well-known, and are able to span distances much greater than any other bridge (the longest is the Akashi-Kaikyo Bridge in Kobe, Japan, which spans 6,532 feet). Most suspension bridges are made with two towers, fixed into solid rock or concrete that support large cables as well as supporting suspenders that support the road. Suspension bridges also have ‘deck trusses,’ which reduce the sway of the road in strong winds and ensure structural integrity.

With these suspenders being attached to the bottom of the bridge, they absorb most of the force of tension. This tension is transferred to the cables and both forces are transferred to the towers on either end.

The towers also take the majority of the force of compression, meaning towers must be well fixed to remain in place. Some of the force is transferred to the anchors fixed on land, but not all suspension bridges have anchors. Because suspension bridges rely so much on suspenders to remain standing, they require frequent maintenance which adds to their long-term cost.

Cable-stayed bridges

Cable-stayed bridges are often confused with suspension bridges because they work on the same principles. However, the towers of cable-stayed bridges are not attached to each other but only to the roadway itself. They also lack suspenders, and the forces acting on the roadway are distributed directly to the tower through cables.

Some designs of the cable-stayed bridge have a single tower that supports the entire bridge, however these work only across short distances. Cable-stayed bridges are somewhat cheaper than suspension bridges, but are not able to cover the same distances. The longest cable-stayed bridge is the Russky Bridge in Vladivostock, Russia, which spans 3,622 feet.

Beam bridges

Beam bridges are the simplest and most commonly used bridge. A beam bridge is composed of a section of bridge resting on two pillars on either end, similar to suspending a board between two containers. Beam bridges can be supported for longer spans with the addition of steel beams, trusses (structures made of triangles that dissipate weight and force), or latticework, which is a framework consisting of a crisscrossed pattern of strips of building material.

Being made into sections, beam bridges can span almost indefinite lengths as long as there are enough pillars to distribute the weight of each section. Most beam bridges, however, span short distances, such as freeway overpasses. The longest beam bridge in the world is the Lake Pontchartrain Causeway in Louisiana, which spans 23.8 miles, with each span about 56 feet long.

Arch bridges

Arch bridges are very simple and are one of the oldest types. Arches are the most efficient shape at redistributing weight, which is what makes these bridges so successful. Some of the oldest arch bridges were built by the Roman Empire between 27 BC and 476 AD. Though not all bridges survived, many of their aqueducts, which used arches similar to bridges, have lasted well into the 21st century and still maintain their structural integrity today.

Because of their shape, arch bridges negate tension stress and evenly distribute compression stress. As with any other kind of bridge, spans too large will collapse. Arch bridges built today are often comprised of steel, which is able to span much greater lengths than the stone used by Romans. Some arch bridges also have the arches above the bridge, rather than building it on an arch. The same principles apply for the over-road arches, though additional supports and suspenders are required to attach the road to the arch. The longest arch bridge is the Chaotianmen Bridge in Chonqing, China at 1811 feet.

By Brandon Hallmark, Go! Staff Writer

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