While bridges come in all sizes and shapes, nearly all of them represent one of five types or variations of the basic bridge types. Typical bridge designs include beam, truss, girder, suspension, arch, cable and cantilever. Forces that come into play in bridge design and engineering include compression, tension or stretch, deck flexibility, torsion or twisting and shear, the force that stresses the bridge materials laterally across the bridge deck. One of the oldest arch bridges still in use today, a testament to its Roman engineering, includes the Ponte dei Quattro Capi bridge which dates back to 62 B.C., and spans half of the Tiber River in Rome Italy.
Beam, Truss and Girder Bridges
The beam, truss and girder bridges work simply, much like laying a plank between two banks. Piers or posts on either end support a flat bridge deck that spans the gap between the posts. The bridge deck consists of beams, like hollow box girders, an open frame or truss that spans the posts or supports on either end. The bridge deck must withstand compression above and tension from below. Most of the covered bridges found in New England, represent these types of bridges made from wood. Economical because of the abundance of wood, beam bridges are not as strong as steel and require constant maintenance.
The Arch Bridge
While the type of bridge plays a role in how well it’s constructed and stands up to wear, tear and weathering, the materials in the bridge also play a part in its longevity. One of the oldest engineered bridges, the arch bridge, supports a deck built above two abutments that serve as the curved arch. Made from masonry and stone, the arch design prevents any one area of the bridge receiving too much tension. With abundant building materials, arch bridges are durable and strong, requiring little to no maintenance. Its drawback is that masonry and stone don’t have great tensile strength.
The first suspension bridges date back to the 15th century and typically span waterways, because the bridge deck requires little to no access from below to build. Tall pillars support this bridge, evenly spaced across the span as needed, from which massive wires on either side sweep from pillar to pillar. From these sweeping wires suspenders hang vertically to hold up the bridge deck. The tension in the cables and the compression from the pillars work together to cancel out the force of gravity, making them strong and efficient. These bridges can span great distances once the pillars are in place, but they are costlier to build, require extensive upkeep and the bridge decks can move and twist when exposed to fierce winds. The Brooklyn Bridge in the state of New York and the Golden Gate Bridge in San Francisco both represent suspension bridges.
Cantilever bridges offer a way to build a continuous bridge across multiple supports to effectively distribute the load evenly. A portion of the bridge provides an anchor that supports a bridge deck that extends to either side of the support that requires precise counterbalance engineering. The advantage of building this bridge design comes during the construction phase. Cantilever designs cost less to build because of their uniformity and they don’t require temporary supports during construction, which helps to speed up the process. But cantilever bridge designs do require precise engineering because the counterbalance weights can affect their strength if incorrect, especially if contractors build the segments slightly differently.