Bridging a Gap and Naming it Too

If you’ve ever had to commute across a river, it’s likely that you’ve become familiar with traveling over bridges (unless you’re Walt Whitman, who takes the ferry) – but would you be able to identify what types of bridges you’ve crossed, other than by their names? Here’s a list (compiled by Historyofbridges.com) of some of the more popular types of bridges and how to identify them by their structures:

Arch bridges, as their name suggests, use an arch underneath the bridge deck to support the brunt of the tension. Mid-span piers help support the arches, as do a series of abutments and pillars that are built strong to carry the weight of the entire bridge. Arch bridges are fixed and unable to move, but they can support any traffic, ranging from pedestrians to water-carrying aqueducts.

Beam bridges are simplistic, connecting the deck between abutments, and are occasionally outfitted with structural piers. The distribution of force for beam bridges moves from vertical force into shear and flexural load that is then transferred to the abutments or piers. This form of bridge is the oldest known to man, originating as wooden logs dropped over ditches and creeks.

Cable-Stayed bridges use deck cables that are connected to vertical pylons erected near abutments or the middle of the span of the bridge. They are similar to suspension bridges with the use of cables, except they connect to pylons in one of two ways: the harp design, where each cable is attached to a different point of the pylon, much like the strings of a harp, or the fan design, where all cables connect to one point at the top of the pylon. They can support almost all traffic, save for heavy rail.

Cantilever bridges appear as a hybrid of an arch bridge, supporting their loads through a diagonal bracing with horizontal beams supported on one end. Most cantilever bridges use a pair of continuous spans placed between two piers, with beams meeting at the center. Some cantilever bridges also use mid-bridge piers as their foundation, allowing them to span in both directions towards other piers and abutments. Unable to span large distances, cantilever bridges will often use multiple beams coming out of each abutment or multiple center piers.

Suspension bridges use cables from vertical suspenders to hold the weight of the deck. They are a popular choice of bridge because of their ability to support weight over a large span, but they also are vulnerable to wind and vibrations due to their relatively unfixed structure, save for any abutments or piers.

Tied arch bridges are similar to arch bridges, except their arches are on top of the bridge deck and are used to transfer the weight of the bridge to the top cord, which is connected to the bottom cords of the bridge foundation.

Truss bridges use a diagonal mesh of triangle-shaped posts above the bridge deck to distribute forces across the structure. Although an individual beam from the bridge can endure dynamic forces of tension and compression, the presence of multiple beams allows the bridge to handle much stronger forces by distributing the load across the entire structure. Truss bridges are typically separated into king and queen posts, depending on if the two diagonal posts are supported by a single vertical post (king) or by two vertical posts and an additional horizontal post (queen).