Stresses On All Bridges
Stress: the basic stresses on all bridges.
Types: types of Bridges and how these basic stresses affect them.
Links: links to examples of students' work and other pages.

Stresses on all bridges: The stresses on all bridges are basically the same: compression and tension. The way the three main different types of bridge structures; beam, arch, and suspension, handle the stresses is what varies. Below are some illustrations of how the different types of structures handle compression and tension.

The basic stresses on all bridges are compression and tension:
Beam bridges come in dozens of different styles. This is a modern beam bridge carrying a hi-way over a pasture for cows.
Truss: The design, location and composition of the truss is what determines the type. In the beginning of the Industrial Revolution, beam-bridge was developing rapidly and designers were coming up with more ideas to design the bridges and install metal bridges. One of the beam bridges was designed by William Howe 1840. - written by Emmanuel

Photo of a Howe truss somewhere in Colorado, taken circa1870. Click on the image to go to the Student Work page, then go to Emmanuel's Page.

Arch: Compression:
Arch bridges are always under compression. The force of compression is pushed outward along the curve of the arch toward the abutments

The tension in an arch is negligible. The natural curve of the arch and its ability to dissipate the force outward greatly reduces the effects of tension on the underside of the arch.
The Suspension Bridge:
A suspension bridge is one where cables (or ropes or chains) are strung across the river (or whatever the obstacle happens to be) and the deck is suspended from these cables. Modern suspension bridges have two tall towers through which the cables are strung. Thus, the towers are supporting the majority of the roadway's weight.
The force of compression pushes down on the suspension bridge's deck, but because it is a suspended roadway, the cables transfer the compression to the towers, which dissipate the compression directly into the earth where they are firmly entrenched.
The supporting cables, running between the two anchorages, are the lucky recipients of the tension forces. The cables are literally stretched from the weight of the bridge and its traffic as they run from anchorage to anchorage. The anchorages are also under tension, but since they, like the towers, are held firmly to the earth, the tension they experience is dissipated.

Almost all suspension bridges have, in addition to the cables, a supporting truss system beneath the bridge deck (a deck truss). This helps to stiffen the deck and reduce the tendency of the roadway to sway and ripple.

Photo panorama of the Brooklyn Bridge, showing bridge in context, circa 1900. Click on the image to go to the Student Work page; then select The Story About The Brooklyn Bridge - written by Derrick.

The force of compression on a suspension bridge releases the pressure on top of the bridge and translates the pressure to the tower, and from there it compresses to the beam where the main supports are. The tension cables run between the two anchorages and stretch the weight from the bridge. They are anchored firmly into the earth. - written by Emmanuel.