As with any engineering project, engineers must consider certain forces and physical effects on the bridge to ensure it safely supports its load. Each type of bridge performs this task differently.
A bridge must handle high amounts of force well to maintain stability. Force takes into account mass and acceleration due to gravity. The bridge's traffic or load, beams and endpoints all exert force on the bridge. The net effects of this force can cause bending or twisting.
Stress describes an amount of force applied over a specific area. When parts of a bridge experience opposing forces at opposite ends, it has tensile stress. If a downward force in the middle opposes upward forces at the end of a piece, it has bending stress. Torsional stress describes opposing circular forces on a bridge or joint.
Three main types of bridges include arch, beam and suspension bridge. Suspension bridges use tall posts from which engineers connect strong cables. These cables support the various stresses on the bridge. Arch bridges work the opposite way by supporting the load from the bottom. Beam bridges use pillars or pylons to support the flat bridge surface.