How do bridges stay up?

Bridges stand by efficiently distributing forces of compression and tension through their structure. For example, in an arch bridge the load compresses the arch into its supports, which transfer forces into the ground. In a suspension bridge, the deck is in tension and hangs from cables; those cables transfer load in tension to towers and anchors which bear compression. In beam or truss bridges, materials in the bridge deck experience bending: the top fibers compress while the bottom fibers stretch. The design ensures that every weight (from vehicles, etc.) is balanced by support forces in beams, cables or arches, and ultimately transmitted safely into the foundations. In essence, bridges use geometry and strong materials so that gravity-induced stress is carried along the structure into the ground.