Forces on Bridges How are bridges designed to withstand the forces that act on them?

Classifying Bridges  Function on bridge: to carry a load across a distance  All loads have a downward force (due to gravity)  Bridges classified by how they carry the weight:  Compression  Tension  Tension/ Compression

Compression Bridges  Compression  “push” force  causes objects to get shorter  Stone and concrete are strong in compression

Compression bridges: Arch

Compression bridge: Arch

Tension bridges  Tension  “pull” force  Causes an object to get longer  Wire rope and changes are strong in tension

Tension bridge: Suspension

Tension/ Compression Bridge  A beam bends under the weight of a load  When the beam bends, the top half is in compression and the bottom half is in compression  The taller the beam, the stronger it is

Tension/ Compression Bridge: Beam

Tension/ Compression: Truss  Problem: as a beam gets taller, it becomes costly and too heavy  Solution: build a truss  Trusses composed of triangles