Arch bridges

• In an arch bridge the arch acts to support a load by converting the downward force into sideways thrust
• Arches can make structures last longer for example old buildings and structures in European countries have lasted as long as they have due to arches
• These structures avoid tension which was turned into compression forces
• The load pushes the units down together
• The thrust runs around the ring of the arch and is then transferred to abutments

Flat lintel vs an arch

• The arches can be made using various materials:

An arch bridge built using trellis

• If it is made of steel it can be built using a trellis like framework
• This  greatly reduces the weight of the structure and decreases the thrust on the abutments
• It enables engineers to build flatter arches with cheaper foundations
• An arch bridge differs structurally from a beam/pier bridge
• An arch is entirely in compression while the underside of a beam is in tension
• A beam bridge is cheaper if material can be found that can carry tensile forces reliably

A modern through arc bridge.

• In a modern through arc bridge
• the road hangs from two parallel arches
• The load is carried in compression arches
• The suspended road is free of compression and tension forces
• However the thrust on the abutments is very large
• Therefore very reliable foundations have to be built on solid rock

Old arch bridge

• The ancient Romans discovered the principle of the arch
• A flat lintel such as shown above can collapse under its own weight
• However when stones/bricks/other material is made into an arch they can carry a load
• The load pushes the masonry down and together
• So that the arch is stable and strong

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