How a bridge carries a load

Where does a bridge send a load?

A bridge does not make a load disappear. Its members redirect the load through a path to supports and then into the ground. Choose a structural type and increase the central load to trace the simplified path.

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Structural typeCentral load40 kNLightHeavy

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Beam decks mainly bend and shear, arches carry compression into their abutments, and suspension cables carry tension to anchorages. Real bridges combine many members and load cases.

In this symmetric model, what happens to each vertical support reaction when the load doubles?

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How the parts work together

Start with a beam
A simply supported beam bends under a center load. In the symmetric model each support carries half the vertical load.
Try an arch
The curved member is drawn in compression. Its outward push is shown as a horizontal thrust at the abutments.
Hang the deck
A suspension cable is drawn in tension and sends vertical reactions to the towers and anchorages.

The main parts

Deck: The roadway or floor that receives the load.
Main member: Its idealized shape makes the dominant load path visible.
Supports: Reactions transfer the load into piers, abutments, or anchorages.

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