Forces & Bridge Design

By Laura Zinszer Physics Teacher

Frederick Douglass High School 2016

Compression• Compression is a “push” force.

• Compression causes an object to get shorter.

• Stone and concrete are strong in compression.

• Compression is represented by red force Vectors.

Compression Bridges - Arch• A bridge that supports a weight in compression is

an arch bridge

• The circular arc in compression supports the road

• The arch can be below the road or above the road

Arch Bridge is in compression.

Tension• Tension is a “pull” force

• Tension causes an object to get longer

• Wire rope and chains are strong in tension

• Tension is represented by blue force vectors

Tension and Compression• Most bridges use a combination of both

tension and compression forces.

• Newton’s 3rd Law states for every action,

there is an equal and opposite reaction.

• This applies to both tension or

compression forces.

tension compression

Beam & Girder 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 tension.

• The taller the beam, the stronger it is.

A Beam Bridge…

Now let’s add vertical rods to help you see what

is going on. They serve no structural purpose.

The top rods are pushed together in

compression

The bottom rods are pulled apart in tension

Beam and Girder Bridge

Suspension Bridge• A suspension bridge features a long cable strung

over towers and anchored on both sides

• Smaller cables are hung from the main cables and connect to the

road deck

• The cables in tension support the deck.

• The towers & piers in compression support the entire span.

Suspension Bridge

Cable-Stayed Bridge• A cable stayed bridge features cables that connect

directly from a tower to the deck or roadbed.

• The cables in tension support the deck.

• The towers & piers in compression support the entire span.

Cable-Stay Bridge

• As a beam gets taller, it can become too heavy and costly.

• Solution: build a truss bridge.

• Trusses function like beams, but are composed of triangles.

• The top of the truss bridge has thick beams for compression.

• The bottom of the truss bridge has thin eye-bar chains for tension.

Truss Bridge

Truss Bridge

• The bottom of the truss bridge is composed of thin eye-bar

chains for tension.

• The top of the truss bridge is composed of thick beams for

compression.