The Beam Bridge
Beam Bridge - a simple sketch
A Beam Bridge is the simplest design for crossing a distance.
It consists of a horizontal beam supported at each end by piers. The weight of the beam pushes straight down on the piers. The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 250 feet.
Modern Beam Bridges
Load and Thrust
Load and Thrust
Load is a consequence of gravity and always acts vertically.
The ground exerts an equal and opposite force upwards on the weight. This force is called thrust.
When a weight is placed on a beam it exerts a load. However, the weight is not directly in contact with the ground.
Hence the load reaches the ground by passing along the beam and the down the piers as shown in the diagram opposite.
Advantages of a Beam Bridge:
- easy to construct
- relatively cheap
- cannot span great distances
If the deck is supported by a number of beams instead of just one then it would be easier to erect and easier to fix.
As a bridge is loaded, the beam bends in a downward direction - sag. When the beam bends, new forces are imposed along the length of the beam:
The top of the beam is compressed (shortened)
The bottom of the beam is tensioned (pulled apart).
As you can see from the diagram, the lines mark equal distance on the beam and the angle of these lines converge at a point above the beam.
Testing to destruction
General rule: material breaks far more readily when it is under tension the when it is compressed.
Elastic Substances: materials that deform under a load and then returns to its original shape when the load is removed. Steel is slightly elastic
Advantages of Elastic Substances:
- when exposed to a massive load they will deform before collapsing entirely
- they give warning (visual inspections can determine if buckling has occurred)
The Bridge Challenge
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Good interactive website for the basics
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