A bridge's job is to keep its weight up and to carry the weight of people and vehicles crossing it. The bridges' weight is referred to as a dead load, while the traffic from pedestrians and vehicles is referred to as the live load. The goal in this section will be to talk about the two main forces involved in bridge construction: compression (pushing) and tension (pulling).
Demonstration: Pushing and Pulling.
One way to see if the forces working on something are pushing or pulling is by seeing what happens to its length. You can demonstrate push and pull by the use of a piece foam rubber or sponge. Draw lines about an inch apart on one long edge of the foam or dampened sponge. Bend the foam or sponge into a U.2The lines on the inside will squeeze close together to simulate pushing. The lines on the outside will stretch apart to simulate pulling. Students should notice that pushing or compression makes things shorter, while pulling or tension makes things longer.
Students can experience these forces for themselves if they stand and do a side bend.
Stand up and bring your right arm over your head stretching to the side and bending the torso to the left. They will feel the stretching of their right side of the body as they pull toward the left. Meanwhile they will also experience pushing or squeezing as the left side of their body especially around the waste squeezes or pushes together on the side.
Trusses are often used to strengthen a bridge. This is a design, which relies on the triangulation of steel and wood segments that give amazing strength by reason of their shape. Demonstrate the strength of the triangle by having students make a square out of straws and a square or Popsicle sticks. Students can take these shapes and exert pressure on each. They should see the square give way and collapse. The triangle will be superior in strength. This principle is used to form trusses that are used in beam bridges to add to their durability and strength. You can follow the illustrations in Appendix B and have students put together their own truss structure for a bridge. They can hang the bridge between two desks and try adding small loads from the bridge. How much weight can you add before the bridge squeaks, sags, or wiggles? If your bridge passes this test try adding books to the top. If it collapses or breaks why did it?
The teaching point here is that the main weakness in a beam bridge is going to be in the middle where the weight has a tendency to sag or in the connections. This is why an unsupported beam bridge that is too long will sag, as it gets wider between piers.
The arch has its weakness in the curve, which is pushing out, but the bottom of the arch where it reaches the ground keeps it from collapsing outward. In a suspension bridge the cables are tied to abutments on either end of the bridge and then they cross over the towers, which support the cables as they stretch over the water. The weight of the cables then transfers the loads to the towers, which transfer them to the ground.