Mechanics

Beam Bridges

The two beams in this exhibit are the same size and the same material. How much one of these beams sags depends on whether the beam is resting on its face or its edge. When you walk across either beam, you create compression and tension within the beam. That is, the fibers in the top of the beam are compressed, while fibers in the bottom of the beam are pulled apart. When the beam is placed on its edge, it has twice as many fibers resisting the forces that are trying to make it sag. However, the beam on its edge only has one half as many wood fibers resisting sideways motion, so it tends to flex from side to side instead of sagging. When the beam is placed on its face, it has twice as many wood fibers resisting side-to-side bending.