Forces that Act on Bridges
February 20th, 2005
Compression:
Compression is a pushing (compressing) force. You can take the middle a straw and squeeze it between two fingers, the straw flattens. However, if you hold each end of a straw and push together it is harder to make the straw compress, or flatten. The shorter a piece of wood is, the more compression it can hold. The longer a piece of wood is, the less compression it can hold.
Tension:
Tension is pulling. It would be hard to break a straw if you held both ends and pulled
apart. Wood also has the ability to resist tension. Tension may be applied parallel to
the grain of the wood, but should be avoided perpendicular to the grain. Wood is very strong
in tension parallel to the grain, but extremely weak in tension perpendicular to the grain.
Torsion:
Torsion is twisting. When you wring out a cloth, you are applying torsion to the cloth. If you take a stick pretzel, twist one end, and hold the other end still, it will break very
easily. If you do that with a baseball bat, it will not break. However, if you take a piece
of licorice and apply torsion to it, the licorice will twist around several times before it
breaks. Each of these materials has a different way of responding to torsion. Bridge
designers must watch for torsion and try to reduce it as much as possible.
Shear:
Shear is a interesting force. It happens when there are two opposing forces acting on the
same point. If you hold a piece of wood with both hands next to each other, and push up with
one hand and down with the other, you are applying shear to that piece of wood. Shear usually occurs horizontally, and not vertically.
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