Forces in an Arch Bridge

imagesI have to say that even though I’ve nearly completed my book about the physics of bridges, the arch bridge is still the one that most fascinates me. The arch is virtually the only type of bridge that exhibits one type of force in play. While other bridges generally have both tension and compression at work on their spans, the force at work on the arch bridge is the force of compression.

The sketch above shows that compression is at work on an arch bridge from the span down the arch and on into the rock. It may appear that the compressive members between the span and the ends of the arch are being supported by the members attaching the arch to the deck struts, but that is not the case. In fact, they are being supported by the arch and then supporting the deck on each side of the portion that is supported by the arch.

All this means is that as a force pushes down on the arch, the arc presses down on the rock or other material that supports the ends of the arch. It may be an arch in rock that is held tight by a keystone or an arch bridge crossing a gulley that is far below. Either way, compression is doing the work to keep that bridge standing!

Want to try it for yourself?

  1. Build two walls of blocks that face each other with a space of about 6 inches between them. Put a piece of looseleaf or printer paper between the walls – being sure the walls are as tall as the paper once it is in there and forms an arch. Then, take a toy truck and drive it across the arch. It will stand because the compressive forces are transferred to the walls.
  2. Take a piece of printer paper and form an arch. Rather than put it between two walls, take a single piece of paper and attach each end of the paper to the bottom of the arch on either edge. Push down on the arch. It’s not going to hold a can, but it will support a weight. You will see that the arch “spreads” as the weight is applied (under comprehension). The paper that is attached at the bottom edges of the arch is acting as if it were walls on either side as the compressive forces are transferred down the sides of the arch.

My new book about the science of bridges is nearly done!

Other articles about the forces at work on bridges:

Cantilever , Suspension , Beam , Arch


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