Bridge Building Series - Arch Bridge

Arch Bridges

Arch bridges have extraordinary strength due to their shape.

Unlike a beam bridge, the weight and the load on an arch bridge are pushed outward and carried along the curve to the support structures at each end called abutments.

An arch bridge carries all loads in compression, without any tensile forces. The stones in an arch bridge stay together by the force of their weight and the compressive force transferred between them. 

The size of the arch directly affects the effectiveness of an arch bridge. The arch is flattened down in very large arch bridges and large tensile forces that must be factored into the bridge design.

A simple arch bridge experiment

Supplies:

·         A ¼ piece of cardstock paper

·         A permanent marker

·         A ruler

·         A stack of books to wedge in the paper

Building instructions:

1.   Draw lines 1/2” apart on the cardboard or cardstock paper. 

2.   Wedge it between a gap created by books, chairs, desks, or other objects.

3.   Tape the ends onto the books, desks, or other objects.

4.   Now, press down on any part of the arched cardboard or cardstock paper. What happened? What is happening with the lines on the cardboard?

Top View

An arch bridge without an additional load.

An arch bridge with some load. It still holds its shape well.

An arch bridge with considerable load. It still holds its shape pretty well.

An arch bridge in danger of collapsing. The arch has flattened out too much.

Bottom View

An arch bridge with very little load. It holds its shape well.

An arch bridge with more load. It holds its shape well, again.

An arch bridge under considerable load. Its arch is beginning to flatten out a bit.

An arch bridge in danger of collapsing. The arch has flattened out too much.

Again, this is a very simple project, but it does an excellent job of demonstrating what an arch bridge experiences under load.