How does a truss bridge work? A simple explanation

Short answer: a truss bridge is a frame of straight pieces joined into triangles. When weight sits on the bridge, each piece is either pushed or pulled along its length, and the triangles stop the frame from squashing out of shape.

Why triangles

Try this: make a square from four sticks with pins at the corners. It folds sideways. Now add one diagonal, so it is two triangles. It can't fold. A triangle's shape is fixed by the lengths of its sides, so it holds.

Pushed and pulled

When weight presses down on the middle of the bridge:

• The top is squeezed (compression).
• The bottom is stretched (tension).
• The diagonals share the work, some pushed and some pulled.

Each stick only carries a push or a pull along its length. That is a very efficient use of material, which is why trusses are so light for their strength.

What makes a truss fail

The sticks being pushed can buckle, and the joints can pull apart. See why bridges fail.

Try it at home

Make a small truss from craft sticks or straws and pins. Press down in the middle and watch which sticks bow. Then see which members are pushed or pulled and truss basics.