How do I know which bridge members are in tension or compression?

Short answer: in a simple truss loaded downward at mid-span, the top chord is in compression, the bottom chord is in tension, and the diagonals alternate between the two. For other shapes, work it out by hand or test a model.

A hand test

1. Build a quick model from scrap sticks and pins.
2. Press down on the load point with a finger.
3. Watch which sticks bow, which are being stretched taut, and which joints open.

A stick that bows is in compression. A stick that stays straight and gets tight is in tension.

A rule for diagonals

Each diagonal runs from a point near the top to a point near the bottom. If it slopes up toward the middle of the span, it is typically in compression, and if it slopes down toward the middle, it is typically in tension, when the load pushes down. The pattern flips between Pratt and Howe trusses.

Why it matters

Compression sticks need to be short, stiff and braced. Tension sticks can be thin, but their joints must hold. Putting the right sticks in the right places saves weight. See choosing sticks.

For exact forces in a real design you need to calculate them. Our contest tools are being built to do that for you.