Truss basics for balsa bridges

A truss is a frame of straight sticks joined at their ends. The sticks form triangles, and a triangle is the one shape that cannot change shape without one of its sides changing length. That is why trusses are stiff for their weight.

Tension and compression

When load pushes down on the middle of the span, every stick in a truss ends up in one of two states:

• Tension: pulled. The stick is being stretched, like a rope.
• Compression: pushed. The stick is being squeezed, like a column.

In a simple truss with load at the middle, the top chord is in compression, the bottom chord is in tension, and the diagonals alternate. You can find out which by drawing a quick sketch or by testing a small model and watching which sticks bow.

Why compression is the problem

A short stick crushes only under a huge push. A long thin stick buckles first: it bows sideways and suddenly can't carry load. Buckling load falls rapidly as the stick gets longer, which is why:

• Make compression members short. Add a cross-brace, or use more panels, so the long unsupported length is smaller.
• Make them stiff. A larger cross-section, or a stiffer, heavier stick, buckles later. See choosing sticks.
• Brace them sideways. A member in a flat truss can buckle out of the plane, and needs bracing between the two trusses.

Tension members are the easy part. They can be thin, as long as the joints at both ends hold.

Spending weight where it counts

You have a fixed mass budget. Put weight where the force is largest, usually the middle of the top and bottom chords, and trim it where force is small, near the supports. A bridge where every stick is the same size is almost always overbuilt in some places and underbuilt in others.

Common truss shapes

• Warren: a zigzag of diagonals with no verticals. Simple and light. Diagonals alternate in tension and compression.
• Pratt: verticals plus diagonals, with the diagonals in tension under a downward load, so the long members are the ones pulled.
• Howe: like Pratt but reversed, which puts the diagonals in compression.
• Deck versus through: whether the roadway rides on top of the truss or runs between its sides. The rulebook's clear-space requirement often decides this.

No shape is best for every contest. The span, the load position and the clear-space rule decide what works.

Test small first

Build a quick half-scale model from scrap and load it by hand. Note which sticks bow, and which joints open first. That is a better teacher than any rule of thumb. Then read why balsa bridges fail.