Truss Structure

Build a square from 4 chopsticks and nudge a corner. Even if you nail it tight, it quickly wobbles into a parallelogram. That's because the angles at the corners change easily. But make a triangle from 3 chopsticks, and no matter how you push or press on a corner, as long as the three side lengths stay the same, it can never be squashed out of shape.

Mathematically, the triangle is the only polygon whose angles are locked into one single shape once its three side lengths are set. Thanks to this, the whole structure holds its form even if the corners are joined loosely with pins instead of being locked with rigid welds.

Builders used this property when they built big bridges and tall towers, packing strong triangles tightly together. With far less material than a square frame needs, you can get a rock-solid skeleton that doesn't wobble.

When a train rolls across a heavy bridge, the deck is pushed downward and wants to sag. A plain plank would bend in the middle and then snap. In a truss, that bending force is split up and spread out as simple pushes and pulls.

Press down on the top point of a triangle, and the two slanted sides get pushed outward and squeezed. That squeezing force is called compression. At the same time, the bottom side is pulled tight in both directions. That stretching force is called tension.

When the load is delivered to the joints, each member mostly handles just its own share of pushing or pulling. That's why relatively thin steel pipes or lumber can share the work and hold up a big weight. One catch: a member under compression that's too thin can bow sideways and snap, which is called buckling. So these members need enough thickness and stiffness.

To be a little more precise, the joints in real building and bridge trusses aren't perfect rotating pins. They're often firmly welded, so small bending forces do appear. But to keep the math simple, engineers first calculate as if the joints were ideal connections carrying only forces along the length of each bar. Then they check separately for the extra forces the joints add.

Using a truss instead of massive solid concrete or steel plate cuts the amount of material dramatically. Less material means a lighter structure, which lets you build a bridge that spans farther without columns.

Also, with an open lattice tower, wind slips right through the gaps in the frame, so even in a fierce storm it cuts down wind resistance. That's one of the reasons the Eiffel Tower and power transmission towers stand firm in strong winds. Some trusses, like those under a covered stadium roof, are blocked from the wind by roofing, though.