The parts of a truss, why it's made of triangles, and the four simple ideas behind every truss calculation
Last updated 2026-09-25
A truss is a structure made of straight members joined at their ends into triangles.
You find trusses in bridges, roofs, cranes and towers. What makes a truss efficient is that each member only pulls or pushes along its own length. It doesn't bend, so a light structure can carry a large load.
Join four members into a square with hinged corners and push on one side: it folds flat into a parallelogram without any member changing length. A triangle can't do that. The only way to change its shape is to stretch or squash one of its sides, and that's exactly what the members resist.
That's why a truss is built from triangles. Each one holds its shape on its own, and triangles joined side by side keep holding it together.
Example
Typical trusses
Four classic truss layouts, same span and load
Any shape works, as long as it's made of triangles. Even a cat:
Example
Cat
A truss shaped like a cat
Triangles alone don't guarantee it, though. A truss built from triangles can still move if its supports are in the wrong place, or if a joint is held from one side only. Truss stability shows how to spot that.
A real truss has bolted or welded joints, members with their own weight, and loads that don't land neatly on the joints. To calculate it by hand, or in any truss solver, you replace it with a simpler model: the ideal truss. Truzme's solver uses exactly these four assumptions.
The members meet at nodes that act like hinges. A node holds the members together, but each member can still turn freely around it, so no bending passes from one member to the next.
Real joints are stiffer than that. For long, thin members the difference is small, because the forces mostly run along the members anyway.
Every load and every support sits on a node. In Truzme you place a load by clicking a node; there's no way to put one in the middle of a member. If a real load sits between two joints, like the weight of a roof covering, you share it out between the nearest nodes.
The members' own weight is left out too. Truzme doesn't add it, so if it matters, add it as loads on the nodes.
Each member runs in a straight line from one node to another. A curved member under the same force would also bend.
This one follows from the first three. A straight member, pinned at both ends, with no load along its length, can only hold on by pulling or pushing along its own line. That force is the member's normal force (also called axial force), written N.
How to tell which one a member is in, even without calculating, is in Tension and compression.
A member in compression has one extra way to fail: a long, thin one can bow sideways and give way before its material runs out of strength. That's buckling.
Once a truss fits these four assumptions, balance alone gives you everything: first the support reactions, then the force in every member, one joint at a time. You need one tool for all of it: splitting a force into its horizontal and vertical parts.