Roof trusses, bridges and pylons all rely on structural members working together. Learn how forces act on each member, and why triangulation makes structures stiff.
A frame structure is built from a skeleton of separate structural members (beams, poles, struts) joined together, rather than a single solid mass. Roof trusses, electrical pylons, and bicycle frames are all frame structures — they use far less material than a solid structure while still carrying heavy loads.
Every member in a frame structure experiences one or more of these forces:
Pylon designs vary widely, but they all solve the same problem: holding heavy power lines high above the ground, safely, in wind. Look closely at any pylon and you'll see a lattice of small triangles rather than large open squares. This is triangulation — a triangle is the only shape that cannot change shape without one of its sides changing length, so triangulated frames resist twisting and bending far better than a frame of squares. The diagonal cross-bracing members convert what would be a wobbly rectangle into two rigid triangles.
| Type | How it spans a gap | Examples |
|---|---|---|
| Beam | A single rigid horizontal member resists bending | Steel I-beams (girders), concrete lintels, beam-and-column bridges |
| Suspension / cable-stayed | Cables in tension hold up a deck | Suspension bridges, cable-stayed bridges |
| Arch | Compression is carried around the curve into the supports | Bridges, dam walls, building doorways |
| Cantilever | A beam fixed at only one end, projecting outward | Balconies, cable-stayed cantilevers |
There are three main ways a structure can fail:
Before anything gets built, an idea has to be worked out and shown to other people — a client, a builder, an assessor. That's the purpose of graphics: technical drawing lets you develop your own idea clearly in your head, and communicate that exact idea to someone else without any confusion.
Technical drawings use different line types so anyone reading them instantly knows what each line means:
Drawings are also scaled up or down from real life so they fit on the page, and dimensioned — every measurement is written on the drawing in millimetres, so the exact size is never in doubt.
A single-view flat 2D drawing shows one face of an object flat-on, drawn to scale with correct line types and dimensions — enough detail for someone to make a simple, flat part.
For a more realistic 3D view, use isometric drawing: sketch on top of an isometric grid, where all lines run vertically or at 30° to the horizontal in two directions. Because the grid keeps every angle consistent, you can produce an accurate-looking 3D sketch quickly, without needing drawing instruments.
For the most realistic, eye-catching impression of a design, technologists use double vanishing point perspective: two vanishing points are marked on a horizon line, and every edge of the object that runs "into the distance" is drawn heading toward one of these two points. This mimics the way parallel lines appear to converge as they get further from your eye — think of looking down a long straight road.
Artistic drawings usually start as a quick freehand sketch, using only a pencil, a ruler for straight reference lines, and blank paper — no measuring instruments needed at this stage. The sketch is then enhanced to look more realistic by adding colour, texture and shading (darker tones on surfaces facing away from the light) and shadows cast onto the ground or nearby surfaces.
Click a member of the truss (on the diagram, or the list) to see whether it's in tension or compression, and why.