Tension and compression

Every truss member is either pulled or pushed. How to tell which, and why it matters

Last updated 2026-09-25

Every member of a truss is either pulled or pushed along its length. Nothing else happens to it (see What is a truss). Those two are called:

• Tension: the member is pulled apart and gets a tiny bit longer. Its force is a positive number, and Truzme draws it red.
• Compression: the member is pushed together and gets a tiny bit shorter. Its force is a negative number, and Truzme draws it blue.

So a member force of −5 means compression, and 3 means tension. Sometimes a member carries nothing at all: 0.

One member, three views

• The member. A member in tension is pulled from both ends. One in compression is pushed from both ends.
• Cut open. Cut a member in tension in two, and the halves are pulling on each other: let go, and they'd spring apart. In compression, the halves are pushing on each other.
• What the joint feels. This is the one to remember. A member in tension pulls the joint toward itself. A member in compression pushes the joint away.

That last view is the whole trick of solving a truss: every joint feels its members pulling or pushing, and those forces have to balance. It's what Method of joints and the force polygon are built on.

The rope test

You can often tell tension from compression without any numbers. Imagine swapping a member for a rope:

• If the structure would still stand, the member is in tension. A rope is perfectly good at pulling.
• If the rope would go slack and the structure would sag or fold, the member is in compression. A rope can't push.

Try it on a shelf bracket: a flat strut sticking out of the wall, and a sloped tie from higher up the wall to its tip.

With the load pushing down, the tie could be a rope: it's in tension. The strut couldn't: it's in compression, and a rope there would just fold. Turn the load around and they swap.

Four simple structures

• Shelf bracket. The tie holds the tip up, the strut stops it from swinging into the wall.
• Triangle. The legs are pushed, the base keeps them from spreading apart. Same numbers as in Support reactions.
• Hung upside down. The same shape, hanging from the ceiling: now every member pulls. Both legs could be ropes.
• Truss bridge. The top is squeezed, the bottom is stretched. The two middle diagonals carry nothing with this load: 0. They aren't useless, though. Move the loads, and they get to work. Members like these are called zero-force members.

Flip the load, flip the colours

Tension or compression isn't a property of a member's shape or position. It's decided by the load. Turn every load around, and every member swaps: tension becomes compression, compression becomes tension, with the same size.

This happens on real buildings too. Wind blowing over a light roof can lift it instead of pressing it down. The members that normally push now pull, and the ones that normally pull now push, so they have to be built for both.

Try it in Truzme: Flip loads in the Quick tools menu of the toolbar turns every load around at once. Watch the colours swap.

"The top chord is always in compression"? No

In the bridge, the top is squeezed and the bottom is stretched. It behaves like a plank laid across two supports: press down in the middle and the top edge gets shorter, the bottom edge longer.

It's tempting to learn that as a rule, but it only holds for things supported at both ends. The shelf bracket is held at one end only, and there it's the other way around: the top member, the tie, is in tension, and the bottom one, the strut, is in compression. Don't guess from where a member is. Look at how the structure is held and loaded, or try the rope test.

Why compression is the dangerous one

A member in tension is being pulled straight. It can only fail by tearing, and that takes a lot.

A member in compression can fail long before that: a long, thin one suddenly bows out sideways. That's buckling, and it's why the two look so different in real structures:

• Tension members can be thin, even a wire or a cable.
• Compression members need to be thick, or short, so they don't buckle.

Which materials can do what

• Rope and cable: tension only. They can't push at all.
• Concrete: very good in compression, but it cracks when it's pulled.
• Steel and wood: good at both.

That's why concrete that has to take tension is reinforced: steel bars are cast into it, in the places that get pulled. The concrete takes the push, the steel takes the pull.

Tension and compression around you

• Bicycle wheel spokes: tension. They're far too thin to push; the wheel's rim is held in place by all of them pulling on it.
• Chair legs: compression. They're pushed between you and the floor.
• Suspension bridge cables: tension. The road hangs from them.
• Stone arches: compression. Every stone is pushed against its neighbours, so the arch holds without any glue.

Common mistakes

• Mixing up the sign. Positive is tension, negative is compression. A negative answer doesn't mean you made a mistake.
• Mixing up the two arrows. A member in tension is pulled from both ends, but it pulls the joints toward itself. Draw the arrow the way it acts on whatever you're looking at: the member or the joint.
• Being afraid of a minus sign. When you solve a truss, you pretend every member is in tension. A negative result just means it's in compression. That's the method working, not a mistake.
• Forgetting that loads can turn around. Wind lifting a roof, or a load that sometimes pulls, can swap tension and compression. A member has to be fine both ways.

Check it in Truzme

Open the triangle, and use Flip loads to watch every member swap colour: