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How can a bridge of thin sticks hold a heavy load? Which sticks are pulled, and which are squashed?
Bridge Stress lab · Last updated: 16 October 2026
How can a bridge of thin sticks hold a heavy load? In this 3D lab, you load a truss bridge and see which sticks are stretched and which are squashed. It reveals why engineers love triangles and how real bridges, cranes and roofs carry enormous weights with surprisingly little material.
What you'll discover
- Triangles keep their shape under load, which makes a framework of sticks stiff and strong.
- Under a load, some members are pulled in tension while others are squashed in compression.
- Thin members cope well with being pulled but tend to buckle when squashed, so compression members need extra care.
The science in plain words
A truss is a framework of straight members joined into triangles. Engineers use triangles because a triangle cannot change shape unless one of its sides changes length. A square frame with hinged corners can easily fold into a diamond, but a triangle stays rigid. By linking many triangles, a truss bridge spreads a load across all its members and down to the supports at each end.
When a weight sits on a simple truss bridge supported at its ends, the bridge tends to sag in the middle. This squeezes the top members together, putting them in compression, and stretches the bottom members, putting them in tension. The diagonal members carry the load between the top and bottom, and each diagonal is either pulled or pushed depending on its direction and position. Moving the load changes which members work hardest.
Materials behave differently under the two kinds of force. A thin stick or steel rod can carry a large pull before snapping, but if you push on its ends it bends sideways, or buckles, at a much smaller force. So engineers make compression members thicker, shorter or braced, while tension members can be slim. Getting this right lets a bridge carry many times its own weight, and the same thinking is used for cranes, roof frames and pylons.
Where it fits
- India, NCERT: Class 8 science, Chapter 5: Exploring Forces
- India, NCERT: Tension and compression in trusses go beyond the NCERT syllabus for classes 6 to 10; the lab builds on Class 8 Exploring Forces.
- United States: Grades 6 to 8
- Next Generation Science Standards: MS-ETS1-1, MS-ETS1-3
Common questions
Why are triangles used in bridges?
A triangle is the only simple shape that cannot change its angles without changing the length of a side. That makes it rigid. Bridges, cranes and roof frames are built from many triangles so they keep their shape and spread loads evenly across all their parts.
What is the difference between tension and compression?
Tension is a pulling force that stretches a material, like a rope in a tug of war. Compression is a pushing force that squashes it, like the legs of a table under a heavy load. Bridge parts are designed to cope with whichever force they carry.
How can a bridge made of sticks hold so much weight?
When sticks are arranged into triangles, a load is shared across many members, and each one is only pulled or squashed along its length, which is where it is strongest. Good designs make the squashed members sturdy so they do not buckle, letting light structures carry heavy loads.
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Try it yourself
The Bridge Stress lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.