Think about the Benjamin Sheares Bridge, or the little bridge over the canal near your block. Cars, buses and lorries roll over it all day. It is heavy. So why doesn't it crack and drop into the water?
The secret isn't that bridges are made of super-strong magic material. The secret is shape — engineers arrange the parts so the weight gets shared out and sent safely down into the ground.
When something heavy sits on a bridge, the parts of the bridge get squashed or stretched. Engineers have special names for these:
A squashing push. Like standing on an empty drink can.
A stretching pull. Like tug-of-war with a rope.
A simple beam is a plank across a gap. Put a weight on it and it bends a tiny bit. Tap the plank to reveal what is happening inside it.
Both forces at once! The top of the beam gets squashed together, and the bottom gets stretched apart. If the material can't handle the stretch, the crack starts at the bottom.
Here are two frames made from the same sticks, joined with loose bolts. Push the top of each one and watch what happens.
Push both, then answer the question below.
A square can squish sideways into a diamond without any stick changing length. A triangle can't — to squash a triangle you would have to make a stick shorter or longer, and stiff steel refuses. That's why truss bridges are covered in triangles! 🔺🔺🔺
An arch bridge is curved. When a heavy truck presses down on the top, the arch doesn't just hold the weight — it passes it along the curve, sideways and down, into the strong ground at each end.
Drag the slider (or use the arrow keys) to move the truck. The orange glow shows where the squashing push is flowing.
This is why old stone bridges have lasted hundreds of years. Stone is brilliant at being squashed (compression) but terrible at being stretched — and an arch is almost all squash. Clever! 🏛️
Engineers work out the heaviest load a bridge will ever carry, then build it much stronger than that. This extra strength is called the safety factor. It's like packing an umbrella even when the sky looks clear. ☂️
Tap Add a lorry and watch the beam bend. Can you find the moment it becomes unsafe?
A bridge needs the right part in the right place. Tap a part below, then tap the empty slot where it belongs.
Weight isn't the only enemy. A bridge must handle three sneaky problems:
Strong gusts can make a bridge sway. Engineers shape the deck so wind slips past instead of shaking it.
Metal grows longer when hot. Sunny Singapore bridges have expansion joints — small gaps so the deck can stretch without cracking.
Lots of people walking in step can make a footbridge bounce. Dampers soak up the wobble.
Tap a problem, then tap the fix you think solves it.
Bridges don't stay up by luck. They stay up because every part has a job, and the shapes send the weight safely into the ground.
Next time you cross a bridge, look for the triangles and arches. You now know their secret. Well done, engineer! 👷♀️👷♂️