Kid Asked

How do bridges hold so much weight?

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For the grown-up

The short answer. By passing the weight to the ground. A flat bridge stands on legs, and the load goes straight down them. A stone arch squeezes: each stone presses on the next, round the curve to the banks. A hanging bridge pulls: the road hangs from steel cables tied to huge blocks. Engineers then build it stronger than its heaviest load needs: the Brooklyn Bridge's cables were made six times as strong.

These pages show three ways a bridge holds its load: on legs, in a curve, and from ropes. Here is what each one is doing, how much stronger than it needs to be a bridge is built, where the pictures stop being true, what to ask, and what to look for on the next bridge you cross.

How a bridge carries a load

  1. The load. A bridge holds up itself and whatever crosses it. A loaded truck on a US Interstate may weigh up to 80,000 pounds, about 36 tonnes. By our own sum that is eighteen or nineteen cars. The weight does not go away. A bridge can only pass it along until it reaches the ground.
  2. Bending. A flat beam held up at both ends sags under a load. Its top is squeezed and its underside is stretched. The wider the gap, the weaker the beam, which is why plain beam bridges rarely span more than about 250 feet (80 m).
  3. Legs. Engineers call them piers. Each one shortens the gap the beam has to cross, and carries its share of the load straight down.
  4. A curve. An arch is squeezed and nothing else. Each stone presses on its neighbours, and the squeeze runs round the curve to the ends. There the arch pushes outward as well as down, so the ground or the walls at its ends (abutments) have to push back.
  5. Ropes. A suspension bridge does the opposite: its main parts are pulled. The road hangs on thin ropes from two thick cables. The cables pass over the tops of the towers and are tied into huge blocks called anchorages. The towers are squeezed. The cables and the anchorages take the pull.

Squeeze and pull

Engineers call the squeeze compression and the pull tension. On these pages a push or a squeeze is drawn in orange, and a pull in blue.

Stone is strong when it is squeezed and weak when it is pulled, so a stone bridge is an arch. Steel wire is very strong when it is pulled, so the longest bridges hang: 2,000 to 7,000 feet between towers, farther than any other kind.

An arch can last. The Pons Fabricius in Rome was built in 62 BC, two stone arches of about 24 metres each, and has been in use ever since.

How strong is strong enough

Engineers add up the weight of the bridge itself and the heaviest traffic it could carry. Then they multiply before doing the sums. In the rules US highway bridges are designed to today, the bridge’s own weight is counted at 1.25 times and the traffic at 1.75 times.

The Brooklyn Bridge went further. Its cables were given a safety factor of six. During the building a contractor was caught supplying bad wire. The engineers worked out that the cables would still be four times as strong as needed, and added 150 wires to each one.

It opened on 24 May 1883. Many New Yorkers doubted that anything so large could be safe, and six days later a crowd panicked on its stairs and people were hurt. On the evening of 17 May 1884 the showman P. T. Barnum walked 21 elephants and 17 camels across. It advertised his circus as much as it tested anything, but it settled nerves. In 2024 the bridge carried about 103,000 vehicles and 29,000 walkers a day.

Bridges are also checked. In the US a highway bridge must be inspected at least every 24 months. The rule dates from 1971. It was written after a bridge over the Ohio River, the Silver Bridge, fell in 1967. Bridges do fail, rarely, and that one changed the rules.

Where the pictures stop being true

The plank
Real beams are girders of steel or concrete, far stiffer than a plank. The bend is drawn many times too big.
“The legs carry the push”
True, and there is more. While the truck is between two legs, the beam there still bends, top squeezed and underside stretched. The short gap keeps the bend small.
“No glue at all”
True of this arch and of some real ones. The Roman aqueduct at Segovia in Spain is some 24,000 granite blocks with no mortar between them. Many stone bridges do have mortar, and Roman builders used concrete too.
The wooden frame
Real. It is called centering. Until the last stone at the top, the keystone, is in, the ring cannot hold itself up. The picture leaves one thing out: an arch pushes outward on its ends, so the banks must be solid.
The loose rope
A made-up test. No bridge is opened with its cables untied, and real towers do not lean like that. The page is there so a child can find out what the big blocks are for.
“A big steel rope”
A main cable is thousands of thin steel wires laid side by side. Each of the Golden Gate Bridge’s two holds 27,572 wires and is 92 cm thick. The wire in both is 80,000 miles long: by our own sum, three times round the Earth. Each anchorage weighs 60,000 tons.
It looks stiff
A big bridge moves. The Golden Gate Bridge’s roadway is built to go up and down by as much as 16 feet (about 5 m) at the middle.
The elephants’ bridge
The Brooklyn Bridge is drawn from the side, with the pointed arches of its towers turned to face you. The big elephant at the back is Jumbo, who came last. The camels are left out.
Three tricks
There are more. A truss is a frame of triangles; the bridge on the first page has one under its road, to keep it from bending and twisting. A cable-stayed bridge runs its cables straight from the tower to the road. Many bridges mix them: the Brooklyn Bridge has hanging cables and slanting stays.

Questions to ask

  • Before you start: “Why do you think a bridge doesn’t fall down?” Let the guess stand, then find out together.
  • “Where does the push go?” On the next bridge, follow it with a finger: down the legs, round a curve, or up the ropes?
  • “Which is stronger when you squeeze it, a stone or a rope? And when you pull it?”
  • “What else hangs from ropes?” A swing, a hammock, a washing line.
  • “What would you send across a new bridge to show it was strong?”

Try it

  • Paper bridge. Lay a sheet of paper across two books and put a toy car on it. Flop. Fold the same sheet into zigzag pleats, like a fan, and try again. Same paper, new shape.
  • Add a leg. Lay a ruler across two piles of books and press the middle. Put a third pile under the middle and press again.
  • Squeeze and stretch. Draw straight lines down the side of a kitchen sponge. Hold its ends and let the middle sag. The lines crowd together on top and spread apart underneath.
  • Be an arch. Two people face each other a big step apart, put their palms together and lean in. Each holds the other up. Feel your feet wanting to slide back: that is why an arch needs firm ground at its ends.
  • Be a hanging bridge. Put two chairs back to back, a little apart, and lay a skipping rope over both backs. Hang a bag from the middle. The chairs tip inward, unless someone holds the ends of the rope down. That is the job of the big blocks.
  • Bridge spotting. Legs, a curve, or ropes up high? Many bridges use two tricks at once.

Look from a safe place

The sidewalk, a park, the back seat of the car. Never from the road, and never from a railway bridge’s tracks.

Where this comes from

With the sound off, the words stay on the page for you to read aloud.

Every word, page by page
  1. Look at this bridge. Trucks. Buses. Cars. So, so heavy! But the bridge does not fall down. How can that be? Let's find out!
  2. Let's make a bridge. Here is a long, flat plank. Push it down in the middle. Boing! It bends. Now here comes a heavy truck. Tap the truck. Crack! Splash! A long, flat plank bends too much in the middle. A bridge needs a trick.
  3. Trick one. Legs! Tap the river to give the bridge legs. Count them! Now send the truck. It holds! The truck pushes down. The legs carry the push down into the ground. And between the legs, the plank hardly bends.
  4. Trick two. A curve! Builders set stones on a wooden frame. Take the wood away. Clatter! The top stone was missing. Put the last stone in the top. Now take the wood away. It stays up! No glue at all. A curve like this is called an arch. Tap the truck. Squeeze! Each stone pushes on the next one. The push runs round the curve, and down into the ground.
  5. Trick three. Hang it up! A big steel rope goes over two tall towers. The road hangs from it. Tap the truck. Oh no! The big rope slips. Its ends are loose. Pull each end down to a big block. Tie it on tight. Now send the truck. Pull! The road pulls on the little ropes. They pull on the big rope. And the big rope pulls on the blocks. The blocks hold on tight!
  6. Long ago, New York got a big new bridge. People were scared it would fall. So one evening, elephants walked across! Tap an elephant. Count them! Twenty-one elephants! And the bridge held. It is still there today. How strong is a bridge? Builders add up all the heavy things it must carry. Then they make it much, much stronger than that.
  7. Now you build the bridge. Draw it with your finger, from one side to the other. You did it! Next time you cross a bridge, look for its trick. Legs? A curve? Ropes up high?

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