3 Building Big Bridges

Building Big Bridges written by John Lockyer

Big bridges carry vehicles and people over water, valleys, roads, and railways. They help people to get from one place to another. Big bridges must be made of materials that won't bend, crack, or crumble. Most big bridges are built of concrete or steel because these materials are strong enough to last a long time.

bridge-building Before a new bridge is built, bridge-builders work out how long and how high it needs to be. They look at the land and any water the bridge will cross. They think about how many vehicles and people will use the bridge. There are many different sizes and shapes of bridges in the world. There are bridges that swing and lift and float, but there are really only four kinds.

beam bridge A beam bridge is made of one or more pieces of concrete or steel. The beams are held up at each end by the ground or by piers, which are the bridge supports. A long beam bridge is held up by piers in the middle. This beam bridge in Sweden carries vehicles across the water. It makes the journey much shorter for the traffic.

arch bridge An arch bridge is built from stone, brick, concrete or metal. Huge blocks and piers are placed at each end and sometimes in the middle of the bridge. After cranes lift the arches onto the blocks, a road is built between the piers. An arch bridge is strong because all the weight on the arch is spread along the curve to the ground. This stone arch bridge in France was built more than two thousand years ago. There were no cranes to help with bridge-building then!

suspension bridge A suspension bridge has a road that hangs from strong steel cables. The cables are held up by tall towers. Each cable is made out of thousands of strands of steel wire. They are fixed to concrete anchors at each end of the bridge. Most long bridges are suspension bridges. This suspension bridge in the IOSA is called the Golden Gate Bridge.