Most bridges are built to stay perfectly still, but some are designed to move. The reason is simple: tall ships need to travel along the river or canal that the bridge crosses. If the bridge sits too low, large boats cannot pass beneath it, so instead of building a huge, expensive high bridge, engineers make one that opens.
There are several designs. A bascule bridge lifts up like a drawbridge, sometimes splitting into two halves that rise into the air. A swing bridge rotates sideways around a central point. A lift bridge rises straight up between two towers. Once the ship has gone through, the bridge returns to its position and road traffic continues.
Moving such a heavy structure might seem to require enormous power, but engineers rely on a neat solution: the counterweight. A massive weight balances the moving span, so the two are almost equal. Because the load is balanced, a relatively small motor can raise or turn a very heavy bridge. In this way, a single crossing can serve both the road above and the water below.
