Science Puzzle
Why Do Bridges Have Gaps in the Road?
Drive over a big road bridge and you'll often feel a double clunk as the wheels cross a strip of metal teeth near each end. Between the teeth there are gaps. Engineers put them there on purpose.
Why would anyone design a bridge with a gap in its road?
The Answer
Because the bridge changes length every day, and the gap gives it room to do so.
Almost every material expands when it warms up and shrinks when it cools, because its atoms jiggle harder and push each other slightly further apart. Steel grows by about 12 millionths of its length for every degree Celsius. That sounds like nothing until you multiply it out. A 100 metre deck that goes from -10 °C on a winter night to 40 °C on a summer afternoon changes temperature by 50 degrees, so it changes length by 100 × 0.000012 × 50 = 0.06 metres. That's 6 centimetres; a dark deck in full sun can get hotter than the air around it, so the real figure can be bigger still.
If the deck were jammed tight between solid land at both ends, it couldn't grow. It would push on the land with an enormous force instead, enough to crack concrete, tear bearings loose or make the deck buckle upwards. So engineers fix one end and let the other slide. The interlocking teeth, called a finger joint, let the gap open in winter and close in summer while still giving tyres something solid to roll over.
Old railway lines did the same thing with small gaps between the lengths of rail, which is where the clickety-clack of old trains came from.
The principle: Thermal expansion. Most materials expand when heated and contract when cooled, and if that movement is blocked the forces are huge, so long structures are built with joints that leave room to move.