why are small gaps built into long bridges?

Answer 1

This is to accomodate dilatation due to high temperature without causing overstress in the components.

Answer 2

All the answers are pretty much correct thus far. Expansion joints give bridges flexibility. So when great forces are placed on the spans, they bend but do not break.

The heat and cold expands and contracts most metals. When just one section expands an inch or two, but there are dozens of sections separated by expansion joints, we are talking about a foot or more of expansion overall. If that expansion were not taken up by the joints, the span would quickly fatigue, buckle and split.

When very large suspension bridges, like the Golden Gate Bridge north of San Francisco, are running at maximum capacity in cars, their spans are carrying a lot of weight. That force could easily fatigue and buckle a span if it were not for the expansion joints taking up the strain caused by that stress.

A few years ago, the GGB celebrated its 50th birthday by allowing pedestrians to walk and stand on it. Under the weight of all those thousands of pedestrians (I was one of them), the span of the bridge actually bent downward appreciably. But because of the expansion joints, it sprang back up to its normal upward bow shape when the foot traffic left the span.

Earthquakes can also damage bridge spans when they twist and shake them. Again, the expansion joints give the spans flexibility to spring back to normal shape after the quake forces cease. Without those joints, the spans might very well be permanently twisted or broken.

Answer 3

They are expansion joints, they permit the deck of a long bridge to lengthen due to excessive heat without bending the deck.

Answer 4

they are expansion joints. Heat from the sun expands the bridge, shrinks back when cools off

Answer 5

I think they are called expansion joints, to allow for temperature changes.

Answer 6

so if there is a quake, there is room for it to move.