Question

why do one side of the iron girders of bridges are placed on rollers

Answer 1

when we have rollers at one end then it is allowed to expand freely without any thermal stress.in summers the bridges may expand due to the increase in the temperature and in winters the bridges may compress due to decrease in the temperature. the rollers help to increase/decrease the length of the bridge without any thermal stress.

Answer 2

It is because of expansion.
I will explain u in detail as follows.

As in the season of summer the sun radiation the heat is very high as it reflects on the iron or on any metal it expands , so therefore iron expands dur to heat.

As the railway tracks also has gap between them because of this.

Hope it helps u and mark me brainliest plz