Question

why there is some gap between sections of a bridge

Answer 1

basically all materials expand upon heating, weather we take examples of metals, water or other materials. this expansion is a function of a material properties (specially the coefficient of thermal expansion), amount of heat supplied, degrees the temperature rises, etc. thermal expansion takes place due to increase in the kinetic energy of constituent molecules as they are heated. so there is some gap between sections of a bridge

Answer 2

(the less detailed, summarised version)

During warm months, they will basically gain heat and expand while in cool months they lose heat and contract. Because of the intense pressure, without gaps, they would crack.

(the way more detailed version)

During warmer months, the sections will gain heat from the surroundings. They will gain thermal energy, which will cause them to have an increase in their average kinetic energy (or temperature) and expand. Similarly, during the cooler months, the sections will lose heat from its surroundings, so they will lose thermal energy, which will cause them to have a decrease in their average kinetic energy and contract. Without the gap between sections, they will expand/contract and will crack from the pressure.

hope it helps =)