Question

why are buildings and dams made broader at the base

Answer 1

Dams and tall buildings are made broader at the bottom because if the area is small then a larger pressure will be exerted and the foundation of the building may also break.

Answer 2

When the water goes deep then pressure increases and the pressure on the surface level is low so the bottom of the dam is strong enough to stand with the pressure.Even when construct we put the basement as strong and construct the walls on the top as thin.That is the logic.