The triangle is the strongest shape due to the rigidity of its sides, which allows them to transfer force more evenly through their sides than other shapes. Triangles are used extensively in the construction of buildings and bridges.
Squares are quite strong, but they collapse if sufficient weight is applied to any of their sides. These sides can be strengthened to resist collapsing by reinforcing them with additional braces. However, these braces will effectively create multiple triangles. Triangles are often stacked together, facing opposite directions, in order to span larger distances. Examples of this can be seen in bridges or large buildings like the Eiffel Tower.