Triangular vs square structures.

Square structures are common in the building industry where large clear spaces are important, such as office buildings. Square structures will collapse easily unless the joints are rigidly connected, as shown on the right.
As long as loads are applied to the joints of triangular structures, the forces act along the members. This makes more efficient use of the material so they are stronger. Look at the animation on the left. Note how the sideways load applied at the top creates tension in one of the sloping members and compression in the other.
You will therefore find triangles in nearly all structures supporting or lifting a huge mass, or tall slender ones.