"Triangles are the most efficient shape to accomodate both vertical and horizontal loads. If you build with only right angle members, then secondary members will be required to handle horizontal loadings to keep from racking the structure. Or you will have substantially larger connections or both. A triangluar structure will generally be less cost, and particularly for bridges, less weight than an equal right angle structure with secondary bracing and larger connections." Read more at this link.

Interactive triangle

Triangles at Math is Fun

Building Strong Shapes with Triangles