Bridge Building Basics
Bridges are purposefully designed, or engineered, using geometric shapes that distribute force, thus making the structure strong enough to bear weight. Force is the "push" from the weight being placed on it. The bridge must bear not only the weight of the materials used to make the bridge, but also the weight of of any traffic travelling across it. One shape that distributes force effectively is a triangle.
But why is a triangle stronger than a square? This is because of how a triangle distributes or carries a force. When a load is placed at any vertex (corner) of a triangle, the force is evenly distributed and the shape remains stable. The force is carried equally to the base on both of its sides.
The square, on the other hand, has a weak center. The top of the square carries the force of the entire load. This is also why a roof of a house is so often trianglular in shape: a square roof will collapse more easily, but a triangular shaped roof will be very strong and stable.
In bridge design, triangles are used in a series or pattern to achieve structural strength. This set of connecting triangles used is called a truss. Connecting multiple triangles together as a truss forms a stronger structure than using just a single triangle.