A bridge must be able to withstand compression, tension, torsion and shear. These forces act on different bridge designs to varying degrees, but all must be considered during construction.
Of the four types of forces a bridge must withstand, compression and tension are the strongest. Compression occurs when two opposite points of the same object are pushed toward one another. In most cases, this results in bending of a rigid object. Tension is the opposite of compression. When two opposite points of the same object have forces applied in opposite directions, tension occurs. In a game of tug of war, a rope pulled taught by the opposing teams is experiencing tension. Torsion is a twisting force. When one point on a object is rotated, torsion is applied. For instance, torsion is applied to a wet washcloth to wring the water from it. Bridges often suffer torsion forces as a result of environmental factors such as wind. When the Tacoma Narrows Bridge collapsed due to winds, it collapsed due to torsion forces. Shear is result of two opposing forces acting on the same point of an object. Usually, these are vertical forces and can occur where a strut or pillar is applying upward force and weigh and gravity are applying downward forces.