To understand how bridges are able to stand under large weights, we must first explore the forces that act on a bridge. Forces can be defined as things that act to change the state of motion of an object, by pushing or pulling

Newton's Three Laws of Motion help to explain forces and the general laws they abide by.

1. An object at rest, or with constant velocity, will remain that way unless acted upon by unbalanced forces
2. Net forces acting on a body will cause a proportional acceleration in that direction to occur, regulated by the equation: F=ma (In this equation F is force, m is mass, and a is acceleration)
3. “For every action there is an equal but opposite reaction”

Whenever an object is touching a surface, and a force is applied from from that object to the surface, the surface exerts a "normal force", or a force perpendicular (normal) to the surface. This idea can be seen through Newton's third law of motion, as an "equal but opposite reaction". The following is an example of how the normal force, Fn, isn't always equal to the force of gravity on the object, Fw.

Now that we have established a base in forces, we can explore how forces affect different bridges.

Different Bridges are made out of different materials, and every type of material has a certain elasticity. Elasticity is its ability to change shape while a force is acting on it, and then move back to its original shape. Additionally, every material has a certain limit of how far its shape can be altered, before it becomes plastic, or unable to regain its original form.

This elasticity allows for different parts of a bridge to experience stress, without breaking, and in turn hold up the bridge as a result of normal force.

This picture depicts some of the main types of stress: Compression, Tension, Bending, Torsion, and Shearing

Compression is a pushing force that tends to shorten a member. Tension is the complete opposite of this with it being a force that tends to lengthen a member. These two forces interact when you have an I-beam. On the top and the bottom of the beam is where the tension and compression is occurring while in the middle there is little force acting on the beam. Due to the fact that there is little or no force in the middle of the beam there is minimal material there.

Bending occurs largely in beam bridges and is basically self explanatory. When a force acts upon the middle of a member or some part of the bridge, the material experiences compression on one side and tension ont he other, creating a bend.

In the modern era, with the materials that we have at our hands, torsion has been eliminated in almost all bridges. It only appears in one type, the suspension bridge. With a hanging road deck, which is characteristic of a suspension bridges, winds can cause a strain produced by twisting. To combat the effect of torsion designers use deck-stiffening trusses to minimize torsion.

Resonance is another interesting force because it is a topic in sound and is also present in bridges. It is a vibration in an object that is caused by an external force that is in harmony with the natural vibration of the original object. It is also present in suspension bridges, with its most famous case being in the Tacoma Narrows Bridge that collapsed in 1940. This was a catastrophic collapse in which the wind was at just the right speed to set the bridge oscillating at its resonant frequency. See the links page for a video. The most common technique used by designers to combat this force is to make the road deck in several sections so the waves cannot traverse the entire length of the bridge.

The final major force involved with bridge design is weather. The Tay Bridge disaster is a famous case in which the bridge was not built properly to its environment. After an investigation took place it was determined that the cast iron columns were of poor quality and this led to its failure. Also, the Tacoma Narrows Bridge failure can be blamed on weather. With the progression of wood to iron to steel designers have been able to build better bridges to withstand the forces of nature.. These forces cannot be completely eliminated but just mitigated.