When building a bridge, engineers need to consider the weight and environment, or load types the bridge will encounter over a long period of time. These factors determine what material should be used to build the bridge as well as the type of structure that will best withstand the loads. Also known as forces, the type of loads considered in bridge construction is vital to its integrity.
The dead load of a bridge is the bridge itself -- all the parts and materials that are used in the construction of the bridge. This includes the foundation, beams, cement, cables, steel or anything else that comprises the parts of the bridge. It's called a dead load because it doesn't move. It may breathe with the seasons or sway with the wind, but those movements are almost imperceptible.
A live load is the moving weight the bridge will hold, such as traffic. It is based on traffic patterns that include the number of cars, trucks and other vehicles that will travel across it at any given time. Certain variables, such as snow, may be calculated into the total live weight for a more accurate estimate. The heaviest possible weight in the most extreme conditions is also a factor despite the rarity of such an occurrence.
Dynamic loads are outside forces that cannot be accurately measured such as wind, vibration and extreme weather. These factors need to be considered in the construction of a bridge to build "breathing" room into the structure. This breathing room allows the bridge to move or adjust to the dynamic loads without collapsing or permanently shifting. As solid as a bridge may seem, it still has the ability to sway when a strong wind is present.
When building a bridge, there are other types of loads that need to be considered that are specific to the terrain in which the foundation will be laid. Environmental factors and weather patterns are also considered when calculating load-bearing needs. The load expectation of a bridge will determine the best design for strength and to ensure its longevity, whether the bridge is to span over large bodies of water or between rising mountaintops.