Structural load is the total weight of a building, bridge or other object. This value includes the weight of the object, any equipment installed in it, and the potential effects of weather and people. Loads created by the weight of the structure are normally called static loads, and loads from occupants or weather effects are called live loads. Architects and engineers must include all possible loads and adequate safety factors in structural design to prevent load failures.
Using a building as an example, the static load of the building can include the steelwork, floors, and interior walls. Any equipment installed to support the building operations, such as heating and air conditioning equipment, lighting and plumbing must be added to static load calculations. These factors only equal the structural load of an empty building, and are far less than the minimum safe design requirements.
Additional loads occur from the furniture, portable office equipment and personal effects brought to the building. The people who work in the building and any visitors are a constantly changing live load that must be structurally supported by the building. Winds, snow or heavy rains can add significant weight to the building structure, and must be included in structural load calculations.
Many governments issue minimum load requirements for different type of building operations. An office building may have different load requirements than a manufacturing operation with large equipment positioned on the floors. Another consideration for industrial buildings is the effect of vibration on the building, and vibration structural load calculations must be performed with additional strengthening of the building and foundations.
Moving equipment also has structural load considerations due to vibration and shock effects. Aircraft have large loads created by the effects of air on the wings and outer surface. Passengers and baggage add additional loads that must be supported by the fuselage or aircraft body, and the wings that lift the entire structure. Turbulence, takeoffs and landings are live shock loads that can add significant stresses over short periods, and must be considered in aircraft design. Similar shock load occur on trucks and cars when they travel over rough roads, and the vehicle frame and suspension needs to absorb these stresses.
Bridges have different structural load considerations, because they are often supported only at each end or with regular support piers or columns. Moving traffic creates bending stresses in the roadway sections that are unsupported, and can result in vibration stresses called harmonics that can damage the structure. Bridges requiring longer unsupported sections often use cables or other supports to transfer loads to foundation piers or main bridge support columns. Cable supports allow the bridge structure to be lower in weight, since the structure itself does not have to support the entire bridge and all live loads.
Weather can create significant loads on structures, and can be a major design consideration in parts of the world where winds are strong or snowfalls are heavy. Wind speeds increase with height above ground, which in hurricane-prone areas can create significant loading against a building's exterior and the interior structure. Heavy rain often occurring during tropical storms can add even more load that must be absorbed by the building. Since the early 20th century, many governments have structural load design requirements for hurricane areas, and are revised from time to time as testing and investigation of storm damage result in a better understanding of wind stress.
