Loads are external forces acting on a structure. Stresses are the internal forces that resist the loads. The following are the loads’ forces and stresses to be considered in designing superstructures of building, bridges and culverts.
- Dead Load
- Live Loads
- Wind Loads
- Snow Loads
- Earthquake Loads
- Hydrostatic and soil pressure
- Impact loads
Dead loads consist of the weight of the various structural members and the weights of any objects that are permanently attached to the structure. Hence, for a building, the dead load include the weights of the columns, beams, and girders, the floor slab, roofing, walls, windows, plumbing, electrical fixtures, and other miscellaneous attachments.
Live loads or imposed loads
Live loads are temporary, of short duration, or moving. These dynamic loads may involve considerations such as impact, momentum, vibration, slosh dynamics of fluids, fatigue, etc. Live loads, sometimes referred to as probabilistic loads include all the forces that are variable within the object’s normal operation cycle not including construction or environmental loads.
Snow loads determined by factors influencing snow & ice accumulation on the structure. Snow loads vary considerably by geographic location. Ask your local bldg. dept. for snow load in your area. Design loadings typically depend on the building’s general shape and roof geometry, wind exposure, and location. Like wind, snow loads are generally determined from a zone map 50-year recurrence intervals of an extreme snow depth.
Buildings and their components are to be designed to withstand the code-specified wind loads. Calculating wind loads is important in the design of the wind force-resisting system, including structural members, components, and cladding, against shear, sliding, overturning, and uplift actions.
Every building and its portions, as a minimum, shall be designed and constructed to resist the effects of earthquake ground motions as prescribed by the following provisions. Additions to existing structures shall also be designed and constructed to resist the earthquake ground motion effects. Special structures including vehicular bridges, transmission towers, piers and wharves, hydraulic structures, and nuclear reactors are beyond the scope of these provisions.
The process of determining earthquake loads can be broken down to the following basic steps:
1) Determining the maximum considered earthquake and design spectra response accelerations
2) Determining the seismic base shear in conjunction with the building or structure’s dynamic characteristics (e.g. fundamental period)
3) Distribution of the seismic base shear within the building or structure
Hydrostatic and soil pressure
When structures are used to retain water, soil, or granular materials, the pressure developed by these loadings becomes an important criterion for their design. Examples of such type of structures include tanks, dams, ships, bulkheads, and retaining walls. Here the laws of hydrostatics and soil mechanics are applied to define the intensity of the loadings on the structure.
Impact load is caused by vibration or impact or acceleration. Thus, the impact load is equal to imposed load incremented by some percentage called impact factor or impact allowance depending upon the intensity of impact.