A structure stands by resisting its own load of beams, columns, foundations etc. The occupancy of the structure, soil pressure, temperature variation, rain or snow fall, earthquake, wind force also provide loads. So a structure has to resist all the loads to stand strongly. A structural designer designs considering all the above loads. The acting loads on a structure are classified into three main categories according to their types.
Live Load vs Dead Load
Live loads are consists of occupancy loads in buildings and traffic loads on bridges. They may be fully or partially in place or not present at all and may change its location. Human, chair, table, computer, bed, furniture etc are live loads. Live loads may change its present location as they are not lifetime part of a structure. So, in structural design live loads are provided a larger safety factor than the others. All the structural loads are expressed in psf (per square feet). The minimum uniformly distributed live loads for a residential building is 40 psf and for a office 50 psf.
Dead loads are those that are constant in magnitude and fixed in location through out the lifetime of the structure. Usually the major part of the dead load is the self weight of the structure. The dead load can be calculated accurately from the design configuration, dimension of the structure and density of the material. The loads of the beams, columns, foundations, slabs etc are the dead loads of a structure.
The major difference between live loads and dead loads is:
The environmental load mainly consist of snow loads, wind pressure and suction, earthquake loads, soil pressures on subsurface portion of structures, forces caused by temperature variation. The total lateral force is distributed to floors over the entire height of the structure in such a way as to approximate the distribution of the force obtained from a dynamic analysis.
The structural safety requires that the strength of the structure be adequate for all load that may foresee-ably act on it. The strength of a structure could be predicted accurately if the loads and their internal effects (moment, shear, axial force) were known accurately. Safety could be ensured by providing a carrying capacity just over the known loads. What a general civil engineer does is providing safety factor to a minimum extend as these loads can't be predicted accurately.