Loads to foundations are generally broken into two broad categories, gravity loads (dead and live)?and lateral loads (wind and earthquake). All loads to foundations are treated as static loads. Live loads, wind loads, and earthquake loads may actually be highly dynamic, but in practice such loads are applied as equivalent static loads rather than as dynamic loads.

Environmental loads are a special type of loading that may occur on structures.?Typical of such loads are snow, ice, sand?accumulation, rain, and other regional environmental hazards. The proper application of such loads is highly dependent on local practices; no attempt is made here to account for all such special applications.?As one may suppose, the gravity loads acting on a building are those due to the fixed mass of the building itself ( dead load) and those due to transient masses ( live load). Gravity loads usually act vertically, but in some cases they may act horizontally (e. g., soil loads against a basement wall). Gravity loads are usually of much longer term than the lateral loads (wind and earthquake), which, by comparison, are extremely transient and erratic.

The distinction between dead loads and live loads is an important one. Concrete structures and components, including the concrete footings, are designed for a different factor of safety for dead load than for live load. Also, earthquake loads are computed using only the dead load of the building; live load is excluded. Reasonable care?must therefore be exercised in distinguishing between the two loads throughout the structural analysis in order that wasteful overdesign or dangerous underdesign does not occur.

Dead loads are those due to the weight of the building components, whether constructed as a part of the structure ( beams, frames, columns) or as appurtenances ( firewalls, parapets, equipment housings). Dead loads cannot be changed during the life of the building except by additional construction or remodeling of the building.

Mechanical equipment, ductwork, carpets, and other such loads are not classified as dead loads. Although they are indeed permanently present, they are subject to renewal, replacement, or alteration during the life of the building. Since their weights are subject to change during the life of the building, they are not classified as dead loads.?In addition, they are not rigidly attached to the building frame; when the building undergoes earthquake motion, these items simply shift around without resisting the motion of the building.

Those loads in a building that are movable are called live loads.?Or alternatively, live loads are those that occur due to the usage or occupancy of the building. Over the life of the building, however, the usage of the building can change sharply as tenants change or as new owners subject the building to new functions. Any new loads must, of course, be kept within the original design values, regardless of what the new function is.

For a starting point, ranges of live load which include a reasonable latitude for future changes are prescribed by the various codes. These ranges of live load are based on rather broad categories for the intended usage of the building; over the years, they have been found by the industry to be generally satisfactory.

The magnitude of the vertical gravity load to be supported by each footing can readily be established, based on known and accepted methods of structural analysis. Since there is no need to find moments, the complexity of the analysis is reduced considerably. It should be apparent that the overall accuracy of such a simplified analysis is limited to the accuracy of the gravity loads themselves.

For proof of the foregoing conclusions, one needs to look no further than the ACI coefficients, a commonly applied method of analysis for regular structures of any material. This widely accepted method does not require any redistribution of vertical load due to flexure. The ACI method is derived analytically and is accurate within its prescribed limits.

The following limitations apply to the simplified analysis:

(1) The building must be reasonably regular; that is, adjacent span lengths may not differ by more than 20%.

(2) Loads must be relatively uniformly distributed.

(3) Live load may not exceed three times the dead load.

(4) The building must be framed from prismatic members, although shear panels and bearing walls may of course be used.

Municipal building codes sometimes require that a beam be capable of taking a randomly placed load of 2 000 lb at any point along its span, presumably to represent a safe or other heavy concentrated load. The end result of this provision is to require an increase in the capacity of the beam in shear when the load is placed very close to the end of the beam.