A building foundation must support different kinds of loads:

•  Dead load, the combined weight of all the permanent components of the building, including its own structural frame, floors, roofs, and walls, major permanent electrical and mechanical equipment, and the foundation itself
•  Live loads, nonpermanent loads caused by the weights of the building’s occupants, furnishings, and movable equipment
•  Rain and snow loads, which act pri-marily downward on building roofs
•  Wind loads, which can act laterally (sideways), downward, or upward on a building
•  Seismic loads, horizontal and vertical forces caused by the motion of the ground relative to the building during an earthquake
•  Loads caused by soil and hydro-static pressure, including  lateral soil pressure loads, horizontal pressures of earth and groundwater against basement walls; in some instances, buoyant uplift forces from underground water, identical to the forces that cause a boat to float; and in others, lateral force flood loads that can occur in areas prone to flooding
•  In some buildings, horizontal thrustsfrom long-span structural systems  such as arches, rigid frames, domes,  vaults, or tensile structures

A satisfactory foundation for a building must meet three general requirements:

1. The foundation, including the un-derlying soil and rock, must be safe against a structural failure that could result in collapse. For example, the foundation for a skyscraper must sup-port the great weight of the building above on a relatively narrow base without danger of overturning.

2. During the life of the building, the foundation must not settle in such a way as to damage the structure or impair its function. (Foundation settlement is discussed more fully in the next section.)

3. The foundation must be feasible, both technically and economically,  and practical to build without adverse effects on surrounding property. For example, New York City’s tallest buildings tend to cluster on the central and southern portions of Manhattan Island, where the un- derlying bedrock is closest to the surface and foundations for such buildings are easiest and least expensive to construct.

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