The function of a foundation is to transfer the structural loads reliably from a building into the ground. Every building needs a foundation of some kind: A backyard toolshed will not be damaged by slight shifting of its foundation and may need only wooden skids to spread its load across an area of the ground surface sufficient to support its weight.
A wood-framed house needs greater stability than a toolshed, so its foundation reaches through the unstable surface to underlying soil that is free of organic matter and unreachable by winter frost. A larger building of masonry, steel, or concrete weighs many times more than a house, and its foundations pierce the earth until they reach soil or rock that is competent to carry its massive loads; on some sites, this means going 100 feet (30 m) or more below the surface.
Because of the variety of soil, rock, and water conditions that are encountered below the surface of the ground and the unique demands that buildings make upon their foundations, foundation design is a highly specialized field combining aspects of geotechnical and civil engineering that can be sketched here only in its broad outlines.
Foundation Requirements for Buildings
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 primarily 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 hydrostatic 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 fl oat; and in others, lateral force flood loads that can occur in areas prone to flooding.
- In some buildings, horizontal thrusts from 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:
- The foundation, including the underlying soil and rock, must be safe against a structural failure that could result in collapse. For example, the foundation for a skyscraper must support the great weight of the building above on a relatively narrow base without danger of overturning.
- During the life of the building, the foundation must not settle in such a way as to damage the structure or impair its function.
- 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 underlying bedrock is closest to the surface and foundations for such buildings are easiest and least expensive to construct.
All foundations settle to some extent as the earth materials around and beneath them adjust to the loads of the building. Foundations on bedrock settle a negligible amount. Foundations in other types of soil may settle much more. As an extreme example, Mexico City’s Palace of Fine Arts has settled more than 15 feet (4.5 m) into the clay soil on which it is founded since it was constructed in the early 1930s. However, building foundation settlement is normally limited to amounts measured in millimeters or fractions of an inch.
Where foundation settlement occurs at roughly the same rate throughout all portions of a building, it is termed uniform settlement. Settlement that occurs at differing rates between different portions of a building is termed differential settlement.
When all parts of a building rest on the same kind of soil, and the loads on the building and the design of its structural system are uniform throughout, differential settlement is normally not a concern.
However, where soils, loads, or structural systems differ between parts of a building, different parts of the building structure may settle by substantially different amounts, the frame of the building may become distorted, floors may slope, walls and glass may crack, and doors and windows may not work properly (Figure 1).
Most foundation failures are attributable to excessive differential settlement. Gross failure of a foundation, in which the soil fails completely to support the building, is extremely rare.