FOUNDATIONS AND PILES
That part of a building or structure located belowthe surface of the ground is called the foundation. Its purpose is to distribute the weight of the building or structure and all live loads over an area of subgrade large enough to prevent settlement and collapse. A pile is a slender structural unit driven into the ground to transmit loads to the underground strata. It transfers loads to the surrounding underground strata by friction along its surface or by direct bearing on the compressed soil at or near the bottom. A bearing pile sustains a downward load and may be driven vertically or otherwise; however, when a bearing pile is driven other than vertically, it is known as a batter pile. Another type of pile is the sheet pile. It is used to resist lateral soil pressure.
The following discussion is intended to introduceand familiarize you with some of the common types of foundations and piles that you may be required to include in your construction drawings.
In general, all foundations consist of three essential parts: thefoundation bed, which consists of the soil or rock upon which the building or structure rests; the footing, which is normally widened and rests on the foundation bed; and the foundation wall, which rises from the foundation to a location somewhere above the
Figure 1-7.—wall and column foundations.
Figure 1-8.—Plan and section of a typical spread footing.
ground. The foundation wall, contrary to its name, may be a column or a pedestal instead of a wall. But, when it is a wall, it forms what is known as acontinuous foundation. Figure 1-7 shows common types of wall and column foundations.
The continuous foundation is the type of foundation that is most commonly used for small buildings. The size of the footing and the thickness of the foundation wall are specified on the basis of the type of soil at the site. Most building codes also require that the bottom of thefooting be horizontal and that any slopes be compensated for by stepping the bottom of the footing.
Another type of foundation is the grade-beamfoundation. A grade beam is a reinforced concrete
Figure 1-9.—Plan and section of a mat foundation.
beam located at grade level around the entire perimeter of a building, and it is supported by a series of concrete piers extending into undisturbed soil. The building loads are supported by the grade beam, which distributes the load to the piers. The piers then distribute the load to thefoundation bed
Aspread foundation, such as the one shown in figure 1-8, is often required where heavy concentrated loads from columns, girders, or roof trusses are located. This type of foundation may be located under isolated columns or at intervals along a wall where the concentrated loads occur. Spread footings are generally reinforced with steel. They may be flat, stepped, or sloped, such as shown in figure 1-7.
Figure 1-9 shows the plan and section of a typicalmat foundation. In this type of foundation, a heavily reinforced concrete slab extends under the entire building and distributes the total building load over the entire site. This minimizes problems created by unequal settlement when the subsoil conditions are uneven. The mat foundation is often referred to as a floating foundation.