Houses and small frame buildings do not need complicated foundation systems. A simple inverted-T foundation is all that is normally needed to support the structure under normal conditions. Larger and more complex buildings impose a heavier burden on the foundation system and need to be carefully designed by structural engineers. Foundations for large commercial buildings perform the same functions as those for light-frame structures. The main difference in the foundations for a commercial building and that for a small residence is often the thickness of the concrete and the amount of reinforcing steel.
Figure 6.13A A. Cross-section of a multistory building displaying incomplete vertical dimensioning.
Foundations are usually constructed of continuous footings, pilings, or grade beams. Continuous or spread foundations are commonly used in residential and light-commercial construction. This type of foundation is based on a footing and wall. Concrete footings form the base of the foundation system and are used to displace the building loads over the soil. Piling foundations are typically used when conventional trenching equipment cannot be used safely or economically. Piling is a form of foundation system that uses columns to support the loads of the structure. They are rarely used for residences or low-rise commercial buildings. Grade beams are reinforced-concrete beams positioned at grade (ground) level below the stem wall to provide a bearing surface for the superstructure. They can be used in place of the foundation to provide added support for a foundation in unstable soil.
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