There are number of factors that control a thickness of strip foundation such as losing condition, types of soil, and depth of foundation. The thickness requirements of strip foundation according to load conditions and depth of foundation are discussed.
Fig.1: Strip Foundation
Thickness Requirements of Strip Foundations
Thickness of Strip Foundation Carrying Light LoadsCommonly, the thickness of strip foundation is equal to the projection from the face of footing or the wall but it should not be less than 150mm. This minimum thickness is imposed to make sure that the strip foundation possess adequate rigidity and consequently be able to bridge weak pockets in soil. In addition to withstand longitudinal force generated by thermal contraction and expansion and movements of moisture of footing wall. If the type of soil underneath foundation is clay, then swelling of clay could be large and exert pressure on the foundation. That it is necessary to impose minimum limit on the strip foundation.
Thickness of Strip Foundation Supporting Heavy LoadsIf strip foundation support large loads, then the thickness of the foundation is controlled by its strength to withstand shear and bending moments which may cause failure of foundation projection. Figure 2 explains bending and shear failure respectively. If reinforcements are not embedded into the strip foundation, then bedding failure of strip foundation would control its thickness.
Fig.2: Bending and shear failure of strip foundationThe bending failure can be avoided by using adequate concrete thickness. it is possible to apply stepped or sloped transition to the specified thickness from the face of the wall to the bottom width. Sometimes, strip foundation is designed conservatively by choosing a thickness that prevents the development of tension on the underside of the strip. Such thickness is commonly equal to two times the projection of the strip. However, a 45 degree distribution of loads at the base of strip foundation is considered. And according to this load distribution, a small amount of tension stress at the base of foundation is acceptable but its magnitude is not known.