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 Loads
Commonly, 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 Loads
If 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 foundation
The 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.
Thickness of Deep and Wide Strip Foundation
If the depth and width of strip foundation is large, economical utilization of concrete need to be considered while the thickness of the foundation is considered. This is because considerable amount of concrete may be utilized that does not contribute to the transferring of loads from the wall to the soil beneath the foundation.
The amount of concrete used in the foundation construction can be decreased by stepping the projection of foundation. However, the construction of formworks for stepped construction would be costly and it could exceed the cost of additional concrete used when stepped projections is not utilized.
With regard to slopping projections of strip foundation, it would improve the economy of foundation unless the slope ratio exceeds one vertical to three horizontal.
If the foundation projection slope is greater than 1 vertical to 3 horizontal, then it needs formwork that clearly increase construction cost.
In the case of heavily loaded or wide strip foundation, it is advised to make a cost comparison between unreinforced strip foundation and reinforced strip foundation. This is because the former would lead to more savings in this case specifically when the depth of foundation is increased to reach blew a layer of weak soil.
Added to that, the cost of concrete used in the case of unreinforced concrete is smaller than that of used in the case of reinforced strip foundation. Because the latter should meet the requirements of application code whereas a ratio of 1:9 concrete can be used for unreinforced concrete foundation in non-aggressive soil.