334 FOUND A TIONS 



Depth of Subfoundation Below Surface of Ground. Foun- 

 dations in earth should be carried to such a depth below the 

 ground surface that frost will not reach them. Nearly all 

 moist earth expands, or heaves, with freezing, and repeated 

 freezing is likely to soften and disintegrate it. It may also be 

 subjected to other disturbances near the surface. The depth of 

 foundations may be dictated by conditions other than frost. 

 Often a good material cannot be found except at greater depths 

 than are necessary to provide against frost. 



The penetration of frost varies with the latitude. In the 

 American Gulf States, ice seldom forms; while in the Lake 

 region, the ground sometimes freezes to a depth of 5 or even 

 6 ft. Ordinarily, in the northern parts of the United States, 

 subfoundations 4 ft. below the ground surface may be con- 

 sidered safe from injury by frost. 



Required Area of Subfoundation. In the case of foundations 

 for ordinary structures where the weight is uniformly distri- 

 buted over the whole of the subfoundation, the required area 

 is equal to the total load coming on the subfoundation divided 

 by the safe load per unit area. If the loads are irregular and 

 the subfoundation is compressible, great care must be taken 

 to secure an even distribution of the loads; otherwise, there is 

 danger of uneven settlement, which may cause cracks. 



Intensity of Pressure and Rule of the Middle Third. Let 

 AB, in the accompanying illustration, which represents the 

 width of a rectangular subfoundation of a length equal to unity, 



be divided into three 

 equal parts, At, tt\, 



C t, p an d hB< and be bi- 



3 | sected at C. If the 



L ! point of intersection 



with AB of the result- 

 ant of all the forces acting on the structure, which point is called 

 the center of pressure, is at C, the intensity of pressure is uniform 



V 



throughout AB and is equal to -j-, V being the vertical com- 

 ponent of the resultant pressure. When the center of pressure 

 is at a point e, at a distance d from C, then the intensity is not 

 uniform, being maximum at A and equal to 



