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FOUNDATION DESIGN CRITERfX' A properly-sized foundation must satisfy the two following criteria with respect to the supporting soil. 1. For soil strength. The bearing pressure created on the base of the foundation by the maximum design load must be less than that which would cause shear failure in the soil. A factor of safety of 2 or more with respect to the soil shear strength is generally used. m 2. For soil compressibility.The bearing pressure created on the base of the foundation by the average sustained load must not produce sufficient consolidation in the underlying soil to result in foun- dation settlement that is detrimental to the safety or utility of the structure. o TERMS AND SYMBOLS N E P =Column load(subscript can be used to denote character of load: P, =sustained load, P. _ normal operating load, Pm = maximum P P design load). W, =Weight of soil located above base of founda- tion prior to excavation.' W, W, W, W, W, W, = Weight of soil located above foundation.' ff j + Wr i 1 W, Wr = Weight of foundation.' A = Area of base of foundation. p =Average bearing pressure acting on soil(sub- SYMBOLS script can be used to correspond to coltfmn load: p,+ pn, Pm)• ' Position of water table must be considered in determining weights. Effective, or buoyant, unit weights should be used below the highest expected water table. BEARING PRESSURES Gross Bearing Pressure, p, for any column load is the total effective pressure acting on the base of the foundation. p = 1IA (P + W, + Wr) Net Bearing Pressure, p', for any column load is the difference between the gross bearing pressure acting on the base of the foundation and the soil pressure existing at that elevation prior to excavation. p'= 1IA (P + W, + Wr- W,) - For analysis with regard to the first design criterion-soil strength,the column load in the above equations should usually be the maximum design load, Pm. Occassionally,the normal operating load,P,,,may also be used. If footing is loaded eccentrically, the increase in edge bearing pressure due to the eccentricity should be computed in the usual manner. For analysis with regard to the second design criterion-soil compressibility,the column load in the above equations should be the sustained load, P,. This load is the dead load plus the average continuous live load. For further references, see pp 506-511, 'Soil Mechanics in Engineering Practice" by Karl Terzaghi and Ralph B. Peck (2nd edition); and pp 564-565, "Fundamentals of Soil Mechanics by Donald W. Ta,lor. COMPUTATION OF BEARING PRESSURES 1McClelland consultants PLATE 37