Overturning 



Lateral forces can cause two additional problems, simple overturning 

 of an installation or excessive bearing pressures on the downhill or 

 downcurrent side of a structure. The maximum overturning moment, M^ , 

 is given by the equation: 



M, (foot-pounds) = z (feet) * F (pounds) + z (feet) * 



F. (pounds) (7) 



As a general rule the following inequality should be satisfied: 



Mj < 1/3 * W , * r . 

 d — sub mxn 



where r_ • : for a multiple spread footing foundation is the smallest 

 of r^> rn, and r,.; for a strip footing it is the lesser of L/3 and 

 D-B/2 (see legend on Figure 3) ; for a crossed strip footing it equals 

 L/3; for a ring footing it equals 0.35 * D; and for a single spread 

 footing it equals 0.29 * D. If this equation is not satisfied then 

 consideration should be given to increasing r , n or reducing M^ by 

 shortening the structure or by similar structural changes. In 

 calculating the actual vertical force for which a footing must be 

 designed, it is necessary to take into account the effect of this 

 overturning moment. The following equations give the total vertical 

 force, F, from which foundations, or foundation elements, may be 

 sized. 



For an individual footing of a foundation with three spread 

 footings 



F (pounds) = 1/3 * W , + M, + r . (8) 



r sub d min 



Force per foot on a ring footing 



? 

 F (pounds per foot) »W,*L+7*M J *D (9) 



sub d 



Force on two parallel strip footings 



F (pounds per foot) = W , + (2 * L) + (2 * M.) + (r . * L) (10) 

 sub d min 



Force on crossed strip footings 



2 

 F (pounds per foot) = W . ■* (2 * L - B) + (12 * Mj * L (11) 

 sub a 



10 



