Force for a single spread footing foundation 



F (pounds) = W , + (32 * M,) -s- B (12) 



sub d 



The maximum force, F, calculated from the appropriate equation (8, 9, 

 10, 11, or 12 above) is used in conjunction with Figure 3 to size the 

 footing. In all of these equations values of either "D" or "B" are 

 required to solve the equation. Trial values of each should be selected, 

 tested with the equation and Figure 3, and then adjusted and rechecked 

 if necessary. 



For foundations on rock the bearing elements may be open frames, 

 crossed angles (such as in Figure 2a) or similar configurations. The 

 overall dimensions may be sized as if one were designing a foundation 

 on clean sand. Most rocky areas are rather irregular, and for this 

 reason the following criterion is suggested: 



r . > (2 * M.) * W . (13) 



mm — d sub 



In general, the most satisfactory foundation configuration on rock is 

 a tripodal arrangement of three foundation elements, sized and configured 

 as discussed above. A ring footing configuration of large overall 

 diameter, D, and small element width, B, has also been successfully 

 used. 



Tiedowns 



In some cases, particularly in shallow water, it can be difficult 

 to overcome the large lateral forces and the resulting overturning 

 moments by simply adding weight. For such situations small tiedowns 

 can be used to develop additional anchorage capacity. Where this 

 approach is selected it is necessary to use either three or four tie- 

 down points, each located near the outside edge or end of a foundation 

 element or, for the case of a single element foundation, spaced evenly 

 around the perimeter. The design capacity, F a , required of each 

 tiedown to resist the overturning moment, is given by the following 

 equation: 



F = M, + r . (14) 



a d mxn 



where r . equals the minimum lateral distance from the CSW to a tiedown. 

 min ^ 

 A number of such tiedown anchor configurations are illustrated in 



Figure 4. These are reasonable solutions only in diver depths since from 



a practical standpoint divers are required for their installation. Other 



work systems could be used at deeper locations; however, these are 



typically quite expensive compared to divers. 



11 



