Interpolating between Figures 7-53 and 7-54 gives 



«m = 0-16 . 

 Therefore, from Equation 7-36, 



M„ = "m w Cr, H^ Dd , 

 m m u 



M^ = (0.16) (64) (0.7) (35)2 (4) (85) = 2,985,000 Ib.-ft., 



say 



M^ = 2,990,000 Ib.-ft. 



************************************* 



Before designing the pile or performing the foundation analysis, a 

 safety factor is usually applied to calculated forces. It seems pertinent 

 to indicate (Bretschneider, 1965) that the design wave is often a large 

 wave, with little probability of being exceeded during the life of the 

 structure. Also, since the experimentally determined values of Cyi^ and 



Cd show a large scatter, values of C^ and C^ could be chosen so that 



they would rarely be exceeded. Such an approach is quite conservative. 

 For the recommended choice of C^^ and C^ when used with the generalized 



graphs, the results of Dean and Aagaard (1970) show that predicted peak 

 force deviated from measured force by at most ±50 percent. 



When the design wave is unlikely to oceur^ it is reaommended that a 

 safety factov of 1,5 be applied to calculated forces and moments and that 

 this nominal force and moment be used as the basis for structural and 

 foundation design for the pile. 



Some design waves may occur frequently. For example, maximum wave 

 height could be limited by the depth at the structure. If the design wave 

 is likely to occur, a larger safety factor, say greater than 2, may he 

 applied to account for the uncertainty in C^ and C^. 



In addition to the safety factor, changes occurring during the 

 expected life of the pile should be considered in design. Such changes 

 as scour at the base of the pile and added pile roughness due to marine 

 growth may be important. For flow conditions corresponding to super- 

 critical Reynolds numbers (Table 7-3) the drag coefficient C^ will 

 increase with inci'easing roughness. 



The design procedure presented above is a static procedure; forces 

 are calculated and applied to the structure statically. Tlie dynamic 

 nature of forces from wave action must be considered in the design of 

 some offshore structures. When a structure's natural frequency of 



7-113 



