FIND: Can equation (7-20) be used to find the forces? 



SOLUTION : 



L = 1^ = ^'^^^ = 39.0 m (128.0 ft) 

 o 2tj 271 



d 1.5 



= 0.0061 



2 2 

 gT 9.8(5) 



which, using Figure 7-68, gives 



L 

 L 



-A = 0.47 



L = 0.47 L = 0.47 (39.0) = 18.3 m (60.0 ft) 

 A o 



D 0.3 



L 18.3 



A 



= 0.016 < 0.05 



Since D/L. satisfies equation (7-21), force calculations may be based on 

 equation (7-20). 



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



The result of the example problem indicates that the restriction expressed 

 by equation (7-21) will seldom be violated for pile force calculations. 

 However, this restriction is important when calculating forces on dolphins, 

 caissons, and similar large structures that may be considered special cases of 

 piles. 



Two typical problems arise in the use of equation (7-20). 



(1) Given the water depth d , the wave height H , and period T , which 

 wave theory should be used to predict the flow field? 



(2) For a particular wave condition, what are appropriate values of the 



coefficients C„ and C,, ? 



U M 



c. Calculation of Forces and Moments . It is assumed in this section that 

 the coefficients C-r, and C^j, ave known and are constants. (For the 

 selection of C and C see Chapter 7, Section III,l,e, Selection of Hydro- 

 dynamic Force Coefficients C and C .) To use equation (7-20), assume that 

 the velocity and acceleration fields associated with the design wave can be 

 described by Airy wave theory. With the pile at x = , as shown in Figure 

 7-67, the equations from Chapter 2 for surface elevation (eq. 2-10), hori- 

 zontal velocity (eq. 2-13), and acceleration (eq. 2-15), are 



n = y cos 



2TTt 



(7-22) 



7-105 



