their action on surface or subsurface moored objects. The frequencies 

 of these loads are usually at the frequency of the waves; typically 0.05 

 to 0.15 Hertz. Cable strumming is caused by currents passing a rela- 

 tively taut cable. The cyclic loads from cable strumming are low magni- 

 tude and can be ignored when using the procedures of this report. 

 Earthquake loading differs from the previous two cases in that the 

 entire soil mass is loaded rather than just the anchor fluke and adjacent 

 soil. This special type of cyclic loading is covered in Section 6.3. 



Cyclic loads must have a double amplitude greater than 5% of the 

 quasi- static anchor capacity to be of concern from a cyclic capacity 

 design standpoint. Cyclic loads below this threshold can be ignored. 



Cyclic loads are characterized by a pure cyclic loading component 

 superimposed on a quasi-static loading component. Cyclic and static 

 magnitudes are expressed as percentages of the static anchor holding 

 capacity. Figure 6-4 gives an example of cyclic loading where the 

 quasi- static load component is 20% of the static capacity and the double 

 amplitude cyclic load component is 40% of the static capacity. 



Two additional characterizations concern the number of load cycles . 

 One, the total number of load cycles in the anchor's lifetime, n™, is 

 used in evaluating cyclic creep potential. The other, the number of 

 cycles, n„, that occur in a period, t ,, marked by low excess pore- 

 pressure dissipation is used to evaluate strength loss or liquefaction 

 failure potential. Cyclically loaded anchors are designed to preclude 

 failures from creep or liquef action. Creep failure is an accumulation of 

 small movements that reduce anchor depth and, hence, capacity until 

 pullout occurs . A liquef action -like failure is characterized by soil 

 strength loss and sudden anchor instability. 



The above guidance is straightforward when the cyclic loads are of 

 relatively uniform magnitude or when a major portion of the cyclic load 

 (one-third) is relatively uniform and significantly larger (50%) than the 

 rest. For cases where distribution of cyclic loads is not uniform and 

 exhibits extreme values, a different approach is required. In general, 

 for these cases, the loading spectrum can be broken into segments of 

 loads of relative uniformity . These segments can be characterized by 



52 



