The established criteria for maximum cyclic loading are presented 

 in spectral format in Figure 6-7. Criteria are presented for two cate- 

 gories of soil type. The more restrictive criterion applies to few sites 

 for which these guidelines are applicable, as most such sites are 

 excluded as hazardous sites as defined in an earlier chapter. The 

 criteria established in Figure 6-7 are applicable to cases where the 

 average quasi-static load is less than 20% of static anchor capacity. 

 For cases when this static load is exceeded, the portion above 20% 

 should be singly added to the double amplitude cyclic load and the 

 analysis continued. This requirement is quite restrictive for longer life 

 anchor systems subjected to significant and long-term cyclic loading; 

 however, cyclic creep of anchors is not well understood and until 

 further data are available, this relatively conservative approach is 

 recommended . 



6.3 EARTHQUAKE LOADING 



Earthquakes cause loads (usually at a frequency of about 2 Hertz 

 and with 10 to 30 significant loading cycles depending upon the magni- 

 tude of the earthquake) that differ from the preceding category in that 

 the cyclic loading is induced relatively uniformly in the entire soil mass 

 by the earthquake energy emanating from the epicenter. The geo- 

 graphical locations of past major earthquakes, and thus likely future 

 ones, are illustrated in Figure 6-8. The maximum accelerations induced 

 in the soil mass by major earthquakes are a function of the earthquake 

 magnitude and the distance of the site from the earthquake epicenter, 

 or more precisely from the causative fault zone. Predictions of these 

 accelerations are summarized in Figure 6-9. 



A major earthquake centered within 60 km (100 miles) of an anchor 

 can temporarily reduce the anchor capacity relative to all types of 

 loads. This possibility exists only for relatively clean, granular soils 

 (granular soils with few fines - clay size or fine silts). Cohesive soils 

 do not lose any significant amount of strength (relative to anchor 

 capacity) in the 30 or less significant loading cycles associated with 



57 



