COMPUTATION OF NUMBER OF EQUIVALENT CYCLES FOR LIQUEFACTION EVALUATIONS

摘要

The "equivalent number of cycles" concept is integral to cyclic liquefaction evaluations, whether applied directly in laboratory evaluations or via magnitude scaling factors in field evaluations. The premise of the concept is that the random motions of an earthquake can be represented by an equivalently damaging number of uniform cycles (n_(ep)), which facilitates laboratory testing and provides a convenient metric for comparing the duration of earthquake motions. The most widely used procedure for computing n_(ep) was developed by the late Professor H.B. Seed and colleagues in the late 1960's to early 1970's and is based on the Palmgren-Miner (P-M) cumulative damage hypothesis developed for metal fatigue evaluations. However, the P-M hypothesis, as originally developed for metals and as adapted by Seed et al., applies to high cycle fatigue conditions. As such, strains are confined to the elastic range of the material. By contrast, low cycle fatigue conditions, such as the case of soils subjected to strong ground shaking, are characterized by significant amounts of plastic strain. The implications of using a high cycle fatigue hypothesis to compute n_(ep) for evaluating liquefaction are examined herein, and an alternative approach to implementing the P-M fatigue hypothesis that appropriately accounts for the non-linear behavior of soil is proposed. The alternative implementation procedure equates energy dissipated in the soil subjected to the actual earthquake motions and n_(ep) uniform cycles.