ASCE GSP 283

Abstract

Scope: The seismic demand imposed by a ground motion is related to its amplitude and duration, where greater demand placed on the soil increases the probability that liquefaction is triggered and that its effects will be damaging. Historically, the seismic demand imposed by an arbitrary ground motion was quantified by describing a uniform series of shear stress cycles that caused an equivalent likelihood of liquefaction. The amplitude of the ground motion is usually represented by the Cyclic Stress Ratio (CSR), and the effects of the duration of ground shaking have usually been represented through an equivalent number of uniform amplitude cycles (N) or through the related magnitude scaling factor (MSF). We review methods used to determine N, as well as important factors known to influence the evaluation of N such as: the weight given to cycles of different amplitudes through the cycle sequence, non-zero crossings, multidirectional shaking, and soil and site conditions. Discrepancies between the current cycle counting methods and the known influential factors are identified. A cycle counting routine is proposed, and we apply it to a small suit of records in the NGA-West2 ground motion database with results presented.