Effects of early-arriving pulse-like ground motions on seismic demands in RC frame structures

Abstract Forward-directivity effects cause most of the seismic energy from the rupture to arrive at the beginning of the motion as a large pulse. This characteristic of ‘early-arriving’ pulses is often adopted to identify pulse-like motions caused specifically by directivity effects. However, not all early-arriving pulses are caused by the forward-directivity effects. Also, current criteria to select near-source directivity pulses mainly include the presence of large pulses in the velocity time series, whether the velocity pulse is early-arriving or located within 20 or 30 km closest distance to the fault. This study is intended to provide important insights into the differences in the response attributes of reinforced concrete (RC) frame structures subjected to early-arriving pulse-like ground motions caused by different physical processes, i.e., forward-directivity versus non-directivity effects. Nonlinear time-history analyses of three generic RC frame structures to two extensive suites of unscaled and scaled ground motions were performed to examine the distinct effects of these two types of ground motions. Results indicate that for the short period (2-story) frame, the mean drift demands by directivity motions are equal to or smaller than that caused by non-directivity motions. For the medium (6-story) and long period (20-story) frames, the directivity-induced motions result in larger demands in the lower half of the structures compared to non-directivity motions, especially for high-intensity ground motions. Spectral analyses of the ground motions provide key information as to why the non-directivity records cause larger demands in the 2-story frame. Simulations using pulse models that represent pulse-like ground motions are carried out to gain additional understanding on the primary findings for the 6-story and 20-story structures.