AMPLIFICATION FACTORS TO ESTIMATE INELASTIC DISPLACEMENT DEMANDS FOR THE DESIGN OF STRUCTURES IN THE NEAR FIELD

The effects of rupture directivity at near-fault sites on the ratio of maximum inelastic displacement demand to maximum elastic displacement demand are investigated. Inelastic displacement ratios are computed for single-degree-of-freedom systems undergoing different levels of inelastic deformation when subjected to 82 earthquake ground motions recorded at distances closer than 15 km from the surface projection of the rupture. It is found that in addition to increments of linear elastic spectral ordinates in the long period spectral region previously identified by seismologists, forward directivity effects can affect the ratio of maximum inelastic displacement demand to maximum elastic displacement demand. Results indicate that inelastic displacement ratios computed from near-fault records are typically larger than those computed from distant records for periods between 0.1 and about 1.3s. Similarly, inelastic displacement ratios corresponding to faultnormal components are, in general, larger than those of fault-parallel components in the same spectral region. From various ground motions parameters investigated that may affect inelastic displacement ratios of structures located in the near field it is found that peak ground velocity and maximum incremental velocity are the most important ones. Results show that structures subjected to ground motions with large velocity pulses may experience maximum inelastic deformations larger than those subjected to ground motions that do not have these pulses, even if linear elastic ordinates in the short period spectral region are similar. Thus, it is concluded that modification of linear elastic design spectra alone may not be enough to adequately control maximum inelastic deformations in structures located near active faults.