Effects of Near-Fault Ground Motions on Frame Structures

Near-fault ground motions have caused much damage in the vicinity of seismic sources during recent earthquakes. These ground motions come in large varieties and impose high demands on structures compared to “ordinary” ground motions. Recordings suggest that near-fault ground motions are characterized by a large high-energy pulse. This impulsive motion, which is particular to the “forward” direction, is mostly oriented in a direction perpendicular to the fault, causing the fault-normal component of the motion to be more severe than the fault-parallel component. This study is intended to evaluate and quantify salient response attributes of near-fault ground motions and to investigate design guidelines that explicitly account for near-fault effects.

In this study the elastic and inelastic response of SDOF (single degree of freedom) systems and MDOF (multi degree of freedom) frame structures to near-fault and pulsetype ground motions is investigated. Generic frame models are utilized to represent MDOF structures. The stiffness and strength of the models are tuned to a story shear distribution based on the SRSS (square root of sum of squares) combination of modal responses. The extent to which these models represent code-compliant structures is evaluated by comparing the dynamic response of the generic frames with that of steel structure models. Near-fault ground motions are represented by equivalent pulses, which have a comparable effect on structural response but whose characteristics are defined by a small number of parameters. The inelastic dynamic response to both near-fault records and basic pulses demonstrates that structures with a fundamental period greater than the pulse period respond differently than shorter period structures. For the former, early yielding occurs in higher stories but the high ductility demands migrate to the bottom stories as the ground motion becomes stronger. For the latter, the maximum demand always occurs in the bottom stories.

Models are proposed that relate the parameters of the equivalent pulse to magnitude and distance by means of regression analysis. A preliminary design methodology is developed based on the equivalent pulse concept, including a procedure that provides an estimate of the base shear strength required to limit story ductility ratios to specific target values. Alternative story shear strength distributions are introduced that can improve the distribution of ductility demands over the height for long period frames. Strengthening of frames with walls that are either fixed or hinged at the base is studied, and it is shown that strengthening with hinged walls can provide effective protection against near-fault effects at all performance levels.