EFFECT OF THE NEAR-FIELD GROUND MOTION PULSE PERIOD ON THE SEISMIC RELIABILITY OF RC/MR FRAMES USING MONTE CARLO SIMULATION

Study of the random nature of structural parameters such as material properties, external loads and geometric dimensions, have led to the development of research on classical methods of modeling. Classical modeling methods based on the deterministic decisions do not meet the designer's functional requirements. In order to reduce the structural capacity uncertainty, new decision-making methods based on probabilities have been developed. Reliability theory is a tool that gives a probabilistic estimate of the structural design safety factors. One of the source of uncertainty is the ground motion characteristics. Near-field ground motion with directivity focusing or fling effects produces pulse-like ground motion that has characteristics different from those of far field records. Large peak ground velocity and displacement, energy concentration in one or a few pulses and also unusual response spectrum shape are the main characteristics of the near field ground motion records. These features have been recently evaluated by many researchers, while the least attention has been paid to the near-field ground motion pulse period. In this study, the effect of near-field ground motion pulse period on the seismic reliability of RC moment resisting frames has been investigated. For this purpose, multi-story RC frames were designed based on the seismic provisions provided by ASCE 7-16. Nonlinear computational model based on concentrated nonlinear springs placed at the ends of beams and columns, has been developed in OpenSees framework. For reducing computational costs, the idea of simplifying near-field ground motion record with an equivalent pulse is taken into consideration. In the past, analytical models of pulses present in the nearfield ground motion have been proposed by researchers. Mavroeidis and Papageorgiou proposed a simple, yet effective, analytical model for the representation of near-field strong ground motions. In order to develop a systematic procedure for assessing near-field effects, Alavi and Krawinkler (2004) tried to relate near-field records to a small number of simple input pulses that can be fully defined by a few parameters. This pulse is fully defined by two parameters, i.e., the pulse period (Tp), which is defined as the duration of a complete velocity cycle, and the maximum ground acceleration (ag,max) or velocity (vg,max = ag,maxTp/4). He and Agrawal (2008) proposed an analytical pulse model for velocity pulses observed in near-field ground motions for a systematic design and assessment of seismic protective systems. The proposed pulse model utilizes pulse period, decay factor, and shape parameters to model both buildup and decaying phases observed in recorded ground motions. In addition, considering structural dynamic systems subjected to digitized excitations, a technique and correspondingly a computational procedure are proposed by Soroushian (2008) for time integration with steps larger than the excitation steps. This method can considerably reduce the total computational cost. In this study Analytical simplified pulse model developed by He and Agrawal (2008) was used to reduce computational costs. He and Agrawal pulse model uses an amplitude-modulated sinusoid of the form given by Equation 1.

Keywords:

Reliability methods , Probabilistic models , Monte Carlo simulation , Equivalent pulses , Near-field ground motions