A HYBRID METHOD FOR ESTIMATING THE INELASTIC RESPONSE SPECTRUM DUE TO DETERMINATION OF TARGET DISPLACEMENT USING THE N2 METHOD

In recent years, nonlinear static analysis (pushover) has been rapidly developed. Simplicity, speed and ease of interpretation of results are some advantages of this method, rather than the nonlinear dynamic analysis procedure, as the most accurate method for seismic analysis. Target displacement or performance point is a fundamental component of each nonlinear static analysis, where displacements and stresses of structures are evaluated. The N2 method is a well-known procedure for determination of the performance point, by which, target displacement is determined through intersecting the capacity curve and corresponding inelastic response spectrum, using a trial and error method. In this procedure, determination of the inelastic response spectrum is an important problem. To achieve an easy and quick analysis in the seismic design of structures, using the concept of a spectrum is common. A response spectrum is a simple plot of the peak or steady-state response (displacement, velocity or acceleration) of a series of SDOF systems with various natural frequencies that are forced into motion by the same base vibration. This concept fairly estimates the characteristics of ground motion and maximum amounts of displacement, velocity and acceleration due to an earthquake. Since, in severe ground motions, structures experience deformations beyond the elastic range, the inelastic spectrum is utilized to account for the inelastic behavior. The shape and magnitude of inelastic response spectra are dependent on the period and damping characteristics of the structure, the hysteresis behavior of used material and some seismic parameters, such as soil condition, earthquake magnitude and epicenter distance. In this paper, with regard to existing equations for determination of the inelastic response spectrum, a combinatory approximate method, based on soil condition and the ductility ratio period of the structure, is developed, by which the inelastic response spectrum is estimated using the elastic design response spectrum. Using this inelastic response spectrum, the target displacement of regular systems is estimated with admirable accuracy.