An investigation of inelastic seismic demand

Abstract

In Turkish Earthquake Code (TEC), ratio between inelastic and elastic systems with same initial stiffness and damping is defined as spectral displacement ratios (also known as inelastic displacement ratios). The main aim of this study is to detect the correctness of the formulation about spectral displacement ratio given in TEC Annex-7c. Inelastic displacement ratios are affected by earthquake magnitude, soil conditions, critical damping ratio, yielding strength reduction factors, the material used in construction and resistance strain relation of the hysteresis. To analyze these parameters, total 555 records of Earthquakes occurred at Türkiye and Taiwan is selected and 21 groups and 6 stacks are formed. In the analysis of parameters, 3 to 6 values are investigated while all other parameters kept constant. It is concluded that distance to source has negligible effect due to its fluctuation between maximum and the minimum for each element. Earthquake magnitude, yield strength reduction factors have effects on inelastic displacement ratios in all constant regions. Soil conditions, damping coefficient, post-yield stiffness have moderate effect on inelastic displacement ratios in constant acceleration region. As critical damping increases, increase in inelastic displacement ratios decreases. Difference between trends of 5 and 20% critical damping in constant acceleration region is between 10-20%, and in other regions lower than 5%. Furthermore, detailed investigation on soil shear wave velocities in same NEHRP classification is done. Effects of different soil shear wave velocities in same NEHRP classification show great difference. For every period of vibration, maximum and minimum of mean inelastic displacement ratios of groups is recorded to construct inelastic displacement ratio boundaries which is dependent on yield strength reduction factors. According to this boundary, TEC (2007) formulation is checked. For low yield strength reduction factors like 2 good matches with boundaries, for higher yield strength reduction factor such as 4, 7 formulation underestimates inelastic displacement ratios and reasons for this are discussed. A formulation related with inelastic displacement ratio is presented at the end of paper.