Spectral Displacement Demands of Stiffness- and Strength-Degrading Systems

Abstract

The effect of stiffness and strength degradation on the maximum inelastic displacement of single-degree-of-freedom (SDOF) systems was investigated. The SDOF model included strength and stiffness degradation with increasing deformation amplitude and upon reversal of loading cycles. Pinching of the hysteresis loops was also considered. Spectral displacements were calculated for oscillators with a myriad of degrading characteristics subjected to twelve ground motions on rock, firm and soft soils. The results show that the maximum displacements of degrading oscillators are, on average, larger than those of nondegrading systems. The displacement amplification varies significantly with the period, strength coefficient, degradation rate, and ground motion considered. Nonetheless, the amplification due to the degradation characteristics of the system is much more important in the short period range where average amplification factors of two or three are credible. The amplification factors proposed by ATC 33 provided conservative estimates for oscillators with periods greater than 0.3 seconds subjected to motions on rock or firm soil. On soft soils, a good correlation was found for periods greater than 1.5 seconds. At shorter periods, the ATC 33 factors underestimate the displacement amplification.