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.

A nonlinear model and an analytical procedure for calculating the cyclic response of nonductile reinforced concrete columns are presented. The main characteristics of the model include the ability to represent flexure or shear failure under monotonically increasing or reversed cyclic loading. Stiffness degradation with cyclic loading can also be represented. The model was implemented in a multipurpose analysis program and was used to calculate the response of selected columns representative of older construction. A comparison of the calculated response with experimental results shows that the strength, failure mode and general characteristics of the measured cyclic response can be well represented by the model.

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