| Strong earthquakes have shown that reinforced concrete buildings designed according to older codes are vulnerable to severe damage due lack of strength and/or substandard reinforcing details. Today, the most common approach for the evaluation of such structures is the nonlinear-static analysis, focusing on estimating the maximum roof displacement. However, the effect of stiffness and strength degradation, often observed in older construction, on lateral displacements has not been systematically studied.; The specific goals of this study were: (i) to investigate the effect of stiffness and strength degradation on spectral displacements, (ii) to develop an equation to estimate the amplification between the inelastic and elastic spectral displacement, (iii) to compute the time history response and amplification factor between the maximum roof displacement of generic reinforced concrete frames and the inelastic spectral displacement of an equivalent single-degree-of-freedom system.; Two force-deformation models with various levels of strength, pinching, and stiffness and strength degradation were used to compute inelastic response spectra more than 100 records on various soil types. Fourteen, two-dimensional frames of 5 and 10 stories with two strength levels and various rates of stiffness and strength decay were also studied. Damping ratios of 2 and 5 percent were considered.; The results showed that the post-peak softening slope and the residual strength of the system have the most significant influence on spectral displacements. The amount of pinching or the hysteresis model type, on the other hand, did not significantly affect spectral displacements. The time history analyses showed that frames with strength degrading characteristics tend to have interstory drifts concentrated in a single story. As a result, the use of a lateral load distribution proportional to the fundamental mode will not capture accurately the actual drift profile and could lead to non-conservative estimates of actual drifts.; The provisions of FEMA-356 were found to be very conservative in estimating the target roof displacement of the generic frames studied here. The methods proposed in this study, while still conservative, provide improved estimates of spectral displacements and lateral drift of reinforced concrete frames with stiffness and strength decay. |
