Inelastic displacement demands in steel structures and their relationship with earthquake frequency content parameters

This paper deals with the estimation of peak inelastic displacements of single-degree-of-freedom (SDOF) systems, representative of typical steel structures, under constant relative strength scenarios. Mean inelastic deformation demands on bi-linear systems (simulating moment resisting frames) are considered as the basis for comparative purposes. Additional SDOF models representing partially-restrained (PR) and concentrically-braced (CB) frames are introduced and employed to assess the influence of different forcedisplacement relationships on peak inelastic displacement ratios. The studies presented in this paper illustrate that the ratio between the overall yield strength and the strength during pinching intervals is the main factor governing the inelastic deformations of PR models and leading to significant differences when compared against predictions based on bi-linear structures, especially in the short-period range. It is also shown that the response of CB systems can differ significantly from other pinching models when subjected to low or moderate levels of seismic demand, highlighting the necessity of employing dedicated models for studying the response of CB structures. Particular attention is also given to the influence of a number of scalar parameters that characterize the frequency content of the ground-motion on the estimated peak displacement ratios. The relative merits of using the average spectral period Taver , mean period Tm, predominant period Tg , characteristic period Tc and smoothed spectral predominant period To of the earthquake ground-motion, are assessed. This paper demonstrates that the predominant period, defined as the period at which the input energy is maximum throughout the period range, is the most suitable frequency content scalar parameter for reducing the variability in displacement estimations. Finally, non-iterative equivalent linearization expressions based on the secant period and equivalent damping ratios are presented and verified for the prediction of peak deformation demands in steel structures. Copyright c ⃝ 2010 John Wiley & Sons, Ltd.

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