This paper assesses the influence of cyclic and in-cycle degradation on seismic drift demands in moment-resisting steel frames (MRF) designed to Eurocode 8. The structural characteristics, ground motion frequency content, and level of inelasticity are the primary parameters considered. A set of single-degree-of-freedom (SDOF) systems, subjected to varying levels of inelastic demands, is initially investigated followed by an extensive study on multi-storey frames. The latter comprises a large number of incremental dynamic analyses (IDA) on 12 frames modelled with or without consideration of degradation effects. A suite of 56 far-field ground motion records, appropriately scaled to simulate 4 levels of inelastic demand, is employed for the IDA. Characteristic results from a detailed parametric investigation show that maximum response in terms of global and inter-storey drifts is notably affected by degradation phenomena, in addition to the earthquake frequency content and the scaled inelastic demands. Consistently, both SDOF and frame systems with fundamental periods shorter than the mean period of ground motion can experience higher lateral strength demands and seismic drifts than those of non-degrading counterparts in the same period range. Also, degrading multi-storey frames can exhibit distinctly different plastic mechanisms with concentration of drifts at lower levels. Importantly, degrading systems might reach a "near-collapse" limit state at ductility demand levels comparable to or lower than the assumed design behaviour factor, a result with direct consequences on optimised design situations where over-strength would be minimal. Finally, the implications of the findings with respect to design-level limit states are discussed
This paper assesses the influence of the cyclic strength and stiffness degradation on the local demands in moment resistant steel frames (MRF), designed according to Eurocode 8. For steel structures, local demand is frequently evaluated by means of plastic rotation of structural members. This local response is relevant both for the design of new structures and particularly for the assessment or safety evaluation of existing structures. Recent studies have illustrated the weaknesses of the local demand prediction methods based on linear analysis. When predictions based on linear analysis were contrasted against nonlinear models, mismatches up to 100% were found. However, those nonlinear analysis approaches have not considered the effect of cyclic strength and stiffness degradation. In this study, degradation models are used and calibrated for assessing structural collapse scenarios. A set of twelve MRF systems, designed according to the provisions of Eurocode 3 and Eurocode 8, are examined using two different modelling approaches: a degrading model and a conventional nondegrading approach. The frames are subjected to different levels of given inelastic demands by means of a scaling procedure using a suite of ground motion records. Plastic rotations are directly measured during the nonlinear analyses and consequently contrasted to quantify the effect of degradation at different levels of induced inelasticity. It is shown that using degrading models results in local inelastic rotations which can be significantly higher, reaching up to 25% more than without degradation. Finally, the implications of the findings on current European seismic design procedures are outlined.
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