Drift and rotation demands in steel frames incorporating degradation effects

This paper is concerned with the assessment of seismic drift demands in steel moment frames designed to comply with Eurocode 8 provisions, with due account for cyclic and in-cycle degradation. In addition to degradation effects, the main parameters examined include the ground motion frequency content as well as the level of constant relative strength or inelasticity. To represent a wide range of structural characteristics, a set of 54 multi-storey frames are considered, in which the number of stories, steel profiles, seismic hazard and compliance criteria are varied. Detailed incremental dynamic analyses are performed on the full set of frames using a suite of 56 far-field ground motion records, which are scaled appropriately to achieve different levels of inelastic demand or equivalent behaviour factors. The seismic performance is evaluated in terms of maximum global and local drifts as well as beam chord rotations. Characteristic results show that maximum response in terms of global deformations and inter-storey drifts is significantly affected by degradation phenomena, along with the ground motion frequency content and the level of inelastic demand. For medium rise typologies subjected to earthquakes with relatively high frequency content, concentration of seismic demand in terms of inter-storey drift is captured by the degrading models and leads to early development of plastic mechanisms. The seismic demand scenarios used in this study, through spectral acceleration-based scaling of ground motions, indicate that the influence of degradation can be significant not only at collapse levels but also at those associated with typical design situations. Finally, based on the extensive analyses carried out in this investigation, expressions for predicting the global and local demands are proposed and discussed

Influence of deterioration modelling on the seismic response of steel moment frames designed to Eurocode 8

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