Permanent seismic drifts in steel moment frames

This paper examines residual drift demands in steel moment-resisting frames incorporating the influence of degradation and ground motion frequency content. Detailed assessments are carried out using 54 multi-storey framed buildings, with a wide range of structural characteristics, which are designed according to the provisions of Eurocode 8. In order to identify the influence of cyclic and in-cycle degradation effects, the analysis is carried out with and without degradation modelling. Incremental dynamic analysis is employed in order to achieve various limits of lateral strength demand, using a suite of 56 ground motion records. It is shown that residual drifts are markedly higher in degrading models in comparison with non-degrading models, with the differences being more pronounced in relatively short period ranges, when higher rates of cyclic deterioration are employed, and for comparatively high lateral strength demand levels. The residual drift demand is also shown to increase with the increase in number of stories, and is often concentrated in the lower levels when degrading models are used. Overall, significant residual drift demands are observed in the structural systems considered, with a high likelihood of exceeding a 0.5% residual drift limit in most cases. Based on the results, two simplified prediction relationships are proposed to estimate the permanent drifts of multi-storey steel moment framed systems. The first is concerned with the design stage based on the results of elastic analysis, whilst the second is associated with post-earthquake structural assessment based on actual measurements of residual drifts.