Title: Modified Modal Superposition for Seismic Capacity-Design Forces in Concrete Structures

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Publication: CIA

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Date: 2/13/2011

Abstract:
Current ductile seismic design philosophy requires plastic hinges to be located based on an estimated first-mode displacement pattern. The influence of higher modes on member forces – particularly shear forces, is often ignored, or is included by modal analysis, or by simple capacity-design rules intended to ensure that shear strength exceeds the maximum feasible shear demand. Recent research, discussed in this paper, into displacement-based seismic design has shown that none of the existing methods produce reasonable estimates of the maximum moments or shear forces that can be developed under design-level seismic attack. It is shown that ductility modifiers, or force-reduction factors should only be applied to the modal combination from the fundamental mode, not the higher modes, and that prediction is greatly improved if member stiffnesses are represented by the effective stiffness to maximum displacement response, rather than by initial elastic stiffness. Simple capacity design measures are presented as an alternative to the proposed modified modal superposition procedure. These are based on extensive inelastic time-history analyses, and recognise the significance of structural form (e.g. wall, frame, dual system etc), and displacement ductility demand.