A Displacement-Based Adaptive Pushover Algorithm

A DISPLACEMENT-BASED ADAPTIVE PUSHOVER ALGORITHM FOR ASSESSMENT OF VERTICALLY IRREGULAR FRAMES RUI PINHO European School for Advanced Studies in Reduction of Seismic Risk (ROSE School) c/o EUCENTRE, Via Ferrata 1, 27100 Pavia, ITALY STELIOS ANTONIOU SeismoSoft, 21 Perikleous Stavrou Str., Chalkis 34100, GREECE ABSTRACT Due to the unvarying nature of the applied displacement loading vector, conventional (non- adaptive) displacement-based pushover analysis can conceal important structural characteristics, such as strength irregularities and soft storeys, should the displacement pattern adopted at the

start of the analysis not correspond to the structure’s post-yield failure mechanism. Consequently, when only non-adaptive static nonlinear analysis tools are available, as has been the case throughout the past, force-based pushover does constitute a preferable choice over its displacement-based counterpart. On the other hand, however, if one is able to apply displacements, rather than forces, in an adaptive fashion, that is, with the possibility of updating the displacement loading pattern according to the structural properties of the model at each step of the analysis, then a conceptually appealing deformation-based nonlinear static analysis tool would be obtained. The present study focuses therefore on the verification of the accuracy of such an innovative displacement-based adaptive pushover method (DAP), to estimate the response characteristics of vertically irregular frames subjected to earthquake excitation. INTRODUCTION A major challenge in performance-based engineering is to develop simple, yet accurate methods for estimating seismic demand on structures considering their inelastic behaviour: the use of nonlinear static procedures, or pushover analyses, is inevitably going to be favoured over complex, impractical for widespread professional use, nonlinear time-history methods. The term ‘pushover analysis’ describes a modern variation of the classical ‘collapse analysis’ method, as fittingly described by Kunnath [1]. The procedure consists of an incremental- iterative solution of the static equilibrium equations corresponding to a nonlinear structural model subjected to a monotonically increasing lateral load pattern. The structural resistance is evaluated and the stiffness matrix is updated at each increment of the forcing function, up to 4 th European Workshop on the Seismic Behaviour of Irregular and Complex Structures Thessaloniki, Greece, 26–27 August 2005 convergence. The solution proceeds until (i) a predefined performance limit state is reached, (ii) structural collapse is incipient or (iii) the program fails to converge. Within the framework of earthquake engineering, pushover analysis is employed with the objective of deriving, with relative ease, an envelope of the response parameters that would otherwise be obtained through a much more complex and time-consuming Incremental Dynamic Analysis (IDA) procedure, as can be construed by Figure 1. IDA is a parametric analysis method by which a structure is subjected to a series of nonlinear time-history analyses of increasing intensity [2], with the objective of attaining an accurate indication of the “true” dynamic response of a structure subjected to earthquake action. 0 20 40 60 80 100 120 0 0 .5 1 1.5 2 2.5 3 drift (%) Ma x B...

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