engleski

Extremes modal combinations for pushover analysis of RC buildings

The pushover method is a simple procedure for the comprehensive nonlinear analysis of structures subjected to seismic action. Application of this method, in accordance with the Eurocode 8 rules, favours application utilizing the first mode. This paper presents the influence of multiple mode combinations in assessing the bearing capacity of reinforced concrete (RC) frames and walls. Herein a procedure is presented, in which the most extreme limit state is defined by the minimum ground acceleration that it causes for a pre- selected shape of the elastic spectra. The extreme bearing value is obtained utilizing the envelope principle. Mode shapes and the size of the period are determined using the linear elastic model, while the limit state of the load bearing system is verified in the nonlinear state of structures. Two possible mode combinations are analyzed: linear combinations and square root combinations. The results of analysis, carried out on specific examples (5-storey structure), show that the influence of higher modes are significantly bigger and that (for observed types of structures) the safety/reliability, indicated by criteria for the target displacement, in accordance with Eurocode 8 (Annex B), is not achieved. Inclusion of higher modes, in some presented examples, decreases the peak ground acceleration by more than two times, which is significantly less favourable than the target displacement criteria in which the safety margin is given as a ratio of 150/100. From the examples tested, the results demonstrate that the extreme form of lateral impact load/s is significantly different than the first mode and is in line with the form that is part of the constant acceleration along the height of the structure.

pushover analysis ; modal combinations

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