Proposing Input-Dependent Mode Contribution Factors for Simplified Seismic Response Analysis of Building Systems

Abstract eng:
When the excitation frequency at the base of a building is close to one of its modal frequencies other than its first mode, the contribution of that mode in comparison with the first mode is much more comparing with other cases of excitation, however, this fact is not taken into accoutn by the common definitions of “Mode Participation / Contribution Factors” in which always the first mode is dominant. If the effect of input frequency is incorporated by some way in the definition of these factors, it will be possible to calculate the approximate structural responses to various dynamic loads, including seismic forces, with no need to time history analysis. For this purpose, in this paper at first various definitions, proposed by different scholars, for modal participation factors have been introduced and discussed briefly. Then a simple MDOF system has been considered subjected to sinusoidal base excitations with various frequencies. For each input, the ratio of the corresponding maximum response of every mode in each degree of freedom of the system to the total response of that degree of freedom has been calculated and compared to those of other modes. Then the responses of the considered system to some seismic inputs with various frequency contents have been calculated and the same ratios have been obtained again to find out the possibility of defining ‘input-frequency dependent mode participation factors’. Numerical results show that this definition is possible and these factors can be used for ‘Simplified Seismic Response Analysis’ of building systems.

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