The main purpose of the present study is to develop an alternative modal combination rule for use in the adaptive pushover analysis. Since the quadratic modal combination rules do not take into account the sign reversals of the modal load vectors in the higher modes, the accuracy of the advanced pushover methods are decreased. The proposed modal combination rule is a direct vectorial addition technique in which the relative contribution of each mode and its sign are taken into account. The proposed modal combination rule is employed within the displacement-based adaptive pushover technique, and an alternative pushover procedure is developed. In order to verify the accuracy of the proposed method, two reference buildings are used, and the obtained results from the proposed method and nonlinear time history analysis are compared. It is concluded that the proposed method can estimate the benchmark responses with remarkable accuracy.

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In this research, the performance abilities of tube type lateral load resistant framed systems are studied in order to assess the dynamic response properties of mid-rise steel structures under effects of far and near-field ground motions. For this purpose, three 10 story structural models with separated framed tube based skeletons were selected and designed. The structural models have been designed according to the Iranian seismic code 2800 (4th edition). The main criterion which was considered to select powerful ground motions for performing nonlinear time history analyses is the existence of energized coherent velocity pulses as well as high amplitude acceleration spikes in the time history of each earthquake record. Assessment of the analytical results should emphasize the importance of both lateral displacement and drift parameters which must be taken into account during the structural design phase. Furthermore, it was concluded that the maximum drift demand is more than 0.035 and the upper level of rotation of rigid connecting zones was obtained more than 5 percentages of a radian. As a general note, it was concluded that the seismic response parameters of mid-rise steel framed tube structures are intensively influenced by those strong earthquake records which contain powerful forward directivity effects and are able to emerge long period pulses in their time histories.