An adaptive pushover procedure based on effective modal mass combination rule

In order to overcome the major drawbacks of conventional pushover methods, researchers have recently been motivated to develop adaptive pushover procedures by which effect of higher modes as well as progressive damage accumulation are taken into account. In spite of their vigorous theory, these novel methods suffer from the quadratic modal combination rules, in which the sign reversals of load vectors in higher modes are neglected and consequently lead to a positive load pattern. In this paper, a displacement-based adaptive modal pushover method, called APAM, based on effective modal mass combination rule is developed in order to include the sign reversals in the load vectors. In this combination rule a modification factor associated to each mode of interest is determined and applied to the corresponding load vector. The modified modal load vectors are algebraically added and subtracted and result in a range of load pattern and thus, multiple pushover analysis is required. These load patterns are independently applied to the structure within an adaptive framework and the envelope of demand values is considered. These modification factors are updated proportional to the instantaneous dynamic characteristic of structure in each step. Another novel aspect of the proposed method is that the target displacement is estimated during the analysis by implementing the concept of capacity spectrum method recommended by ATC 40. In order to assess the accuracy of this method in predicting the seismic responses, the proposed methodology is applied to three different moment-frame buildings. The obtained results demonstrate that APAM procedure provides well estimation of important seismic demand parameters.