A simplified method to estimate the seismic performance of existing steel moment resisting frame buildings is presented. Emphasis is placed on the estimation of the level of damage in beam-to-column moment connections. The proposed method makes use of fragility functions for pre-Northridge moment connections that allow the estimation of the probability of reaching or exceeding various damage states as a function of the peak interstory drift ratio. The fragility functions were developed based on a statistical study of a large number of experimental data on the testing of welded beam-to-column moment connections to cyclic loading. Of particular interest is the estimation of the probability of fracture in pre-Northridge welded beam-tocolumn connections. In the proposed method peak interstory drift ratios at each story are estimated by using a simplified continuous model consisting of a flexural beam laterally coupled to a shear beam. The method is evaluated by estimating the seismic performance of two instrumented building that recorded the 1994 Northridge earthquake and experienced different levels of damage. Estimated performance is then compared to observed performance documented in detailed reconnaissance reports. Results show that the method provides relatively good results by providing an indication of the level of damage in the building and of the stories where damage may have occurred.
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