A sensitivity study is first performed on some damage indices of simple building systems. It is shown that the maximum softening index composed of the fundamental vibration period becomes insensitive to damage severity when damage occurs in high-rise buildings or at the upper stories of buildings. Directly identifying the story lateral stiffness is then necessary in order for damage diagnosis. Through a nonlinear regression analysis to the total sum of squares of the errors between recorded and predicted accelerograms, story parameters are identified using a model of shear building or non-shear frame in the case of full measurement. When the beams of a frame are not rigid, using the story stiffness reduction, indices from the identified results of a shear frame model to assess damage will underestimate the damage severity at the lower stories but overestimate that at the upper stories. In the case of incomplete measurement, an equivalent system with less number of degree-of-freedoms is proposed to identify the story lateral stiffness. The damage location and severity of an original system can be reasonably assessed by the identified story stiffness reduction indices of the equivalent system. Finally, the equivalent story lateral stiffness coefficients of a real building triggered by several events are identified to view their variations before and after a powerful earthquake.
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