PERFORM-3D was used to study the seismic collapse capacity of steel frame structures with dropped floors.Considering different floor dropping conditions and structural measures,two models of ordinary steel frame structures and four models of steel frame structures with dropped floors(two of which are strengthened with tie beams)were designed,and the fragility analysis and seismic collapse resistance assessment of the above steel frame models were carried out by using incremental dynamic analysis methods.Results show that the number of dropped stories has little influence on the seismic collapse capacity of steel frame structures with dropped floors.For the dropped floor steel frame reinforced with tie beams,the seismic collapse resistance and seismic collapse safety reserve of upper grounding side are improved,but they are reduced for the dropped floor side.Increasing the number of tie beams does not have a significant effect on this type of structure,but it does change its damage pattern.In the upper grounding side collapse mode,the ground floor of the upper grounding is the weakest floor on the upper grounding side;in the drop side collapse mode,the secondt floor of the upper grounding is the weakest floor on the drop side.
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