Study On Seismic Collapse Capacity Of 3-D Steel Frames Supported By Foundations With Multiple Elevations

With the decrease of the exploitable flat area and the frequent occurrence of strong earthquakes,the mechanical performance of mountain buildings has been widely concerned,but at present it is only limited to reinforced concrete buildings.A large number of studies have found that the comprehensive economic benefits of steel structure are obvious,and the state has also made it clear in the "13th five-year plan" that it will vigorously support steel structure buildings.Based on this,the concept of " steel frames supported by foundations with multiple elevations(SFME)" came into being.Existing researches on mountain buildings have gained a deep understanding of seismic performance,seismic design,seismic measures and collapse resistance.However,it is still to be considered whether the SFME has the same seismic laws.So,it is necessary to SFME for quantitative analysis.Through comparing with ordinary steel space frame found out the problems existing in the SFME,and provides some references about the SFME for the future of seismic design,safety performance assessment,and earthquake disaster loss prediction.In order to study the seismic collapse capacity of SFME under strong earthquakes,six steel frame models,including one ordinary steel frame and five SFMEs,were designed according to the current codes for ordinary steel frames in China.In addition,in order to analyze the seismic collapse performance in different directions,the ordinary steel frame and the SFME with one dropped-bay and one dropped-story are added,with 8 examples in total.Incremental dynamic analysis was used to analyze the IDA curves and vulnerability curves under two different collapse limit states for different dropped-stories,different dropped-bays and different slope-directions.Finally,the collapse modes of each example are studied,and the collapse probability of each example in different stories is calculated.Through this study,the main conclusions are as follows:(1)In this paper,IDA curves of all examples are divided into three types: softening type,alternate type and hardening type.The starting sections of the three types of curves are straight lines,indicating that the structure is in unyielded condition.Subsequently,due to the inconsistent dynamic response of the structure,the yield mechanism of the stories in each model is inconsistent,resulting in the differences in the elastoplastic stage.(2)By comparing the steel frame models with different dropped-stories,it is found that the seismic behavior of the steel frame is consistent in the analysis of the collapse point of the IDA 50% fractal curve and the spectral acceleration value corresponding to the collapse probability of 50% in the vulnerability curve.Specifically,for the lower embedding side,the more dropped-stories there are,the better the seismic collapse capacity of the lower embedding side will be.Even when the number of dropped-stories accounts for 33%~50% of the total story height,the seismic collapse capacity of the lower embedding side begins to exceed the ordinary steel frame.For the upper embedding side,the more dropped-stories,the weaker the seismic collapse capacity of the upper embedding side.However,the CMR analysis is different from the previous two aspects.Since the natural vibration period of each structure is taken into account,the seismic collapse safety margin of the lower embedding side decreases with the increase of the number of dropped-stories.(3)By comparing the steel frame models with different number of dropped-bays,it is found that the seismic behavior of the steel frame is consistent in three aspects: collapse point of the IDA 50% fractal curve,spectral acceleration value corresponding to the collapse probability of 50% in the vulnerability curve,and CMR analysis.The concrete performance is as follows: for the lower embedding side,when the number of dropped-bays accounts for less than about 50% of the total building bay,the seismic collapse capacity and safety margin of the lower embedding side is weaker than that of the ordinary steel frame,and the more the number of dropped-bays,the weaker the seismic collapse capacity.When the dropped-bays ratio exceeds about 50% of the total bay,the lower embedding side is improved,but it is still weaker than the ordinary steel frame.For the upper embedding side,the more the number of dropped-bays,the weaker the seismic collapse capacity and safety margin of the upper embedding side.(4)The seismic collapse capacity of SFME is lower than that of ordinary steel frame due to the weak upper embedding position and significant torsion effect.Collapsed on two kinds of limit state,in the longitudinal slope direction,lower embedding side on the seismic collapse safety margin is higher than upper embedding side,and transverse slope direction,lower embedding side on the seismic collapse safety margin below of upper embedding side.This indicates that the collapse failure modes of the SFME are different in different directions,and suggested that seismic analysis in both directions should be taken into account in the design of the SFME.(5)In consideration of the upper embedding side,the first story above the upper embedding side of the SFME is the weak position to collapse,while the other stories are less tendency to collapse.Therefore,the seismic collapse capacity of the structure is almost entirely determined by the first story above the upper embedding side.It is suggested that more attention should be paid to the seismic design of the structure.In addition,in the case of lower embedding side,the second story above the upper embedding side of the SFME is the most important weak position of collapse,in which the collapse modes of models with different number of dropped-bays are similar,while new collapse modes will appear in models with different number of dropped-stories.