Influence Of Bracing Layout On Seismic Collapse Capacity Of Step-terrace Steel Frame Structure

The step-terrace structure is widely used in mountain construction because it could better conform to the mountain and reduce the amount of excavation and filling.With the promotion of steel structure by the state,more and more steel structures appear in the project.At the same time,some scholars have begun to pay attention to the study of the step-terrace steel frame structure However,the applicability of existing research on seismic performance,seismic design,seismic measures and collapse capacity about stepterrace structure or steel frame structure remains to be discussed.Based on the existing results,setting bracing to the structure could effectively improve the seismic performance.The concept of " bracing-step-terrace steel frame structure" is proposed and qualitative research on the effect of different bracing layout on seismic collapse capacity of stepterrace steel frame is conducted.In this paper,the study on seismic collapse capacity of step-terrace steel frame and bracing step-terrace steel frame is researched based on incremental dynamic analysis method.Meanwhile,finite element models are established through the SAP2000 software to discuss the influence of bracing arrangement.During the research,the variation of the IDA curve and collapse fragility curve under the action of a single ground motion are investigated.The study summarizes the probability of each model reaching different limit states under a specific ground motion intensity.The structural collapse mode and the probability of collapse mode are also analyzed.The main contents and conclusions are as follows:Four prototype structures are designed using PKPM software,and the corresponding spatial model is established through SAP2000.Based on the step-terrace steel frame model,four bracing-step-terrace steel frame models with different bracing layout are built.Moreover,the IDA method,the collapse judgment standard and the analysis index of seismic collapse capacity are also introduced.The distribution of IDA curve is obtained by comparing IDA curves under different single ground motion.In the initial stage,IM varies linearly with DM whether it is an ordinary steel frame or a step-terrace steel frame structure.With the DM increasing,there is an obvious "bifurcation" phenomenon in the IDA curve between the falling side and the grounding side of step-terrace steel frame structure.The greater the number of failing layers,the earlier and more obvious the "bifurcation" phenomenon is.Comparing the IDA quantile curves and the fragility curves of the eight models,the results show that with the increase of the number of falling layers,the seismic collapse capacity of the falling side gradually increases,but the seismic collapse capacity of the grounding side gradually decreases.As the same time,setting bracing could effectively improve seismic collapse capacity whether the falling side or the grounding side of the bracing-step-terrace steel frame structure.Basically,with the increase of the number of support arrangements,the collapse resistance of the structure gradually increases.The damage probability matrix and collapse margin ratio of the examples are carried out.It reveals that the seismic collapse probability of the falling side of the step-terrace steel frame is always lower than that of the ordinary steel frame.The probability decreases as the number of bracing layer increases.For another part of the step-terrace steel frame structure,the seismic collapse probability of the grounding side is always greater than that of the ordinary steel frame and when the number of bracing layer increases,the probability increases.Setting bracing could effectively improve seismic collapse capacity of structure and reduce the probability of seismic collapse.However,the seismic collapse probability of the model does not decrease continuously with the increasing number of bracing layer.The distribution of seismic collapsed weak layer is obtained by the analysis of the collapse modes and collapse probabilities.The results show that the seismic collapsed weak layer of the ordinary steel frame structure appears on the second layer.As to stepterrace steel frame structure,different collapse judgment method has different results.When the collapse limit state of the falling side is used as the criterion,the collapsed weak layer is the second layer of the falling side.When the collapse limit state of the grounding side is used as the criterion,the grounding layer of the grounding side is the weak layer.Furthermore,the layout of bracing has less influence on the distribution of seismic collapsed weak layer.