Progressive Collapse Assessment Of Plane And 3D L-shaped Reinforced Concrete Frame Structures Based On Pulldown Analysis

In the alternate path method,it is allowed to use Nonlinear Static(NS)analysis instead of Nonlinear Dynamic(ND)analysis for progressive collapse assessment.In order to simulate the dynamic amplification effect,the standard NS method needs to amplify the uniform distributed load in the bays immediately adjacent to the removed column and at all floors above it,so it is inconvenient for high-rise complicated structures.The pulldown analysis uses an additional downward pull at the top of the removed column to simulate the dynamic effect,making it easier to implement.However,since the pulldown analysis has been put forward,there have been few related studies.For reinforced concrete frame structures,whether the pulldown analysis can replace the ND analysis for progressive collapse assessment still needs further research.In addition,NS analysis and pulldown analysis require dynamic increase factor(DIF)to determine the extent of load amplification.Presently,the displacement parameters and internal force parameters are mainly used to study DIF and obtain empirical DIF calculation formulas through curve fitting.The displacement parameter formulas are independent of the current load level of the structure;the internal force parameter formulas are not convenient for reinforced concrete structures and high-rise complicated structures.And there are few systematic studies about the dynamic effect of the irregular structures.Thus,this thesis carried out the following research work:Firstly,introducing the progressive collapse experiment of reinforced concrete plane frame conducted by Professor Yi Weijian and using SAP2000 software to simulate it.The simulation results are basically consistent with the test results,so the numerical model of the reinforced concrete frame structure can be established based on SAP2000 finite element analysis software to simulate its progressive collapse behavior.Secondly,by utilizing SAP2000,ND analysis,NS analysis and pulldown analysis are applied to the progressive collapse assessment of several plane and three-dimensional L-shaped reinforced concrete frame structures.By taking the plastic rotation predicted by ND analysis as the accuracy standard,the accuracy and response characteristics of pulldown analysis and NS analysis used in plane and three-dimensional L-shaped reinforced concrete frame structures are compared in four aspects of length of spans,seismic precautionary intensity,the number of failure stories and loading magnitude.The results show that the main difference between the pulldown analysis and NS analysis is that when the extent of plasticity is small under light load.As the load increases,the extent of plasticity increases,and the relative error of static analyses will quickly decrease.The two static analyses are then basically equivalent.Therefore,pulldown analysis can be used as a static alternative to ND analysis for progressive collapse analysis,as with NS analysis.Therefore,the DIF of pulldown analysis needs to be studied next.Finally,by utilizing SAP2000,the reciprocal of the load factor(LF)corresponding to the yielding of the remaining structure’s pushdown analysis curve under the current load,1/LFy,is used as the parameter to analyze the influence of the nonlinearity of the reinforced concrete structures on the DIF,and curve fittings of these data points are plotted to derive empirical DIF calculation formulas which defines the DIF as a function of 1/LFy.These parameter formulas can not only reflect the current load level,but also be more convenient to use,especially for high-rise complicated structures.By comparing the DIFNS and DIFP calculation formulas for the two static analyses,it can be seen that for three-dimensional frames,DIFNS and D/FP+1 are basically equal;while for plane frames,only when 1/LFy>0.7,DIFNS and DIFP+1 are basically equal.Therefore,for three-dimensional frames,we can use DIFP+1 as DIFNS to realize the mutual conversion between the two static analyses.By comparing the plane and threedimensional frames’ DIF calculation formulas in the two static analyses,we can see that plane and three-dimensional DIF fitting curves are very consistent in the two static analyses.Therefore,the plane and three-dimensional structures’ DIF calculation formulas can be unified into the three-dimensional frames’DIF calculation formulas.