Study On Direct Analysis Method And Its Application Based On Second-order Notional Load Method

Steel frame is one of the widely used structural systems in practical engineering.Due to the influence of installation deviation and other factors,there are initial geometric imperfections（initial lateral displacement,initial bending）,residual stress and other initial imperfections in the actual steel structure.The traditional design method of steel frame structure takes the ideal structure as the object and carries out the design through the effective length factor.However,it is found that this design method has some shortcomings,such as inaccurate calculation of internal force,cumbersome calculation process,difficult determination of effective length of complex structure,failure to consider initial imperfections and so on.Since the 21 st century,the direct analysis method of steel structures has developed rapidly and has been incorporated into the GB 50017-2017.In this method,the initial imperfections are included in the analysis,and the calculation software is needed to complete the calculation.Among them,for the initial geometric imperfections,the current standard for design considers them equivalent to notional load,which is called " notional load method".However,the notional load method considering initial geometric imperfections in the current direct analysis method is based on the first-order analysis,which is inconsistent with the fact that the direct analysis method belongs to the second-order analysis.In this thesis,the direct analysis method of steel structure is studied by the combination of theoretical analysis and numerical simulation.Firstly,the notional load calculation formula in the standards for design of steel structure of China,the United States and Europe is theoretically analyzed.In the direct analysis,based on the initial imperfection consideration method（imperfection direct simulation method）of the software,the notional load method in the steel structure design standards of China,the United States and Europe is compared to reveal the shortcomings of the notional load method in the current standards;Secondly,using the second-order analysis theory,based on the equal maximum second-order deflection and maximum second-order bending moment,the notional horizontal load and notional uniform load are derived respectively,and the second-order notional load method is proposed.Under the condition of determining the comparison benchmark,the second-order notional load method proposed in this thesis and the notional load method in the standards for design of steel structure of China,the United States and Europe are used to compare the direct analysis results of plane rigid frames;Finally,the second order notional load method proposed in this thesis and the initial imperfection consideration method in the standard for design of steel structure of China are used to compare the direct analysis results of three-dimensional frames.The research results show that the derivation principle of the notional load calculation formula in the current standards of China,the United States and Europe is the same that influence of axial force is not considered and there are problems when the notional uniform load of the initial imperfections of equivalent members is applied to the structure,and the calculation result of direct analysis by using the notional load method considering the initial imperfections is too small and the design is unsafe;The second-order notional load method can’t be equivalent based on the maximum secondorder deflection,but should be equivalent based on the maximum second-order bending moment.In plane rigid frames,the second-order notional load method based on the equivalent second-order bending moment proposed in this thesis has high accuracy;When the second-order notional load method based on second-order moment equivalence is applied to three-dimensional frames,the method in this thesis has high accuracy when the direction of wind load is the most unfavorable to the structure,and the research in this thesis can provide a reference for the application of direct analysis method.