The Progressive Models Method For Ultimate Bearing Capacity Of Ribbed Box Section Members Including Local Buckling Effects

Aiming at ribbed box-section members which are widely used in steel bridges,some factors affecting their stability behavior are discussed in the research order of plate-stiffened plate-member,and the local buckling effect is transmitted step by step through using the material simplification method,the progressive models method(PMM)for ultimate bearing capacity of ribbed box-section members is established subsequently.The research work of this dissertation extends progressive collapse method in the field of ship structures and the traditional beam-column theory,the main research work are as follows:(1)Smith imperfection mode is assumed in plates,the influences of element type,mesh size,material constitutive model,boundary condition,non-linear finite element solving way and thick plate effect on the results analyzed by finite element method are studied.And both non-linear finite element model modeling methods based on solid and shell elements are given.On this basis,the database of compressive material model of steel plates is reconstructed by shell finite element model.(2)Aiming at the typical ribbed box-section compression bar with only one flat rib on the center of each walled plate,on the basis of results of no-ribs box-section members analyzed by elastic buckling theory and grillage theory,the rotational spring stiffness on unloading edge of weak stiffened plate in ribbed box-section members is analyzed through folded plate theory,and the influencing mechanism of the rotational spring stiffness is further studied by this theory.Subsequently,influences of the rotational spring restraint on the ultimate bearing capacity of the weak stiffened plate in the ribbed box section are studied by non-linear finite element method.(3)For stiffened plates which have several ribs and a continuous span,based on the comparison results of several typical finite element models commonly used to analyze stiffened plate,the influence mechanism of transverse support effect and end rotation effect of stiffened plate is preliminarily discussed.Besides,based on the database of plate compressive material model,a new beam-column theoretical model of stiffened plate,in which geometric defects and residual stresses are included,is established,and the corresponding calculation code is programmed,and the program is verified by shell finite element method.(4)Effective section method in which Smith imperfections is concluded is established,and PMM suitable for ribbed box-section members is established by combining plate material database and shell finite element method.This method takes the interaction between plates and the supporting action of diaphragm at the end of stiffened plate into account.Numerical examples are used to verify the PMM as it is used to analyze typical ribbed box-section.(5)Taking EGongyan Railway Special Bridge as engineering background,the stability performance of narrow steel box girder under eccentric load is studied in depth based on model test,finite element analysis and proposed PMM,and the conclusions can be used as reference for similar projects.In addition,the rationality and accuracy of the PMM for analyzing the more complex ribbed box-section member is preliminarily verified by the experimental and finite element models results.The PMM established in this thesis provides a new method for analyzing the ultimate bearing capacity of ribbed box-section members,which is helpful for more clearly understanding of the global-local buckling behavior of such members.