The Buckling Mode Analysis Of Cold-formed Steel Based On The Direct Strength Method

Cold-Formed Steel Structural has a lot of advantage such as high strength, lightweight, high degree of factory, low labor intensity of construction and so on. It is moreeconomic than the traditional hot rolling steel, so the application of Cold-Formed SteelStructural is more and more widely. But with the constant improvement of the coldbending section material strength, the thickness of the cold bending profiles becomesmore and more thin, so that a new kind of buckling mode which is named as distortionalbuckling gradually be taken seriously. Distortional buckling is different from the localbuckling and overall buckling. There are no specific clauses for calculating the loadcapacities of distortional buckling in current national specification “Technical Code forCold-Formed Thin-Walled Steel Structures”(GB50018-2002). GB50018implementsthe traditional Effective Width Method(EWM) in calculating the load capacities of localbuckling.Effective width method is a method of calculating the section’s effective width tocalculate bearing capacity. But the calculation is becomes complex when the section iscomplex. The Direct Strength Method (DSM) is a new design method of thecold-formed thin-walled member. Since DSM is a method of calculating the reducedstrength, it is unnecessary to calculate the section’s effective width and only the grossproperties of the section are required. DSM can be used in the calculation of localbuckling and distortional buckling. It’s critical for DSM to calculateσl-elastic criticalstress of local buckling andσd-elastic critical stress of distortional buckling.Generally speaking, there are two kinds of calculation methods: analytic methodand numerical method. Analytic method can be used for simple section, that is,the solveis according to the equilibrium equations and the deformation coordination equation.But the calculated expression ofσlandσdis too complex. Is not convenient to use inthe design. Elastic buckling critical stress of plate can also be determined by numericalsolution such as Finite element method（FEM）（e.g., ANSYS program) and Finite StripMethod（FSM）（Such as CUFSM program）. For commonly used cross section thenumerical solution can also provide the proper solution of interaction panel. Theadvantages of this method is to improve the precision and applicability.The current good method of calculatingσlandσdis numerical method whichcan obtain the approximative results. The Finite Strip Method is by far the most popular and general numerical method. But the FSM didn’t make a clear distinction betweenthis three buckling modes, so the critical load is not a pure solution of model.Sometimes there is difficult apply the result to the Direct Strength Method.In order to solve this problem, the text obtained the pure buckling mode throughthe theoretical derivation of single constraint equations. To implement local buckling,distortional buckling and overall buckling and separate these three buckling modal,webuild a model and applying the corresponding constraint equations through thelarge-scale general finite element analysis program which named ANSYS. The resultcalculate by ANSYS is same to the result from cFSM which Illustrate the correctness ofthe method put forward by the paper. By comparing,we found the result calculate bycFSM is conservative.The article establish the constraint equation based on mechanics defines of sectionswhen buckling. So there is no assumption of the deformation (FSM assume that barlength only half wave buckling). So the proposed method is more accurate and simplemore generality.Paper puts forward the method of finite element analysis of the buckling modeanalysis, and providing a new platform for the buckling mode analysis.Paper collected a lot of test data at home and abroad, and use the method of thispaper analysis the test section to get a single buckling modes of local buckling criticalstress σcrl’, single buckling modes of distortional buckling critical stress σcrd’andsingle buckling modes of global buckling critical stress σcre’. Then fitted regressionwith the test results. Thus puts forward a practical calculation formula of compressivebar distortional buckling. The formula is simple and practical, can be used for referencein the design and analysis.