| Cold-formed thin-walled steel is used increasingly in a wide range oflight-weight construction including steel storage rack columns, low-rise, andindustrial factory building due to its many advantages compared with hot-rolledsteel. At present, the effective section method is adopted to predict the memberbearing capacity for the design specification of cold-formed thin-walled steel inmost countries. The computation of the effective width of complex section is moredifficult and meanwhile it does not take account of the disadvantage influence of thedistortional buckling. To overcome this problem, the direct strength method wasdeveloped by Schafer and Pek z. In recent years, the cold-formed thin-walled steellipped channel section columns with an intermediate stiffener in the web are widelyused in civil engineering which can increase the local buckling bearing capacity ofthe member. In this case, the distortional buckling may be one of failure modes todecide the member capacity.In order to accommodate plumbing, electrical, and heating conduits in the walls,the holes are pre-punched in the web of lipped channel section and can alter thebuckling mode and ultimate bearing capacity of the members. The buckling modesand critical elastic buckling stress as well as ultimate bearing capacity of thespecimens with perforations are significantly different from the members withoutperforations. The majority of existing test data on cold-formed thin-walled steelcolumns with holes generally focus on the simple lipped channel sections withoutintermediate stiffener in the web. Thus, it is important to do some experiments andtheoretical investigation in order to know the stability behavior of lipped channelsection columns with holes in the stiffened web.In this dissertation, the stability behavior of cold-formed thin-walled steellipped channel section columns with and without holes in V-shaped stiffened web,which were made of Q345steel, was analyzed by experiments, finite elementmethod, finite strip method, and theoretical method.A total of20cold-formed thin-walled steel lipped channel short andintermediate length columns with and without holes in V-shaped stiffened webincluding two different cross-sections were tested. Detailed measurements of initialgeometric imperfections, cross-section dimensions, and material properties wereconducted. All of the specimens failed in distortional buckling. At post-ultimaterange, a noticeable interaction between distortional and flexural buckling bendingabout the weak axial was observed for intermediate length columns. The openings in the web result in the change of buckling modes of the members and also change thenumbers of distortional buckling half-wave of intermediate length columns.Experimentally, slotted web holes accelerated the development of distortionalbuckling deformation, which resulted in the decrease of ultimate load and increaseof lateral displacement of the flange tips. Moreover, it is found that the lagerdistortional initial imperfections of the columns would lead to the lower bearingcapacities and greater lateral deformations induced by the distortional buckling. Inaddition, all of the tested specimens are simulated by means of the finite elementmethod, and the results of the finite element accord well with that of theexperiments.On the basis of experiment verification, a comprehensive of finite elementparametric analysis were performed for Q345material cold-formed thin-walled steellipped channel columns with and without holes in V-shaped stiffened web. Thestability behavior of these specimens was studied by changing the column length,stiffener height, web width, flange width, lip width, holes distance, and boundaryconditions. The ultimate bearing capacities and failure modes were compared bychanging the above parameters. The result of finite element parametric analysis is asupplement to the experiment data that can provide a theoretical base forestablishing the design formulae.The calculation of elastic buckling stress of cold-formed thin-walled member isa key problem using direct strength method. When the elastic distortional bucklingstress of the members was calculated with flange model, it was necessary todetermine the rotational restraint provided by the web to the flange. However, thecurrent calculation formula for the rotational restraint was complicated, and oneiteration was required in the solution process. In order to simplify the calculationprocess, a simplified calculation method for the rotational restraint of cold-formedthin-walled steel lipped channel section column was proposed. Based on Lau andHancock’s formula, the proposed method was derived in combination with theregulations concerning the local buckling coefficients of the web and flange in the―Technical code of cold-formed thin-wall steel structures (GB50018-2002)‖, and itwas then modified according to the results obtained by Lau and Hancock’s formula.Compared with finite strip analytical values, it is shown that the simplified methodcan calculate the rotational restraint accurately and conveniently, and can be usedfor the calculation of cold-formed thin-walled steel lipped channel section columns.The holes influence on elastic distortional buckling stress of cold-formedthin-walled steel lipped channel columns could be approximated by modifying thewhole cross-section thickness. In this paper, closed-form prediction of elastic local buckling stress and elastic distortional buckling stress were provided and shown toagree well with finite strip method or finite element method.A suggested direct strength method to estimate distortional buckling bearingcapacity for axially compressed cold-formed thin-walled steel lipped channelcolumns with and without holes in V-shaped stiffened web had been presented, andthe values calculated by the proposed method were in good agreement with theresults of experimental and parameter analysis. The design method is very useful forcode revision in the future.In finite element parametric analysis, it was found distortional and localinteractive buckling may occur in some specimens. Previous scholars proposed thatthe ratio of elastic distortional buckling stress to local buckling stress effecteddistortional and local interactive buckling. This paper pointed out that the length ofspecimens was another important factor. Based on the finite element analysis data,the design method to consider the distortional and local interactive buckling forcold-formed thin-walled steel lipped channel columns with and without holes inV-shaped stiffened web was proposed. It is shown that the data acquired bycalculation formulae is safe and reliable. |
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