Tests And Direct Strength Design On Cold-Formed Steel Channel Beams With Web Holes

In order to facilitate plumbing,electrical facilities and heating conduits going through beams,columns and walls,holes are commonly placed in the webs of cold-formed steel members which have effects on the buckling behaviors of these members.In the past decades,researchers at home and abroad successively conducted numerous experimental and theoretical investigations on the bearing capacities and failure modes of cold-formed steel compression members with web holes,and the bearing capacity formulas of cold-formed steel columns with web holes were proposed based on effective width method,which were listed in the relevant specifications.In recent years,the studies on buckling behaviors and direct strength method(DSM)of cold-formed steel columns were carried out successively as the DSM was proposed and listed in relative specifications of some countries such as North America and Australia,but the relative studies on cold-formed steel beams were reported rarely.In 2016,the direct strength method for flexural members with web holes corresponding to distortional buckling and local-global buckling interaction stipulated in North American Specification for Design of Cold-formed Steel Structural Members(NAS)(2016)for the first time,in which the DSM formalus for members corresponding to local-global buckling interaction lack of the support of relative experimental data.Therefore,studies on the buckling behaviors of cold-formed steel channel beams with web holes were carried out and the reliability of direct strength method for cold-formed steel channel beans with web holes stipulated in current NAS(2016)was assessed in this paper.The research results can provide reference for engineering design and revision of the relevant specifications in the future.Tests of 20 groups of cold-formed steel beams with various hole height-to-web depth ratios and lip lengths were carried out under pure bending and impure bending in this study,in which the pure bending tests and impure bending tests were 10 groups respectively,and two kinds of lip lengths corresponding to 20mm and 40mm and five kinds of hole height-to-web depth ratios corresponding to 0,0.2,0.4,0.6 and 0.8 were considered.It is shown that the web holes are significant influence factor on buckling behaviors of cold-formed steel channel beams.The web holes change the failure modes of specimens from only distortional buckling or only local buckling to distortional-local buckling interaction controlled by distortional buckling or local-distortional buckling interaction controlled by local buckling under pure bending;the oversize web holes make local buckling intensified and distortional buckling weakened under impure bending.Compared with the specimens without holes,the moment capacities of pure bending specimens with web holes decrease,and the decrease amplitude of moment capacities increases with the increase of the hole height-to-web depth ratio,while the moment capacities of impure bending specimens with web holes increase slightly or level off at first and then decrease,and the decrease process is decreasing slowly at first and then decreasing dramatically.Tests were simulated by ANSYS finite element program and the simulation results show good agreement with the test results in terms of failure modes,moment capacities and moment-deflection curve,therefore,it was verified to adopt ANSYS finite element program to analyze such specimens with web holes.On this basis,the finite element models verified by tests were adopted to carry out numerous finite element parametric analyses by changing the web height5 sheet thickness and other parameters,and the failure modes,the critical elastic buckling stresses and the moment capacities of members obtained from finite element parametric analyses lay the foundation for the following study of critical elastic buckling stress solution method and direct strength method.According to the critical elastic buckling stresses of cold-formed steel channel beams with web holes under pure bending obtained from finite element parametric analyses,the accurateness of critical elastic distortional buckling stresses for this kind of pure bending beams with web holes predicted by finite strip program CUFSM based on discounted-thickness method was verified,and the existed approximate prediction formulas of critical elastic local buckling stresses for this kind of pure bending beams with web holes was revised.Based on the experimental results,the finite element parametric analysis results and the prediction method of critical elastic buckling stresses,the revised DSM formulas for cold-formed steel channel beams with web holes under pure bending and impure bending were proposed corresponding to failure mode controlled by distortional buckling and local buckling respectively.