Research On Flexural Capacity Of ∑-shaped Doublelimb Built-up Cold-formed Thin-walled Closed Section Beam

Due to the setting of stiffeners,the section moment of inertia,the effective width and the effective area of the plate increase,and the bearing capacity of the cold-formed thin-walled steel beam stiffened with web and flange is greatly improved than that of the unstiffened members.In this paper,the flexural capacity of ∑-shaped double-limb built-up cold-formed thin-walled closed section beam is studied,the influence of section parameters and flange stiffening on the flexural capacity of the beam is analyzed,and the current code is evaluated and revised.In order to verify the reliability of the finite element modeling method,the bending test of cold-formed thin-walled steel beams with web stiffeners abroad was simulated.The test results were verified,and the establishment process of the finite element model is described in detail.The failure mode,bending moment mid span deflection curve and ultimate capacity of the specimens obtained from the finite element analysis are compared with the test results,and it is found that the data are in good agreement.After verifying the accuracy of the finite element method,the finite element model of the∑-shaped double-limb built-up cold-formed thin-walled closed section beam is established.The parameters that may affect the flexural capacity of the beam are analyzed,such as screw spacing,ratio of web stiffener depth to flange width,ratio of web stiffener height to web height,ratio of section height to width,ratio of flange width to thickness,ratio of flange stiffener depth to flange length,ratio of flange stiffener width to flange width.The ultimate bearing capacity of the beam with different parameters is studied.The results show that when the screw spacing is between 150-500 mm,the influence on the ultimate bearing capacity of the built-up beam is small,but when the screw spacing is large,the built-up beam is more likely to be damaged in the middle of the span,leading to a rapid decline in the bearing capacity.Combined with the engineering practice,it is suggested that the screw spacing should be 300 mm.Compared with the beam without web and flange stiffeners,it is found that both web stiffeners and flange stiffeners can significantly improve the flexural capacity of the built-up beam.The ultimate flexural capacity increases with the increase of the ratio of web stiffener depth to flange width,the ratio of web stiffener height to web height,the ratio of section height to width,the ratio of flange stiffener depth to flange length,and decreases with the increase of flange width thickness ratio.The ratio of flange stiffener width to flange width has little effect on the ultimate bearing capacity.The ultimate bending capacity of flange stiffeners decreases with the increase of the ratio of height to width,and increases with the decrease of the ratio of flange width to thickness.The effective width method and direct strength method are used to calculate the flexural capacity.It is found that the direct strength method has the largest calculation result,which is closest to the finite element value,but the estimation of some models is not safe.However,the results of the effective width method in China and the United States are too conservative to make full use of the bearing capacity of this section.According to the bending modulus reduction method,the parameters affecting the bending capacity of the split beam are fitted,and the calculation formula of the bending modulus reduction coefficient is obtained.Based on the American direct strength method,the modified direct strength method is obtained,which is suitable for the calculation of the flexural capacity of this section.The two methods are in good agreement with the finite element values,which can provide reference for engineering practice.