Research On The Influence Of Residual Stresses On The Stability Of Castellated Beams

The castellated beam is a new type of beam which is commonly produced by flame cutting the web of an I-section beam or H-section beam along with a predefined line and then welding the halves following a certain pattern.Compared to the conventional I-section beams,castellated beams possess great advantages in terms of light weight,high load bearing capacity,large flexural stiffness,and substantial material savings,etc.During the production of the castellated beam,the intense localized heating in the cutting and welding processes causes severe thermal gradients,and the restraint from the adjacent region during the cooling process will result in the interior self-balanced thermal residual stresses,thus,the residual stress distribution of castellated beam is different from that of the conventional I-section beam and H-section beam.In addition,the web of castellated beam is vulnerable to the local buckling failure as the web height is significantly increased compared with the parent plain-webbed I-section beam.The transfer of shear force is changed by the web openings,which causes different failure modes in castellated beams.The residual stress distribution proposed by related scholars is still limited.However,the residual stress distribution of the flanges has significant influence on the stability of castellated beams,in addition,the influence of the residual stress distribution of the web on the local buckling of web posts cannot be ignored.To date,research on these issues is still not deep enough.Based on the existing research work,experimental study,numerical analysis and theoretical analysis are integrated in this paper to systematically investigate the influence of residual stress on the stability of castellated beams.The main research work is shown as follows.(1)Based on the symmetric conditions,a partial finite element model of castellated beams with hexagonal holes was established using the commercial finite element software ANSYS.The cutting and welding processes were simulated by the life and death unit technology,thus the residual stress distribution of every plates was obtained,which was found in good agreement with the test results.Parametric study was conducted using the verified model to investigate the influence of sectional properties,such as flange thickness,flange width,and web thickness.The residual stress distribution of castellated beams was proposed based on the existing residual stress distribution pattern of traditional I-section beams.(2)According to the existing studies on the stability of castellated beams,the main factors that influenced the stability of castellated beams were investigated.The probability distribution and impact factor sensitivity ofthe bending-torsional buckling critical load for simply supported castellated beams subjected to pure bending were obtained by using the Monte Carlo method.Existing test results for castellated beams in terms of the lateral torsional buckling behavior were compared with the calculated results by the Eurocode EN 1993-1-1.Based on the partial safety factor for resistance in Chinese Specification GB 50068-2001,the reliability index of the lateral torsional buckling behavior of castellated beams under various combination of loading conditions was calculated.The influence of different initial geometric imperfection and residual stress distribution on the lateral torsional buckling behavior of castellated beams was investigated using the verified finite element model.Calculation method for the lateral torsional buckling behavior of castellated beams considering the residual stress was proposed based on the analysis results of the finite element method and the comparison of existing codes.(3)The localized failure models and corresponding main theoretical calculation methods for the castellated beams were presented,and the judgement for whether the castellated beams failed by the local buckling of web posts or not was analyzed.A finite element model was developed and verified against the experimental results,based on which the main factors of influence on the critical local buckling resistance of castellated beams was investigated.According to the numerical results,the theoretical formulas of shear coefficient k considering the coupling effect of each parameter were obtained by fitting,and then the design formula which accounted for the residual stress was proposed for calculating the local buckling resistance of castellated beams.(4)An experimental study was conducted to investigate the distortional buckling mode of castellated beamsas the experimental investigations were still limited.The castellated beam specimens were fabricated from the hot-rolled parent profiles HN200×100 and HN250×125.All the specimens were simply supported and loaded at the mid-span.To investigate the lateral distortional and restrained distortional modes of castellated beams,specimens were lateral braced at the end supports and at the upper flange in the mid-span.The distortional buckling strength of castellated beams,load versus displacement curves of the cross section atthe mid-span(1/2 point)and one quarter span(1/4 point),and strain of the longitudinal weld at the central axis of web posts were monitored.And the distortion diagram of web posts was obtained according to the measured load-displacement curves.The analysis results show that the specimens fail in the distortional buckling.(5)The initial stress file was compiled based on the obtained residual stress distribution,which was then implemented in the finite element model.The commercial finite element software ANSYS was employed to develop a three-dimensional analysis model for the castellated beams,which considered the material nonlinearity,geometric nonlinearity,and residual stress distribution.Parametric study was conducted to investigate the influence of different parameters on the distortional buckling strength of castellated beams using the verified model.According to the distortional buckling theory of I-section beams,the calculation method for the distortional buckling strength of castellated beams was proposed in which the influence of web distortion was considered by the effective torsional and warping stiffness.The calculated results of the proposed model,AS4100 Specification,and AISC Specification were compared with the test results to verify the reliability of the proposed model,which could provide references to the design of the distortional buckling strength for castellated beams.