Study On Lateral Torsional Buckling Behavior Of H-shaped Beam Welded By Q460GJ Structural Steel

High-strength structural steel has been widely used in building structures and bridges all over the world,in which good benefits have been achieved.Relevant experimental and numerical study has been carried out.Available results demonstrate that high-strength steel members exhibit superior global stability to conventional steel members.High-performance structural steel plates,GJ steel for short,have better total performance as compared with conventional low-alloy high-strength steel,and thus it has been applied to critical parts or the whole of many high-rise or large-span structures.The draft version of Code for Design of Steel Structures in our country has extended its scope of application to Q460 grade steel and also incorporated Q345 GJ steel in it.Nevertheless,the associated design provisions are proposed based on the study of conventional structural steel,which hinders the full use of the advantages of high-strength and high-performance steel.To date,investigations on GJ steel have been conducted for many years,whereas the study on the global stability behaviour of high-strength GJ steel flexural members has just started.Supported by the research grant entitled “Performance-based calculation model study on 460 MPa high strength GJ steel”(No.51578089)from the National Natural Science Foundation,the thesis intends to conduct experimental tests,numerical simulations and theoretical analyses on the global buckling behaviour of welded H-shaped Q460 GJ steel beams and to recommend design methods.The following aspects have been completed in the study.(1)Residual stresses in eight welded Q460 GJ H-sections was measured by using sectioning method,and the effects of height-width ratio,section height,width-thickness ratio and thickness of steel plates on residual stress distribution were discussed.A model for residual stress distribution in H-sections welded by Q460 GJ heavy plates was proposed.(2)The effects of supports and restraints at the loading point on the global buckling resistance of flexural members were analyzed systematically.A new testing method and the corresponding testing setup were developed to overcome the major limitation of previous studies,which could be used for quantifying the global buckling resistance of simply-supported beams loaded by a point load at the mid-span with or without lateral restraints.Moreover,formulae considering the coupling effect were derived for calculating the vertical and lateral displacements and the rotation of beam section at the measuring point.(3)By using the patented test setup developed by the research group,experimental tests were carried out on 16 laterally-unrestrained welded Q460 GJ H-sections(with 8 doubly-symmetric sections and 8 singly-symmetric sections)and 14 laterally-unrestrained welded Q460 GJ H-sections(with 8 doubly-symmetric sections and 6 singly-symmetric sections),to investigate the ultimate load resistance and failure mode.(4)Numerical simulations were conducted for welded Q460 GJ H-beams,through which the influence of initial geometric imperfections,residual stresses on the global buckling resistance of Q460 GJ steel beams was studied.Comparisons between numerical results and calculated values from various design codes suggested that the design method incorporated in the draft version of Code for Design of Steel Structures(GB50017-201X)could be used for determining the global buckling resistance of welded Q460 GJ H-sections with good accuracy.(5)The statistical parameters for the uncertainty of calculation mode was obtained when the global buckling resistance of welded Q460 GJ H-sections was calculated in accordance with the recommended method in GB50017-201 X.Based on the requirements specified in Unified Standard for Reliability Design of Building Structures(GB50068-201X)(draft version),partial safety factors for the global buckling resistance of beams made of Q460 GJ steel heavy plates were calculated under various combinations of loads,and finally the recommended values of the partial safety factor and the design strength were proposed after statistical analyses.