Research On The Overall Buckling Behavior And Design Method Of High Strength Steel Columns Under Axial Compression

With the development of the steel production and manufacture process, highstrength steel structures have been increasingly applied to building and bridgeconstructions in recent years, due to their advantages in structural safety, architecturalfunction, economical benefit and resource saving etc. The overall buckling behavior ofhigh strength steel compression columns may be characterized differently from that ofnormal strength steel, because of their different material properties, manufacturingprocesses and effects from initial imperfections such as initial geometric defect andsectional residual stresses. However, investigations on high strength steel columnsinternationally have just started, while in China there is no relevant research available.The current design code for steel structures GB50017-2003in China allows the use of420MPa high strength steels, and the overall buckling design provisions for columns inthis standard are mainly based on the relevant researches of normal strength steelstructures, which is inadequate to mobilise the advantage of high strength steelstructures. Consequently, the solution on high strength steel columns is in urgent need.As one part of the research work for improving current Chinese steel structuredesign code, a large amount of experimental and numerical investigations were carriedout to study the overall buckling behavior of high strength steel members under axialcompression in this dissertation, and the design method and calculation formulae wereproposed. The main research works covered in this dissertation include:(1) As one of the most important prerequisite for understanding the columnbuckling behavior, the residual stress distribution and magnitude of35different sectionswere measured by using the sectioning method, including three types of steel grades (i.e.420MPa,460MPa and960MPa) and three types of section shapes (i.e. hot-rolled equalleg angle, welded I-shape and box sections). The effects of the width-thickness ratio,plate thickness, weld type, human operating error, steel grade, interaction amongsectional component plates were clarified.(2) According to the experimental investigation conducted in this dissertation aswell as many other researches, residual stress distribution models for different steelgrades (i.e.420MPa,460MPa) and various section types (i.e. hot-rolled equal leg angles, welded I-shape and box sections) were proposed. In addition, the unified model whichcan be applied to the above steel grades welded I-shape and box sections was alsosuggested. The detailed calculation formulae for compressive residual stress magnitudeswere given, which can consider the effect of sectional dimensions (i.e. width-thicknessratio and plate thickness) very well.(3) Experimental investigations were carried out to clarify the overall bucklingbehavior of420MPa steel hot-rolled equal leg angle columns,460MPa and960MPasteel welded I-shape and box section members. Totally84columns were tested. Initialimperfections such as the residual stress, initial bending and loading eccentricity weremeasured. Based on test results the buckling deformation and capacity were investigated.(4) The finite element model was established, which considered the geometricnonlinear property, initial geometric imperfection and residual stress. By comparing thenumerical simulation results with experimental results in both present dissertation andother researches, the finite element model was validated. A large amount of parametricanalysis were carried out to investigate the effects from initial geometric imperfections,residual stresses to steel material properties such as the yield strength and yield-tensilestrength ratio. It was indicated that effects of initial imperfections were less severe forhigh strength steel columns, and thus the nondimensional buckling strength of suchmembers becomes significantly higher than that of normal strength steel columns.(5) A large number of columns with various section dimensions and slendernesswere calculated by using the finite element model, including those fabricated fromvarious steel grades (i.e.235MPa,420MPa,460MPa,690MPa and960MPa), and theoverall buckling strengths of totally930columns were obtained, which were comparedwith design values according to different steel structures specifications (i.e.GB50017-2003, Eurocode3and ANSI/AISC360-10). As the results, relevant designcurves from current design codes for different grades of high strength steel columnswere suggested. Furthermore, new column curves were proposed by the nonlinearfitting from numerical results, which were based on the expression form of columncurve formulae in both Chinese and European codes. For the convenience of applying inpractical engineering, a modification factor on the base of current design column curvesin the Chinese code was also proposed, as well as its detailed calculation formula.