BEHAVIOUR AND DESIGN OF STAINLESS STEEL COLUMNS by Shameem Ahmed The University of New South Wales August 2017 BEHAVIOUR AND DESIGN OF STAINLESS STEEL COLUMNS By Shameem Ahmed A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy School of Engineering and Information Technology The University of New South Wales Australian Defence Force Academy Canberra ACT 2610, Australia August 2017 Abstract Current design standards do not appropriately recognise the characteristic nonlinear stress-strain behaviour with significant strain hardening offered by stainless steel. Loss of effectiveness due to local buckling is dealt with effective width method, which is unjustified for stainless steel as there is no obvious yield plateau. With recent developments, the continuous strength method (CSM) was shown to accurately predict cross-section resistances of stocky sections exploiting the benefit of strain hardening. The current research extended the scope of CSM for slender sections retaining the same base curve, and also develop CSM based design guidelines for predicting the buckling resistance of stainless steel columns. A new concept of using an equivalent elastic deformation capacity εe,ev, defined as the elastic strain at ultimate load, was proposed for slender sections. A comprehensive FE study was carried out to establish relationships between εe,ev and buckling strain εcsm. Proposed relationships allowed using the same CSM base curve for both stocky and slender sections, and hence, cross-section resistances could be directly determined using CSM buckling stress fcsm and gross cross-sectional properties. Once CSM was successfully implemented for all cross-section types, the current study extended its scope to the member level. A series of new buckling curves was proposed for cold-formed RHS and SHS columns using available test results and generated FE results. The proposed technique combined fcsm with Perry type buckling curves. Once the proposed buckling formulas performed well for cold-formed hollow sections, the flexural buckling of welded I columns were thoroughly investigated through experimental and numerical methods. Valuable experimental evidences on column buckling of welded sections were added to currently available limited data. An extensive FE analysis supplemented the test results, and all available results were used to develop CSM based buckling curves for welded I-sections considering the effects of residual stresses and other important parameters. All developed formulas were shown to produce accurate, consistent and reliable predictions. i Acknowledgements First and foremost, I express my heartiest gratitude to my supervisors Dr Mahmud Ashraf and Dr Safat Al-Deen, for giving me the opportunity to work on this project and their untiring support during my study. They trained me to be an independent researcher and taught me the very important traits a researcher should possess. Their useful suggestions, kind supports, precious discussions and constant encouragements at the various stages throughout the research work are highly appreciated. Without their support and guidance, this work would not have been possible. I also extend my appreciation to all the laboratory and workshop staffs at the School of Engineering and Information Technology at UNSW Canberra, especially Jim Baxter, David Sharp, Douglas Collier, Mark Dumbrell and Nick Baxter for their help in tackling all experimental challenges. I would like to give special thanks to my colleagues Mohammad Anwar-Us-Saadat, Shayani Mendis and Xin Li for giving hands during experiments. I would also like to thank my colleagues and officemates Md. Ashraful Ismal, Md. Abdul Kader, Biruk Hailu Tekle, Zhengliang Liu and Zongjun Li their support, encouragement and friendship. I am also grateful to Md. Shahidul Islam, Shahana Ferdous, Shahariar Jahan, Shawkat Mozumder, Shafin Rahman and Jamil Ashraf for their support and encouragement throughout my study and stay in Canberra. Special thanks are due to my parents and all my families for their endless patience, encouragement, continuous support and help during this study. Finally, I take this opportunity to express my gratitude to the University of New South Wales, Canberra for the financial support in the form of TFS scholarships ii Table of Contents Table of Contents Abstract ............................................................................................................................. i Acknowledgements .......................................................................................................... ii Table of Contents ........................................................................................................... iii List of Figures ................................................................................................................. vi List of Tables ................................................................................................................ xiv Notation ......................................................................................................................... xvi Chapter 1 Introduction ................................................................................................... 1 Background .............................................................................................................. 1 Material content and classification .......................................................................... 3 Applications in the construction industry ................................................................ 5 Research objectives ................................................................................................. 8 Thesis outline ........................................................................................................ 10 Chapter 2 Literature Review ....................................................................................... 12 Introduction ........................................................................................................... 12 Material model ...................................................................................................... 12 Corner strength enhancement due to cold-work .................................................... 17 Residual stress ....................................................................................................... 20 Geometric imperfections ....................................................................................... 23 Existing codes ........................................................................................................ 27 Design guidelines for cross-section resistance ...................................................... 28 Design guidelines for flexural buckling resistance ............................................... 29 Continuous Strength Method ................................................................................. 32 Direct Strength Method ....................................................................................... 38 Summary ............................................................................................................. 41 Chapter 3 The Continuous Strength Method for Stainless Steel Slender Sections. 42 Introduction ........................................................................................................... 42 Experimental results .............................................................................................. 43 Development of the finite element model ............................................................. 43 Verification of the developed FE model ............................................................... 49 Parametric study .................................................................................................... 57 iii Table of Contents CSM for slender sections ...................................................................................... 59 Resistance of slender cross-sections against compression and bending ............... 67 Performance of the proposed design technique ..................................................... 68 Reliability analysis ................................................................................................ 78 Worked out Examples ......................................................................................... 80 3.10.1 Example I: Axial Compression resistance .................................................... 80 3.10.2 Example II: In-plane bending resistance ...................................................... 81 Conclusions ......................................................................................................... 82 Chapter 4 Buckling Resistance of Stainless Steel Hollow Columns ......................... 84 Introduction ........................................................................................................... 84 CSM based design approach for buckling resistance ............................................ 85 Development of the finite element model ............................................................. 87 Verification of the FE model ................................................................................. 90 Parametric study .................................................................................................... 94 Effect of λp, e , n and H/B on column curves ........................................................ 96 Correction factors for e and