M.Ing. (Civil Engineering) / The results of a study presented on the behaviour of cold-formed stainless steel beams subjected to shear taking into account elastic shear buckling, inelastic shear buckling and shear yielding, as well as an investigation into the interaction relationship between shear and bending are presented. From this investigation the results obtained show good relation to the theory. The local shear buckling stress was experimentally determined and it was found that for unreinforced beam webs the shear buckling coefficient is that of an infinitely long plate, namely k=5,34. A good agreement between the experimental ultimate shear strength and the predicted ultimate shear strength was found. Stainless steel beams comprising of lipped channels were manufactured and tested to failure. The types of stainless steels used in this investigation were Types 304, 316, 430 and Type 3CR12 corrosion resiting steel, a modified Type 409 stainless steel. The stress-strain relationship for stainless steels differs from that of carbon steel in that stainless steel is a gradual yielding material. It was concluded in this investigation that Gerard's plasticity reduction factor, Gs/G o, should be used as a plasticity reduction factor in calculations concerning shear. It was found that present design criteria are adequate.

Cold-formed stainless steel sections have been increasingly used in architectural and structural applications. Yet the high price of stainless steel limits the application to construction projects. The lean duplex stainless steel (EN 1.4162) offers an opportunity for stainless steels to be used more widely due to its competitive in price, good mechanical properties and corrosion resistance. The lean duplex stainless steel is a relatively new material, and research on this material is limited. Currently, the lean duplex stainless steel is not covered in any design specification, and no design rules are available for such material. Therefore, the behaviour and design of cold-formed lean duplex stainless steel members are investigated in this study. The investigation focused on columns, beams and beam-columns of square and rectangular hollow sections. Both experimental and numerical investigations were performed and reported. Design rules for cold-form lean duplex stainless steel members are proposed. The experimental investigation included material tests, column tests, beam tests and beam-column tests at room and elevated temperatures ranged from 24 – 900 °C. The test specimens were cold-rolled from flat strips. The test program consists of two square hollow sections and four rectangular hollow sections. Coupon specimens were extracted from each hollow section, and their material properties were obtained from tensile coupon tests at room temperature and elevated temperatures. In this study, a modified design rule was proposed to predict the cold-formed lean duplex stainless steel material properties at elevated temperatures. The local and overall geometric imperfections were measured. A total of 38 column tests were conducted. The effective column length ranged from 75 to 1660 mm in order to obtain a column curve for each test series. The test program for beams included 10 pure bending tests, and the bending capacities of the specimens were determined. A total of 37 beam-column specimens were compressed between pinned ends at different eccentricities in order to obtain an interactive curve for each series of test. Numerical investigation on columns, beams and beam-columns at room temperature as well as elevated temperatures are also presented. Accurate finite element models were developed and verified against the experimental results for columns, beams and beam-columns at room temperature. The structural members at elevated temperatures were simulated by replacing the material properties with those obtained at elevated temperatures. Extensive parametric studies were carried out, including 150 columns, 126 beams and 150 beam-columns at room temperature, as well as 180 columns, 125 beams and 195 beam-columns at elevated temperatures. Column, beam and beam-column strengths obtained from the experimental and numerical investigations as well as available data were compared with the design strengths calculated using American, Australian/New Zealand, European specifications for stainless steel structures of duplex material, since lean duplex material is not covered by these specifications. In addition, direct strength method for carbon steel and stainless steel as well as continuous strength method for stainless steel were assessed for cold-formed lean duplex stainless steel. Modified design rules were proposed. The reliability of the current and modified design rules was evaluated using reliability analysis. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

The structural behaviour of single shear bolted connections and double shear bolted connections of cold-formed stainless steel at elevated temperatures and post-fire condition has been investigated in this study. The current design rules on bolted connections of cold-formed stainless steel are mainly based on those of carbon steel, and are applicable for room (ambient) temperature condition only. These design rules may not be applicable for elevated temperatures. Therefore, design guidelines should be prepared for bolted connections of cold-formed stainless steel structures at elevated temperatures. The key findings of the investigation are described in the following paragraphs. A total of 25 tensile coupon tests were conducted to investigate the material deterioration of three different grades of stainless steel at elevated temperatures. The stainless steels are austenitic stainless steel EN 1.4301 (AISI 304) and EN 1.4571 (AISI 316Ti having small amount of titanium) as well as lean duplex stainless steel EN 1.4162 (AISI S32101). Totally 434 tests on bolted connections of stainless steel were performed in the temperature ranged from 22 to 950 ºC using both steady state and transient state test methods. The test results were compared with the nominal strengths calculated from the American Specification, Australian/New Zealand Standard and European codes for stainless steel structures. In calculating the nominal strengths of the connections, the material properties at elevated temperatures were used in the design equations for room temperature. It is shown that the nominal strengths predicted by these specifications are generally conservative at elevated temperatures. A total of 78 cold-formed stainless steel single shear and double shear bolted connections were tested in post-fire condition. The test results were compared with those tested at room temperature. Generally, it is found that the bolted connection strengths in post-fire condition cooling down from 350 and 650 ºC are higher than those tested at room temperature for all three grades of stainless steel. Finite element models for single shear and double shear bolted connections were developed and verified against the experimental results. Static analysis technique was used in the numerical analyses. Extensive parametric studies that included 450 specimens were performed using the verified finite element models to evaluate the bearing resistances of bolted connections of stainless steel at elevated temperatures. Design equations for bearing resistances of cold-formed stainless steel single shear and double shear bolted connections were proposed based on both the experimental and numerical results in the temperature ranged from 22 to 950 ºC. The bearing resistances of bolted connections obtained from the tests and the finite element analyses were compared with the nominal strengths calculated using the current design rules and also compared with the predicted strengths calculated using the proposed design equations. It is shown that the proposed design equations are generally more accurate and reliable in predicting the bearing resistances of bolted connections at elevated temperatures than the current design rules. The reliability of the current and proposed design rules was evaluated using reliability analysis. The proposed design equations are recommended for bolted connections assembled using cold-formed stainless steels. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

M.Ing. / Because cold-formed stainless steel is a new type of light steel material and dose not have a long history of utilisation in structures, there are many issues that need to be researched and discussed. Making a more thorough investigation and study of cold-formed stainless steels is essential. As a numerical analysis tool, the finite element method proves to be useful in structural analysis. The buckling modes of cold-formed stainless steel members, such as local, flexural, torsional and torsional-flexural buckling, are well known and well documented in design specifications. Distortional buckling is a special kind of buckling mode, which is less well known. Researchers have recently paid more attention to this problem. For stainless steel structures, it is necessary to investigate their behaviour when distortional buckling occurs. In this project, the distortional buckling of cold-formed stainless steel columns under axial compression is investigated. The finite element method is used to analyse and calculate different buckling modes, especially distortional buckling. This is compared to experimental results and other theoretical predictions. The ABAQUS finite element code is used throughout. Finite element modelling is very important prior to processing and analysis. ABAQUS models are created to study distortional buckling. The initial imperfection of structural members is taken into account with these models, using specific sine wave descriptions with respect to different structural parameters. A dynamic processing approach is chosen in the finite element analysis. The effectiveness and accuracy of these models have been verified by both experimental tests and theoretical calculations. Buckling mode and behaviour are predicted and analysed in terms of the finite element models and processes. Suggestions are made for buckling analysis and design based on the research results.

M.Ing. / During all manufacturing processes it is crucial to use certain design criteria and guidelines. Special care should be exercised when the final product of a manufacturing process is used in the automotive industry, because the failure of such a component may have tragic consequences. The failure of a bus chassis in the public transport sector is an example of a case where the failure of a product can have serious consequences. In recent years it has become common practice to use corrosion-resisting steel in the manufacture of these vehicles. The reason for this is the corrosion caused by a prolonged service life and adverse conditions such as salted road surfaces (The salt is used to melt the ice that forms on roads, particularly in European countries). These bus structures consist of tubes of varying size and geometry, and the manufacturing process of these tubes is considered in the present investigation. In a tube manufacturing process the design criteria may consist of such properties as the tube size and geometry, the thickness of the sheet that is used and the radius of the corners of the tube. Design criterion is also dependent upon the material that is used. The change in mechanical properties of the material during a manufacturing process is an important consideration during the establishment of design guidelines. The purpose of this investigation is to study the effects of particularly the cold forming manufacturing process on the mechanical properties of the material. The material used is 3CR12 corrosion resisting steel, a proprietary alloy also known as Type 1.4003, that was developed by Columbus joint venture as a cheaper alternative to stainless steels. 3CR12 is not a substitute for stainless steel but it is an alternative to treated mild steel, providing a cost-effective solution to corrosion. An experimental investigation is conducted into the forming of 40mm 3CR12 square tubes and normal plate bending of 3CR12. Various different wall thicknesses and bend radiuses are considered. A numerical investigation consisted of simulating the above-mentioned manufacturing processes using non-linear finite element analysis and then comparing its results to the experimental investigation. The experimental investigation showed that substantial work hardening occurred in the corner regions of the tube during forming. A loss of up to 70% of 3CR12's ductility may result in the corner regions. The work hardening at the inside of the tube was found to be higher than at the outside. A region of very little work hardening near the middle of the tube wall thickness was also identified (neutral axis). This neutral axis also seems to shift slightly more to the inside of the tube with thicker wall sections. The numerical analysis confirmed the experimental observations. An excellent correlation between the experimental and numerical results was achieved.