Some plasticity studies relating to thin-walled beams
In the field of cold formed structural steels, in which the load carrying members consist of thin-walled sections, failure often occurs due to the development of local buckles which are initiated in the compression elements. The work presented in this thesis details experimental and theoretical investigations conducted to study the behaviour of some thin-walled beams which fail mainly due to local buckling. The ultimate load carrying capacity and collapse behaviour of plain channel, lipped channel and zed section beams in simple bending were examined and the results were then used in an extension of the theory to predict the behaviour of indeterminate beams of similar cross-section which experience plastic moment redistribution before ultimate collapse. The findings from an experimental investigation on the effects of strain hardening on the material strength of cold formed sections are also included in this thesis. The thesis begins with a short introduction, followed by a review of relevant published literature, which focuses mainly on the use of the plastic mechanism approach in the theoretical analysis of failure modes in thin-walled structures. The theoretical method of using an elastic buckling analysis in conjunction with a separate plastic analysis to estimate the behaviour of thin-walled beams in the entire range of loading history, from the initial linear elastic characteristics to the collapse behaviour of the beams as they are loaded beyond the ultimate load carrying capacities, is described in chapter 2. Two theoretical methods of using the results from the study of the collapse characteristics of beams in simple bending to predict the behaviour of multi-spanning beams are presented in chapters 2 and 3. These theories are then applied in models for plain channel, lipped channel and zed section beams in chapters 3, 4, and 5 respectively. Details of the rather extensive experimental investigations carried out to examine the behaviour of beams of the selected sections are then presented in chapter 6. The resulting predictions generated by the theoretical models which are based on the findings from these experiments are compared with the experimental load-deflection results in chapter 7 of this thesis. Results based on conventional simple elastic and plastic theories and the BS 5950 : Part 5 : 1987 are also presented for comparison. Generally, good agreement was found between the results of current theory and the experimental findings. The experimental work on the effects of strain hardening on the yield strength of cold formed sections is detailed in chapter 8 where the results and observations are included. Discussion of the presented work is carried out in detail in chapter 9, which also summarises the conclusions drawn from the studies and some recommendations for further research and extension of the analysis. The publications cited in this thesis are listed in a bibliography. These are arranged in alphabetical order with reference to the authors. The thesis is then concluded by three appendices which supplement the main text.