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.

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Cellular beams are widely spread structural elements in the modern construction industry having as main characteristic the web openings. Cellular beams are produced through a fabrication process from solid beams and have bigger section depth than the parent beam. In buildings design with need of open large spaces cellular beams can be used in long span, up to 20 metres, without the necessity of including columns. The most common protection material applied in cellular beams is intumescent coating, giving the advantages of allowing the passage for technical services through the web openings and adding an aesthetic look to the beams. This PhD research investigates the behaviour of short-span and long-span composite protected with intumescent coating cellular beams in fire conditions, performing experimentally and analytical studies. The experimental process performed in FireSERT laboratory, tested under ISO-834 standard fire curve three composite protected with water based intumescent coating for 60 minutes cellular beams with different geometries but same 4.5 metres span. The furnace fire tests showed that the temperature distribution on protected cellular beams in non-uniform and the failure modes are web post buckling and Vierendeel bending. The collected results were compared with other furnace tests, which were previously emulated in the same conditions, although using unprotected beams. These comparative analyses showed that on the protected beams the temperature increase is slower and maximum deflections lower.

Due to increase in the application of glass fibre reinforced polymer (GFRP) beams based structural systems for rehabilitation of existing and construction of new bridges there is a requirement for identification of critical components of these structural systems and the assessment of damage in this sort structural. The application of vibration-based damage detection (VBDD) techniques has been identified as one of the universal technique that is promising in damage assessment in composite beams. The current study aimed at using vibration-based damage detection technique for assessment of damage exhibited by FRP composite beam structure. This thesis addresses the experimental and numerical study of damage assessment of FRP composite beams using vibration data. Dynamic measurements were carried out considering different specified boundary conditions and the effect of supports on the modal parameters and the effect of damage would be obtained. Another set of experiments were conducted on a Fixed-Free beam with unspecified boundary conditions at the free end, where a mass and a spring attached to the free end and beams were tested for different damage scenarios. Triaxial accelerometers were placed at selected locations based on the result of simple FE beam model. The beams were excited in the two (vertical and horizontal) directions by means of a hammer but only the vertical direction was taken into consideration at this point. The responses at different points along the beam were recorded at different levels of damage and fed into an FIT analyzer. Post-processing of data was done with the stochastic subspace identification (SSI) technique. The effect of the damage on the natural frequencies, mode shapes, and damping ratio were determined. Finite element models for the intact, reference and damaged beams were constructed and used to support the dynamic measurements. FE model updating was applied to different five boundary conditions in order to evaiuate their effect. Damage assessment techniques based on selected methods such as modal frequency change, direct mode shapes, change in modal curvature, damage index method and model updating methods were developed and applied to the beams. A unique test configuration was introduced by combining fixed-free beam with undefmed boundary at the free end. The spring and masses were attached to the free-end, this was implemented for different damage scenarios. The outcome of this implement experiment was discussed and analysed.

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