This thesis deals with the seismic behaviour of steel frames with particular focus on structures that employ tubular members as either columns or bracing elements. It addresses a number of design and assessment issues at the local (connection), frame, and overall (system interaction) levels. At the connection level, two experimental investigations on: (i) blind-bolted and angle connections, and (ii) combined channel/angle connections, are presented. The main behavioural patterns and the effects of key design parameters on the connection performance are examined. Refined mechanical models able to estimate the response of these connecting details are developed. These mechanical models are subsequently employed to perform parametric studies based on which simplified design-oriented expressions for the estimation of stiffness, strength and ductility are suggested. The susceptibility to low-cycle fatigue within critical connection components and the predictions of available fatigue damage models are also assessed. At the frame level, an evaluation of the inelastic demands on moment-resisting, partially-restrained and concentrically-braced steel structures is performed and equivalent linear models for the estimation of peak deformations are proposed. Particular attention is given to the influence of a number of scalar ground-motion frequency content parameters on the estimation of peak displacements. Additionally, simplified models based on rigid-plastic dynamics, and implemented within response history analysis, are proposed. It is shown that such rigid-plastic models can predict global deformations with reasonable accuracy. At the system interaction level, a comparative assessment of the peak response of one-way, two-way and mixed framing configurations under bi-directional earthquake loading is studied by means of idealized 3D simplifications and refined 2D models. This enables a detailed quantification of the contribution of gravity frames to the reduction of seismic risk and highlights the benefits of proper secondary frame design in mitigating the probabilities of dynamic instability. Finally, the findings of the thesis are summarized and future research areas are identified.

Elliptical hollow sections (EHS) have been recently added to the family of hollow steel sections available to the construction industry. Their merits include different flexural rigidities about the two principal axes offering efficient bending resistance about the major axis and an aesthetically pleasant appearance. A number of recent practical applications have emerged, which are outlined in this thesis. Previous research on elliptical hollow sections has mainly focused on the cross-section level with a set of design rules for cross-section classification and shear resistance being proposed. The current study reviews the existing cross-section classification limits for both circular and elliptical tubular sections and investigates member instability of EHS in bending (lateral torsional buckling) and under combined axial compression and bending. Reliability analyses to establish a set of reliable design rules for elliptical hollow sections in the Eurocodes and other international structural design codes have been also performed. The key components of this research include laboratory testing, numerical modelling, and development of statistically verified design guidance. A series of experimental studies were undertaken to investigate the buckling response of elliptical hollow section members in bending and under combined axial load and bending. In total, 8 beams, 6 columns and 27 beam-columns were tested; the test results were then used to calibrate finite element models. Parametric studies were performed utilizing the validated numerical models. Based on the experimental and numerical findings, reliability analyses were undertaken to verify design rules for elliptical hollow sections. It is envisaged that these design rules will be incorporated into future revisions of Eurocode 3.

Current structural steel design codes, such as EN 1993-1-1, were developed on the basis of a bi-linear (elastic, perfectly-plastic) material model, which lends itself to the idea of cross-section classification. This step-wise design concept is a useful, but somewhat artificial simplification of the true behaviour of structural steel since the relationship between the resistance of a structural cross-section and its slenderness is, in reality, continuous. The aim of this study is therefore to develop a more efficient structural steel design method recognising this relationship and rationally exploiting strain-hardening, whilst maintaining, where possible, consistency with current design approaches. As part of the present study, laboratory tests were carried out on cold-formed and hot-rolled steel hollow sections. A total of 6 simple beams and 12 continuous beams (with two configurations) and corresponding material coupon tests were conducted. These experimental results were added to existing collected test data to develop and calibrate a new structural steel design method. The test results indicated that capacities beyond the yield load in compression and the plastic moment capacity in bending could be achieved due to strain-hardening. The new design approach, termed the continuous strength method (CSM), enables this extra capacity to be harnessed. The developed deformation based steel design method employs a continuous ‘base curve’ to provide a relationship between cross-section slenderness and deformation capacity in conjunction with a strain-hardening material model. The material model is elastic, linear-hardening and has been calibrated on the basis of collected stress-strain data from a range of structural sections. The CSM has been developed for both statically determinate and indeterminate structures utilising both experimental data and that generated through sophisticated numerical modelling. Comparisons between test results and predictions according to EN 1993-1-1 and the proposed method were made. The results revealed that the CSM provides a more accurate prediction of test response and enhanced structural capacity over current design methods.

The economic use of cold-formed steel members means that buckling and the possible loss of effectiveness it produces are important features of design. Cross-sectional instabilities in laterally-restrained cold-formed steel beams include local and distortional buckling. The prediction of the true buckling behaviour of cold-formed steel beams accounting for all governing features such as geometrical imperfections, spread of yielding, postbuckling etc. has been possible with the development of advanced numerical modelling. In this thesis, the finite element (FE) method (ABAQUS) has been used to develop numerical analyses to study the buckling behaviour of laterally-restrained cold-formed steel lipped Z-section beams. The FE models were verified against a series of four-point bending tests available from previous research, with special references to material and geometrical nonlinearities. Two sets of analyses have been conducted: FE analyses allowing for both local and distortional buckling and ones allowing for local buckling while distortional buckling is restrained using appropriate boundary conditions. For the former, the controlling buckling mode (local, distortional or combined) at different stages of loading (up to, at and beyond maximum load) has been realized. Comparing the results of two sets of analyses, the effect of distortional buckling on performance for different geometric proportions has been studied. The effect of the lip size, flange width, angle of inclination of the edge stiffener (lip), size and position of the intermediate stiffener and material strength as well as the interaction between them on both the ultimate strength and the buckling of cold-formed Z-section beams has been investigated. Limits for optimum design of the section were proposed. Depending on the geometric properties and material strength of the section, transitions between local, distortional and combined local/distortional buckling were observed. The lip/flange interaction including the interaction between the edge stiffener (lip) and the intermediate stiffener was the key governing feature of behaviour. The effect of the linear moment gradient and sharply varying bending moment on both the ultimate strength and the buckling of cold-formed Z sections was investigated. The latter occurred in two-span continuous beams subject to uniformly distributed loading. The results of moment gradient cases were compared with those of pure bending cases. The suitability of the design treatments available in Eurocode 3 (EC3) for local, combined local/distortional and distortional buckling of cold-formed Z-section beams was assessed. Overall, the EC3 predictions for cross-sectional bending resistances were unconservative. Shortcomings were identified and some suggestions for improvements were made. This included improvements in plate buckling factors for edge-stiffened compression flanges.

In this thesis, experimental investigations together with analytical and numerical work on the understanding and modelling of failure in laminated composites are presented. Failure of carbon fibre reinforced plastics is investigated using acoustic emission. Signals are collected for various test configurations which give rise to specific failure modes. The signals are then analysed using pattern recognition techniques and the fast Fourier transform. An identi cation of the failure modes with their acoustic signatures is proposed using the fast Fourier transform, which was found to be the most suitable technique. The failure modes in longitudinal compression are then studied using microscopy techniques and finite element modelling. Experimental observations show that failure results from an interaction between shear-driven compressive failure and kinkband formation. Micromechanical finite element analyses are used to explain the experimental observations. The interaction of shear-driven compressive failure and kinking captured by the model is used to explain the variation in characteristics typically measured in failure envelopes for combined longitudinal compression vs. in-plane shear. Based on the experimental and the numerical results, a failure criterion for fibre kinking and splitting is developed and used to predict failure envelopes for combined longitudinal compression vs. in-plane shear. The model correlates well with the numerical predictions and experimental results. The R-curve effect observed in mode I intralaminar matrix crack growth and its specimen-dependence are then investigated. Relationships between crack extension and crack opening displacement are obtained for the Double Cantilever Beam (DCB) and Compact Tension (CT) specimens. Measured R-curves are used with the previous relationships to define a trilinear cohesive law. The cohesive law is implemented in finite element models and the load versus displacement curves predicted for the DCB and CT specimens show that the R-curve effect is numerically well captured.

The use of tubular columns in conjunction with reinforced concrete flat slabs provides structurally efficient solutions which avoid undesirable failure modes such as those associated with shear. This thesis is concerned with the development of a tubular column-to- flat slab connection system that enables reliable performance under seismic loading conditions. During this research a novel detail which features a gap around the column is proposed and developed; hence only the structural steel shearhead establishes the connection. The exposed parts of the shear arms (fuses) are designed to yield prior to punching shear failure, in a way that utilises the favourable features of steel in terms of the response to seismically induced loads. The proposed connection could serve as a primary lateral resisting system within all building configurations in regions of low to moderate seismicity or as a secondary system in areas of signi cant seismicity. In order to provide validation for the proposed details as well as associated numerical and design procedures, a purpose-built rig which is suitable for large scale testing of structural sub assemblages under combined gravity and uniaxial lateral loading, has been designed and constructed, and subsequently employed for a number of tests. Test results and numerical analyses are presented with respect to a conventional con guration, as well as for the proposed, partially embedded connection. The latter is shown to offer enhanced ductility compared with traditional forms. The results are used to demonstrate the favourable inelastic performance of the proposed detail in terms of ductility, low degradation effects and increased energy dissipation capabilities. Complementary small scale slab panel tests are also used to further optimise the composite behaviour of the proposed detail. Additionally, a closed form solution based on plastic limit analysis which can serve as a basis for a simplified design approach is proposed. Finally, the main findings from the experimental and analytical investigations are highlighted, and recommendations for future research are outlined.

Subsequent to the World Trade Centre collapses in 2001, general interest into structural robustness and progressive collapse has been significantly heightened. It is recognised that the current prescriptive rules employed in the design of building structures to resist progressive collapse need to be replaced by performance-based approaches. A continuing research program at Imperial College London aims at the development of a complete design method that will address the basic features of progressive collapse whilst being tractable in terms of complexity for routine use in practice. An important step towards that objective is the proper treatment and understanding of the fundamental mechanics of the problem. The current study is motivated by that requirement and seeks to build on previous developments at Imperial to explore the progressive collapse response of steel and composite buildings on a quantitative basis. At first, the study is dedicated to the development of a simplified model for representation of the connection behaviour. The model is incorporated into a slope-deflection approach and an analytical method for prediction of the nonlinear static response of steel and composite beams following column removal – i.e. a common design scenario for progressive collapse – is derived. The method is carefully validated and applied in a detailed study of the response of axially restrained beams in progressive collapse, where the most important structural parameters and their effects on performance are identified. Based on those outcomes, the behaviour of bare steel and composite grillage systems following sudden column loss is subsequently appraised. It is concluded that progressive collapse resistance depends on the interplay between the connection moment capacities and ductility. Performance may also be enhanced by compressive arching action in the presence of axial restraint; however, for average values of connection ductility, failure typically occurs prior to the development of significant tensile catenary action. Therefore, it is suggested that design methods for progressive collapse should be primarily oriented towards the prediction of appropriate values for the connection moment capacities.

The focus of the research reported in this thesis is the quantification of colloid transport through porous media, which is implicated in the contamination of soil and shallow groundwater. The limited spatio-temporal resolution of existing experimental techniques prevents the formulation of testable hypotheses for colloid transport within· complex (environmentally-relevant) porous media and therefore restricts further progress in the understanding of colloid movement in the environment. The research presented here responds to this premise by developing, testing and applying a high spatial and temporal resolution, meso-scale (millimetre to decimetre) fluorescence imaging system for measurement ofcolloids transport in porous media. A review of existing literature highlights the restrictions placed on the parameterisation of theoretical models by the low resolution of experimental datasets such as breakthrough curves and retained mass profiles. Previous applications of meso-scale imaging techniques do not extend to quantitative measurement of colloids in unsaturated or transient flow through porous media. In Chapter 2 an existing UVfluorescence imaging system is modified and calibrated to enable quantification of changing colloid mass distributions over time in saturated pore conditions. In Chapter 3 the technique, Time Lapse Fluorescence Imaging, is further extended to enable quantification oftwo-dimensional spatial variations in pore saturation, simultaneously with measurement of colloid mass. A data analysis method is developed to enable quantification of the colloid removal efficiency (ratio of deposition rate to colloid flux) at any point in a variably saturated flow field. In Chapter 4, the methods developed in previous chapters are applied to the problem of colloid mobilisation during drainage ofa previously-saturated quartz sand. The descriptive power of the novel meso-scale fluorescence imaging techniques. presented provides important objective data that will help to extend the theoretical framework for colloid deposition to non-uniform, unsaturated flow and to constrain transport models for colloids in the vadose zone.

No description available.

No description available.