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Sandwich materials, consisting of two thin, stiff facings separated by a low density core, can be used to produce structures that are both light and flexurally rigid. Such assemblies are attractive for applications in transport and construction. However, their optimisation is rarely straightforward. Not only is this due to the complex equations that govern their mechanics, but also because multiple design variables and objectives are often present. The work in this thesis identifies population-based optimisation techniques as a novel solution to this challenge. Three of these techniques have been developed in MATLAB specifically for this purpose and are based on particle swarm optimisation (sandwichPSO), ant colony optimisation (sandwichACO), and simulated annealing (sandwichSA). To assess their suitability, a benchmark problem considered the application of these techniques to a multiple objective sandwich beam optimisation. Optimised for stiffness mass and cost, a selection of 16 materials for both facing and core were available. Several constraints were also present. The sandwichACO technique demonstrated superior ability as it was able to obtain all optimal solutions in most cases. However, the sandwichPSO and sandwichSA techniques struggled to identify local optimum solutions for the multi-ply, fibre-reinforced polymer sandwich facing laminates. A further case study then applied sandwichACO to the optimisation of a sandwich plate for a rail vehicle floor panel. In addition to the benchmark, the problem was extended to include 40 materials. Also, the material and thickness of the top face was allowed to be different to the bottom. Furthermore, orthotropic fibre-reinforced facing constructions were included, as well as a localised load constraint. A broad range of optimal solutions were identified for the applied minimum mass and cost objectives. Sandwich constructions provided a significant (approximately 40%) saving in both mass and cost compared to the existing plywood design. More significant mass saving designs were also identified (of over 40%), but with a cost premium. Overall, population-based techniques have demonstrated successful application to the design of sandwich materials and structures.

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The aim of this thesis is to investigate the local and global buckling behaviour of delaminated composite plates using exact stiffness analysis. Several attempts are made to model delamination with the accuracy of detailed 3D finite element analysis (FEA) but substantially improved computational efficiency. Investigation of local buckling behaviour is performed using the exact stiffness program VICONOPT, giving good comparative results and substantially less solution times compared to those of FEA. Extending this approach to global buckling behaviour poses limitations and difficulties in retaining computational efficiency. Several techniques are introduced to study global buckling behaviour while requiring less solution time than FEA. The advantages and disadvantages of these techniques are discussed. Finally, an improved smeared stiffness method is derived which results from simplification of the total potential energy expression for the plate. This simplification avoids expensive computational effort while maintaining results of good accuracy (within 2%-3% of FEA results). This method can be employed for modelling delaminations of different shape and size located anywhere in the composite plate.

The use of adhesively bonded composite reinforcements for strengthening metallic structures is a relatively new technique that has been introduced in the past few years. The technique was first applied for crack patching of aircraft components, with the first civil engineering applications on steel bridge members, which have been damaged due to corrosion or fatigue, being more recent. The finite element (FE) method was employed in order to analyse a steel plate containing a crack and reinforced with an adhesively bonded composite patch. The role of parameters such as the crack size and the properties (modulus and thickness) of the patch and the adhesive was examined. Results in tenns of the stress magnification factor at the crack tip and the shear and peel stress distributions at the steel/adhesive interface were obtained. Following this, the FE results were used in order to develop an analytical model for estimating the stress magnification factor (YJ for patched cracks. The effects of patch pre-stressing and thennal loading due to mismatch in thennal expansion coefficients of the materials involved were also examined. Patch debonding was also investigated. Two different cases were considered, namely a patched plate without a crack and a patched plate containing a crack. For the latter, apart from debonding being initiated from the patch extremities (patch end debonding), crack mouth debonding was also considered. The effect of debonding on Yp was determined. Following this, debonding was modelled as a crack located at the steel/adhesive interface and a fracture parameter, namely the energy release rate G, was obtained at the interface crack tip. An analytical model for G pertaining to patch end debonding was also developed. An experimental study was carried out in order to validate the model proposed for Yp' Fatigue tests of patched cracked specimens were performed and crack growth data was obtained and compared with predictions using the analytical model for Yp inside the well known Paris crack growth law. Prior to this, the tensile properties for steel and composite and the fatigue properties of steel were determined from tests. The results indicate that the models obtained can be used for the design of composite patch repairs of steel members. Therefore, they were included in a design methodology, which is also presented. The latter could form the basis of a more ,comprehensive future design guide, or even complete existing guides, such as the CIRIA . guide (Cadei et a1. 2004).

Due to environmental concerns and increasing asphalt prices concrete pavements are seen as a sustainable alternative for road construction. Steel fibres are used as reinforcement for concrete pavements due to ease of construction, as well as improvement in the post-cracking, tensile/flexural and fatigue behaviour of the concrete. However, cost and method of construction are two major barriers for their use. Recycled fibres obtained from post-consumer tyres are a new alternative due to their lower cost and potential environmental benefits. The roller compacted concrete technique is also an alternative that enables road construction with the use of conventional asphalt equipment. These were the two main innovations being investigated by the FP6 EU Project Ecolanes. Understanding the durability of recycled steel fibre reinforced concrete (SFRC) is very important before these technologies can be used in real structures. This thesis addresses the issue of long-tenn behaviour of recycled SFRC, based on an experimental programme divided in two main studies: I) the mechanical properties (compressive and flexural behaviour), pore structure (porosity, density and free-shrinkage) and transport mechanisms (penneability, sorptivity and diffusivity) and 2) the main deterioration processes affecting the perfonnance of concrete pavements, corrosion (accelerated by means of wet-dry cycles in chloride solution), freeze-thaw (accelerated by continuous submerged freezing and thawing cycles) and fatigue (accelerated by flexural cyclic loads). A probabilistic analysis in terms of service life design has also been developed. Recycled fibres can increase the flexural strength of the concrete by up to 70% compared to plain concrete and they can significantly enhance the post-cracking behaviour. Recycled fibres, when added 2-6% by mass, do not affect the pore structure and the transport mechanisms of the concrete. Exceptions apply when contents around 6% by mass lead to compaction problems or affect the rheological properties of the concrete. Recycled fibres improve the fatigue resistance by allowing approximately 30% higher stresses than plain concrete for an endurance life of 2 million cycles. Fibres also contribute to slowing down the advanced stage of freeze-thaw degradation of concrete. Both fatigue and freeze-thaw are enhanced since these fibres control different stages of crack propagation. When subjected to wet-dry cycles, the fibres appear to be well protected inside the concrete and the main consequences are only in terms of superficial rust. The coupled benefits of mechanical and long-term performance of recycled SFRC make it a promising alternative for concrete pavements, especially in blends with industrially produced fibres. If these advantages are taken into account in the design of concrete pavements, a 20% reduction in the thickness of the concrete pavements should be expected, leading to less use of natural resources and to a further 10% reduction in costs.

This thesis describes an investigation into the wind loading on access scaffolds erected around a cubical building, clad by impermeable sheeting or permeable debris netting. The subject was investigated experimentally by tests in a wind-tunnel and theoretically using computational fluid dynamics techniques. The results were verified from the wind tunnel tests and computational analyses on the Silsoe Experimental Building (SEB) using data from the full-scale tests made in 1993-94 at Silsoe, U.K. The lower portion of the Atmospheric Boundary Layer exhibits different flow properties to the upper elevations. A procedure is presented for modelling the atmospheric surface layer flow properties in a boundary-layer wind-tunnel at useful model scales. The full-scale data available from the cubical 6m x 6m x 6m SEB was used to validate the results presented in this study. A model scale of 1:30 were used both for experiments in a wind-tunnel and in the computational analyses undertaken in the study. Pressure data obtained from the wind-tunnel experiments on the SEB model were compared to full-scale data with good agreement. These data were also compared with various computational fluid dynamics techniques available commercially and the conclusions drawn on the use of the different techniques. The wind-tunnel simulations on an SEB model and on a sheet/elevated sheet clad scaffold models were undertaken based on a duplication of the turbulence intensities and small-scale turbulence of the incidence flow. It is very difficult to achieve equality of Reynolds number in the wind-tunnel as it is very difficult to achieve exactly the same integral scales of turbulence. Two different types of terrain and inflow boundary conditions were simulated in the wind-tunnel for the models and results are reported here. Large suctions (separation of flows) occur near the leading edges and roof corners. The modelling of these phenomena in the wind-tunnel remains a problem. Because of the limited space near the corners and leading edges, it is difficult to make reliable measurements by introducing probes in these areas. This difficulty can be overcome by modelling the flow with Large Eddy Simulation (LES) numerical techniques. However, the disparity between the large and small scales, especially under extreme wind conditions, makes it extremely difficult to resolve the entire range of dynamic scales. The pressure force on bare pole access scaffolds are further influenced by the presence of the building façade which induces a shielding effect. A 2-D model of bare pole scaffolds surrounding the SEB using CFD techniques was successfully achieved whereas a 3-D model could not be produced because of the limitations of the meshing-software GAMBIT available to the author. Cladding increases the wind loads on scaffold structures above the pressure force on bare pole access scaffolds. To determine the wind forces on net/sheet clad scaffolds the Silsoe Experimental Building was used as a base model and simulated scaffolds erected around it. Although, sheeting/netting exhibits aero-elastic behaviour under wind load, an assumption was made to treat the cladding (sheeting/netting) surrounding the scaffold as being made of static solid thin plates. Models were tested in a wind-tunnel and the same assumptions were used in the computational fluid dynamics analyses. For the sheet clad scaffolds, two models were made, one with sheeting touching the ground and the other with an elevated sheet surrounding the building. These models were tested in a wind-tunnel to determine the pressure coefficients on the outer and inner faces of the sheeting. The permeability of the two types of net were successfully obtained from wind-tunnel tests. The simulated data from the wind-tunnel tests were used as input for different computational techniques with good agreement. A new procedure was developed to extend the computational model to net clad scaffolds (both elevated and touching the ground) with the netting simulated as porous media. The author presents new results of the pressure coefficients on sheeted scaffolds obtained using CFD and wind-tunnel techniques and also CFD results on netted scaffold structures. This thesis is the result of research undertaken to assess various methods available for the numerical simulation of turbulent fluid flow using the Fluent Software Package and to see their applicability in computational wind engineering. Investigations have concentrated on analysing the accuracy and numerical stability of a number of different turbulence models including both widely available models and state of the art techniques. Furthermore, Large Eddy Simulations using the dynamic kinetic energy sub-grid-scale model have been completed on some models, in order to account for the four dimensional nature of turbulent flow and to show the best correlation between wind-tunnel, full-scale and sheeted scaffolds. The author has detailed and tested all the above techniques and gives recommendations on the appropriate turbulence model to be used for successful computational wind engineering. Finally the author has given recommendations on the wind pressures to be used in analysing the scaffold structures.

The composite structures studied in this thesis involve concrete slabs with profiled steel sheeting supported by steel beams. The composite interaction is realized by providing shear connectors welded to the top flange of the steel beams. The columns are normally noncomposite. The steel beam-to column connections are either designed as pinned or designed as rigid. Recent research has shown that semi-rigid joint behaviour may be achieved in composite joints by considering only a small amount of reinforcement over the connections. However due to the lack of design provisions and a simple and effective analytical method, semi-rigid frame design is rarely adopted by engineers. In this thesis the behaviour of semi-rigid composite connections, composite beams, and composite frames is investigated. Simple and effective finite element analysis models for composite joints, composite beams and composite frames are proposed. The models are validated against published tests on composite joints and beams. Three composite frames are analyzed and the results are compared with different proposals. Satisfactory agreements are achieved. Design recommendations are promoted for semi-rigid composite frames. Seismic analysis of composite frames is performed using the proposed frame model. The influence of semi-rigid joints on the overall performance of composite frame is investigated. The proposed method is so simple and straightforward that it provides an effective tool for both static and dynamic analysis of composite semi-rigid structures.