This thesis presents an investigation into the thermo/hydro/mechanical interactions occurring in two large scale experiments with different geometry setup designed for deep geological disposal of nuclear waste. The analysis of flow and deformation through unsaturated soils was performed using the proposed constitutive model. The following mechanisms are accommodated moisture flow in liquid and vapour forms, dry air flow including the movement of bulk air and the transport of dissolved air in the pore water, heat transfer by conduction, convection and latent heat of vapourisation, and the elasto-plastic deformation of unsaturated soils. The governing differential equations are solved spatially using a finite element technique and temporally using a finite difference technique. The numerical model was subsequently applied to two large scale experiments: AECL's Buffer/Container Experiment and AECL's Horizontal Canister In-room geometry. The analyses involved modelling the coupled thermo/hydro/mechanical interaction between the engineered buffer and the host rock, and the swelling phenomena at the buffer/rock interface. A comprehensive set of material parameters was determined for use in the numerical code. A theoretical formulation was developed to describe a time-dependent adsorption/desorption process associated with the micro/macro swelling phenomena occurring in bentonite-based buffer materials. This was implemented within the numerical model. The transient swelling behaviour of bentonite-based buffer materials was found to be influential in the simulation of an underground repository, in particular the resaturation behaviour in expansive clay. Sensitivity analyses on critical parameters identified for both geometry setups are presented and shown to affect the resaturation processes to varying degrees. This investigation has provided valuable insights into the repositories' behaviour.

Civil engineering design problems are typically approached using traditional techniques i.e. deterministic algorithms, rather than via stochastic search such as evolutionary algorithms. However evolutionary algorithms are adept at exploring fragmented and complex search spaces, such as those found in design, but do require potential solutions to have a 'representation' amenable to evolutionary operators. Four canonical representations have been proposed including: strings (generally used for parameter based problems), voxels (shape discovery), trees and graphs (skeletal structures). Several authors have proposed design algorithms for the conceptual layout design of commercial office buildings but all are limited to buildings with rectangular floor plans. This thesis presents an evolutionary algorithm based methodology capable of representing buildings with orthogonal boundaries and atria by using a 3-section string with real encoding, which ensures the initialisation and evolutionary operations are not too disruptive on column alignments encoded via the genome. In order to handle orthogonal layouts polygon- partitioning techniques are used to decompose them into rectangular sections, which can be solved individually. However to prevent the layout becoming too discontinuous, an 'adjacency graph' is proposed which ensures column line continuity throughout the building. Dome geometric layout design is difficult, because every joint and member must be located on the external surface and not impinge on the internal void. This thesis describes a string-based representation capable of designing directly in 3D using surface area and enclosed volume as the major search parameters. The representation encodes support and joint positions, which are converted into a dome by constructing its corresponding convex hull. Once constructed the hull's edges become the structural members and its vertices the joints. This avoids many of the problems experienced by the previous approach, which suffers when restrictive constraints such as the requirement to maintain l/8th symmetry are removed. The aim of this thesis is to investigate how some civil engineering design problems, in particular structures, can be represented using evolutionary algorithms (EA) and contains two, independent experimental chapters on building layout design and geometric dome design (an introduction to EAs and design is also provided). Civil engineering design problems are typically approached using traditional techniques i.e. deterministic algorithms, rather than via stochastic search such as EAs. However EAs are adept at exploring fragmented and complex search spaces, such as those found in design, but do require potential solutions to have a 'representation' amenable to evolutionary operators. Four canonical representations have been proposed including: strings (generally used for parameter based problems), voxels (shape discovery), trees and graphs (skeletal structures). Several authors have proposed design algorithms for the conceptual layout design of commercial office buildings but all are limited to buildings with rectangular floor plans. This thesis presents an evolutionary algorithm based methodology capable of representing buildings with orthogonal boundaries and atria by using a 3-section string with real encoding, which ensures the initialisation and evolutionary operations are not too disruptive on column alignments encoded via the genome. In order to handle orthogonal layouts polygon- partitioning techniques are used to decompose them into rectangular sections, which can be solved individually. However to prevent the layout becoming too discontinuous, an 'adjacency graph' is proposed which ensures column line continuity throughout the building. Dome geometric layout design is difficult, because every joint and member must be located on the external surface and not impinge on the internal void. This thesis describes a string-based representation capable of designing directly in 3D using surface area and enclosed volume as the major search parameters. The representation encodes support and joint positions, which are converted into a dome by constructing its corresponding convex hull. Once constructed the hull's edges become the structural members and its vertices the joints. This avoids many of the problems experienced by the previous approach, which suffers th when restrictive constraints such as the requirement to maintain 1/8 symmetry are removed.

This thesis presents three new three-dimensional constitutive models for cementitious materials. All three models use embedded damage planes and adopt the theory of contact mechanics to describe the characteristic behaviour of cracks formed in concrete and other cementitious materials. The first of these is a smooth frictional contact model which incorporates a simplified Mohr-Coulomb yield surface to capture plastic slip planes in concrete. The aim of the model is to accurately represent the behaviour of smooth construction joints in large concrete structures. The second proposed model is the dual-surface contact model. The model employs two contact surfaces, each of which nominally represents a different component of concrete composite, i.e. coarse aggregate particles and mortar. The third model is the 'embedded planes with local plasticity contact' model (EPLPC). The model adopts a yield surface, which is similar to the damage surface in strain space, to capture plastic embedment on crack surfaces. This model, as with the dual-surface contact model, is developed to simulate crack opening-closing, as well as the behaviour of aggregate interlock. The models are integrated with a hardening/softening frictional plasticity component that uses a smoothed triaxial plastic yield surface developed from that used by Lubliner et al. (1989). Each of the proposed models is implemented with a consistent tangent stiffness operator and return mapping algorithm, similar to that of the Closest Point Projection algorithm. The models are coded in Fortran77 and implemented in a constitutive driver program, and also a finite element software package LUSAS. The models are assessed using a series of stress/strain paths at the constitutive level, and also validated against a range of experimental data. These include data from uniaxial and multiaxial compressive tests, uniaxial tensile tests with and without unloading-reloading cycles, and also tests in which shear load is applied on open cracks.

This thesis describes the experimental work undertaken to investigate the possibility of using oil lake residue to stabilise Sabkha soils in Kuwait. Sabkha is a problematic salt-encrusted soil deposited under arid conditions which cannot be used for construction in its natural condition. The oil lake residue being considered is the waste hydrocarbons resulting from the destruction of oil wells in Kuwait during the Gulf War, 1990. The oil lake residue covers an overall area of 24 km2, and represents an environmental hazard that needs to be eliminated. The experimental programme included laboratory and field testing of physical properties and strength, consolidation and leaching aspects of the natural and oil mixed Sabkha soils. A soil survey of a large area was undertaken to select representative soil samples. Sabkha soils from four main locations were selected for the detailed experimental testing. Oil residue was added to the Sabkha soils at different percentages ranging from 0% to 10%. The focus of the experimental work was towards physical and mechanical behaviours due to low clay (less than 0.5%) and organic (less than 2%) contents of the soil and high content of non-polar compounds (85%) in the oil residue. Results showed that the addition of oil residue reduced the friction between the soil particles in the range of 5% to 28% and the facilitated sliding over each other resulted in an increase in the density of the compacted Sabkha soils of between 2% to 8.5%. The UCS increased in the range of 34% to 504% of the natural values. The shear strength slightly increased with oil addition since the internal friction decreased and the cohesion intercept values increased in the range of 45% to 150%. The adsorbed oil residue on the cemented soil lumps acted as a waterproofing agent that reduced both salt dissolution by 56% of the natural soil and the long term coefficient of permeability in the range of 73% to 88%. Under soaked conditions, the improvement in strength properties were pronounced. The natural Sabkha soil disintegrated upon soaking while oil mixed Sabkha maintained its integrity. Yield stress increased in the range of 25% to 60% from the values of natural soils and compression index and collapsibility decreased in the stabilised Sabkha. Field testing on 5% stabilised compacted test beds revealed an additional increase in CBR and UCS above that of the laboratory results. Density and shear strength of field samples showed similar results as those obtained by laboratory testing. Leachability of oil from the 5% oil stabilised Sabkha soil measured in the laboratory under a hydraulic gradient of 40 was less than 1 mg/1, which can be considered very low. Leachability of oil residue in the field was undetectable. The main conclusion of the work is that the addition of 5% oil residue improved the performance of Sabkha soils especially under soaked conditions, which may result in its applications in the construction industry. These results are encouraging. However the acceptance of oil residue as a stabilising material necessitates further research and this future programme is briefly mentioned.

A probabilistic based assessment procedure for the prediction of the remaining service life of reinforced concrete bridge beams and slabs subject to reinforcement corrosion has been developed. By modelling the corrosion of reinforcement as a two stage process consisting of an initiation period and a propagation period, analytical methods have been developed and used to estimate the probability of failure with time for a number of limit states. The most common form of deterioration has been identified as the corrosion of reinforcement due to the ingress of chloride ions from de-icing salts applied during the winter period. Fick's second law of diffusion has been used to model the transportation mechanism of chloride ions in concrete. Usually the effect of any spatial variation that may be exhibited by the model parameters is neglected. However in this instance the probability of the onset of corrosion with time has been evaluated using random field theory. This allows the spatial variability exhibited by some of the model parameters to be incorporated into the analysis. Subsequent to the onset of corrosion the loss of the steel cross-sectional area and eventual spalling of the concrete will adversely affect the structural performance of the bridge elements. An algebraic formulation based on simple material properties has been proposed to evaluate the load bearing capacity of reinforced concrete beams subject to general or localized corrosion. In each case the resulting probability of failure with time for a number of limit states has been evaluated. One of the most commonly used methods of assessing the load bearing capacity of slabs is yield line theory. In conventional yield line analysis the development of inplane membrane forces is neglected. However, laterally restrained bridge deck slabs have been known to support loads well in excess of the design loads. Hence an analytical model has been formulated to predict the enhanced ultimate load bearing capacity of laterally restrained concrete slabs. Using the proposed model the reliability of a deteriorating solid bridge deck slab spanning in one direction and clamped at both ends has been evaluated. Various degrees of reinforcement corrosion and available surround stiffness have been considered.

No description available.

High strength steels (HSS), which are generally considered to be those with yield strengths over 460 MPa, are being increasingly utilised in construction, particularly in high rise structural applications and where long and column-free spans are an important design requirement. In place of ordinary carbon steels, the use of HSS can enable structural elements with smaller cross-sections, resulting in significant material savings. However, compared to normal strength steels, the structural use of HSS is still quite rare. The European design code EN 1993-1-12 provides design rules for HSS up to S700, but was conceived as a simple extension of the rules in EN 1993-1-1 for normal strength steels. In order to contribute to the existing limited HSS data pool and to verify and develop the current Eurocode 3 design rules, a comprehensive experimental programme on hot-finished S460 and S690 square and rectangular hollow sections has been carried out. The testing programme covered different structural aspects at the material, cross-section and member levels and consisted of 40 tensile coupon tests, 11 compressive coupon tests, 11 stub column tests, 11 full section tensile tests, 22 in-plane bending tests, 12 eccentrically loaded stub column tests, 30 long column tests, as well as measurements of geometrical imperfections and residual stresses. Numerical models, validated against the test results, were also developed to examine the cross-section and member behaviour, and subsequently employed in a comprehensive parametric study in order to generate further data. Based on the combined test and numerical data set, as well as experimental results reported in the literature, the current HSS design rules in Eurocode 3, including the slenderness limits for cross-section classification, effective width equation, N-M interaction curves and column buckling curves, were assessed by means of reliability analyses in accordance with Annex D of EN 1990. To realise the potential of HSS in long span structures, a novel structural form was also examined, namely an HSS truss with prestressing cables housed within the tubular bottom chord. A total of 4 prestressed trusses, made of S460 square hollow sections with different prestress levels, were tested under static downward loading. The truss test results showed the enhanced structural efficiency brought about by the addition of prestressing cables and by the application of prestress. Additionally, 12 tensile and 10 compressive member tests with cables, representing the bottom chord of the truss under gravity and uplift loading, respectively, were carried out to investigate the behaviour of individual prestressed cable-in-tube members. Analytical models and numerical models were also established to compare with the test behaviour and to contribute to the development of design rules for prestressed cable-in-tube systems.

Thin-walled, metallic structures are widely used across many engineering industries and are a popular choice due to their high load bearing capacity to self weight ratios. Interactive buckling is a common and potentially dangerous form of instability in these structures. The current work aims to investigate interactive global-local buckling in an I-section compression strut with rigidly rotating flange-web joints, using primarily an analytical approach. The analytical approach uses the Rayleigh-Ritz method, combined with continuous displacement functions to formulate a system of ordinary differential and integral equations, describing the equilibrium states of the strut. Initially, weak axis global-local buckling interaction is considered where both the flange and the web components of the cross-section contribute to the local buckling mode, owing to the rigidly rotating flange-web joint. The solutions are validated using a finite element (FE) model, showing excellent comparisons. The strut is then considered to be braced in the weak axis, thus susceptible to strong axis global-local buckling interaction. The strong axis global buckling mode and local buckling of the flange and web components are first considered separately, revealing a neutrally stable and stable post-buckling response respectively. The buckling modes are then combined in an analytical model, enabling them to act simultaneously; it is found that the critical, global buckling mode has a neutrally stable post-buckling path, which then becomes highly unstable when the local buckling mode is triggered and mode interaction is observed. The solution is validated against an FE model and shows excellent comparisons. Imperfection sensitivity of the strut is then investigated, revealing that the structure is sensitive to both global and local initial geometric imperfections. The shape of the local imperfection to which the strut is most sensitive is also identified and it shows a greater sensitivity when both global and local imperfections are present simultaneously. The solutions for an example strut with imperfections is compared to an FE model, again showing excellent comparisons. Parametric studies are conducted to investigate the effect of varying the geometry of the strut. Both the strut length and cross-section height are varied in independent studies, identifying the geometries that give rise to the most interactive, and therefore most undesirable, behaviour in the structure. The implications of the identified behaviour on the design of similar structures is discussed. The post-buckling behaviour of a thin-walled I-section strut, buckling under either weak or strong axis global-local mode interaction with rigidly rotating flange-web joints has therefore been established at a fundamental level, using an analytical approach.

The research presented in this thesis addresses the influence of non-structural masonry infill on the resistance of multi-storey buildings to progressive collapse under sudden column loss scenarios. In particular, the mechanical response of infilled frames in peripheral bays is investigated within the scope of a design-oriented robustness assessment framework. A ductility-centred progressive collapse assessment methodology recently developed at Imperial College is employed as a rational procedure to quantify structural robustness for sudden column loss. This allows due consideration of structural redundancy, ductility, strength, dynamic effects and energy absorption capabilities in a unified manner. In this way, robustness quantification is shifted from typical code recommendations into a sound performance-based mechanical assessment. The realistic contribution of masonry panels towards collapse arrest is examined considering the results from full-scale laboratory tests and accurate numerical simulations. Novel real-scale tests were performed on different two-bay frames with brick-masonry infill subjected to incremental pushdown deformation, capturing the dominant deformation mode actually found following removal of an edge column. In these physical tests, it was observed that the failure mechanisms and damage patterns displayed by the infilled frames under pushdown deformation are similar to those activated by lateral pushover loading. Chiefly, clear evidence of diagonal cracking and shear sliding, eventually culminating in crushing of the compressed corners were recorded during the tests. Different infill configurations were tested, including central openings and initial gaps between masonry and frame elements. Overall, a global stable response was observed even at the expense of severe damage in the masonry panels. Importantly, a monotonic supply of energy absorption was noticed with increasing vertical deformation, which translates into considerable robustness reserve associated with the confined infill walls. Secondly, advanced mesoscale finite element simulations were employed in order to capture the complex frame-infill interaction in the early stages of pushdown response, where it has been observed in a realistic case study that progressive collapse is effectively arrested at small dynamic deformations, with minimal damage to the masonry panels and surrounding structural elements. Finally, application of the robustness assessment framework allowed a critical comparison between the collapse resistance arising from secondary mechanisms typically considered in this context (such as floor membrane and beam catenary effects) and that related to the presence of masonry infill. While the former are quite effective at relatively large deformations, the latter is shown to add substantial contribution at small displacements. The conclusions in this thesis are particularly relevant within the context of retrofitting operations for robustness enhancement of existing structures, as a result of the growing demand for upgraded resilience of urban infrastructure. On the other hand, due account for masonry infill subject to proper quality control during the construction process is recommended for rational robustness design of new buildings.

The investigation of the collapse of “7 World Trade” as part of the events of 11 September 2001 in New York City (Gann, 2008) indicated that connections were among the most vulnerable elements of steel-framed or composite buildings, and their characteristics can determine whether such buildings survive in extreme scenarios such as fire. In this case total collapse of the building was triggered by the fracture of beam-to-column connections caused largely by thermal expansion of long-span beams. This emphasized the importance of investigating the complex mechanisms through which forces are transferred from the adjacent parts of a structure to the connections under fire conditions. The Cardington fire tests in 1995-96 (Newman, 2000) provided ample evidence that both shear buckling of beam webs and beam bottom-flange buckling, near to the ends of steel beams, are very prevalent under fire conditions. Both of these phenomena could affect the force distribution at the adjacent column-face connection bolt rows, and therefore the sequence of fracture of components. However, there is a distinct lack of practical research investigating the post-buckling behaviour of beams of Classes 1 and 2 sections adjacent to connections at elevated temperatures. In this PhD thesis, the development of analytical models of pure beam-web shear buckling and a combination of both beam-web shear buckling and bottom-flange buckling of beams of Classes 1 and 2 sections are reported. The analytical models are able to predict the post-buckling behaviour of the beam-end buckling panels in the vicinity of beam-column connections at elevated temperatures. A transition criterion, to distinguish between cases in which pure beam-web shear buckling occurs and those in which the instability is a combination of shear buckling and bottom-flange buckling, has been proposed, including a calculation procedure to detect the transition length between these two buckling modes. A component-based buckling element has been created and implemented in the three-dimensional structural fire analysis software Vulcan. The influence of the buckling elements on the bolt row force redistribution of the adjacent connections has been investigated in isolated beams and a simple two-span two-floor frame. It is expected that the buckling element will be involved in more complex performance-based frame analysis for design, and that it will be used with an explicit dynamic procedure to simulate local and progressive collapse of whole buildings.