M.Com. (Economics) / Please refer to full text to view abstract

In this thesis, some nonlinear effects associated with the buckling behaviour of plated steel structures are examined using a modified finite strip method. To include the effects of plasticity over parts of the cross-section, a more general stress-strain relationship than previously included has been used. The method is also extended to account for the large deflection behaviour of perfect and imperfect plates in the elastic range. The only restriction on the method presented here is that the buckling mode varies sinusoidally in the .;, longitudinal direction, which implies either that the ends of the structure are simply supported or that the wavelength of the buckled mode is small in comparison with the overall length of the structure. The present study may be divided into three parts. In the first part the small deflection theory is used to determine the stiffness and stability matrices of ~ individual strip and these are assembled to form an overall stiffness matrix, representing a structure which may be under concentric load, eccentric load or pure bending. In some cases a structure with an overall initial imperfection is considered. The Wittrick-Williams Algorithm is used to obtain the smallest critical buckling load. The method is applicable to the analysis of various structures such as isolated plates, stiffened panels, rolled sections and stiffened box-girder bridges. To check the accuracy of the method a comparison with some published theoretical and experimental results is undertaken. Secondly, a parametric study for stiffened panels, columns, and beams is presented. For the stiffened panels, the effect of seven parameters (slenderness ratio, residual stress, dimensions and shape of the stiffener, mode of buckling, the longitudinal boundary conditions, and the yield stress) has been investigated. Approximate design curves for the optimum dimensions of panels stiffened by flat stiffeners are given. The capability of the method for the analysis of a stiffened box-girder in bending is also shown. The effect of seven parameters (dimensions and shape of the cross-section, the slenderness ratio, the material yield stress, the residual stress, the initial overall imperfection and the eccentricity of the applied load) on the inelastic buckling of columns and beams has been studied. All the results are given in nondimensional graphs or tables. Finally large deflection plate theory is applied to study the post-buckling behaviour of both perfect plates and those with initial imperfections. The work in this section is restricted to the elastic state. The longitudinal axial compression is assumed to act on the plate through two rigid bars at the ends, and various in-plane boundary conditions for the longitudinal unloaded edges have been considered. The Newton-Raphson method is used for the solution of the non-linear equations.

A number of reinforced concrete slabs have been exposed to blast and impact loading in order to access modes of slab behaviour under these extreme dynamic loadings. Two sizes of specimens were used; small scale slabs modelled the large slabs at 1: 2.5 scale. Impact loads were produced by a free falling hammer impacting coaxially onto a cylindrical bar of steel placed at rest in the centre of the slab. The steel bar was instrumented with electrical strain gauges which recorded the stress pulses produced by the impact. Blast loads were produced using explosive charges made of Plastic Explosive PE4. In most cases the charge used was hemispherical in shape and was placed centrally above the slab at close range standoffs, i.e. up to 10 times the radius of the charge. Additional blast tests were conducted in order to monitor the transient and spatial pressure distribution across the slab by using the pressure gauges placed in replica steel slab. Transient deflections of the slabs under both types of load were obtained using long stroke displacement transducers, whilst transient strains in the steel reinforcement of the slabs were obtained using electrical resistance strain gauges bonded to the steel bars at mid span point. A rotating prism high speed camera was used to film the damage on some of the small scale specimens at rates of up to 10,000 pictures per second. The Hopkinson pressure bar tests were used to obtain the dynamic characteristics of the concretes used at high rates of loading. Differeent concrete mixes were used for the 1:1 and 1:2.5 scale slabs. An analytical function of the spatial and transient blast pressure distribution based on the detonation pressure of PE4 was established. This is in close agreement to experimentally measured results. The nature of the local and overall failure are discussed, and the time sequence of the slab failure is established for the case of explosive loading. The crack pattern that occurs soon after the explosion in area of local failure has been established from the high speed films whilst the overall deflected shape was obtained from the displacement vs time records. After test scab sizes and slab perforations were used to establish a relation between the slab thickness, amount of explosive and the slab damage in respect to scabbing and perforation. The displacement records and the shape of after test damage provided the basis for comments on "gravity neglected - the ultimate strength" modelling law that was employed in this research.

The modelling of unbounded domains is an important consideration in many engineering problems, for example in fluid flow, electro-magnetics, acoustics and solid mechanics. This thesis focuses on the problem of modelling elastic solids to infinity, with the specific purpose of modelling dynamic soil-structure interaction (DSSI). However, the reader should be aware that the techniques presented may also be adapted to address those other physical phenomena. The need for techniques to model the soil domain to infinity and a qualitative introduction into the problems associated with dynamic soil-structure interaction are outlined in chapter 1. This is done to illustrate why such an abstract mathematical concept of modelling infinite domains has an important role to play within the design process of large, safety critical, civil engineering structures. A brief review of a number of alternative ways of addressing this problem is given in chapter 2. Their relative strengths and weaknesses along with the typical applicability of the techniques is discussed. A consequence of this review is the identification of a very promising rigorous approach [59] which is singled-out for further study. A detailed explanation of this (Consistent Infinitesimal Finite Element Cell Method, CIFECM) method is then given in chapter 3. Attention is restricted to the use of the technique for solving the 3-D vector wave equation in the time domain. The features of the non-linear dynamic finite element code, into which the CIFECM has been incorporated, is highlighted in chapter 4. The non-linear (microplane) material model for quasi-brittle materials is described along with the solution strategy employed. It should be mentioned that the soil is treated within this thesis as drained linear elastic medium. The method of coupling the CIFECM into the dynamic equation of force equilibrium for both directly applied and transmitted loading regimes is detailed. Application of the code follows in chapter 5; firstly by introducing the simplest test problem of one finite element coupled with one CIFECM element to model a surface foundation. Comparisons are made between the dynamic displacements resulting from the method and standard FE solutions obtained from the use of extended meshes and fixed boundary conditions, along with a study of the influence input variables. Following these examples a larger (more realistic) engineering problem is tacked involving the simulation of an aircraft impact on a reinforced concrete nuclear containment vessel. This represents the first use of the method in a 3-D nonlinear structural analysis problem. The results illustrate the practical implications of including DSSI in the analysis. III In chapter 6, a series of general observations on the method are made with an assessment of its value together with a discussion on its wider application to other engineering fields. Possible future developments to make the method more computationally efficient are finally suggested.

The thesis is concerned with investigating the role of mathematical programming in expressing the general theory of and facilitating effective computation for elasto-plastic skeletal structures subjected to deterministic sources of dynamic excitations, Static-kinematic duality, a common feature in the static analysis of structures, is extended to dynamic systems through the adoption of d'Alembert's principle. This allows the full use of graph theoretic methods for describing the fundamental structural relations in both mesh and nodal forms. For structures whose dynamic characteristics can be effectively described by a rigid-plastic constitutive law, mathematical programming formulations are presented. They are compared and contrasted with existing formulations, especially those associated with impact loading. Elasto-plastic structures are studied and their dynamic response is shown to be given by the solution of a differential linear complementarity problem. Four equivalent formulations are presented and are solved numerically through the use of direct integration methods. The effects of change of geometry may also be important in the dynamic analysis of structures. Firstly, for relatively small displacements, the method of fictitious forces is shown to lead to alternative mesh and nodal formulations. For large displacements, only the nodal method proves to be effective. An incremental differential linear complementarity problem is obtained and a suitable numerical solution procedure is proposed. Finally, a perturbation technique is established for solving the resulting differential equations and differential linear complementarity problems. It is proved that this technique is more general and flexible than the direct integration methods.

Firstly rather accurate approximations to the p value functions of the common Standard CUSUM test and the OLS-based CUSUM test for structural change are derived. Secondly alternative boundaries for both tests are suggested and their properties are examined by simulation of expected p values. It turns out that the power of the OLS-based CUSUM test for early and late structural changes can be improved, whereas this weakness of the Standard CUSUM test cannot be repaired by the new boundaries. / Series: Working Papers SFB "Adaptive Information Systems and Modelling in Economics and Management Science"

Concrete is still the most widely used construction material of modern times. In very recent years attempts have been made by using steel fibre reinforcement to improve the inherent weaknesses that concrete possesses such as its low tensile strength and the tendency to shrink on drying and to creep under stress. In this context, the use of steel fibre reinforcement together with high strength concrete corbel joints has been investigated. This study came after fibre reinforced concrete had received wide recognition for its crack and deformation control, ductility and energy absorption characteristics. In the present study, the fracture behaviour and deformation characteristics of plain conventionally reinforced concrete corbels with and without steel fibre reinforcement has been investigated. The different types of steel fibres used and other experimental materials are described in chapter 3, whereas chapter 2 gives a review of the old and current design approaches used for concrete corbel design. In chapter 4 the deformation, cracking and ultimate strength of plain high strength concrete corbels has been studied with different cube strength ranged between 25 to 90 N/mm². In chapter 5 a proposed theory to predict the ultimate strength of high and normal strength concrete corbels, conventionally reinforced, has been derived. The influence of steel fibre reinforcement on the performance of conventionally reinforced concrete corbels has been studied in chapter 6. Melt extract steel fibres were used in the majority of the corbels together with other types such as crimped, hooked and plastic fibres (polypropylene). In the same chapter 6, the theory has been extended to account for the strength gained by fibre addition. The effect of steel fibre reinforcement on the shear transfer strength has been studied in chapter 7. The theory proposed in chapter 5 has been further extended to predict the shear strength of 'push-off' type of specimens of plain and fibre reinforced concrete, with conventional steel reinforcement.

Interferon (IFN) regulatory factor family member 4 (IRF4) is a transcription factor that serves specific roles in transcriptional regulation of IFN responsive genes and is indispensable in B- & T-cell differentiation. IRF4 like the other members of the family has two major domains- the N-terminal DNA binding domain (DBD) essential for its recognition and binding to the Interferon Stimulated Response Element DNA sequence and a C-terminal Interferon activation domain (IAD) thought to maintain IRF4 in an auto-inhibited inactive state and is also critical in its activation. A putative unstructured linker connects the DBD and IAD. Activation in most members of the IRF family requires phosphorylation to induce homo and hetero-dimerization. In contrast, IRF4 functions primarily through ternary complex formation involving different proteins including PU.1 and MyD88. The IRF4IAD has a C-terminal auto-inhibitory region (AIR) that has been proposed to physically impede the DBD from interacting with DNA in the absence of its binding partner. To understand the activation mechanism in molecular detail we determined the crystal structure of the IAD of IRF4 and also performed small-angle X-ray scattering (SAXS) studies. Our data reveals that the surface electrostatics of IAD and presence of additional loops confers exclusivity to IRF4 in the IRF family. SAXS studies suggest that the AIR is structured and makes interactions with the putative linker. We also performed analytical ultracentrifugation studies, fluorescence anisotropy binding experiments and SAXS studies on full-length IRF4 as well as on constructs where the first 20 residues, exclusive to IRF4 or the AIR were removed. We observe that the first 20 residues are critical in decreasing the binding affinity of full-length IRF4 to DNA. In addition, the putative linker of IRF4 connecting the N- and C-termini appears to be a folded domain and interacts with AIR. Also, overall full-length IRF4 appears as an elongated molecule and the N- and the C-terminal domains are arranged on either ends of full-length IRF4. Moreover, there are no signs of huge conformational changes in the protein during the activation process. Taken together, based on our data we propose that there is no auto-inhibited state for IRF4. Furthermore, it is the binding affinity of full-length IRF4 that is increased in the presence of its binding partner most likely through modest conformational changes.

In summary, the main points to support the view that dextran synthesis is unessential to the dental caries process are presented below: 1 . Dextran-fed, microbe-bearing rats do not experience dental caries. 2 . Lactobacillus casei 4646 has no extracellular dextransucrase enzyme or other extracellular enzymes which release glucose or fructose from sucrose . 3. L. casei 4646 does not synthesize extracellular polysaccharide of the dextran class when grown on sucrose-containing medium. 4. L. casei 4646 does not exhibit in vitro, adherent growth on stainless-steel wire when grown on sucrose-containing medium . 5. L. casei 4646 does not cause dental plaque formation on the molars of sucrose-fed gnotobiotic rats (Rosen, Lenney, and O'Malley, 1968). 6. L. casei 4646 induces dental caries of the pit and fissure class when introduced as a monoinfecting agent in sucrose-fed gnotobiotic rats (Rosen, Lenney, and O'Malley, 1968). These observations advocated rejection of the hypothesis that extracellular dextran is an absolute requirement in the dental caries process. Therefore, dextranase will be an effective anticaries agent only against organisms such as Streptococcus E-49 (Fitzgerald, Keyes, Stoudt, and Spinell, 1968) which depend on the synthesis of dextran from sucrose for tooth colonization. This position predicts that dextranase treatment of sucrose-fed gnotobiotic rats infected with Lactobacillus casei 4646 would have no effect on the rats' caries experience.

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