published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Civil Engineering / Doctoral / Doctor of Philosophy

Over the last twenty years there has been a dramatic rise in subsidence claims for residential buildings. This has led to growing concern amongst insurance companies and the structural engineers who investigate these cases on their behalf. Their attention has begun to focus more on the engineering management of these cases. A review of the mechanism of subsidence damage to residential buildings and existing subsidence management procedures has shown that existing management procedures are often ad-hoc and uncoordinated, have been criticised for being inadequate, and have lead to solutions which have either been inappropriate or have failed to solve the problem. The review concludes that there is major scope for improving the decision-making process, by ensuring that it is based on a sound technical framework. This thesis describes an innovative approach to the engineering management of subsidence cases based on knowledge-based system (KBS) techniques. The Subsidence CAse Management System (SCAMS) is applicable to the multi-task domain and consists of three main components which address important facets of subsidence management. These are the diagnosis of a subsidence problem, the choice of an appropriate course of investigations, and the specification of effective remedial measures. SCAMS was developed using a KBS building shell- Kappa PC. The data structure adopted was object-oriented and incorporates production rules. The knowledge acquisition was based on multiple sources of knowledge using a variety of techniques. The system implementation was designed to result in a user-friendly system, and incorporates a highly graphical interface. Evaluation of the system was undertaken using test cases from industry. The system proved highly effective in all the tests and, in some cases, achieved more accurate results than the human expert. SCAMS represents a significant advance over existing approaches to the engmeenng management of subsidence cases. It provides many benefits to the construction industry including greater consistency in the evaluation of subsidence damage, improved quality of engineering judgement, reduced repair costs and specific guidance to -engineers for individual cases.

Precast, prestressed concrete panels (PCPs) are used in 85% of bridges in Texas. The goal of this thesis is to reduce collinear cracking (cracks propagating parallel to strands) in PCPs. One way to reduce collinear cracking would be to reduce the initial prestress force. In design, TxDOT conservatively assumes total prestress losses of 45 ksi. Based on eight panel specimens, instrumented and fabricated at two different precast plants in Texas, actual prestress losses were measured as at most 25 ksi. This difference (about 20 ksi) is consistent with a reduction in initial prestress force from 16.1 kips per strand to 14.4 kips per strand. Another way to reduce collinear cracking would be to provide additional transverse reinforcement in the end regions of the panels. By comparing crack spacings and crack widths in current and modified panel specimens, it was found that additional reinforcement consisting of one or two #3 bars placed transverse to strands at panel ends would effectively control collinear cracking in PCPs. / text

The main objectives of the project were to investigate the interactions of carbon, titanium and aluminium contents, grain size and cold working, with precipitation reactions which occur during heat treatments similar to those experienced as a consequence of commercial PWR Steam Generator (SG) tube processing. In order to carry out this investigation commercial material was supplemented by a range of experimental casts. The selected casts allowed the investigation of the following compositional variables: (i) the effect of C in alloys free from Ti or Al, (ii) the effect of C in alloys containing both Ti and Al, and (iii) the effect of T! in alloys containing constant C and Al contents. As a result of the experimental programme considerable progress has been made in clarifying the complex structure-property relationships which occur in Alloy 600. The microstructures observed have been characterised and understood in terms of thermal and mechanical treatments. Studies involving analysis of the mechanical properties, have led to a clear understanding of the effects of grain size, precipitate type and distribution and residual cold work on mechanical behaviour. It has been shown that the activation energy for normal grain growth increases with increasing carbon and titanium contents. Explanations of the mechanical properties have been discussed in terms of grain size, dislocation hardening and solid solution hardening and it has been possible to understand the overall material properties with respect to these. The kinetics of precipitation have been studied in depth, since it is the precipitation of chromium carbide which ultimately results in the principle mode of material degradation, namely, intergranular corrosion. A range of carbide precipitation 'C-curves' have been established and related to thermal and mechanical processing. Accelerated corrosion testing has provided an insight into the relationship between precipitation and structure on the stability of material in potentially corrosive environments.

The dynamic response of subsurface cracks in fiber reinforced composites is analytically studied. The response of layered half-space and three-layered plate with two interface cracks excited by a plane SH-wave and line load respectively are studied by formulating the problem as integral equations in the frequency domain. The governing equations along with boundary, regularity and continuity conditions across the interface are reduced to a coupled set of singular integral equations by using Betti's reciprocal theorem along with the Green's functions. In addition, the transient response of an orthotropic half-space with a subsurface crack subjected to inplane line load at an arbitrary angle is analyzed. Two new Green's functions for the uncracked medium are developed and used along with the representation theorem to derive the scattered field. Satisfaction of the traction free condition at the crack surfaces gives rise to a system of singular integral equations. Singular integrals involved in the analysis are computed numerically by removing the poles. Part of the integrals containing the poles are then obtained analytically by using residue theorem. The solution of singular integral equations are obtained by expanding the unknown crack opening displacements (COD) in terms of a complete set of Chebychev polynomials. The problem is first solved in the frequency domain, the time histories are then obtained numerically by inverting the spectra via Fast Fourier Transform (FFT) routine. Numerical results are presented for isotropic and anisotropic materials for several different crack geometries. The results show significant influence of crack geometries and material properties on the COD and surface response of composites.

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