Up to a decade ago fixed offshore platforms in the Gulf of Mexico were inspected below the water line only after extreme loading from hurricanes or ship collisions. However. increased investment in new platforms has led to oil companies introducing regular inspection routines. and some countries have laid down regulations for instrumentation and inspection of offshore install~ tions. to ensure safe operation and a feedback of data to designers. The conventional techniques used for inspection of offshore structures include: (a) visual inspection by divers and use of underwater television with video and sound recording; (b) magnetic particle and ultrasonic crack detection. methods. These all require diver access. and are feasible in moderate water depths and in areas with long and predictable periods of calm weather. Neither of these conditions is met in the northern North Sea. There is a demand for inspection methods which will monitor the performance of platforms under any weather conditions. and. in particular under storm conditions. The methods must give unambiguous warning about failures which could endanger the installations. Furthermore. they should depend on only a small number of transducers all positioned above the waterline for easy access and maintenance. 1 The purpose of this work was to investigate a method of detecting damage from changes in the natural frequencies of a structure. The main objective was to investigate what changes in natural frequencies would occur when a primary load-carrying memner became detached from a typical fixed offshore platform. The platform design was a four-leg. K-braced platform for which approximate dimensions were available. and which operated in 70 metres of water. A 1/20th scale model was also designed and the structural analysis programme ICES STRUDL II used to compute the natural frequencies and mode shapes of both the full scale and model platforms. Before STRUDL II was applied to the quite complex offshore platform. analyses were carried out on two simple structures to gain expe:ience in using the programme. Natural frequencies and mode shapes were computed and compared with measured values and with analytical solutions where applicable. This preliminary work also gave experience in dynamic response testing. The sensitivity of the natural frequencies of the platform was investigated by removing a single diagonal in one K-panel. and again computing the natural frequencies of the structure. The brace was then replaced and the computations were repeated with other single braces removed in turn. The reductions in the frequencies with each member removed could then be calculated. There were large changes in some of the modes and the pattern of the frequency reductions gave an indication of the location of the f~ilure. 2 The computational study was repeated for the model platform and frequency reductions similar to those for the full scale platform were found. The model platform was built and sinusoidal force excitation was applied to determine its natural frequencies andlmode shapes. Comparison of computed and measured frequencies was generally good. Two K-braces in different planes and at different elevations were severed and repaired in turn; the changes in natural frequencies compared well with the computed predictions. In the third stages of the project a dedicated Time Series Analyser became available and a number of tests were carried out with random excitation. The results for frequencies and mode shapes were in good agreement with those obtained with sinusoidal excitation. This demonstrated the feasibility of obtaining natural frequencies and mode shapes from response to ~andoM excitation. It was concluded for the project that changes in natural frequencies could provide a viable method of detecting gross damage in offshore structures. 3

Strengthening of existing structures has become an important aspect of civil engineering. Various methods exist and have been developed in the last few decades one of which has become increasingly popular- strengthening using fibre reinforced polymers. Their excellent mechanical properties and resistance to different environmental conditions make them a viable alternative to the traditional materials like concrete and steel. In this study the effect of elevated temperatures on the behaviour of FRP strengthened reinforced concrete beams is investigated. The behaviour of reinforced concrete beams strengthened with FRF laminates has been investigated by different researchers in the last two decades. Ultimate load, failure modes and improved techniques for strengthening have been the main areas of interest. As a result of the extensive research several classifications and design guidelines have been proposed to ensure better performance and prevent premature failures of the systems. One important aspect of the strengthening is the susceptibility of polymers to significant and rapid reduction of their strength when exposed to elevated temperatures and fire. Little research has been done up-todate on the residual properties of FRP strengthened systems after heating and cooling. Three experiments have been conducted for the purpose of this study. The bond strength of CFRP strengthened systems is first investigated for temperature range of 20°C to 300°C. The behaviour of small-scale strengthened beams during the heating process is presented next. The third experiment is then designed to determine the residual flexural capacity of the heated and cooled minibeams. The results are presented and compared indicating residual strength of the systems to 30% compared to their performance at room temperature

Presented in this thesis is an evaluation of a novel box beam system of Pul- truded Fibre Reinforced Polymer (PFRP) shapes. The flat-pack modular beam system consists of separate PFRP flange and web shapes joined together with a new method of mechanical fastening. It is based on the first generation Star- tlink building system, conceived by UK engineers in 1999. The Startlink building system is introduced, and classified within the scope of Modern Methods of Con- struction (MMC), and its merits are discussed. In the context of MMC a critical review by the author finds that, although the proposed 1999 generation Startlink system offers design flexibility, it will probably have a limited market potential. The novel use of the steel MlO Unistrut connection method as a means of fastening distinct PFRP shapes in a building system is characterised. Individ- ual connector design parameters for joint stiffness and resistance are identified and determined, under pure shear loading. The results of a series of physical tests show no significant loss of stiffness or strength with long term environmen- tal exposure. Values of key mechanical properties for design calculations are recommended. A 400 x 200 x 2848 mm prototype PFRP box beam assembly is fabricated from two flange and two web panel-type shapes, cut from existing off-the-shelf PFRP shapes. This is 60 mm deeper than the largest single PFRP shape that could be used as a beam. The assembly is joined at the web-flange junction with M10 Unistrut connectors set at various spacing's, in the range 50 to 400 mm. These connectors carry the longitudinal shear that is generated between the joined shapes when the modular assembly is in flexure, Theoretical deflections, cal- culated using a modified form of a partial-interaction analysis model developed for composite concrete and steel structures, are predicted for the assembly ac- counting for the finite shear stiffness of the web-flange connection. A series of 16 four-point bending load tests on the beam assembly, across two load arrange- ments, show that its performance is linked to the designated spacing of the M10 Unistrut connections. The flexural rigidity and degree of interaction present in the assembly are determined from analysis of vertical deflections and longitudi- nal strains, as the beam is deformed. The influence of secondary effects, due to the poor tolerances achieved in the hand fabrication of the beam's assembly, are found to greatly affect the ability of the deflection analysis to give the required measured deflections. Comparison of the effective joint shear rigidities obtained from theory and testing indicates a higher individual connection stiffness in the prototype beam than previously determined by way of the individual Unistrut connector characterisation. It is found through the combined analytical and physical testing research that the M10 Unistrut connection method can only provide the necessary joint shear stiffness and resistance to the 400 mm deep beam if the connector spacing, along the four joints, is ≤ 50 mm. The total number of connectors this represents in the beam is likely to make this modular construction approach too expensive for it to be commercially viable. Although the M10 connector could be used to fabricate beams of lesser depths, since the number of connectors will then be reduced, these beams would find it difficult to compete with the available off-the-shelf PFRP beam shapes, of up to 300 mm deep. There is however scope to use the Unistrut method of connection to provide longitudinal shear resistance in building systems where, for example, a floor panel is to be stiffened by a channel shaped beam and the overall depth is ≤ 300 mm. The research work contained in this thesis has contributed to a radical change in the PFRP product offerings now proposed in the 2006 generation Startlink building system.

For the development of performance based design on a proper scientific basis the use of the concept of risk is inevitable. However, the application of this concept to actual structural design is not simple because of the large ranges of probability and consequences of events which exist. This is compounded by a plethora of different actions that can be taken to reduce the probabilities of the events and also the magnitude of the consequences. It is the reduction in the magnitude of these consequences which is essentially the goal of design. This work aims to address the challenges posed by the application of the concepts of performance based design for structures in fire. Simple methodologies have been developed for the assessment of the consequences of an extreme event. These methodologies are based upon fundamental behaviour of structures in fire. A methodology has been developed which can be used to assess the capacity/deflection behaviour through the complete thermal deflection of floor slabs. This takes into account positive effects on the capacity of floor slabs of the membrane stress at the slabs boundaries at low deflections as well as the final capacity provided by the tensile membrane action of the reinforcement mesh at high deflections. For vertical stability of structures in fire, analytical equations to describe the behaviour of floor systems at the perimeter of a building are developed. From these equations, the resulting pull-in forces on external columns can be calculated as well as the resulting horizontal load applied to the column. From this, a simple stability assessment is proposed which can be used to assess the consequences of multiple floor fires on tall buildings. These analytical methodologies are brought together in a risk based frame- work for structural design which can be used to identify areas in a building or structural components which pose a high residual risk. These elements can be qualitatively ’ranked’ according to their relative risk and appropriate measures taken to reduce the risk to an acceptable level. The framework is illustrated via 2 case studies. The first is of a typical small office building, and the second is of a prestige office development.

Barely Visible Impact Damage (BVID) can occur when laminated composite material is subject to impact, i.e. from runway debris or dropped tools, and may result in a significant reduction in the compressive strength of composite structures. A component containing BVID subjected to compression may fail via a number of mechanisms. However, it is assumed that the impact damage problems to be modelled will fail by delamination buckling leading to propagation of damage away from the original site. This precludes problems where the initial mechanism of failure is via kink banding or buckling of the full laminate. An analytical model is presented, for application to various composite structures, which predicts the level of compressive strain below which growth of BVID following local buckling of a delaminated sublaminate will not occur. The model is capable of predicting the critical through-thickness level for delamination, the stability of delamination growth, the sensitivity to experimental error in geometric measurements of the damage area and additionally establishes properties desirable for laminates optimised for damage tolerance. Problems treated with the model are split into two impact categories; ‘face’ (i.e. an out-of-plane skin impact) and ‘free edge’ (i.e. an in-plane stiffener edge impact) and two compressive loading regimes; ‘static’ and ‘fatigue’. Analytical results for static and fatigue compression of face impacted plates show an agreement of threshold strain to within 4% and 17% of experimental values respectively. In particular, for impacts to the skin under a stiffener subject to static loading the model is accurate to within 5%. An optimised laminate stacking sequence has shown an experimental increase of up to 29% in static strength can be achieved in comparison to a baseline configuration. Finally, compression testing has been undertaken on three coupons in order to validate an analysis of static free edge problems. Analytical results are, on average, within 10% of experimental results. An optimised laminate is theoretically predicted to increase static compression after free edge impact strength by at least 35%.

This research uses a novel construction material (lime-pozzolan concrete) and real-world project (a school) as a vehicle for investigating the implementation or applied-innovation process in construction. The implementation of new technologies at a product-level is recognised to be an antecedent of technological change in the construction industry. A ‘real world’ construction project aiming to implement a novel lime-pozzolan concrete in the field, has been used as a process-tracing case study. Rigorous analysis of this case study project, expressly focusing on project-level communication, has shown the implementation of innovative and sustainable materials to be a complex, socio-technical process. With the aim of identifying opportunities to improve project-level design processes in order to support the uptake of innovation and sustainable solutions, twelve high-level theories have been built on twenty-five emergent themes. Collectively these insights demonstrate that implementation processes, once initiated, are experiential, social, contextual, active, interactive, temporal, intentional and mutually constituted phenomena. On the strength of empirical findings this thesis argues for a radical shift in managerial attention from the outcome of the process to the process itself; specifically focused on the experience of the design team as process participants. Laboratory testing and initial field trials have demonstrated the technical feasibility of producing structural grade lime-pozzolan concretes with 28-day compressive strengths of up to 50MPa. The lime-pozzolan concretes were ternary combinations of hydraulic lime (NHL5), ground granulated blastfurnace slag (GGBS) and silica fume (SF). The use of NHL5 in conjunction with pozzolanic materials has been shown to be a viable ‘low-carbon’ alternative to CEMI or CEMIII/A in certain circumstances, although this work has demonstrated that the potential savings in the embodied CO2 and energy of lime-pozzolan concretes are highly dependent on the boundaries of the analysis. Moreover the potential for lime-pozzolan concrete with a lower still CO2 and energy intensity than any concretes tested to date has been identified.

The research covered in this thesis is concerned with the effects of the behaviour and load carrying capacities of classes of steel beam structures with various shapes of web openings. A comprehensive investigation on non-composite and partially concrete encased perforated steel Isections with large web openings positioned along the centre-line of the beams was undertaken. This thesis enhances the current knowledge on these classes of perforated beams, as previous research has shown that these beams are susceptible to various failure modes, due to the existence of large web openings. Currently, perforated steel beams with large web openings are utilised in most engineering applications such as infrastructure, ship building and aeronautical engineering. The most significant benefits of using such beams are the achievement of reductions in weight and accommodation of services within their structural depth of floor systems. Specifically, in building applications, service integration eliminates the internal columns and supports, produces lighter structures which leading to reduced construction and installation time and results in cost effective structural forms and uses. However, many uncertainties are associated with perforated beams as well as non-standard methodology is used for their assessment. Perforated beams with standard circular, hexagonal and elongated web openings are most widely used nowadays, whilst various non-standard web opening shapes, such as `elliptical', are introduced through this thesis for first time. These new pioneering web opening shapes improve the structural performance of the perforated beams when examined under two critical failure modes (i. e. shear-'Vierendeel' mechanism and web-post buckling). Moreover, the manufacturing procedure of the `elliptical' web openings show great advantage in comparison with the manufacturing way of the more popular perforated beams with circular web openings (i. e. cellular beams). Also, other web opening shapes are reported and examined in this thesis. Furthermore, the novelty of the work seems to consist of the treatment of web openings of somewhat greater web opening depth than those usually considered and the introduction of a new class of composite concrete-steel beam. Despite the abundant experimental work on perforated steel beams that has been conducted by researchers throughout the years, the results are not comprehensive, due to the complexity of the beam configuration and the large number of variable parameters. Therefore, using commercially available finite element (FE) software, numerical analyses were verified by comparison to a new experimental programme designed to test each of the new structural forms. The numerical programme was then used to undertake extensive parametric studies to isolate some of the geometric and material properties that influence the failure modes associated with each of the new forms of structural systems. The main parameters under consideration are the web opening depth (noted usually as diameter), the critical opening length of the top and bottom tee-sections, the web opening spacing, the steel flange and web thicknesses, the concrete strength and contact properties between the steel and concrete of the newly formed composite beams. Detailed study of plastic hinges formation (i. e. high stress concentrations) was also employed in the vicinity of the web openings, by conducting both experimental and finite element (FE) investigation. This research study should now lead to better management of the use of perforated beams with large web openings as the profound difference between the novel and the conventional perforated beams is demonstrated. Useful practical applications of the so-called structural forms would be of particular interest in the general engineering, not just because of their superior structural performance, but also because of their low cost in manufacturing and usage. Another contribution is the investigation of the partial steel encasement with the concrete in-fill, on the percentage of enhancement of the steel perforated beams with web openings under high shear forces as well as on the distinction which is drawn between the conventional and the new composite beams. Finally, a further indirect outcome of this research thesis is the excellent agreement between the experimental and FE analyses as well as the data that can be used by future researchers to widen the above research to various engineering applications.

In this thesis a theoretical analysis is presented for statically loaded structural hollow section (SiuS) lattice girder joints having one compression bracing member and one tension bracing member welded to a rectangular hollow section chord member. A set of joint failure modes are established for gapped and overlapped bracings and the research is aimed towards establishing the yield and ultimate loads of such joints with the yield line method as the main analytical tool. The results of 150 joint tests, conducted both in isolation and in complete trusses at testing centres in three different countries, have been used to verify the theories proposed. A study of the parameters which are believed to influence the strength and behaviour of rectangular hollow section joints has also been made. Finally, a computer program has been written in Fortran to provide an automatic assessment of the strength of welded lattice girder joints having a rectangular hollow section chord member and either rectangular or circular bracing members.

In this thesis, the collapse mode of failure of tapered steel beams is examined for three different loading cases. Firstly, for the case when the beam is loaded inside the tip; secondly, when the beam is loaded outside the tip, and thirdly, when the beam is loaded at the tip. The theoretical collapse mode of failure presented here provides an identical collapse load whether obtained from lower or upper bound solutions. Fourteen tests on tapered steel beam specimen were carried out to examine the collapse modes of failure and their ultimate strengths for steel tapered beams loaded inside, outside and at the tip. The experimental collapse loads and their collapse modes of failure are compared with theoretically predicted collapse loads and the proposed collapse mechanisms respectively. The first elastic yield load of the tapered web panels is assessed on the basis of 'circular Arc Theory' simplified by Davies et al. (6) and is compared with the predicted theoretical collapse loads. Conclusions are drawn relating to the plastic collapse modes of failure and ultimate strengths of tapered steel beam.

This Thesis describes the development of an analysis for inelastic columns, with cross-sections composed of one or more materials, loaded with axial load and biaxial moments. The column can have both rotational and directional restraints at its ends. The analysis has been programmed for a computer and subsequently tested against published results for steel columns, reinforced concrete columns, and concrete-encased steel composite columns and shown to give good agreement. A test rig with an axial load capacity of 2MN and capable of testing full-scale columns of any practical length has been designed and built. Columns with elastic and elastic-plastic rotational restraints or, pin-ends or any combination can be tested and column end-moments of up to 50 kNm can be applied through the beams. One important feature of the test rig is sets of crossed knife-edges, which give both major and minor axis rotational freedom and thus allow accurate positioning of the axial load. Eight elastically restrained concrete-encased steel composite no-sway columns have been tested, three with biaxial restraints and loadings, using the rig. The results from the tests have been compared with predictions using the computer program and agreement between the observed and predicted results for ultimate loads, deflections, and end-moments is good. The behaviour of column lengths within rigid-Jointed no-sway frames with both plastically and elastically designed beams has been studied. For the case of a column with elastic restraints design proposals have been checked and shown to be conservative. When the beams are designed plastically it is recommended that a conservative approach should be adopted until further research has been carried out into this topic.