When subjected to a combination of moment and shear force, a reinforced concrete (RC) beam with either little or no transverse reinforcement can fail in shear before reaching its full flexural strength. This type of failure is sudden in nature and usually disastrous because it does not give sufficient warning prior to collapse. To prevent this type of shear failure, reinforced concrete beams are traditionally reinforced with stirrups. However, the use of stirrups is not always cost effective since it increases labor costs, and can make casting concrete difficult in situations where closely-spaced stirrups are required. The use of steel fiber reinforced concrete (SFRC) could be considered as a potential alternative to the use of traditional shear reinforcement. Concrete is very weak and brittle in tension, SFRC transforms this behaviour and improves the diagonal tension capacity of concrete and thus can result in significant enhancements in shear capacity. However, one of the drawbacks associated with SFRC is that the addition of fibers to a regular concrete mix can cause problems in workability. The use of self-consolidating concrete (SCC) is an innovative solution to this problem and can result in improved workability when fibers are added to the mix. The thesis presents the experimental results from tests on twelve slender self-consolidating fiber reinforced concrete (SCFRC) beams tested under four-point loading. The results demonstrate the combined use of SCC and steel fibers can improve the shear resistance of reinforced concrete beams, enhance crack control and can promote flexural ductility. Despite extensive research, there is a lack of accurate and reliable design guidelines for the use of SFRC in beams. This study presents a rational model which can accurately predict the shear resistance of steel fiber reinforced concrete beams. The thesis also proposes a safe and reliable equation which can be used for the shear design of SFRC beams.

No description available.

The quantitative description of rough fracture surfaces of concrete has been an important challenge for many years. Looking at the fracture surface of a concrete specimen, one realizes that the self-affine geometry of crack faces results from the stochastic nature of the crack growth. This is due to the heterogeneous nature of concrete that makes the crack tortuous leading its way through weak bonds, voids, mortar and getting arrested on encountering a hard aggregate forming crack face bridges. These mechanisms contribute to the tendency of the crack to follow a tortuous path. The self-similarity contained in the tortuous fracture surface of concrete makes it an ideal candidate to be considered as a fractal. Further, the softening response itself has been treated as a singular fractal function by earlier investigators. The very process of cracking and microcracking, could be considered very close to the stick and slip process and therefore as a fractal. Therefore modeling a crack as a fractal and characterizing it by a fractal dimension have become the focus of research in recent years. Due to randomly distributed discontinuous flaws and high heterogeneity of the internal structure of concrete, mechanical properties also randomly vary. Under the effect of the same external force, the stress intensity factors to which different points in the concrete are subjected are different. Hence the microcracks induced by the external force are distributed discontinuously and randomly. Therefore in the present study the effect of the random nature of the microcracks in the fracture process zone of concrete is investigated using both fractal and probabilistic approach. The most probable fractal dimension of a network of micro cracks is obtained as a function of the branching angle ‘α’ of the microcracks, considered as a random variable. Further, an ensemble of cracks is synthetically generated using Monte Carlo technique imposing a constraint that the random deviations do not exceed the maximum size of the aggregate. Such tortuous cracks are analyzed by extending Fictitious Crack Model (FCM) proposed by Hillerborg et al [37]. A numerical study is carried out to examine the influence of certain important fracture parameters on the beam response of plain concrete beams. The contents of this thesis are organized in seven chapters with references at the end. Chapter-1 summarizes the historical development of fracture mechanics. A brief review of the basic concepts of fracture mechanics theory is presented. In chapter-2 a brief review of literature on fracture mechanics of concrete is presented. An overview of the analytical models, numerical models and fractal models till date has been presented in a systematic way. In chapter-3 the fracture processs zone has been modeled as a fractal following the work of Ji et al [118]. The contribution here has been to improve the work of Ji et al [118] (which considers the region of microcracks as a fractal tree) by considering the branching angle as a random variable. Mean fractal dimension thus obtained is found to match well with the experimental results available in the literature. In chapter-4 FCM, as proposed by Hillerborg et al [37] has been modified to be applicable to cracks with varying inclined faces by considering both horizontal and vertical components of the closing forces. The theoretical aspects of the modified FCM have been described in detail. The procedure for the determination of influence co- efficient matrices for a random tortuous crack in mode-I and mixed-mode along with a fractal crack has been explained. In the subsequent chapters the study has been taken up in two parts. In the first part only one generator of the fractal tree considered by Ji et al [118] has been analyzed by FCM to obtain load-deformation responses and fracture energy. In part two, a random tortuous crack, as already defined earlier has been analyzed both in mode-I and mixed mode using FCM. In chapter-5 plain concrete beams with one generator of fractal tree has been analyzed. The influence of the branching angle on the post-peak response of (P-δ) curves and fracture energy has been obtained. In chapter-6 a random tortuous crack has been analyzed in mode-I by FCM. The analysis reveals the influence of maximum aggregate size upon the pre and post-peak behaviour in support of the experimental findings. The nominal stress at peak is found to depend on the characteristic dimension of the structure thereby confirming the size effect. Further fracture energy values have been obtained by the work of fracture method and the results show good agreement with the results obtained in the literature. In chapter-7 a random tortuous crack has been analyzed in mixed mode by FCM. While modeling, symmetry has been assumed only to facilitate computational work though it is known that loss of symmetry affects the peak load. However analysis of the whole beam can be handled by the code developed in the thesis In chapter-8 a summary of the research work is presented along with a list of major observations and references at the end.

Until recently, the hot-rolled steel members have been recognized as the most popular and widely used steel group, but in recent times, the use of cold-formed high strength steel members has rapidly increased. However, the structural behavior of light gauge high strength cold-formed steel members characterized by various buckling modes is not yet fully understood. The current cold-formed steel sections such as C- and Z-sections are commonly used because of their simple forming procedures and easy connections, but they suffer from certain buckling modes. It is therefore important that these buckling modes are either delayed or eliminated to increase the ultimate capacity of these members. This research is therefore aimed at developing a new cold-formed steel beam with two torsionally rigid rectangular hollow flanges and a slender web formed using intermittent screw fastening to enhance the flexural capacity while maintaining a minimum fabrication cost. This thesis describes a detailed investigation into the structural behavior of this new Rectangular Hollow Flange Beam (RHFB), subjected to flexural action The first phase of this research included experimental investigations using thirty full scale lateral buckling tests and twenty two section moment capacity tests using specially designed test rigs to simulate the required loading and support conditions. A detailed description of the experimental methods, RHFB failure modes including local, lateral distortional and lateral torsional buckling modes, and moment capacity results is presented. A comparison of experimental results with the predictions from the current design rules and other design methods is also given. The second phase of this research involved a methodical and comprehensive investigation aimed at widening the scope of finite element analysis to investigate the buckling and ultimate failure behaviours of RHFBs subjected to flexural actions. Accurate finite element models simulating the physical conditions of both lateral buckling and section moment capacity tests were developed. Comparison of experimental and finite element analysis results showed that the buckling and ultimate failure behaviour of RHFBs can be simulated well using appropriate finite element models. Finite element models simulating ideal simply supported boundary conditions and a uniform moment loading were also developed in order to use in a detailed parametric study. The parametric study results were used to review the current design rules and to develop new design formulae for RHFBs subjected to local, lateral distortional and lateral torsional buckling effects. Finite element analysis results indicate that the discontinuity due to screw fastening has a noticeable influence only for members in the intermediate slenderness region. Investigations into different combinations of thicknesses in the flange and web indicate that increasing the flange thickness is more effective than web thickness in enhancing the flexural capacity of RHFBs. The current steel design standards, AS 4100 (1998) and AS/NZS 4600 (1996) are found sufficient to predict the section moment capacity of RHFBs. However, the results indicate that the AS/NZS 4600 is more accurate for slender sections whereas AS 4100 is more accurate for compact sections. The finite element analysis results further indicate that the current design rules given in AS/NZS 4600 is adequate in predicting the member moment capacity of RHFBs subject to lateral torsional buckling effects. However, they were inadequate in predicting the capacities of RHFBs subject to lateral distortional buckling effects. This thesis has therefore developed a new design formula to predict the lateral distortional buckling strength of RHFBs. Overall, this thesis has demonstrated that the innovative RHFB sections can perform well as economically and structurally efficient flexural members. Structural engineers and designers should make use of the new design rules and the validated existing design rules to design the most optimum RHFB sections depending on the type of applications. Intermittent screw fastening method has also been shown to be structurally adequate that also minimises the fabrication cost. Product manufacturers and builders should be able to make use of this in their applications.

Made available in DSpace on 2015-03-26T13:28:09Z (GMT). No. of bitstreams: 1 texto completo.pdf: 8690976 bytes, checksum: 44c939a6112d27b6f8696be3efc2044f (MD5) Previous issue date: 2012-03-16 / The growth of steel construction in Brazil in recent years has naturally been accompanied by an increasing demand for resources that support the entire design process of steel structures, from its conception to its realization. In this context, there has been a resurgence of interest in alveolar beams, due to technical and aesthetic advantages that these elements offer, motivated in part by beginning production of steel rolled I profiles in Brazil, from 2002. Generally, the alveolar beams are manufactured from rolled steel I-profiles, by cutting the web along, according to a sinuous symmetrical line so that the halves so obtained can be displaced relative to one another and welded, originating a new beam with a series of web holes, with the same mass as the original profile, but with greater stiffness in the plane of bending, suitable for large spans. Although the alveolar beams are an old solution, abandoned due to rising manufacturing costs in a given time, most recently returned to appear as a competitive solution, due to technological advances in automation of cutting and welding of structural steel. Given the wide range of possibilities available today for the manufacture of alveolar beams, facilitated by the resources of design and computer aided manufacturing, this study aimed to procedures to define the geometric characteristics of alveolar steel beams to floor systems and roof systems, due to the lack of technical updates on this topic. Finally, we present a series of examples of applying the criteria and procedures proposed for various possible situations in the usual structures. / A expansão do segmento da construção metálica no Brasil tem sido acompanhada de uma demanda crescente por recursos que subsidiem todo processo de projeto de estruturas de aço, desde sua concepção até sua materialização. Nesse contexto, observa-se o ressurgimento do interesse pelas vigas alveolares, em função das vantagens técnicas e estéticas que esses elementos oferecem, em parte motivado pelo início da produção de perfis I de aço laminados no Brasil, a partir de 2002. Geralmente, as vigas alveolares são fabricadas a partir de perfis I de aço laminados, cortando-se sua alma longitudinalmente, segundo um traçado sinuoso simétrico, de modo que as metades obtidas podem ser deslocadas uma em relação à outra e soldadas, dando origem a uma nova viga com uma série de furos na alma, com praticamente a mesma massa do perfil original, porém com maior rigidez no plano de flexão, apropriada para grandes vãos. Embora as vigas alveolares sejam uma solução antiga, abandonada devido ao aumento dos custos de fabricação em determinada época, mais recentemente voltaram a figurar como uma solução competitiva, devido aos avanços tecnológicos em automação de corte e solda de aço estrutural. Tendo em vista a ampla gama de possibilidades atualmente existente para a fabricação de vigas alveolares, facilitadas pelos recursos de projeto e fabricação assistidos por computador, este trabalho teve como objetivo desenvolver critérios e procedimentos para o prédimensionamento de vigas alveolares de aço, em função da carência de prescrições técnicas atualizadas sobre o tema. Ao final, é apresentada uma série de exemplos de aplicação dos critérios e procedimentos propostos, para diversas situações possíveis nas estruturas de edificações estruturadas em aço.

Thesis (PhD)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: This study involves the analysis of crystallization and melting behavior of polymers in solution, using Solution crystallization by laser light scattering (Scalls) as analytical tool. Various other techniques such as differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR) and dynamic light scattering (DLS) were used for further characterization of the polymer materials. For the first time the solution behavior (crystallization and dissolution kinetics) of two biopolymers namely, poly(lactic acid) (PLA) and poly(butylene succinate) (PBS), was monitored by Scalls. Non-isothermal solution crystallization and dissolution behavior of both enantiomers (D and L) PLA and their blends were studied. It was found that addition of the D- enantiomer (PDLA) to the L- enantiomer (PLLA) in solution resulted in the formation of stereocomplex crystals (SC) and the nucleation-effect of the crystals were intensified with an increase in PDLA content leading to an earlier onset of crystallization and increased crystallization peak area. DSC analysis confirmed the formation of SC during solution crystallization. Overall, results obtained by Scalls provided promising information regarding PLA crystallization kinetics which significantly influences practical applications of this biopolymer. Binary blends consisting of PLLA and PBS were prepared in various blend ratios. Scalls was used to investigate the non-isothermal crystallization and melting behavior of these PLLA/PBS blends from dilute solutions. Addition of PBS up to 30 wt% increased crystallization temperatures of PLLA through a nucleation effect in the molten PBS state. Results indicated that the crystallization rate of PLLA can be improved by additions of small amounts of PBS, and this is of significant interest for practical applications due to the low crystallization rate that PLLA is known to exhibit. DSC thermograms showed an increase in PLLA crystallization exotherm at PBS loadings below 50 wt%, providing further evidence of the enhancement in PLLA crystallization kinetics. A series of linear polyethylene samples with varying molecular weights were studied by Scalls. The effect of molecular weight (Mw) on the solution crystallization temperature (Tc) were monitored and compared to literature where the conventional Crystaf technique was used. Experimental Scalls data correlated well with reported theoretical models. Dissolution studies were also done to study the effect of Mw on the solution melting temperature (Tm) and for the first time, an equation was generated to model solution melting temperatures of linear polyethylenes in Scalls. The solution behavior of a PE-1-octene linear low-density polyethylene (LLDPE) copolymer in a range of solvents including trichlorobenzene (TCB), ortho-dichlorobenzene (o-DCB), decalin and xylene were successfully studied. Scalls allowed for rapid analysis of crystallization and dissolution during a single experiment and offered clearer insight into crystallization kinetics when using different solvents for fractionation. Peak temperatures for analyses in TCB and o-DCB were almost identical but remarkably different to those observed when decalin and xylene was used. The study revealed that solvent type plays an important role in the solution behavior of polymers as well as ultimately the interpretation of these results. / AFRIKAANSE OPSOMMING: Hierdie studie behels die ontleding van kristallisasie en smelt gedrag van polimere in oplossing, met behulp van Oplossing kristallisasie deur laser lig verstrooiing (Scalls) as analitiese instrument. Verskeie ander tegnieke soos differensiële skandeer kalorimetrie (DSC), fourier-tranform infrarooi spektroskopie (FTIR) en dinamiese lig verstrooiing (DLS) was gebruik word vir verdere Karakterisering van die polimeer materiaal. Vir die eerste keer is die oplossingsgedrag (kristallisasie en smelting kinetika) van twee biopolimere naamlik, poli(melksuur) (PLA) en poli(butileen suksinaat) (PBS), deur Scalls gemonitor. Nie-isotermiese oplossing kristallisasie en smelt gedrag van beide enantiomere (D en L) van PLA en hul mengsels was ge-analiseer. Daar is gevind dat byvoeging van die D-enantiomeer (PDLA) tot die L-enantiomeer (PLLA) in oplossing gelei het tot die vorming van stereokompleks kristalle (SC) en die nukleasie-effek van die kristalle is versterk met 'n toename in PDLA inhoud wat gelei het tot 'n vroeër aanvang van kristallisasie en toename in kristallisasie piek-area. DSC analise het die vorming van SC gedurende oplossing kristallisasie bevestig. Algeheel, die resultate wat verkry was deur Scalls het belowende inligting oor PLA kristallisasie kinetika verskaf wat aansienlik praktiese toepassing van hierdie biopolimeer beïnvloed. Binêre mengsels bestaande uit PLLA en PBS was voorberei in verskillende mengsel verhoudings. Scalls was gebruik om die nie-isotermiese kristallisasie en smelt gedrag van hierdie PLLA/PBS mengsels, vir die eerste keer, in verdunde oplossings te ondersoek. Byvoeging van PBS tot 30 wt% het ‘n toename in kristallisasie temperature van PLLA veroorsaak deur 'n nukleasie-effek in die gesmelte PBS toestand. Resultate dui aan dat die tempo van PLLA kristallisasie versnel kan word deur byvoegings van PBS in klein hoeveelhede, en dit is van beduidende belang vir praktiese toepassings weens die lae kristallisasie-tempo waarvoor PLLA bekend is. DSC termogramme het 'n toename in PLLA kristallisasie eksoterm aangedui by PBS inhoud onder 50 wt%, wat 'n verdere bewys is van die verbetering in PLLA kristallisasie kinetika. 'n Reeks lineêre poliëtileen monsters met wisselende molekulêre massas was met Scalls geanaliseer. Die effek van molekulêre massa (Mw) op die oplossing kristallisasie temperatuur (Tc) was gemonitor en vergelyk met literatuur waar die konvensionele Crystaf tegniek gebruik is. Eksperimentele Scalls data het goed gekorreleer met gerapporteerde teoretiese modelle. Smelting studies is ook gedoen om die effek van Mw op die oplossing smelt temperatuur (Tm) te bestudeer en vir die eerste keer, is 'n vergelyking ge-genereer vir die modellering van oplossing smelt temperature van lineêre poliëtileen met behulp van Scalls. Die oplossing-gedrag van ‘n PE-1-okteen lineêre lae-digtheid poliëtileen (LLDPE) kopolimeer was suksesvol bestudeer in 'n verskeidenheid oplosmiddels, insluitend trichlorobenseen (TCB), ortodichlorobenseen (o-DCB) , decalin en xileen. Scalls het toegelaat vir vinnige analise van die kristallisasie en smelting in 'n enkele eksperiment en het dieper insig gegee oor die kristallisasie kinetika tydens fraksionering met die gebruik van verksillend oplosmiddels. Piek temperature vir analises in TCB en o-DCB was byna identies, maar merkwaardig verskillend van dié wat waargeneem was in die geval waar decalin en xileen gebruik was. Hierdie studie het getoon dat die tipe oplosmiddel 'n belangrike rol speel in die oplossing-gedrag van polimere en uiteindelik asook die interpretasie van hierdie resultate. Verder blyk Scalls na ‘n nuttige metode vir die ontleding van polimere in oplossing te wees.

Three projects are described in this thesis that combine microfabrication techniques with optical micromanipulation. The aim of these projects is to use expertise in microlithography and optical tweezing to create new tools for Lab-on-Chip devices. The first project looks at the creation of microgears that can be moved using an optical force. The microgears include one dimensional photonic crystal that creates birefringence. This allows the transfer of angular momentum from a circularly polarised light beam to the microgear, making them spin. The microgears are simulated, fabricated and tested. Possible biological applications are suggested. The second project looks at creating microchannels to perform micromanipulation experiments in. Different methods of fabricating the microfluidic channels are compared, and the resulting chambers are used to find the maximum flow rate an optical sorting experiment can be performed at. The third project involves using a thin photoconductive layer to allow the optical control of an electrical force called dielectrophoresis. This light induced dielectrophoresis (LIDEP) allows similar control to optical tweezing but requires less irradiance than optical tweezing, allowing control over a larger area with the same input optical power. A LIDEP device is created and experiments to measure the electrical trap size that is created with a given optical spot size are performed. These three projects show different microfabrication techniques, and highlight how well suited they are for use in optical manipulation and microfluidic experiments. As the size of objects that can be optically manipulated matches well with the size of objects that can be created with microfabrication, it seems likely that many more interesting applications will develop.

This thesis describes the design and implementation of several novel, nanosecond pulsed, intracavity optical parametric oscillators for the generation of terahertz radiation. The application of the intracavity approach in the context of terahertz optical parametric oscillators has been demonstrated for the first time, and the pump wave energy required was thereby reduced by an order of magnitude. The terahertz wave was tunable from under 1THz up to 3THz with a free running linewidth of ~50GHz and pulse energies up to ~20nJ (pulses were a few nanoseconds in duration). The terahertz beam profile was of Gaussian shape and could be focussed down to 2.3 times the diffraction limited spot size (M² values of 2.3 and 6.7 in the components of the beam parallel and perpendicular to the silicon prism array output coupler respectively). Developments of this intracavity source with regard to the linewidth are also reported. Implementation of etalons in the optical (laser and OPO) cavities was shown to be a promising technique that brings the terahertz linewidth down below 1GHz (close to the transform limit of nanosecond pulses) while retaining the tuning range and beam characteristics of the free running system. Close to Fourier transform limited pulses were obtained (<100MHz linewidth) via an injection seeding technique, although with significantly increased system complexity. A deleterious effect caused by the mode beating of a multimode host laser was also discovered, in that sidebands were induced on the seeded downconverted wave. This has wider implications in the field of intracavity OPOs. Finally, quasi-phasematching techniques implementing periodically poled lithium niobate were investigated as a way to lower the downconversion threshold energy requirement (by collinear propagation of the optical waves), and also to extract the terahertz wave rapidly from the (highly absorbing in the terahertz region) lithium niobate crystal. The existence of two phasematching solutions arising from the bidirectionality of the grating vector was identified as a serious design constraint in the context of an OPO where either solution can build up from noise photons, and so prefers the solution with the lowest walkoff of the downconverted waves - possibly resulting in unextractable terahertz radiation. Quasi-phasematching with an orthogonal grating vector (with identical but opposite phasematching solutions) was demonstrated and cascaded downconversion processes observed and characterised. These cascaded processes are permitted by the collinearality of the optical waves and may allow efficiency improvements through overcoming the quantum defect limit. This research has resulted in four peer reviewed papers in respected journals, and the intracavity terahertz OPO has been licensed to a company who have commercialised the technology (M Squared Lasers, Glasgow).

published_or_final_version / abstract / Civil Engineering / Master / Master of Philosophy

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