Ultimate limit states design of concrete railway sleepers

The railway industry has been slow to adopt limit states principles in the structural design of concrete sleepers for its tracks, despite the global take up of this form of design for almost every other type of structural element. Concrete sleeper design is still based on limiting stresses but is widely perceived by track engineers to lead to untapped reserves of strength in the sleepers. Limit design is a more rational philosophy, especially where it is based on the ultimate dynamic capacity of the concrete sleepers. The paper describes the development of equations and factors for a limit design methodology for concrete sleepers in flexure using a probabilistic evaluation of sleeper loading. The new method will also permit a cogent, defensible means of establishing the true capacity of the billions of concrete sleepers that are currently in-track around the world, leading to better utilisation of track infrastructure. The paper demonstrates how significant cost savings may be achieved by track owners.

Critical comparative evaluation of design methodologies for the ultimate limit state of fatigue in concrete structures

La aparición de la fatiga ha sido ampliamente investigada en el acero y en otros materiales metálicos, sin embargo no se conoce en tanta profundidad en el hormigón estructural. Esto crea falta de uniformidad y enfoque en el proceso de verificación de estructuras de hormigón para el estado límite último de la fatiga. A medida que se llevan a cabo más investigaciones, la información sobre los parámetros que afectan a la fatiga en el hormigón comienzan a ser difundidos e incluso los que les afectan de forma indirecta. Esto conlleva a que se estén incorporando en las guías de diseño de todo el mundo, a pesar de que la comprobación del estado límite último no se trata por igual entre los distintos órganos de diseño. Este trabajo presentará un conocimiento básico del fenómeno de la fatiga, qué lo causa y qué condiciones de carga o propiedades materiales amplían o reducen la probabilidad de fallo por fatiga. Cuatro distintos códigos de diseño serán expuestos y su proceso de verificación ha sido examinado, comparados y valorados cualitativa y cuantitativamente. Una torre eólica, como ejemplo, fue analizada usando los procedimientos de verificación como se indica en sus respectivos códigos de referencia. The occurrence of fatigue has been extensively researched in steel and other metallic materials it is however, not as broadly understood in concrete. This produces a lack of uniformity in the approach and process in the verification of concrete structures for the ultimate limit state of fatigue. As more research is conducted and more information is known about the parameters which cause, propagate, and indirectly affect fatigue in concrete, they are incorporated in design guides around the world. Nevertheless, this ultimate limit state verification is not addressed equally by various design governing bodies. This report presents a baseline understanding of what the phenomenon of fatigue is, what causes it, and what loading or material conditions amplify or reduce the likelihood of fatigue failure. Four different design codes are exposed and their verification process has been examined, compared and evaluated both qualitatively and quantitatively. Using a wind turbine tower structure as case study, this report presents calculated results following the verification processes as instructed in the respective reference codes.

Ultimate flexural limit states analysis of prestressed concrete sleeper

Railway is one of the most important, reliable and widely used means of transportation, carrying freight, passengers, minerals, grains, etc. Thus, research on railway tracks is extremely important for the development of railway engineering and technologies. The safe operation of a railway track is based on the railway track structure that includes rails, fasteners, pads, sleepers, ballast, subballast and formation. Sleepers are very important components of the entire structure and may be made of timber, concrete, steel or synthetic materials. Concrete sleepers were first installed around the middle of last century and currently are installed in great numbers around the world. Consequently, the design of concrete sleepers has a direct impact on the safe operation of railways. The "permissible stress" method is currently most commonly used to design sleepers. However, the permissible stress principle does not consider the ultimate strength of materials, probabilities of actual loads, and the risks associated with failure, all of which could lead to the conclusion of cost-ineffectiveness and over design of current prestressed concrete sleepers. Recently the limit states design method, which appeared in the last century and has been already applied in the design of buildings, bridges, etc, is proposed as a better method for the design of prestressed concrete sleepers. The limit states design has significant advantages compared to the permissible stress design, such as the utilisation of the full strength of the member, and a rational analysis of the probabilities related to sleeper strength and applied loads. This research aims to apply the ultimate limit states design to the prestressed concrete sleeper, namely to obtain the load factors of both static and dynamic loads for the ultimate limit states design equations. However, the sleepers in rail tracks require different safety levels for different types of tracks, which mean the different types of tracks have different load factors of limit states design equations. Therefore, the core tasks of this research are to find the load factors of the static component and dynamic component of loads on track and the strength reduction factor of the sleeper bending strength for the ultimate limit states design equations for four main types of tracks, i.e., heavy haul, freight, medium speed passenger and high speed passenger tracks. To find those factors, the multiple samples of static loads, dynamic loads and their distributions are needed. In the four types of tracks, the heavy haul track has the measured data from Braeside Line (A heavy haul line in Central Queensland), and the distributions of both static and dynamic loads can be found from these data. The other three types of tracks have no measured data from sites and the experimental data are hardly available. In order to generate the data samples and obtain their distributions, the computer based simulations were employed and assumed the wheel-track impacts as induced by different sizes of wheel flats. A valid simulation package named DTrack was firstly employed to generate the dynamic loads for the freight and medium speed passenger tracks. However, DTrack is only valid for the tracks which carry low or medium speed vehicles. Therefore, a 3-D finite element (FE) model was then established for the wheel-track impact analysis of the high speed track. This FE model has been validated by comparing its simulation results with the DTrack simulation results, and with the results from traditional theoretical calculations based on the case of heavy haul track. Furthermore, the dynamic load data of the high speed track were obtained from the FE model and the distributions of both static and dynamic loads were extracted accordingly. All derived distributions of loads were fitted by appropriate functions. Through extrapolating those distributions, the important parameters of distributions for the static load induced sleeper bending moment and the extreme wheel-rail impact force induced sleeper dynamic bending moments and finally, the load factors, were obtained. Eventually, the load factors were obtained by the limit states design calibration based on reliability analyses with the derived distributions. After that, a sensitivity analysis was performed and the reliability of the achieved limit states design equations was confirmed. It has been found that the limit states design can be effectively applied to railway concrete sleepers. This research significantly contributes to railway engineering and the track safety area. It helps to decrease the failure and risks of track structure and accidents; better determines the load range for existing sleepers in track; better rates the strength of concrete sleepers to support bigger impact and loads on railway track; increases the reliability of the concrete sleepers and hugely saves investments on railway industries. Based on this research, many other bodies of research can be promoted in the future. Firstly, it has been found that the 3-D FE model is suitable for the study of track loadings and track structure vibrations. Secondly, the equations for serviceability and damageability limit states can be developed based on the concepts of limit states design equations of concrete sleepers obtained in this research, which are for the ultimate limit states.

Regeneration limit of classical Shannon capacity

Since Shannon derived the seminal formula for the capacity of the additive linear white Gaussian noise channel, it has commonly been interpreted as the ultimate limit of error-free information transmission rate. However, the capacity above the corresponding linear channel limit can be achieved when noise is suppressed using nonlinear elements; that is, the regenerative function not available in linear systems. Regeneration is a fundamental concept that extends from biology to optical communications. All-optical regeneration of coherent signal has attracted particular attention. Surprisingly, the quantitative impact of regeneration on the Shannon capacity has remained unstudied. Here we propose a new method of designing regenerative transmission systems with capacity that is higher than the corresponding linear channel, and illustrate it by proposing application of the Fourier transform for efficient regeneration of multilevel multidimensional signals. The regenerative Shannon limit -the upper bound of regeneration efficiency -is derived. © 2014 Macmillan Publishers Limited. All rights reserved.

Alternate structural system of incorporating flat bed rail wagons as low volume road bridge decks

The stimulus for this project rose from the need to find an alternative solution to aging superstructures of road-bridge in low volume roads (LVR). The solution investigated, designed and consequently plans to construct, involved replacing an aging super-structure of a 10m span bridge with Flat-Bed Rail Wagon (FBRW). The main focus of this paper is to present alternate structural system for the design of the FBRW as road bridge deck conforming to AS5100. The structural adequacy of the primary members of the FBRW was first validated using full scale experimental investigation to AS5100 serviceability and ultimate limit state loading. The bare FBRW was further developed to include a running surface. Two options were evaluated during the design phase, namely timber and reinforced concrete. First option, which is presented here, involved strengthening of the FBRW using numerous steel sections and overlaying the bridge deck with timber planks. The idea of this approach was to use all the primary and secondary members of the FBRW in load sharing and to provide additional members where weaknesses in the original members arose. The second option, which was the preferred option for construction, involved use of primary members only with an overlaying reinforced concrete slab deck. This option minimised the risk associated with any uncertainty of secondary members to its structural adequacy. The paper will report selected results of the experiment as well as the design phases of option one with conclusions highlighting the viability of option 1 and its limitations.

Advanced analysis of steel frame structures comprising non-compact sections

During the past decade, a significant amount of research has been conducted internationally with the aim of developing, implementing, and verifying "advanced analysis" methods suitable for non-linear analysis and design of steel frame structures. Application of these methods permits comprehensive assessment of the actual failure modes and ultimate strengths of structural systems in practical design situations, without resort to simplified elastic methods of analysis and semi-empirical specification equations. Advanced analysis has the potential to extend the creativity of structural engineers and simplify the design process, while ensuring greater economy and more uniform safety with respect to the ultimate limit state. The application of advanced analysis methods has previously been restricted to steel frames comprising only members with compact cross-sections that are not subject to the effects of local buckling. This precluded the use of advanced analysis from the design of steel frames comprising a significant proportion of the most commonly used Australian sections, which are non-compact and subject to the effects of local buckling. This thesis contains a detailed description of research conducted over the past three years in an attempt to extend the scope of advanced analysis by developing methods that include the effects of local buckling in a non-linear analysis formulation, suitable for practical design of steel frames comprising non-compact sections. Two alternative concentrated plasticity formulations are presented in this thesis: the refined plastic hinge method and the pseudo plastic zone method. Both methods implicitly account for the effects of gradual cross-sectional yielding, longitudinal spread of plasticity, initial geometric imperfections, residual stresses, and local buckling. The accuracy and precision of the methods for the analysis of steel frames comprising non-compact sections has been established by comparison with a comprehensive range of analytical benchmark frame solutions. Both the refined plastic hinge and pseudo plastic zone methods are more accurate and precise than the conventional individual member design methods based on elastic analysis and specification equations. For example, the pseudo plastic zone method predicts the ultimate strength of the analytical benchmark frames with an average conservative error of less than one percent, and has an acceptable maximum unconservati_ve error of less than five percent. The pseudo plastic zone model can allow the design capacity to be increased by up to 30 percent for simple frames, mainly due to the consideration of inelastic redistribution. The benefits may be even more significant for complex frames with significant redundancy, which provides greater scope for inelastic redistribution. The analytical benchmark frame solutions were obtained using a distributed plasticity shell finite element model. A detailed description of this model and the results of all the 120 benchmark analyses are provided. The model explicitly accounts for the effects of gradual cross-sectional yielding, longitudinal spread of plasticity, initial geometric imperfections, residual stresses, and local buckling. Its accuracy was verified by comparison with a variety of analytical solutions and the results of three large-scale experimental tests of steel frames comprising non-compact sections. A description of the experimental method and test results is also provided.

An experimental research investigation into disused flat rail wagons as bridges with applications for low volume rural roads in Australia

An experimental laboratory investigation was carried out to assess the structural adequacy of a disused PHO Class Flat Bottom Rail Wagon (FRW) for a single lane low volume road bridge application as per the design provisions of the Australian Bridge Design Standard AS 5100(2004). The investigation also encompassed a review into the risk associated with the pre-existing damage in wagons incurred during their service life on rail. The main objective of the laboratory testing of the FRW was to physically measure its performance under the same applied traffic loading it would be required to resist as a road bridge deck. In order to achieve this a full width (5.2m) single lane, single span (approximately 10m), simply supported bridge would be required to be constructed and tested in a structural laboratory. However, the available clear spacing between the columns of the loading portal frame encountered within the laboratory was insufficient to accommodate the 5.2m wide bridge deck excluding clearance normally considered necessary in structural testing. Therefore, only half of the full scale bridge deck (single FRW of width 2.6m) was able to be accommodated and tested; with the continuity of the bridge deck in the lateral direction applied as boundary constraints along the full length of the FRW at six selected locations. This represents a novel approach not yet reported in the literature for bridge deck testing to the best of the knowledge of the author. The test was carried out under two loadings provided in AS 5100 (2004) – one stationary W80 wheel load and the second a moving axle load M1600. As the bridge investigated in the study is a single lane single span low volume road bridge, the risk of pre-existing damage and the expected high cycle fatigue failure potential was assessed as being minimal and hence the bridge deck was not tested structurally for fatigue/ fracture. The high axle load requirements have instead been focussed upon the investigation into the serviceability and ultimate limit state requirements. The testing regime adopted however involved extensive recording of strains and deflections at several critical locations of the FRW. Three locations of W80 point load and two locations of the M1600 Axle load were considered for the serviceability testing; the FRW was also tested under the ultimate load dictated by the M1600. The outcomes of the experimental investigation have demonstrated that the FRW is structurally adequate to resist the prescribed traffic loadings outlaid in AS 5100 (2004). As the loading was directly applied on to the FRW, the laboratory testing is assessed as being significantly conservative. The FRW bridge deck in the field would only resist the load transferred by the running platform, where, depending on the design, composite action might exist – thereby the share of the loading which needs to be resisted by the FRW would be smaller than the system tested in the lab. On this basis, a demonstration bridge is under construction at the time of writing this thesis and future research will involve field testing in order to assess its performance.

Performance load testing and structural adequacy evaluation of road bridge decks

Many ageing road bridges, particularly timber bridges, require urgent improvement due to the demand imposed by the recent version of the Australian bridge loading code, AS 5100. As traffic volume plays a key role in the decision of budget allocations for bridge refurbishment/ replacement, many bridges in low volume traffic network remain in poor condition with axle load and/ or speed restrictions, thus disadvantaging many rural communities. This thesis examines an economical and environmentally sensible option of incorporating disused flat rail wagons (FRW) in the construction of bridges in low volume, high axle load road network. The constructability, economy and structural adequacy of the FRW road bridge is reported in the thesis with particular focus of a demonstration bridge commissioned in regional Queensland. The demonstration bridge comprises of a reinforced concrete slab (RCS) pavement resting on two FRWs with custom designed connection brackets at regular intervals along the span of the bridge. The FRW-RC bridge deck assembly is supported on elastomeric rubber pads resting on the abutment. As this type of bridge replacement technology is new and its structural design is not covered in the design standards, the in-service structural performance of the FRW bridge subjected to the high axle loadings prescribed in AS 5100 is examined through performance load testing. Both the static and the moving load tests are carried out using a fully laden commonly available three-axle tandem truck. The bridge deck is extensively strain gauged and displacement at several key locations is measured using linear variable displacement transducers (LVDTs). A high speed camera is used in the performance test and the digital image data are analysed using proprietary software to capture the locations of the wheel positions on the bridge span accurately. The wheel location is thus synchronised with the displacement and strain time series to infer the structural response of the FRW bridge. Field test data are used to calibrate a grillage model, developed for further analysis of the FRW bridge to various sets of high axle loads stipulated in the bridge design standard. Bridge behaviour predicted by the grillage model has exemplified that the live load stresses of the FRW bridge is significantly lower than the yield strength of steel and the deflections are well below the serviceability limit state set out in AS 5100. Based on the results reported in this thesis, it is concluded that the disused FRWs are competent to resist high axle loading prescribed in AS 5100 and are a viable alternative structural solution of bridge deck in the context of the low volume road networks.

Nanostructures for sensors, electronics, energy and environment

Nanoscale science is growing evermore important on a global scale and is widely seen as playing an integral part in the growth of future world economies. The daunting energy crisis we are facing could be solved not only by new and improved ways of getting energy directly from the sun, but also by saving power thanks to advancements in electronics and sensors. New, cheap dye-sensitized and polymer solar cells hold the promise of environmentally friendly and simple production methods, along with mechanical flexibility and low weight, matching the conditions for a widespread deployment of this technology. Cheap sensors based on nanomaterials can make a fundamental contribution to the reduction of greenhouse gas emissions, allowing the creation of large sensor networks to monitor countries and cities, improving our quality of life. Nanowires and nano-platelets of metal oxides are at the forefront of the research to improve sensitivity and reduce the power consumption in gas sensors. Nanoelectronics is the next step in the electronic roadmap, with many devices currently in production already containing components smaller than 100 nm. Molecules and conducting polymers are at the forefront of this research with the goal of reducing component size through the use of cheap and environmentally friendly production methods. This, and the coming steps that will eventually bring the individual circuit element close to the ultimate limit of the atomic level, are expected to deliver better devices with reduced power consumption.

Tailoring the resonance of bilayer graphene sheets by interlayer sp3 bonds

Graphene-based resonators are envisioned to build the ultimate limit of two-dimensional nanoelectromechanical system due to their ultrasensitive detection of mass, force, pressure and charge. However, such application has been greatly impeded by their extremely low quality factor. In the present work, we explore, using the large-scale molecular dynamics simulation, the possibility of tailoring the resonance properties of a bilayer graphene sheet (GS) with interlayer sp3 bonds. For the bilayer GS resonator with interlayer sp3 bonds, we discovered that the sp3 bonds can either degrade or enhance the resonance properties of the resonator depending on their density and location. It is found that the distribution of sp3 bonds only along the edges of either pristine or hydrogenated bilayer GS, leads to a greatly enhanced quality factor. A quality factor of ~1.18×105 is observed for a 3.07×15.31 nm2 bilayer GS resonator with sp3 bonds, which is more than 30 times larger comparing with that of a pristine bilayer GS. The present study demonstrates that the resonance properties of a bilayer GS resonator can be tuned by introducing sp3 bonds. This finding provides a useful guideline for the synthesis of the bilayer GS for its application as a resonator component.

Numerical simulation of idealised three-dimensional downburst wind fields

Convective downburst wind storms generate the peak annual gust wind speed for many parts of the non-cyclonic world at return periods of importance for ultimate limit state design. Despite this there is little clear understanding of how to appropriately design for these wind events given their significant dissimilarities to boundary layer winds upon which most design is based. To enhance the understanding of wind fields associated with these storms a three-dimensional numerical model was developed to simulate a multitude of idealised downburst scenarios and to investigate their near-ground wind characteristics. Stationary and translating downdraft wind events in still and sheared environments were simulated with baseline results showing good agreement with previous numerical work and full-scale observational data. Significant differences are shown in the normalised peak wind speed velocity profiles depending on the environmental wind conditions in the vicinity of the simulated event. When integrated over the height of mid- to high rise structures, all simulated profiles are shown to produce wind loads smaller than an equivalent 10 m height matched open terrain boundary layer profile. This suggests that for these structures the current design approach is conservative from an ultimate loading standpoint. Investigating the influence of topography on the structure of the simulated near-ground downburst wind fields, it is shown that these features amplify wind speeds in a manner similar to that expected for boundary layer winds, but the extent of amplification is reduced. The level of reduction is shown to be dependent on the depth of the simulated downburst outflow.

Analysis of steel structures for first- and second-order element (displacement and force) solutions

Finite element method (FEM) relies on an approximate function to fit into a governing equation and minimizes the residual error in the integral sense in order to generate solutions for the boundary value problems (nodal solutions). Because of this FEM does not show simultaneous capacities for accurate displacement and force solutions at node and along an element, especially when under the element loads, which is of much ubiquity. If the displacement and force solutions are strictly confined to an element’s or member’s ends (nodal response), the structural safety along an element (member) is inevitably ignored, which can definitely hinder the design of a structure for both serviceability and ultimate limit states. Although the continuous element deflection and force solutions can be transformed into the discrete nodal solutions by mesh refinement of an element (member), this setback can also hinder the effective and efficient structural assessment as well as the whole-domain accuracy for structural safety of a structure. To this end, this paper presents an effective, robust, applicable and innovative approach to generate accurate nodal and element solutions in both fields of displacement and force, in which the salient and unique features embodies its versatility in applications for the structures to account for the accurate linear and second-order elastic displacement and force solutions along an element continuously as well as at its nodes. The significance of this paper is on shifting the nodal responses (robust global system analysis) into both nodal and element responses (sophisticated element formulation).

Modelagem do comportamento dinâmico de passarelas tubulares em aço e mistas (aço-concreto)

A experiência dos engenheiros estruturais e os conhecimentos adquiridos pelo uso de materiais e novas tecnologias, têm ocasionado estruturas de aço e mistas (aço-concreto) de passarelas cada vez mais ousadas. Este fato tem gerado estruturas de passarelas esbeltas, e consequentemente, alterando os seus estados de limite de serviço e último associados ao seu projeto. Uma consequência direta desta tendência de projeto é o aumento considerável das vibrações das estruturas. Portanto, a presente investigação foi realizada com base em um modelo de carregamento mais realista, desenvolvido para incorporar os efeitos dinâmicos induzidos pela caminhada de pessoas. O modelo de carregamento considera a subida e a descida da massa efetiva do corpo em cada passo. A posição da carga dinâmica também foi alterada de acordo com a posição do pedestre sobre a estrutura e a função do tempo gerada, possui uma variação espacial e temporal. O efeito do calcanhar do pedestre também foi incorporado na análise. O modelo estrutural investigado baseia-se em uma passarela tubular (aço-concreto), medindo 82,5m. A estrutura é composta por três vãos (32,5 m, 20,0 m e 17,5 m, respectivamente) e dois balanços (7,5 m e 5,0 m, respectivamente). O sistema estrutural é constituído por perfis de aço tubular e uma laje de concreto, e é atualmente utilizada para travessia de pedestres. Esta investigação é realizada com base em resultados experimentais, relacionando a resposta dinâmica da passarela com as obtidas via modelos de elementos finitos. O modelo computacional proposto adota as técnicas de refinamento de malha, usualmente presente em simulações pelo método de elementos finitos. O modelo de elementos finitos foi desenvolvido e validado com resultados experimentais. Este modelo de passarela tubular permitiu uma avaliação dinâmica completa, investigando especialmente ao conforto humano e seus limites de utilização associados à vibração. A resposta dinâmica do sistema, em termos de acelerações de pico, foi obtida e comparada com os valores limites propostos por diversos autores e padrões de projeto. As acelerações de pico encontradas na presente análise indicou que a passarela tubular investigada apresentou problemas relacionados com o conforto humano. Por isso, foi detectado que este tipo de estrutura pode atingir níveis de vibrações excessivas que podem comprometer o conforto do usuário na passarela e especialmente a sua segurança.

Modelagem numérica de elementos tracionados em aço inoxidável com parafusos defasados

Atualmente, a utilização do aço inoxidável em elementos estruturais é considerada uma solução cara para os problemas da engenharia estrutural. Todavia, mudanças de atitudes dentro da construção civil, uma transição global para um desenvolvimento sustentável e redução em impactos ambientais têm seguramente provocado um aumento na utilização do aço inoxidável. As normas de projeto de aço inoxidável atuais são, em grande parte, baseadas em analogias assumidas com o comportamento de estruturas desenvolvidas com aço carbono. Todavia, o aço inoxidável apresenta quatro curvas não-lineares tensão versus deformação (tensão e compressão, paralela e perpendicular a laminação do material), sem patamar de escoamento e região de encruamento claramente definidos, modificando assim, o comportamento global das estruturas que o utilizam. Em elementos estruturais submetidos a forças axiais de tração, a ruptura da seção líquida representa um dos estados limites últimos a serem verificados. Com o objetivo de se avaliar a resistência a tração de elementos estruturais aparafusados em aço inoxidável S304, este trabalho apresenta um modelo numérico baseado no método dos elementos finitos através do programa Ansys (versão 11). A não-linearidade do material foi considerada através do critério de plastificação de Von Mises e curvas tensão versus deformação verdadeira. A não-linearidade geométrica foi introduzida no modelo através da Formulação de Lagrange atualizado. O modelo numérico foi calibrado com resultados experimentais obtidos em ensaios de laboratório, a partir de ligações aparafusadas alternadas rígidas, onde não se ocorre nenhuma rotação entre os membros, transferindo nenhum momento fletor, apenas esforço normal e cisalhante.