Repositório Colecção: DEC_PhD
http://hdl.handle.net/10362/1037
DEC_PhD2018-02-24T05:45:04ZCaracterização das propriedades de madeiras africanas, Câmbala e Sapelli
http://hdl.handle.net/10362/27870
Título: Caracterização das propriedades de madeiras africanas, Câmbala e Sapelli
Autor: Cunha, Carlos José Parreira da
Resumo: Câmbala e Sapelli são duas espécies de madeiras tropicais africanas com possível utilização em estruturas. O escasso conhecimento científico generalizado das propriedades mecânicas de madeiras tropicais constitui uma limitação na sua utilização em elementos estruturais. A atual normalização europeia para a classificação mecânica das espécies é baseada em madeiras de resinosas, com implícita aceitação para as madeiras de folhosas. Por sua vez, o novo EUTR (European Union Timber Regulation) vem impor critérios de sustentabilidade na importação de madeiras tropicais, originando o aparecimento de novas espécies no mercado Europeu. Foi objetivo da presente dissertação a definição de modelos alternativos expeditos, de caracterização mecânica para espécies tropicais, baseados em provetes de pequenas dimensões.
Para cada espécie, prepararam-se mais de 400 provetes de pequenas dimensões, isentos de defeitos, que foram submetidos a ensaios de ultrassons e de flexão, de acordo com as normas ISO 3131 e ISO 3133. Os resultados experimentais obtidos encontram-se nos intervalos previstos na bibliografia para as propriedades das espécies em estudo.
Procedeu-se, também, a uma campanha de ensaios à flexão em provetes de dimensões estruturais, segundo a norma EN 408, tendo-se extraído destes, provetes de pequenas dimensões e mesoprovetes, que foram ensaiados à flexão e à tração paralela às fibras, respectivamente.
Foram estudadas as possíveis correlações existentes entre as diferentes propriedades das madeiras, quer dentro da mesma campanha de cada ensaio, quer entre campanhas de ensaios.
Das correlações estudadas, entre as propriedades das diferentes campanhas, obtiveram-se valores significativos para a caracterização das madeiras, que serviram de base para a elaboração de novas propostas de modelos para a aferição das propriedades características de referência de madeiras tropicais.
Em conclusão, o presente estudo pretende potenciar o uso das novas madeiras tropicais em estruturas, ao determinar as principais propriedades mecânicas, através de modelos alternativos.2017-12-01T00:00:00ZViability and Applicability of Simplified Models for the Dynamic Analysis of Ballasted Railway Tracks
http://hdl.handle.net/10362/21663
Título: Viability and Applicability of Simplified Models for the Dynamic Analysis of Ballasted Railway Tracks
Autor: Rodrigues, André Filipe da Silva
Resumo: The numerical models of the railway track are fundamental tools for the study of their
dynamic behaviour, with implications for the safety and comfort of rail transport and the
degradation and need for maintenance of the track. The importance of these models has
increased alongside the speed and capacity of the railway vehicles over the last decades.
Although the use of three-dimensional finite element models is becoming common
practice, simplified models are still relevant, due to their simplicity of implementation
and results interpretation, and low computational cost. However, the general validity of
these models has not yet been demonstrated in the relevant literature.
The present thesis aims to establish the applicability and viability of such simplified
models in the analysis of the dynamic behaviour of the ballasted railway track. The
following questions are considered:
1. Are these models able to approximate the real rail displacement due to the passage
of rail vehicles, despite their simplicity?
2. If yes, for which situations (i.e., track properties and loading conditions) can they
be used reliably?
3. In these situations, is it possible to define adequate parameters for the simplified
models based on the track’s geometry and mechanical properties?
To that end, three linear elastic models are implemented: a detailed three-dimensional
finite element model, a one-dimensional beam in discrete supports model, and a
one-dimensional beam on elastic foundation model. Transient and steady-state dynamic
solutions for a load moving at moderate and high speed are obtained. The vertical displacement
of the rail is chosen as the reference to measure the equivalence between the
models, since it is a common element between all models and is the interface between
the load and the track.
The three-dimensional model is validated by comparison with published experimental
measurements. Its results cover a representative range of the properties of the ballast
and subgrade, and are used as a reference to calibrate the simplified models using genetic
algorithms and non-linear programming.
It is concluded that a good approximation to the reference solution can be achieved,
particularly when the load moves slower than the velocity of propagation of the elastic waves in the soil. For high velocities and/or soft soils, the wave propagation becomes
more relevant to the dynamic behaviour of the track, and the simplified models become
less reliable.
Following a review of the existing literature, theoretical expressions for the determination
of the parameters of the simplified models are proposed. It is concluded that these
are suitable for the beam on discrete supports model, but not for the beam on elastic
foundation model, whose optimum parameters are less consistent across the different
properties of the track and load speeds.2017-06-01T00:00:00ZCharacterization of the Turbulent Structure in Compound Channel Flows
http://hdl.handle.net/10362/20776
Título: Characterization of the Turbulent Structure in Compound Channel Flows
Autor: Azevedo, Ricardo Manuel Martins de
Resumo: The main goal of this investigation is to characterize the turbulent structures in compound channel flows considering two geometrical conditions, a simple asymmetric compound channel and the same compound channel but with the placement of rods on the upper bank.
For the simple asymmetric compound channel, three different water depths were
analyzed, one corresponding to deep flows and two corresponding to shallow flows. For the compound channel with rods, three different spacing between elements were studied, including two different water depths for each spacing condition.
The measurements were taken with a 2D Laser Doppler Velocimiter, at 9.0 m from the
inlet for simple compound channel. For the compound channel with rods, three cross-section around 9.0 m from the inlet of the channel were measured, corresponding to locations downstream of the rod, in the middle of two rods and upstream of the rod. The measurements were performed under quasi-uniform flow condition and streamwise and vertical instantaneous velocity components were obtained.
The raw data was filtered and processed in order to estimate the time-averaged
velocities U and W, the turbulent intensities U' and W', Reynolds stress u'w' , streamwise integral length scale Lx, turbulence dissipation rate ε and Taylor’s micro scale x. Taylor's frozen field hypothesis was adopted in order to transform the time record into a space record, using a convection velocity Uc. The autocorrelation function was built and the integral length scale estimated using three different stop methods of the integral: the second zero of the autocorrelation function, the first minimum, and assuming the integral length scale as the wavenumber value when the autocorrelation function reaches 1/e (this was concluded to be the most consistent method). For estimating the dissipation rate, the following methods were used: from the third order structure function, from the second order structure function and finally, and from the energy spectrum of the velocity (this was concluded to be the most consistent method).
In the case of a simple compound channel, the deep flows are characterized by macro
vortices with streamwise axis between the interface and main channel and between interface and floodplain, having a notable separation in the "main channel vortex" and the "floodplain vortex" meeting, due the double shear layer of the streamwise depth-averaged velocity. Shallow flows are characterized by macro vortices with vertical axis confined between the interface and main channel and originated by the depth average velocity gradient between the main channel and floodplain. A clear linear relation exists between the streamwise integral length scale, Lx, the dissipation rate, ε, and the streamwise turbulent intensity U’. Contrary to 2D fully developed open-channel flow equations that relation appears to be constant for all water depths.
For the compound channel with rods, new turbulent structures are generated due the interaction between rods and flow. Downstream of rods, the horseshoes-vortex system is perfectly observed and a strong descendant flow dominate both sides of rod, turning invalid the universal laws for 2D fully developed open-channel flows. The integral length scale presents almost constant values in the vertical direction, which indicates that the wakes generated by the rods influence the entire water column. The turbulent microscale and dissipation rate acquire a streamwise variation due to the vortex propagation in the downstream direction, both presenting higher values than the ones corresponding to 2D flows.2016-09-01T00:00:00ZRobustness Analysis of Structures in Post-Earthquake Scenarios Considering Multiple Hazards
http://hdl.handle.net/10362/20212
Título: Robustness Analysis of Structures in Post-Earthquake Scenarios Considering Multiple Hazards
Autor: Ribeiro, Filipe Luís Alves
Resumo: Recent earthquakes have highlighted that the consideration of isolated seismic events, although necessary, may not be sufficient to prevent building collapse. In fact, the occurrence of a large number of aftershocks with significant intensity, as well as the occurrence of tsunamis, fires, and explosions, poses a safety threat that has not been addressed properly in the design and assessment of building structures over the last decade. Although research has been developed in order to evaluate the impact of multiple and/or cascading hazards in structural safety and economical losses, there is no established framework to perform such analysis. In addition, the available numerical tools lack a unified implementation in a widely used software in order to allow for the development of large numerical simulations involving these hazard events.
This work proposes a probabilistic framework for quantifying the robustness of structures considering the occurrence of a major earthquake (mainshock) and the subsequent cascading hazard events, namely fire and aftershocks. These events can significantly increase the probability of collapse of buildings, especially for structures that are damaged during the mainshock. In order to assess the structural performance under post-earthquake hazards, it is of paramount importance to accurately simulate the damage attained during the earthquake, which is strongly correlated to the residual structural capacity to withstand cascading events. In this context, the influence of ground motion characteristics, namely ground motion duration, has been identified as one of the parameters that may induce significant bias on damage patterns associated with the mainshock. Thus, ground motion duration influence on structural damage is analyzed in this work.
Steel moment resisting frame buildings designed according to pre-Northridge codes are
analyzed using the proposed framework. These buildings are representative of the design practice in the US and Europe for decades, and the conclusions of this work can be significant in the assessment/retrofit of thousands of buildings. Fragility curves and reliabilitybased robustness measures are obtained using the proposed framework. The fragility curve parameters obtained herein can be used in the development of future probabilistic-based studies considering post-earthquake hazards. The results highlight the importance of the post-earthquake hazard events in the structural safety assessment. Further work is needed in order to better characterize these hazards as to include them in the code-based design and assessment methodologies.2017-01-01T00:00:00Z