Fisuración y deformaciones en tabiquerías

A diferencia de otros parámetros, el efecto de la existencia de huecos en la aparición y desarrollo de los procesos de fisuración en los paños de fábrica no ha sido considerado por las distintas normativas existentes en la actualidad. En nuestros días se emplea una variada gama de tipologías de elementos de cerramiento para realizar las particiones en las obras de edificación, cada una de ellas con características mecánicas diferentes y distinta metodología de ejecución, siendo de aplicación la misma normativa relativa al cálculo y control de las deformaciones. Tal y como expresamos en el Capitulo 1, en el que se analiza el Estado del Conocimiento, los códigos actuales determinan de forma analítica la flecha probable que se alcanza en los elementos portantes estructurales bajo diferentes condiciones de servicio. Las distintas propuestas que existen respecto para la limitación de la flecha activa, una vez realizado el cálculo de las deformaciones, bien por el método de Branson ó mediante los métodos de integración de curvaturas, no contemplan como parámetro a considerar en la limitación de la flecha activa la existencia y tipología de huecos en un paño de fábrica soportado por la estructura. Sin embargo se intuye y podríamos afirmar que una discontinuidad en cualquier elemento sometido a esfuerzos tiene influencia en el estado tensional del mismo. Si consideramos que, de forma general, los procesos de fisuración se producen al superarse la resistencia a tracción de material constitutivo de la fábrica soportada, es claro que la variación tensional inducida por la existencia de huecos ha de tener cierta influencia en la aparición y desarrollo de los procesos de fisuración en los elementos de partición o de cerramiento de las obras de edificación. En los Capítulos 2 y 3 tras justificar la necesidad de realizar una investigación encaminada a confirmar la relación entre la existencia de huecos en un paño de fábrica y el desarrollo de procesos de fisuración en el mismo, se establece este aspecto como principal Objetivo y se expone la Metodología para su análisis. Hemos definido y justificado en el Capítulo 4 el modelo de cálculo que hemos utilizado para determinar las deformaciones y los procesos de fisuración que se producen en los casos a analizar, en los que se han considerado como variables: los valores de la luz del modelo, el estado de fisuración de los elementos portantes, los efectos de la fluencia y el porcentaje de transmisión de cargas desde el forjado superior al paño de fábrica en estudio. Además se adoptan dos valores de la resistencia a tracción de las fábricas, 0.75MPa y 1.00MPa. La capacidad de representar la fisuración, así como la robustez y fiabilidad ha condicionado y justificado la selección del programa de elementos finitos que se ha utilizado para realizar los cálculos. Aprovechando la posibilidad de reproducir de forma ajustada las características introducidas para cada parámetro, hemos planteado y realizado un análisis paramétricos que considera 360 cálculos iterativos, de cuya exposición es objeto el Capítulo 5, para obtener una serie representativa de resultados sobre los que se realizará el análisis posterior. En el Capítulo 6, de análisis de los resultados, hemos estudiado los valores de deformaciones y estados de fisuración obtenidos para los casos analizados. Hemos determinado la influencia que tiene la presencia de huecos en la aparición de los procesos de fisuración y en las deformaciones que se producen en las diferentes configuraciones estructurales. Las conclusiones que hemos obtenido tras analizar los resultados, incluidas en el Capítulo 7, no dejan lugar a dudas: la presencia, la posición y la tipología de los huecos en los elementos de fábricas soportadas sobre estructuras deformables son factores determinantes respecto de la fisuración y pueden tener influencia en las deformaciones que constituyen la flecha activa del elemento, lo que obliga a plantear una serie de recomendaciones frente al proyecto y frente a la reglamentación técnica. La investigación desarrollada para esta Tesis Doctoral y la metodología aplicada para su desarrollo abre nuevas líneas de estudio, que se esbozan en el Capítulo 8, para el análisis de otros aspectos que no han sido cubiertos por esta investigación a fin de mejorar las limitaciones que deberían establecerse para los Estados Límite de Servicio de Deformaciones correspondientes a las estructuras de edificación. SUMMARY. Unlike other parameters, the effect of the existence of voids in the arising and development of cracking processes in the masonry walls has not been considered by current Codes. Nowadays, a huge variety of enclosure elements types is used to execute partitions in buildings, each one with different mechanical characteristics and different execution methodology, being applied the same rules concerning deflection calculation and control. As indicated in Chapter 1, which analyzes the State of Art, current codes analytically determine the deflection likely to be achieved in structural supporting elements under different service conditions. The different proposals that exist related to live deflection limitation, once performed deformations calculation, either by Branson´s method or considering curvatures integration methods, do not consider in deflection limitation the existence and typology of voids in a masonry wall structured supported. But is sensed and it can be affirmed that a discontinuity in any element under stress influences the stress state of it. If we consider that, in general, cracking processes occur when masonry material tensile strength is exceeded, it is clear that tension variation induced by the existence of voids must have some influence on the emergence and development of cracking processes in enclosure elements of building works. In Chapters 2 and 3, after justifying the need for an investigation to confirm the relationship between the existence of voids in a masonry wall and the development of cracking process in it, is set as the main objective and it is shown the analysis Methodology. We have defined and justified in Chapter 4 the calculation model used to determine the deformation and cracking processes that occur in the cases analyzed, in which were considered as variables: model span values, bearing elements cracking state, creep effects and load transmission percentage from the upper floor to the studied masonry wall. In addition, two masonry tensile strength values 0.75MPa and 1.00MPa have been considered. The cracking consideration ability, robustness and reliability has determined and justified the selection of the finite element program that was used for the calculations. Taking advantage of the ability of accurately consider the characteristics introduced for each parameter, we have performed a parametric analyses that considers 360 iterative calculations, whose results are included in Chapter 5, in order to obtain a representative results set that will be analyzed later. In Chapter 6, results analysis, we studied the obtained values of deformation and cracking configurations for the cases analyzed. We determined the influence of the voids presence in the occurrence of cracking processes and deformations in different structural configurations. The conclusions we have obtained after analyzing the results, included in Chapter 7, leave no doubt: the presence, position and type of holes in masonry elements supported on deformable structures are determinative of cracking and can influence deformations which are the element live deflection, making necessary to raise a number of recommendations related to project and technical regulation. The research undertaken for this Doctoral Thesis and the applied methodology for its development opens up new lines of study, outlined in Chapter 8, for the analysis of other aspects that are not covered by this research, in order to improve the limitations that should be established for Deflections Serviceability Limit States related to building structures.

Pinus uncinata Ramond ex DC.: Conservation and proposal of management at its southwestern limit of distribution (Castillo de Vinuesa, Soria, Spain)

Pinus uncinata forms forests in the centre and southwest of the Alps and in the subalpine Pyrenees (at around 1700 – 2600 m) (Costa Tenorio et al., 1997). The species reaches the southwestern limit of its distribution at the top of Mount Castillo de Vinuesa (Soria, Spain). The small population on this mountain occupies just 66 ha, but is very important from a geobotanical viewpoint since it is just one of two populations (the other being in the Sierra de Gúdar range in Teruel, Spain) isolated from the main area where the species is found in the Iberian Peninsula (The Pyrenees)

Holocene distribution of woody taxa at the westernmost limit of the Circumboreal/Mediterranean boundary: evidence from wood remains

Información paleoecológica relativa a la distribución de táxones arbóreos durante el holoceno, basado en restos fósiles leñosos.

Degradation in seawater of structural adhesives for hybrid fibre-metal laminated materials

The adhesives used for applications in marine environments are subject to particular chemical conditions, which are mainly characterised by an elevated chlorine ion content and intermittent wetting/drying cycles, among others.These conditions can limit the use of adhesives due to the degradation processes that they experience. In this work, the chemical degradation of two different polymers, polyurethane and vinylester, was studied in natural seawater under immersion for different periods of time.The diffusion coefficients and concentration profiles of water throughout the thickness of the adhesiveswere obtained.Microstructural changes in the polymer due to the action of water were observed by SEM, and the chemical degradation of the polymer was monitored with the Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The degradation of the mechanical properties of the adhesive was determined by creep tests withMixed Cantilever Beam (MCB) specimens at different temperatures. After 180 days of immersion of the specimens, it was concluded that the J-integral value (depending on the strain) implies a loss of stiffness of 51% and a decrease in the failure load of 59% for the adhesive tested.

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.

Precipitated iron: a limit on gettering efficacy in multicrystalline silicon

A phosphorus diffusion gettering model is used to examine the efficacy of a standard gettering process on interstitial and precipitated iron in multicrystalline silicon. The model predicts a large concentration of precipitated iron remaining after standard gettering for most as-grown iron distributions. Although changes in the precipitated iron distribution are predicted to be small, the simulated post-processing interstitial iron concentration is predicted to depend strongly on the as-grown distribution of precipitates, indicating that precipitates must be considered as internal sources of contamination during processing. To inform and validate the model, the iron distributions before and after a standard phosphorus diffusion step are studied in samples from the bottom, middle, and top of an intentionally Fe-contaminated laboratory ingot. A census of iron-silicide precipitates taken by synchrotron-based X-ray fluorescence microscopy confirms the presence of a high density of iron-silicide precipitates both before and after phosphorus diffusion. A comparable precipitated iron distribution was measured in a sister wafer after hydrogenation during a firing step. The similar distributions of precipitated iron seen after each step in the solar cell process confirm that the effect of standard gettering on precipitated iron is strongly limited as predicted by simulation. Good agreement between the experimental and simulated data supports the hypothesis that gettering kinetics is governed by not only the total iron concentration but also by the distribution of precipitated iron. Finally, future directions based on the modeling are suggested for the improvement of effective minority carrier lifetime in multicrystalline silicon solar cells.

Can seed production and restricted dispersal limit recruitment in Pinus pinaster Aiton from the Spanish Northern Plateau?

Natural regeneration faces increasing difficulties in dry forests from the Mediterranean basin, including for normally well-regenerating species such as maritime pine (Pinus pinaster Aiton). In this paper, we studied female fertility, seed dispersal and spread rates in P. pinaster from the Spanish Northern Plateau, where natural regeneration failure is a main concern for forest managers. For this purpose we periodically collected data from seed traps and trees located at two core locations across several years. We found significant variation in interannual cone production, with the best seed trees being the same across years. In addition, we found highly skewed distributions of female reproductive effort and large fertility differences across stands located few kilometres away. Annual seed dispersal kernels fitted lognormal or 2Dt models depending on the stand analysed, with median dispersal distances between 14 and 25 m. Kernels fitted for maximum dispersal periods showed an outstanding intraseasonal variation of median dispersal distances, from 10 to 54 m, in association to variable patterns of rainfall and maximum wind speed. The amount of seed produced appeared to be enough to guarantee the natural regeneration of the stands during the typical 20-year regeneration period. Colonisation simulations concluded that Mediterranean maritime pine has a notable dispersion capacity, which is strongly influenced by levels of fecundity and, especially, by the number and frequency of long-distance dispersal events. The latter play a key role in tree dispersion processes through enlarging the occupied area and fostering the invasion of abandoned crop land.

Asymptotic analysis of vertical geothermal boreholes in the limit of slowly varyng heat injection rates

Theoretical models for the thermal response of vertical geothermal boreholes often assume that the characteristic time of variation of the heat injection rate is much larger than the characteristic diffusion time across the borehole. In this case, heat transfer inside the borehole and in its immediate surroundings is quasi-steady in the first approximation, while unsteady effects enter only in the far field. Previous studies have exploited this disparity of time scales, incorporating approximate matching conditions to couple the near-borehole region with the outer unsteady temperatura field. In the present work matched asymptotic expansion techniques are used to analyze the heat transfer problem, delivering a rigorous derivation of the true matching condition between the two regions and of the correct definition of the network of thermal resistances that represents the quasi-steady solution near the borehole. Additionally, an apparent temperature due to the unsteady far field is identified that needs to be taken into account by the near-borehole region for the correct computation of the heat injection rate. This temperature differs from the usual mean borehole temperature employed in the literatura.

Monte Carlo limit cycle characterization

The fixed point implementation of IIR digital filters usually leads to the appearance of zero-input limit cycles, which degrade the performance of the system. In this paper, we develop an efficient Monte Carlo algorithm to detect and characterize limit cycles in fixed-point IIR digital filters. The proposed approach considers filters formulated in the state space and is valid for any fixed point representation and quantization function. Numerical simulations on several high-order filters, where an exhaustive search is unfeasible, show the effectiveness of the proposed approach.

Limit load instabilities in structural elements

Este trabajo analiza distintas inestabilidades en estructuras formadas por distintos materiales. En particular, se capturan y se modelan las inestabilidades usando el método de Riks. Inicialmente, se analiza la bifurcación en depósitos cilíndricos formados por material anisótropo sometidos a carga axial y presión interna. El análisis de bifurcación y post-bifurcación asociados con cilindros de pared gruesa se formula para un material incompresible reforzado con dos fibras que son mecánicamente equivalentes y están dispuestas simétricamente. Consideramos dos casos en la naturaleza de la anisotropía: (i) Fibras refuerzo que tienen una influencia particular sobre la respuesta a cortante del material y (ii) Fibras refuerzo que influyen sólo si la fibra cambia de longitud con la deformación. Se analiza la propagación de las inestabilidades. En concreto, se diferencia en el abultamiento (bulging) entre la propagación axial y la propagación radial de la inestabilidad. Distintos modelos sufren una u otra propagación. Por último, distintas inestabilidades asociadas al mecanismo de ablandamiento del material (material softening) en contraposición al de endurecimiento (hardening) en una estructura (viga) de a: hormigón y b: hormigón reforzado son modeladas utilizando una metodología paralela a la desarrollada en el análisis de inestabilidades en tubos sometidos a presión interna. This present work deals with the instability of structures made of various materials. It captures and models different types of instabilities using numerical analysis. Firstly, we consider bifurcation for anisotropic cylindrical shells subject to axial loading and internal pressure. Analysis of bifurcation and post bifurcation of inflated hyperelastic thick-walled cylinder is formulated using a numerical procedure based on the modified Riks method for an incompressible material with two preferred directions which are mechanically equivalent and are symmetrically disposed. Secondly, bulging/necking motion in doubly fiber-reinforced incompressible nonlinearly elastic cylindrical shells is captured and we consider two cases for the nature of the anisotropy: (i) reinforcing models that have a particular influence on the shear response of the material and (ii) reinforcing models that depend only on the stretch in the fiber direction. The different instability motions are considered. Axial propagation of the bulging instability mode in thin-walled cylinders under inflation is analyzed. We present the analytical solution for this particular motion as well as for radial expansion during bulging evolution. For illustration, cylinders that are made of either isotropic incompressible non-linearly elastic materials or doubly fiber reinforced incompressible non-linearly elastic materials are considered. Finally, strain-softening constitutive models are considered to analyze two concrete structures: a reinforced concrete beam and an unreinforced notch beam. The bifurcation point is captured using the Riks method used previously to analyze bifurcation of a pressurized cylinder.

An experimental study of the breakage of liquid bridges at stability limit of minimum volume

An experimental apparatus to study the breaking process of axisymmetric liquid bridges has been developed, and the breaking sequences of a large number of liquid bridge configurations at minimum-volume stability limit have been analyzed. Experimental results show that very close to the breaking moment the neck radius of the liquid bridge varies as t1/3, where t is the time to breakage, irrespective of the value of the distance between the solid disks that support the liquid column.

The extended Betz–Lanchester limit

A proposal for an extended formulation of the power coefficient of a wind turbine is presented. This new formulation is a generalization of the Betz–Lanchester expression for the power coefficient as function of the axial deceleration of the wind speed provoked by the wind turbine in operation. The extended power coefficient takes into account the benefits of the power produced and the cost associated to the production of this energy. By the simple model proposed is evidenced that the purely energetic optimum operation condition giving rise to the Betz–Lanchester limit (maximum energy produced) does not coincide with the global optimum operational condition (maximum benefit generated) if cost of energy and degradation of the wind turbine during operation is considered. The new extended power coefficient is a general parameter useful to define global optimum operation conditions for wind turbines, considering not only the energy production but also the maintenance cost and the economic cost associated to the life reduction of the machine.