A procedure for determining the load carrying capacity in cold-formed steel arches

En este trabajo se presenta una metodología de cálculo para la determinación de la carga última de arcos circulares con perfiles trapezoidales conformados en frío. Estos elementos de cubierta, sometidos fundamentalmente a esfuerzos de compresión y de directriz curva, deben ser dimensionados teniendo en cuenta la inestabilidad por pandeo. Actualmente, ninguna normativa describe un procedimiento de cálculo para la consideración del pandeo en elementos curvos. La metodología aquí planteada se basa en la filosofía de las curvas europeas de pandeo establecidas en los Eurocódigos. Así, se ha obtenido un coeficiente reductor empírico que considera los efectos del pandeo conjuntamente con imperfecciones geométricas y mecánicas en función de la esbeltez del arco considerado. Para ello, es necesario determinar la carga última experimental de al menos tres arcos, cuyos radios y luces se encuentren lo suficientemente diferenciados.This paper deals with a new methodology for the estimation of the strength of circular arches with cold-formed trap - ezoidal section. These cover elements, mainly subjected to compression stress and with circular curvature should be dimensioned taking into account the possibility of buckling instability. Currently, no standard describes a method for calculating the buckling consideration of curved elements. The methodology proposed herein shares the same philosophy of the European buckling curves method established in the Eurocodes. Thus, an experimental coefficient considering the effects of buckling and geometrical and mechanical imperfections depending on the slenderness of the considered arch have been obtained. For this, it was necessary to determine the experimental strength of at least three arcs, which radius and spans are sufficiently differentiate

Aprendizaje Centrado en el Proyecto de Estructuras Adaptados a la Enseñanza Universitaria

The establishment of the European higher education area (EHEA) and the Bologna process tries to improve student’s motivation and the development of personal skills, communication, teamwork, etc. through theoretical and practical similar experiences to those they will find in their future career. In this regard, this work pretends to perform the design and calculation of structures by a realistic project adapted to the academic field. The project must follow a series of guidelines established, so that students have to analyse possible solutions, make decisions, as well as, develop teamwork skills. At the end of the course, all the groups make a public presentation of their project and the students act as participants in the evaluations of their peers, thus encouraging a constructive critical discussion of the projects and the decision-making made.Con la instauración del marco educativo común en Europa y del proceso de Bolonia se pretende mejorar la motivación de los estudiantes y desarrollar sus habilidades personales, de comunicación, de trabajo en equipo, etc. mediante experiencias teórico-prácticas similares a las que encontrarán en su futura carrera profesional. Para ello, en este trabajo se propone la realización del diseño y cálculo estructural de un proyecto realista adaptado al ámbito académico. Se establecen una serie de condicionantes que debe cumplir el proyecto, de manera que los alumnos tienen que analizar posibles soluciones, tomar decisiones, así como, desarrollar habilidades de trabajo en equipo. A final de curso todos los grupos realizan una exposición pública de sus trabajos y los propios alumnos son los que evaluarán los trabajos de sus compañeros de manera que se fomente una discusión crítica-constructiva de los proyectos y de la toma de decisiones realizada

Influencia del número de llaves en la resistencia a cortante de juntas secas postensadas

La resistencia a cortante en las juntas secas es una parte importante del diseño de estructuras de dovelas postensadas. Sin embargo, la formulación de las distintas normativas no se ajusta al comportamiento de las juntas con llaves múltiples. En este trabajo se analiza el comportamiento en fractura de las juntas secas con llaves bajo solicitaciones de cortante, centrándose en la influencia del número de llaves en la resistencia de la junta y la tensión tangencial media. Para ello, se modeliza en elementos finitos cuatro tipos de junta, de una a siete llaves. Los resultados muestran que la tensión tangencial media transferida por la junta disminuye al aumentar el número de llaves, siendo este efecto menor conforme aumenta la tensión de compresión actuante en la junta. En este trabajo se presenta una fórmula de ajuste de los resultados numéricos obtenidos incluyendo un factor que considera la dependencia del número de llaves.The shear strength of dry joints plays an important role in the design of prestressed segmental structures. However, the formulations of different design codes do not conform to the behavior of multiple-keyed joints. This paper ana - lyzes the fracture behavior of dry keyed joints under shear loading, focusing on the influence of the number of keys on the joint capacity and its average shear stress. It has been developed a finite element model of four different types of joints, with a number of keys varying between one and seven. The results show that the average shear stress transferred across the joint decreases with the number of keys but this effect is less appreciated as the prestressing stress increases. In this paper it is presented a formula of adjustment of the obtained numerical results including a factor with the dependence on the number of keys

Modelling of the behavior of post-tensioned concrete structures subjected to long term thermal loads

Congreso celebrado en la Escuela de Arquitectura de la Universidad de Sevilla desde el 24 hasta el 26 de junio de 2015.The effect of the reduction of concrete material strength and the deformations and stress level due to the non-uniform temperature distribution within a structural element such as a storage tank has a coupled negative effect on the post-tensioned concrete design. Heated strands embedded in a heated concrete element will elongate, hence reducing the post-tensioning force. This reduction will reduce the confinement of concrete, which added to the temperature gradient derived tension results in a greater level of damage and cracking. This study sets up a strongly non-linear model that attempts to account for all these effects. The results show that cracking localization grows surrounding the tendon duct, producing a sharp decrease in the post-tensioning force and a significant reduction of global stiffnes

Effect of mix design on the size-independent fracture energy of normal- and high-strength self-compacting concrete

Self-compacting concrete has a characteristic microstructure inherent to its specific composition. The higher content of fine particles in self-compacting concrete relative to the equivalent vibrated concrete produces a different fracture behavior that affects the main fracture parameters. In this work, a comprehensive experimental investigation of the fracture behavior of self-compacting concrete has been carried out. Twelve different self-compacting concrete mixes with compressive strength ranging from 39 to 124 MPa (wider range than in other studies) have been subjected to three-point bending tests in order to determine the specific fracture energy. The influence of the mix design and its composition (coarse aggregate fraction, the water to binder ratio and the paste to solids ratio) on its fracture behavior has been analyzed. Moreover, further evidence of the objectivity of the size-independent fracture energy results, obtained by the two most commonly used methods, has been provided on the self-compacting concrete mixes.Influencia de la composición de la mezcla sobre la energía de fractura de hormigones autocompactantes de resistencias media y alta. Los hormigones autocompactantes tienen una microestructura interna inherente a su composición específica. Su mayor contenido de partículas finas, en comparación con hormigones vibrados equivalentes, provoca un comportamiento diferente en fractura que afecta a los principales parámetros de fractura. En este trabajo, se ha realizado una amplia investigación experimental del comportamiento en fractura de hormigones autocompactantes. Así, se han realizado ensayos de flexión en tres puntos para determinar sus propiedades de fractura sobre 12 hormigones autocompactantes de diferente composición, con resistencias a compresión que van desde 39 hasta 124 MPa (mayor que en otros estudios). De esta forma, se ha analizado la influencia de la dosificación del hormigón y su composición (contenido en árido grueso, relación agua-cemento y pasta-sólidos) sobre su comportamiento en fractura. Además, se ha validado, para hormigones autocompactantes, la objetividad de los resultados obtenidos mediante los dos métodos habitualmente empleados para la determinación de la energía de fractura.Ministerio de Economía y Competitividad BIA2016- 75431-

Mesoscale Characterization of Fracture Properties of Steel Fiber-Reinforced Concrete Using a Lattice–Particle Model

This work presents a lattice–particle model for the analysis of steel fiber-reinforced concrete (SFRC). In this approach, fibers are explicitly modeled and connected to the concrete matrix lattice via interface elements. The interface behavior was calibrated by means of pullout tests and a range for the bond properties is proposed. The model was validated with analytical and experimental results under uniaxial tension and compression, demonstrating the ability of the model to correctly describe the effect of fiber volume fraction and distribution on fracture properties of SFRC. The lattice–particle model was integrated into a hierarchical homogenization-based scheme in which macroscopic material parameters are obtained from mesoscale simulations. Moreover, a representative volume element (RVE) analysis was carried out and the results shows that such an RVE does exist in the post-peak regime and until localization takes place. Finally, the multiscale upscaling strategy was successfully validated with three-point bending tests.Ministerio de Economía y Competitividad BIA2013-48352-

Analysis of the mechanical and fracture behavior of heated ultra-high-performance fiber-reinforced concrete by X-ray computed tomography

Artículo premiado 2º Trimestre 2019 en la Escuela Técnica Sup. de Ingeniería de SevillaThis work analyzes the effects of temperature (300 °C) on mechanical and fracture behavior of an ultra-high-performance steel-fiber-reinforced concrete. The deterioration of the pore structure due to thermal damage of the fiber-reinforced concrete and its un-reinforced matrix was analyzed by X-ray computed tomography. Complementarily, a thermogravimetric analysis was performed to relate the observed phase changes, due to dehydration and decomposition, with the deterioration of pore structure. Additionally, an analysis of their mechanical and fracture properties was also done at room temperature and 300 °C. Finally, a connection between the damage within the concrete matrix and its corresponding mechanical behavior was established. From the results, it has been ascertained that the propagation of thermal damage within the matrix affects the mechanical and fracture behavior in different ways depending on the pore-size. The presence of fibers modifies the pore structure and consequently the evolution of the thermal damage in the ultra-high-performance concrete, inferring its mechanical and fracture behavior.Ministerio de Economía y Competitividad BIA2016-75431-RCzech Academy of Sciences Project No. 16-18702

Analysis of the tensile fracture properties of ultra-high-strength fiber-reinforced concrete with different types of steel fibers by X-ray tomography

This study is concerned with the analysis of the tensile properties of an ultra-high-strength fiber-reinforced concrete manufactured with short and long steel fibers. The analysis involve using different techniques - from mechanical tests to X-ray computed tomography - to relate the observed variation of the mechanical properties of the mixes with their porosity. A comprehensive study of the porosity distribution was conducted on the basis of the analysis of X-ray computed tomography images and porosimetry. The study shows the influence of the type of fiber used in the reinforcement on the pore size and distribution of the concrete matrix and consequently on its tensile properties. The tensile properties are obtained with an inverse analysis method available in the literature that yielded the first-cracking tensile strength (f t ) and the ultimate tensile strength (f tu ) and related with the inner structure of the concrete matrix. Our results prove that the tensile properties, especially the first-cracking strength, vary depending on the fibers used. These findings help mix designers make a decision about the type of fibers that should be used when a high first-cracking tensile strength is needed and make it possible to quantify the effect of the fiber.Ministerio de Economía y competitividad BIA2016-75431-RMinisterio de Economía y Competitividad DPI2015-66534-RAcademia de Ciencias de la República Checa 16-18702

A Finite Element‑Based Methodology for the Thermo‑mechanical Analysis of Early Age Behavior in Concrete Structures

This paper presents a general procedure based on fracture mechanics models in order to analyze the level of cracking and structural safety in reinforced concrete elements at early ages, depending on the stripping time. Our procedure involves the development of a thermo-mechanical numerical model based on the finite element method that accounts for the change in the mechanical properties of concrete with time. Moreover, fracture mechanisms are analyzed by means of a material damage model, which is characterized via specific experimental results obtained for standard specimens and notched beams under three-point bending testing. The loading conditions are both thermal and mechanical, and are obtained from the hydration process for a given concrete dosage. The presented methodology allows for the determination of the optimal stripping time, whereas it helps assessing the analysis of the cracking and the stress states of the elements under consideration. A practical application, namely the analysis of a retaining wall, is used to validate our methodology, showing its suitability in engineering practice.Ministerio de Economía y Competitividad BIA2016-75431-

Probabilistic Flexural Fatigue in Plain and Fiber-Reinforced Concrete

The objective of this work is two-fold. First, we attempt to fit the experimental data on the flexural fatigue of plain and fiber-reinforced concrete with a probabilistic model (Saucedo, Yu, Medeiros, Zhang and Ruiz, Int. J. Fatigue, 2013, 48, 308–318). This model was validated for compressive fatigue at various loading frequencies, but not for flexural fatigue. Since the model is probabilistic, it is not necessarily related to the specific mechanism of fatigue damage, but rather generically explains the fatigue distribution in concrete (plain or reinforced with fibers) for damage under compression, tension or flexion. In this work, more than 100 series of flexural fatigue tests in the literature are fit with excellent results. Since the distribution of monotonic tests was not available in the majority of cases, a two-step procedure is established to estimate the model parameters based solely on fatigue tests. The coefficient of regression was more than 0.90 except for particular cases where not all tests were strictly performed under the same loading conditions, which confirms the applicability of the model to flexural fatigue data analysis. Moreover, the model parameters are closely related to fatigue performance, which demonstrates the predictive capacity of the model. For instance, the scale parameter is related to flexural strength, which improves with the addition of fibers. Similarly, fiber increases the scattering of fatigue life, which is reflected by the decreasing shape parameter.Ministerio de Economía y Competitividad BIA2016-75431-RMinisterio de Economía y Competitividad BIA2015-68678-C2-1-RJunta de Comunidades de Castilla-La Mancha PEII-2014-016-