Julián Carrillo

Backbone Model for Performance-Based Seismic Design of RC Walls for Low-Rise Housing

The walls of modern low-rise economic housing in several Latin American countries are typically thin and have a low concrete strength, web steel ratios that are smaller than the minimum prescribed by current codes, and web shear reinforcement made of welded-wire mesh. In light of these particular wall characteristics, research was aimed at developing a performance-based backbone model capable of predicting the seismic behavior of reinforced concrete (RC) walls for one- and two-story housing. The selected tri-linear model is associated with three limit states: diagonal cracking, peak shear strength, and ultimate deformation capacity. The model was developed on the basis of the observed response of 39 quasi-static and shake table experiments. Iterative nonlinear regression analyses were performed for deriving the semi-empirical equations in this study. The paper also discusses the adequacy of some existing models to predict the seismic behavior of RC walls and the limitations of the proposed equations.

DISPLACEMENT DUCTILITY FOR SEISMIC DESIGN OF RC WALLS FOR LOW-RISE HOUSING

The paper compares and discusses displacement ductility ratios of reinforced concrete walls typically used in one- and two-story houses. Ductility is investigated by assessing response measured on 39 walls tested under shaking table excitations and quasi-static lateral loads. Variables studied were the height-to-length ratio and walls with openings, type of concrete and, steel ratio and type of web reinforcement. An equation to estimate the available ductility of a wall is proposed. Based on statistical analysis of data, values of displacement ductility capacity are recommended. Displacement ductility ratios can be used to compute both strength modification and displacement amplification factors for code-based seismic design.

Propiedades mecánicas del concreto para viviendas de bajo costo

La resistencia a cortante de muros de concreto y los nuevos materiales y tecnicas de construccion han posicionado a la vivienda industrializada de concreto como una opcion eficiente para proporcionar seguridad ante eventos sismicos, para incentivar la conservacion del medio ambiente y para promover la reduccion de los costos de construccion, operacion y mantenimiento. Con el proposito de desarrollar ayudas de diseno que promuevan la utilizacion de diferentes tipos de concreto, se llevo a cabo un estudio experimental para caracterizar las propiedades mecanicas de los concretos de peso normal, peso ligero y autocompactable. El programa experimental incluyo el ensayo de 603 especimenes en forma de cilindros y vigas. En el estudio se determinaron las propiedades mecanicas de los tres tipos de concreto sometidos a esfuerzos de compresion, tension y flexion. A partir de las tendencias de los resultados experimentales, se proponen correlaciones numericas para estimar las propiedades mecanicas basicas de los concretos, tales como modulo de elasticidad, resistencia a tension indirecta y resistencia a flexion. Adicionalmente, en el estudio se proponen recomendaciones respecto a la resistencia minima a compresion del concreto para vivienda y a la edad de descimbrado de los muros. Las recomendaciones de este estudio se podrian implantar facilmente en un reglamento para construccion de vivienda de baja altura y de bajo costo.

Flexural mechanical properties of steel fiber reinforced concrete under corrosive environments

En este articulo se evalua la influencia de dos ambientes corrosivos a corto plazo, y de la dosificacion de fibras sobre el desempeno a flexion del concreto reforzado con fibras de acero, CRFA. El programa experimental comprendio el ensayo de 54 especimenes de concreto reforzado con fibras de acero que tienen relacion longitud/diametro de 65 y dosificaciones de fibras de 30 kg/m3 y 60 kg/m3. En cuanto a los ambientes corrosivos, cilindros y vigas fueron sometidos a la accion de un medio acuoso y un medio de solucion NaCl al 3.5%, por un periodo de 60 dias. Los resultados fueron comparados con aquellos cilindros y vigas que permanecieron en condiciones inalteradas. Para dicho tiempo de exposicion que corresponde a la fase de iniciacion de la corrosion, se observo que los iones cloruro presentes en el medio salino (NaCl al 3.5%) ocasionan degradacion en el desempeno mecanico del concreto con fibras de acero, por ejemplo, perdida aproximada del 10% en la resistencia a flexion y disminucion del 11% de la tenacidad en flexion. No obstante para la fase de iniciacion de la corrosion, la exposicion al medio salino provoca un incremento de la capacidad de deflexion del CRFA que puede mejorar su ductilidad y la capacidad de adherencia entre la matriz y las fibras de acero embebidas. Finalmente, se han propuesto ecuaciones que permiten describir el efecto de ambientes acuosos y salinos en la fase de iniciacion de la corrosion sobre el CRFA sometido a esfuerzos de flexion.

Herramienta didáctica para ensayos cuasi-estáticos de pequeños especímenes estructurales

The paper proposes an educational tool for applying quasi–static loads during tests on small structural elements and systems. The magnitude of forces applied to prototypes is initially evaluated using simulations carried out in SolidWorks and Labview. Actuators having capacities of 300 kN, 50 kN and 20 kN were then selected and connected to the load frame of the Lab. Design and assessment of the elements of the hydraulic system, such as the pump, valves and the electric power board, were then performed. After that, the design of the load and displacement control system and the acquisition system, were designed. These systems involve a proportional controller and a connector block to acquire the signals defined by the system, respectively. It is expected that the proposed system promotes academic and research activities on the performance of small structural models subjected to different types of quasi-static forces.

Efecto del ion cloruro sobre las propiedades mecánicas a compresión del concreto reforzado con fibras de acero RC-65/35-BN

The influence of two corrosive environments on a short term and the fiberdosage on the compressive mechanical properties of RC-65/35-BN steelfiber reinforced concrete, SFRC, is assessed in this study. The experimentalprogram comprised the test of 54 cylindrical-type SFRC specimens havingsteel fibers characterized by a length/diameter ratio of 65 and fiber dosagesof 30 and 60 kg/m3. Regarding the exposure environments, 18 cylinderswere subjected to the action of a watery environment, 18 cylinders weresubjected to an environment of 3.5 % NaCl solution (chloride ion) and 18cylinders were kept in unaltered conditions, during 60 days. A reductionof 2 % in the compressive strength, a decrease of 6 % in the modulus ofelasticity, and an increase of 13 % in the Poisson’s ratio of the SFRC wasobserved for this phase of corrosion initiation. These results demonstratedthat, on a short time, the corrosive environments do not affect significantlythe mechanical properties under compressive stresses of the SFRC usedin this study.

Strength Degradation Model for Low-Rise Reinforced Concrete Walls Derived from Dynamic and Quasi-Static Tests

Results from a previous experimental program demonstrated that loading rate, strength mechanisms associated with the failure mode, low-cycle fatigue, and cumulative values of displacement, and dissipated energy strongly affect the degradation properties of reinforced concrete (RC) walls for low-rise housing. Thus, data obtained from quasi-static (QS) cyclic tests should not immediately be assumed to represent a conservative lower bound on a specimens capacity. Aimed at numerically correlating results measured during dynamic and QS-cyclic testing, this paper proposes a strength degradation model. The model is readily applicable to seismic design or assessment of performance of existing and new structures. A seismic demand model is also proposed for correlating the intensity and duration of a given earthquake-induced movement to parameters that define the degradation model. Correlations are deemed useful for interpreting the results of QS-cyclic tests and for calibrating hysteretic and behavioral analytical models obtained from QS testing.

Reinforcement contribution to the behavior of low-rise concrete walls

Based on steel strains recorded during shake table tests of six wall specimens, the effect and contribution of steel reinforcement to peak shear strength and displacement capacity of low-rise concrete walls is assessed and discussed. The experimental program included four variables such as wall geometry, concrete type, web steel ratio and type of web reinforcement. Wall response was assessed through effective steel strains in vertical reinforcement, efficiency factors of wall reinforcement, contribution of web horizontal reinforcement to wall shear strength, and the effect of type of web reinforcement to wall displacement.

MODELING OF CONCRETE DWELLINGS BASED ON RESULTS FROM AMBIENT VIBRATION TESTS

Based on both ambient vibration measurements and numerical models using the wide-column analogy and the finite element method, vibrations periods of low-rise concrete dwellings are estimated in this study. A brief review of parametric and nonparametric techniques is presented and discussed, and in-situ measurements techniques are critically compared. The results of ambient vibration tests are used to calibrate numerical models. Advantages, drawbacks and accuracy of parametric techniques, as well as particular recommendations to build numerical models for assessing the elastic behavior of low-rise concrete wall dwellings, are proposed.