Evaluación experimental del método de puntales y tensores aplicado a muros de concreto de baja altura

El método de puntales y tensores (P-T) es una herramienta práctica para el diseño sísmico de elementos de concreto reforzado. Con el propósito de evaluar el método de P-T propuesto en el reglamento vigente ACI-318 se llevó a cabo una investigación experimental y analítica con muros de concreto de baja altura. En el programa experimental se incluyeron cuatro especimenes diseñados para fallar por cortante, los cuales se ensayaron dinámicamente en mesa vibradora. Las variables estudiadas fueron el tipo de concreto (peso normal y celular), la cuantía de acero (0,125 y 0,25%) y el tipo de refuerzo a cortante en el alma de los muros (barras corrugadas y malla de alambre soldado). Las propiedades de los muros fueron típicas de viviendas mexicanas de baja altura. Cuando se comparó la resistencia calculada con la resistencia medida se encontró que el método de puntales y tensores del reglamento ACI-318 hace una estimación adecuada de la capacidad a cortante de los muros estudiados. Sin embargo, el modo de falla a cortante del muro, la velocidad de aplicación de carga, el número de ciclos y la energía disipada a- cumulada, pueden afectar significativamente la degradación de resistencia de muros de concreto reforzado de baja altura.The strut-and-tie method (S-T) is a practical tool for the seismic design of reinforced concrete elements. Experimental and analytical research with low-rise concrete walls was carried out for assessing the S-T method proposed by the current ACI-318 building code. Four specimens designed to fail during shear and shaking table tests were included in the experimental programme. The variables studied consisted of the type of concrete (normal and cellular weight), the amount of steel web (0.125% and 0.25%) and the type of web reinforcement against shear (corrugated bars and welded wire mesh). Wall properties were typical of low-rise housing in Mexico. When the calculated shear strength was compared with the measured one it was found that the S-T method proposed by the ACI-318 building code suitably estimated the shear capacity of the models being studied. However, the wall’s shear failure mode, loading rate, the number of cycles and the cumulative energy dissipated would noticeably affect the degradation in strength of low-rise, reinforced concrete walls

Deformation analysis of concrete walls under shaking table excitations

Deformation analysis of structural elements is a valuable tool to investigate the relationship of lateral force versus shear, sliding and flexural deformations, to assess the strength mechanism of failure mode, to assign suitable values of lateral stiffness, to estimate the contribution of steel reinforcement to strength and displacement capacities, to calibrate analytical models and, to propose appropriate parameters for performance-based seismic design. An experimental and analytical study was conducted to assess the components of deformation of low-rise reinforced concrete walls. The experimental program included shaking table tests of six low-rise concrete walls. The method and the instrumentation schemes to allow accurate assessment of the three components of deformations for squat walls and walls with openings are proposed. It was found that the proposed method produce consistent results for the tests evaluated

Evaluación experimental de los factores de amortiguamiento en muros de concreto para vivienda

En viviendas de concreto de baja altura se emplean usualmente muros delgados con baja resistencia del concreto, cuantías mínimas de refuerzo y mallas de alambre soldado para refuerzo a cortante en el alma. Con base en la respuesta sísmica medida durante el ensayo en mesa vibratoria de seis muros de concreto de baja altura se evalúa el factor de amortiguamiento que se utiliza usualmente en el análisis dinámico especificado en los reglamentos. Las variables estudiadas fueron el tipo de concreto (peso normal y ligero), la cuantía —0,125% y 0,25%— y el tipo de refuerzo a cortante en el alma —barras corrugadas y malla de alambre soldado— y el efecto de aberturas (puerta y ventana). Tomando en cuenta las ventajas de los sistemas de identificación en el dominio de la frecuencia, las propiedades dinámicas se evaluaron utilizando el método de ajuste de amplitud de la función de transferencia dinámica. En el artículo se discute el efecto del modo de falla sobre el factor de amortiguamiento y se propone una ecuación para estimar el amortiguamiento asociado a un determinado valor del periodo de vibración. Se observó que el factor de amortiguamiento del 5%, supuesto en el análisis dinámico de viviendas de concreto en el intervalo de comportamiento elástico lineal, es consistente con los valores medidos.Thin walls having low concrete strength, minimum web steel ratios and web shear reinforcement made of welded wire meshes are commonly used in low-rise concrete housing. The damping factor commonly used for code-based dynamic analysis was evaluated based on seismic response measured during shake table tests of six low-rise concrete walls. The variables studied were the type of concrete (normal and lightweight), the web steel shear ratio (0.125% and 0.25%), the type of web shear reinforcement (deformed bars and welded wire meshes) and the effect of openings (doors and windows). Dynamic properties were evaluated considering the advantages of frequency-domain system identification, using the dynamic transfer function amplitude fitting method. The effect of failure mode on damping factor is discussed. An equation for estimating damping associated with a particular vibration period value is also proposed. It was observed that a 5% damping factor (assumed for dynamic analysis of concrete housing in the range of lineal elastic behaviour) was consistent with the values measured here

Degredation properties of reinforced concrete walls with openings

This paper compares the performance of four isolated reinforced concrete (RC) walls with openings: two prototype walls tested under quasi-static cyclic (QSC) loading, and two models tested under shaking table excitation. The variables studied were the web steel ratio, the type of web reinforcement, and the testing method. By means of the measured response, it was verified that loading history of the QSCtesting ignores the foremost dynamic effects observed in structures subjected to earthquake loads. When dynamic and QSC responses werecompared, it was apparent that stiffness and strength degradation properties depend on the loading rate, the strength mechanisms associated to the failure modes, number of cycles, and cumulative parameters such as ductility demand and energy dissipated. We deducted that data obtained from QSC tests cannot always be safely assumed to be a lower limit of the expected capacity. Stiffness and strength degradation models for RC walls with openings subjected to earthquake-type loading are also proposed

Degredation properties of reinforced concrete walls with openings

This paper compares the performance of four isolated reinforced concrete (RC) walls with openings: two prototype walls tested under quasi-static cyclic (QSC) loading, and two models tested under shaking table excitation. The variables studied were the web steel ratio, the type of web reinforcement, and the testing method. By means of the measured response, it was verified that loading history of the QSC testing ignores the foremost dynamic effects observed in structures subjected to earthquake loads. When dynamic and QSC responses were compared, it was apparent that stiffness and strength degradation properties depend on the loading rate, the strength mechanisms associated to the failure modes, number of cycles, and cumulative parameters such as ductility demand and energy dissipated. We deducted that data obtained from QSC tests cannot always be safely assumed to be a lower limit of the expected capacity. Stiffness and strength degradation models for RC walls with openings subjected to earthquake-type loading are also proposedEn este artículo se compara el comportamiento de cuatro muros aislados de concreto reforzado (CR) con aberturas: dos muros prototipo ensayados bajo carga cuasi-estática cíclica (CEC) y dos muros ensayados bajo excitación de mesa vibratoria. Las variables estudiadas fueron la cuantía de acero en el alma, el tipo de acero de refuerzo en el alma y el método de ensaye. A partir de la respuesta medida se verificó que la historia de carga de los ensayos CEC ignora los efectos dinámicos fundamentales observados en estructuras sometidas a cargas sísmicas. Cuando las respuestas dinámicas y CEC cíclicas se compararon, se observó que las propiedades de degradación de rigidez y resistencia dependen de la velocidad de aplicación de carga, los mecanismos de resistencia asociados a los modos de falla, el número de ciclos y los parámetros acumulados de la demanda de ductilidad y energía disipada. Por lo tanto, los datos obtenidos a partir de ensayos CEC no siempre se pueden suponer de forma segura como un límite inferior de la capacidad esperada. En el artículo también se proponen modelos de degradación de rigidez y resistencia para muros de CR con aberturas sometidos a carga del tipo sísmic

Experimental evaluation of the strut-and-tie method applied to low-rise concrete walls

The strut-and-tie method (S-T) is a practical tool for the seismic design of reinforced concrete elements. Experimental and analytical research with low-rise concrete walls was carried out for assessing the S-T method proposed by the current ACI-318 building code. Four specimens designed to fail during shear and shaking table tests were included in the experimental programme. The variables studied consisted of the type of concrete (normal and cellular weight), the amount of steel web (0.125% and 0.25%) and the type of web reinforcement against shear (corrugated bars and welded wire mesh). Wall properties were typical of low-rise housing in Mexico. When the calculated shear strength was compared with the measured one it was found that the S-T method proposed by the ACI-318 building code suitably estimated the shear capacity of the models being studied. However, the wall’s shear failure mode, loading rate, the number of cycles and the cumulative energy dissipated would noticeably affect the degradation in strength of low-rise, reinforced concrete walls