The mechanism governing shear strength in reinforced concrete members without transverse reinforcement subjected to both bending and tensile stress is complex. Further IntroductionAxial loading is a parameter that affects the shear strength of reinforced concrete beams without transverse reinforcement. Members may be stressed axially by tensile or compressive forces, which respectively decrease or increase their shear strength. However, the quantitative effect of tensile axial loads on shear strength and ductility has been studied far less than the effect of compressive loads. The shortage of experimental research may raise doubts about justifying the formulas used to quantify the effect of tensile axial loads on shear capacity in members without transverse reinforcement.The partial collapse of a depot at Wilkins Air Force Base, Shelby, Ohio, USA, on 3 August 1955 spurred the revision of the ACI equations for calculating shear. The pathology reports came to the conclusion that the concrete contracted because of low temperatures, causing tensile stresses that lowered the shear capacity of the beam that collapsed. The effect of tensile axial loads on the shear strength of a structural member was studied for the first time in 1957 by Elstner and Hognestad, in research work conducted for the Portland Cement Association (PCA) [1]. Their tests reproduced one of the inner portal frames that collapsed at Shelby at a scale of 1:3.From the theory that the effects of temperature and shrinkage had contributed to the collapse, two tests were conducted by applying tensile stresses of 1.27 and 1.61 MPa to portal frames. An extra reference test was performed without any axial load. Applying the tensile axial loads lowered the shear capacity of the reference test by more than 1 MPa in both cases. The results of these tests led to the conclusion that the existence of tensile axial forces in the collapsed section, which was lightly reinforced (with a longitudinal ratio of nearly 0.45 %), could have lowered the shear strength by more than 50 %. These findings were published in 1962 and the conclusions were incorporated into the ACI code formulas for predicting shear stresses in reinforced beams in 1963. The formula proposed back then has been retrained essentially unaltered in the ACI code to date.Further tests were run by Mattock [2] at the University of Washington in 1969 on the occasion of changes to the ACI sectional design procedures for shear, and a smaller series of tests was conducted two years later by the same researcher in conjunction with Haddadin and Hong [3] . The present study was undertaken in order to revisit those tests on the effect of such tensile axial loads on the shear strength of beams without transverse reinforcement.In addition, the tests performed by This database [7] is a compilation of the tests in which beams without transverse reinforcement were tested to tensile shear failure under tensile axial loads. The general criteria for selecting these tests for inclusion in the database are: reinforced concre...

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Una técnica de refuerzo de vigas y forjados consiste en la disposición de perfiles metálicos bajo los elementos a reforzar y retacados a ellos. En general, este refuerzo se concibe como pasivo: los perfiles no entran en carga en tanto no se incrementan las acciones sobre el elemento reforzado.  La alternativa que se plantea en este trabajo es un planteamiento activo: introducir fuerzas (por ejemplo, mediante gatos, barras roscadas, etc.) entre el perfil y el elemento a reforzar, y retacando posteriormente el perfil a la pieza en los puntos de introducción de las fuerzas, manteniendo la predeformación obtenida en el perfil mediante mortero, calzos metálicos, etc.  El presente expone los aspectos técnicos básicos a considerar en este tipo de refuerzos, tanto en el diseño como en su posterior ejecución y control. En relación con este último, la fiabilidad del cálculo de flechas en perfiles metálicos permite un control exhaustivo de las fuerzas introducidas, tanto al reforzar como a lo largo de la vida de la estructura, mediante la auscultación de las flechas en el perfil durante el proceso y la posterior etapa de servicio.  Se muestran en este trabajo algunos ejemplos de refuerzo desarrollados recientemente con esta técnica.

The paucity of experimental research and several incongruences in boundary conditions of actual sectional design procedures of empirical nature may raise doubts about the validity of the minimum value for the shear strength of reinforced concrete members in Eurocode 2 (EC‐2). Although a minimum reinforcement due to bending is supposed before shear strength checking, EC‐2 Commentary gives an explanation about a conservative value for a minimum on the basis of the tensile strength of the concrete. This manuscript highlights the gaps in the state of present knowledge and incorporates a new proposal.

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