 Printer EmailThe prediction of the ultimate shear strength of reinforced concrete (RC) beams is very important especially when this value is used in the practical design. An unconservative value of the shear capacity may lead to an unexpected and at early stage brittle failure of the structural RC beam so, a great deal of research efforts have been put recently on the proper explanation, modeling and simulation of the shear crack phenomena [1], [5]. Although the current design codes for shear in RC beams rely almost entirely on the test results, a number of simplified methods are given in [1], based on theory of plasticity and the relevant effectiveness factors. It is fair to state that the mechanism of the brittle type diagonal tensile failure of RC beams with no shear reinforcement (stirrups) is complex and not fully understood (see Fig. 1).
This behaviour is common to slender beams – these are beams with ratio a/h (shear span to effective height) between 2 and 4. There are of course few other important factors such as reinforcement ratio and others. Diagonal shear failure starts with the development of few vertical flexural cracks at the midspan, followed by a destruction of the bond between the reinforcement steel and concrete at the zone of the support (see Fig. 1.). Thereafter, without ample warning of a failure, one major diagonal crack develops (we call it critical crack) at about (1.5-2)d distance from the support. Depending on some material or geometric parameters of the beam, the critical crack may extend to the top of the compression fibers and then stabilize. That happens at comparatively small deflection and is considered to be the shear capacity of the RC beam under consideration. Comments on some results developed in work Considerable efforts have been devoted in recent years of developing numerical methods and models to simulate the real behaviour of quassi-brittle materials, such as mortar, concrete and bricks used in civil engineering structures. Traditionally, the numerical models are based on the finite (FEM) or boundary (BEM) elements and are classified into two groups: “smeared” crack approach and “discrete” crack approach. In the smeared crack models the fracture or crack is represented in a smeared over a finite area manner. Without going into detail we shall mention that the present research team has developed an extensive numerical research on RC shear beams using smeared approach and ANSYS software program [2]. A 3D brick concrete element was used and reinforcement bars were modeled again by smearing out the steel over the concrete elements. For comparison the RC beam tested and numerically examined by Hibino at al. (see Fig. 2.) was used.
The conclusion drawn from the research published in [2} are as follows: Author: S. Parvanova1, K. Kazakov1, I. Kerelezova11, G. Gospodinov and M. P. Nielsen2 |
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