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ABSTRACT: In this paper the behavior of prestressed reinforced concrete beams and in particular the shear resistance is analyzed. Even if several different theoretical approaches and numerical models have been proposed over the last years to calculate the shear capacity of prestressed reinforced beams, the problem is not fully solved. A general agreement among researchers is not achieved and also formulations implemented in codes are mostly based on experimental test data. In this paper it is demonstrated that the last formulations contained in Eurocode 2(EN 1992-1-1: 2004) bettered the prediction of the shear capacity of prestressed beams by a new formulation
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of the coefficient αc. NLFE analyses have been carried out to simulate the behavior of the prestressed beamstested in laboratory by Levi and Marro (Levi and Marro, 1992) and to check the effectiveness of αc values recommended by Eurocode 2 (EN 1992-1-1: 2004). The PARC constitutive model (Belletti, 2001), theoretically formulated at the University of Parma and implemented in the user’s subroutine UMAT.for is adopted to carry out NLFE analyses with ABAQUS Code.27712
1 INTRODUCTION
In this paper the shear failure mechanism and the main parameters that influence it are investigated for prestressed reinforced concrete beams with shear reinforcement.For prestressed members there are two primary modes of cracking in shear: web shear and flexure shear. For moments less than the cracking moment, the section is not cracked and the shear strength is controlled by web shear. Web shear cracking initiates in location of high shear that are also subjected to low flexural stress. This cracking mode is typically observed in the end regions of thin-webbed 论文网
prestressed members and occurs when the principal tensile stresses in the web exceed the tensile strength of the concrete. Several parameters have been identified as having a significant influence on the contribution of the shear resistance mechanism and thus the ultimate
shear capacity even if until now no unified theory exists that is capable of fully describing the complex behavior of reinforced concrete elements subjected to shear. According to the previous version of Eurocode 2(ENV 1992-1-1: 1991) two alternative methods to calculate the ultimate shear capacity were initially proposed: the standard method and the variable inclination method (Walgreen, 2002). According to the standard method the shear capacity is the sum of two
terms: one is the shear reinforcement term, based on a truss mechanism with concrete struts inclined of 45◦ to the member axis and the other one is the concrete term regarding the effects of the uncracked compression area, the dowel action of the longitudinal reinforcement and the aggregate interlock. In reality macro-cracks that develop after increasing loads don’t necessarily coincide with the direction of the initial micro-cracks formed at the beginning
of the loading and non linear stress distribution takes place also before the formation of visible cracks. This phenomenon induces a transfer of shear force across the cracks that consequently influence the cracking angle which becomes important in the evaluation of the shear capacity (Vecchio and Collins, 1988, DeiPoli and Gambarova, 1987). According to these considerations
only the variable inclination method is proposed in the current version of Eurocode 2 (EN
1992-1-1: 2004) in which struts could have an inclination θ to longitudinal beam axis ranging from limit values (1≤ cot θ 2.5), Fig. 1.The new Eurocode 2 formulation, based on variable
inclination method, is a simple model suitable for practicing engineers and for everyday work. Some researchers affirm (Cladera and Mari, 2007) that the variable inclination method is a gross oversimplification of the complex problem of shear resistance of RC members that neglects important key variable. For this reason other international Codes have been taken Figure 1. Variable inclination truss model and notation for shear reinforced members.into account in this paper for the evaluation of shear capacity, such as Canadian Standards Associations (CSA Commettee, 2004) which consider as well as EC2 a variable angle truss model, and the ACI formulation (ACI Committee, 2005) which considers a 45◦ truss angle model. The formulation proposed are better investigated in the following paragraphs and compared with analytical and experimental results.