Shear Strength Degradation Due To Ductility Demand in R.C. Columns and Beams.

Abstract eng:
A clear distinction can be made between brittle shear failure, occurring before the flexural strength of the column has been attained, ductile shear failure that occurs after that a flexural plastic hinges has been activated, and plastic rotations increased. The shear strength reduction is due to degrading of several resisting mechanism:- aggregate interlock due to reduction of the roughness of the crack surface by the smoothing action of the cyclic load, and bond slippage; dowel action due to cover rupture, hoops and longitudinal rebar plastic strain, and eventually buckling of the latter; strength of chord and web concrete due to compression softening and development of cracking for cyclic load. Several studies addressed the shear strength reduction due to ductility demand on the basis of smeared cracking non-linear models, such as the Modified Compression Field Theory (MCFT). However, despite their success in modeling several structural type behaviors, they do not appear suitable to handy provide relationships required for designers or to be implemented in software for seismic analysis of whole structure. In the past, many equation for modelling the reduction of shear strength due to ductility demand were proposed and included in design code, all of them based on the evaluation of the shear as the sum of the tensile concrete, truss mechanism and external axial force contribution. However, these models are not consistent with the model included in the present Eurocode 2 and Eurocode 8 for shear strength for static action, that is derived neglecting the contribute of tensile concrete by using the truss mechanism (or the equivalent stress fields approach) with variable concrete strut inclination. In this context a proposal is formulated that allows to evaluate the residual shear strength of reinforced concrete columns for an assigned ductility demand by limiting the range of the deviation angle between the inclinations of the yield θ and the crack line θ I,, taking also into account the reduction of the compressive concrete strength due to cyclic action. To this aim, a previous model derived on the basis of the stress-field approach proposed by Bach et al. is updated. Firstly, aiming at stressing how such limit on the inclination of the web compressed concrete stress field modifies the strength domains of RC members, the effects of the progressive reductions of the deviation angle δ on N-M-V domains are shown. The results prove that the progressive reduction in the yield surface inclination (angle θ) causes a major reduction in the maximum shear strength; by contrast, it does not have any influence on the ultimate bending moment. Then the expression of the limitation of the angle of inclination of the web concrete stress fields is derived on the basis of indication contained in Eurocode 8 for evaluating the shear strength reduction in transversally under-reinforced existing concrete structure.

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