Shear strength model for RC joints, consistent with the shear design rules for prismatic members in the second-generation Eurocodes

Abstract

The CEB/FIP 1985 seismic model code, the 1994 ENV and the 2004 EN versions of Eurocode 8, as well as the 2019 draft of the future Eurocode 8, have adopted different models for the shear strength of RC joints, without paying much attention neither to the shear design rules applying at the time to RC structures designed to non-seismic actions, nor to the large volume of available test results. In the light of current experimental knowledge, these provisions have moved from a high average safety margin in the past to a safety deficit in recent proposals for future. Moreover, the scatter of their predictions renders some of them meaningless. A new rational model is proposed for the cyclic shear strength of RC joints, fully consistent with the shear design rules in the new-generation Eurocodes 2 and 8 for RC members. As such, it has been adopted in the 2020 draft of the second-generation Eurocode 8. It considers the stress fields in the concrete and the reinforcement as continuous and uniform within the joint’s volume, uses first principles of mechanics (principal stresses and strains, constitutive relations, equilibrium) and searches for the inclination of the compression field direction and of the principal compressive strain in the joint which maximizes its shear resistance. The model gives unbiased, low-scatter predictions of the shear strength for over 650 shear-critical joint specimens. Taking the average of the beam and column widths as effective width of the joint gives overall better agreement with test results than the approach in the current Eurocode 8 and its predecessors. The shear force at cracking of the uncracked joint should be considered as lower limit to its ultimate shear strength. If applied with design values of material strengths, the model gives values of the joint’s shear strength which exceed the actual, experimental strength in 2% of the cyclic tests of interior joints and in 5% of those of exterior ones.