Keywords: push-off specimens, reinforced concrete, shear-friction, strength.

The interface between two concrete bodies that can slip relative to each others needs to be designed for the transfer of shear. There are numerous methods that can be used to design such interfaces. The bulk of these models are mainly empirical or semi-empirical equations which were developed to fit the experimental results obtained from testing push-off reinforced concrete specimens.

A recently developed model for calculating the strength and the mode of failure of reinforced concrete membrane elements subjected to in-plane stresses has been shown to offer a favorable combination of generality, accuracy and simplicity. This model, named the simplified model for combined stress-resultants (SMCS) was originally developed for membrane elements subjected to in-plane stresses and was generalized to apply to beams subjected to combined shear and flexure and to combined torsion and flexure. It relates the ultimate shear strength to the amount and strength of the orthogonal reinforcement and the concrete compressive strength. This paper proposes that this model can be used to calculate the shear-transfer strength without the need for modification. Existing models for shear-transfer capacity account for the clamping steel and the concrete strength, while the proposed SMCS accounts in addition for the effects of the reinforcement parallel to the shear transfer interface. The strength of the push-off specimens is related to the shear strength of a membrane element at the shear interface, and the effects of the compressive stresses on these elements are conservatively neglected.

The calculations of the SMCS model are compared against the experimental results from 114 normal-weight push-off specimens with concrete compressive strengths ranging from 16 to 121 MPa. A very good agreement between the observed and calculated shear strength is obtained.

A comparison was conducted between the results from numerous theoretical models and the experimental results from a series of 18 large-scale concrete push-off specimens reinforced with relatively low amounts of reinforcement in the direction parallel to the shear transfer interface. The comparison showed that numerous methods which do not consider the effects of this reinforcement can be excessively un-conservative if this reinforcement is relatively low. The proposed SMCS model and the Mattock model provided the best fit of these results, while the ACI code equations were more conservative.

The proposed model is applicable to a wide range of concrete strengths and levels of reinforcement. On the other hand, many of the existing models were developed based on experimental results from specimens with normal-strength concrete and relatively larger amounts of reinforcement in the direction parallel to the shear interface. The paper showed that these models can be considered as special cases of the more general SMCS.

The comparison between the calculations of the theoretical models and the experimental results from the 114 push-off specimens showed that the SMCS provides very accurate calculation of the shear-transfer strength. The proposed model offers a favorable combination of accuracy, simplicity and generality.

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