Design of slender and squat reinforced concrete members with shear reinforcement

As proposed in several design standards for structural concrete, including Eurocode 2 (EN:1992-1-1:2004) and fib's Model Code 2010, the shear design of slender members with shear reinforcement can be performed by means of consistent models based on suitable stress fields. This is for instance the case of the variable-angle truss models, where the acting shear force is equilibrated by an inclined compression field and the shear reinforcement in tension. The application of such design expressions to squat members or when gravity loads are applied on the upper chord near to the supports, has however been observed to lead to overly conservative results. This is due to the fact that a direct strut can develop in the concrete between the load and the support without the need of being suspended through shear reinforcement. In addition, this leads to a more favorable cracking state and thus to enhanced shear resistance. Despite the fact that such phenomena can be also formulated in a consistent manner with the stress field method, empirical corrections of the resistance of slender members have traditionally been adopted to design beams when significant loads act close to supports. In this paper, the benefits of designing such cases on the basis of tailored stress fields are demonstrated. Such an approach provides a smooth transition between slender and squat members and, when compared to tests, leads to a significantly lower Coefficient of Variation than empirical corrections. On that basis, simple design expressions are formulated in an effort to serve as a revision for the next generation of design codes (Model Code 2020 and 2nd generation of Eurocode 2).