Review of Basic Assumptions for the Shear Design

摘要

There are two basic assumptions for the shear force V_c in ACI 318-02:1) V_c is the shear force at cracking; and 2) V_c is the same for members with and without shear reinforcement. By reviewing tests, it is shown that neither assumption is valid. Therefore, different terms have to be defined for the concrete contribution V_(ct) of the shear capacity for members without shear reinforcement and V_c for members with stirrups. The size effect has to be considered for V_(ct) for members without shear reinforcement, but plays a minor role for V_c for members with stirrups.rnChapter 4, "Truss model versus V_c-term" shows that there is a clear relationship between the angle θ of the inclined struts and a V_c-term. A more realistic model for the state of stress in the web, however, is described by the "truss model with crack friction," where the crack angle is different from the strut angle or angle of principal compression. There, the term V_c has a clear mechanical meaning as the vertical component V_f of friction forces along the inclined crack. In the web, an inclined biaxial tension-compression-field exists, so that the usual truss model is superimposed with a truss model with inclined concrete ties. Because the crack angles are considered, the shear design for prestressed concrete beams leads to a lower value for the angle θ and less amount of stirrups than that for reinforced concrete beams.