Review of Basic Assumptions for the Shear Design

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Title: Review of Basic Assumptions for the Shear Design

Author(s): K.-H. Reineck

Publication: Special Publication

Volume: 265

Issue:

Appears on pages(s): 367-384

Keywords: codes; concrete ties; concrete contribution; crack friction; shear design; structural concrete; truss model.

Date: 10/1/2009

Abstract:
There are two basic assumptions for the shear force Vc in ACI 318-02: 1) Vc is the shear force at cracking; and 2) Vc 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 Vct of the shear capacity for members without shear reinforcement and Vc for members with stirrups. The size effect has to be considered for Vct for members without shear reinforcement, but plays a minor role for Vc for members with stirrups. Chapter 4, "Truss model versus Vc-term" shows that there is a clear relationship between the angle q of the inclined struts and a Vc-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 Vc has a clear mechanical meaning as the vertical component Vf 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 q and less amount of stirrups than that for reinforced concrete beams.