The ACI design code is dangerously unconservative when applied to the shear design of large concrete beams and one-way slabs constructed without stirrups. The ACI method is capable of accurately predicting Vc of members without stirrups only if their height is similar to the original experimental set used to calibrate the equation. To provide an adequate level of safety, the current ACI equation for Vc must be replaced with an expression that can account for the size effect in shear.
The primary mechanism of shear transfer in slender beams constructed without stirrups is aggregate interlock. It is incorrect to assume that the entire shear force is transferred in the compression zone. Clearly, then, any shear design provision that claims to be theoretically sound must be based on a theory that recognizes the critical importance of aggregate interlock in shear transfer. In this regard, the SMCFT represents a clear improvement over the ACI method.
The risks of basing design code provisions on empirical relationships rather than sound theory have been demonstrated in this paper. Developers of structural design codes owe it to the profession to base provisions wherever possible on rational theory rather than risky empiricism.
Figure 8: Calculation of Shear Carried in Uncracked Compression Zone, Specimen SB-10-N-2 References
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