The latest version of the ACI Building Code requires five equations to prescribe the limiting horizontal shear stress for differing amounts of reinforcing steel. The test results from the 16 beams tested in this study indicate that a more consistent limit can be obtained by replacing four of the present equations with a parabolic equation modified from the one used in the PC/ Design Handbook. The proposed equation combines the effect of concrete strength and clamping stress. It is equally applicable for lightweight and semi-lightweight concrete. The test results indicate that an as-cast surface with he coarse aggregate left protruding from the surface, but without special efforts to produce a rough surface, can develop adequate shear resistance and simplify production of the precast concrete beams. Also, the tests show that stirrups are typically unstressed and ineffective until horizontal shear stresses exceed 1.5 to 2 MPa (220 to 290 psi).
