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For the purposes of this discussion, one-way (or vertical) shear is defined as the shear force perpendicular to the axis or span of the member. Also, the discussion in this section is limited to precast concrete members that span horizontally and are governed by the ACI 318 Specification. Note, one-way shear is how it is defined in the ACI 318 code, it is also known as beam action shear.
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Basic Equation
The shear resistance of precast concrete components must meet the following requirement (22.5):
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For non-prestressed members, the analysis uses the appropriate equation based on the sign of the of the axial load (compression – 22.5.6, tension – 22.5.7, or none – 22.5.5). Note that both the ‘tension’ and the ‘compression’ equations become the ‘no axial load’ equation when the axial load is equal to zero. For both no axial force and axial compression, both the detailed and the simplified methods are used, and the program selects the greater of the two values (the axial tension section does not include a detailed method for the calculation of Vc).
The option exists to limit d to the same minimum as prestressed, 0.8h. Although this does not exist in the code, this can give users closer to expected results for layers with compression reinforcement. Often beams with compression reinforcement do not develop a compression block deep enough to put them in compression, for these cases the d from flexure is often very low.
Prestressed Members
For horizontal prestressed members, the analysis uses the traditional Vci/Vcw equations to determine the shear capacity of the concrete (22.5.8.3). The approximate method for calculating Vc as shown in the equations tabulated in the ACI 318 code (Table 22.5.8.2) is typically reserved for members that span vertically and are therefore unavailable for horizontal members in this analysis. The nominal concrete shear strength determined using this approach includes the upper and lower limits as noted in ACI 318.
Minimum Shear Strength
The concrete shear strength is also assumed to have a minimum strength equal to the plain concrete shear strength as defined in Table 14.5.5.1.
Calculation of Vs
When no transverse reinforcement has been defined, the amount of shear required to be resisted by shear reinforcement, Vs, is calculated by:
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