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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 lowBeams without any mild reinforcement in tension (that is an undefined d value) will use 0.8h in place of d. This is not supported by the code and typically only happens at the ends of the beam, where shear is not typically checked.
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.
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