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Base shear V is calculated using NBC 4.1.8.11.(2)
V = S(Ta) Mv Ie W / (Rd Ro)
where
S(Ta) is determined from 4.1.8.4(9) as
S(Ta) = Sa(Ta) F(Ta, PGAref, Site Class)
where the Sa and F are determined for discrete values of Ta as described in Spectral Response Accelerations and Site Coefficients. Interpolation is used to evaluate S at Ta, as described in 4.1.8.4.(9).
Maximum Base Shear
As per 4.1.8.11.(2).(d), unless the Site Class is F, or you entered an Rd < 1.5, the program limits the base shear to the larger of the following expressions
2/3 S(0.2) IE W / (RdRo)
S(0.5) IE W / (RdRo)
Minimum Base Shear
The program does not check for a minimum base shear based on 4.1.8.11.(2).(a), below, because Ta = 4.0 is far higher than the fundamental period allowed in wood frame structures designed by Shearwalls.
S(4.0) Mv Ie W / (Rd Ro)