Does STAAD always consider the axial force in both compression and tension when computing the design output for Chapter H of AISC 360-10? I have a member in a structural model with an axial compression load resulting in a ratio, Pr/Pc, of 0.203 and a minor axis bending ratio, Mry/Mcy, of 1.92. Reviewing the design output, STAAD reports that Eq. H1-1b controls for flexural tension using a tensile force of 0 resulting in a DCR of 1.92. However, the member has a compressive axial force and STAAD reports the flexural compression DCR as 1.91 using Eq. H1-1a. The difference between the DCR values from the two equations is small but I am trying to determine if STAAD is relying on the scenario (tension vs. compression) with the non-zero axial force as controlling when the results from the two equations are compared or if STAAD is considering the equation with the non-zero axial force as a viable design equation. AISC 360-05 returns the same results.
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