Per AASHTO LRFD code requirements unintended eccentricity is handle by the reduction factors 0.85 (spiral) and 0.80 (tied). The program however ignores this requirement and goes back to the old ASD code practice of increasing the moments by considering a minimum eccentricity. Why is this done and how is it acceptable?
In LEAP, the reduction factors 0.85 (spiral) and 0.80 (tied) are considered to place the cap on the usable of the column for unintended eccentricity. On the other hand, the min eccentricity is still being used in the column design. Typically, the moment due to the min eccentricity is triggered in the biaxial bending analysis. However, if the min eccentricity is triggered, it usually generates slightly more conservative in column design. Hope this clarifies.
Not entirely.
AASHTO LRFD 7th ed. C4.5.3.1 and C4.5.3.2.1. For “large deflection theory” a second order analysis is performed and…
“Further application of moment magnification factors is neither required nor appropriate. The presence of compressive axial forces amplifies both out-of-straightness of a component and the deformation due to nontangential loads acting thereon, thereby increasing the eccentricity of the axial force with respect to the centerline of the component.”
What benefit are we getting from the program when we do a second order analysis if it's going to go back and apply magnified moment minimum eccentricity requirements?