Moment Magnification - Sway Frames - ACI 318-08

[EngineerRam]

Hi,

The equation for moment magnification in a column in a sway frame in ACI 318-08 (same in 11) is M1 = M1ns + (deltas)(M1s), where deltas is the sway moment magnifier. My question is why does this equation not consider deltans, the nonsway moment magnifier? Shouldn't this be multiplied to M1ns, the end moment due to gravity loads?

Thanks

 

[ahypek]

The equation is used to approximate second-order effects by doing only a first-order analysis. This is assuming that the second-order effects are small which would mean that the second-order effects from gravity loads are almost completely negligible. If the calculated magnified moment exceeds a certain limit, you will have to perform a rigorous second-order analysis which includes all of these effects.