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May be my questions seem stupid but just for the sake of disucssion today I have posted this to Mr. Arshad Khan, Civilea forum and SEFP forum. Looking forward to arrange all the answers by you guyz and write a tutorial Dear all, I have following question regarding slenderness in ETABS 1) Why moment magnification method and 2nd order elastic method both are used by ETABS. I mean in ACI there are 3 methods to do (any of them) then why we put modifiers and then ETABS calculates internally the moment magnification factors. 2) If structure is braced then how to stop etabs from calculating moment magnification? 3) If drift limits are defined for example in ASCE H/400 then why is the need to perform P-delta. If we control the drifts to be H/400 still we need to perform p-delta? 4) We take wind drifts for 10 year return period. that is multiplying 0.7 to W of 50 year year..to convert this into service level....What type of modifiers are required to be put in ETABS to take drift or deflections on this 10 year Wind.. Serive modifiers Beam=0.5 & Column=1.0 or Ultimate modfiers Beam=0.35 & Column=1.0 I mean as per ACI we can multipy the deflections from ETABS to 1.43 if we use strength level modifiers?

I need to dig again not sure about load combination but I think you are right about only considering EQX and EQY with eccentricity.

Yeah, except Ig=0.5 for beam is not correct value for checking drifts against wind loading. It will overestimate drifts; 1.0 (for beams) is allowed by code but one can use value closer to one, like 0.8 to have conservative estimates of drifts. Drifts should be checked against all load cases involving lateral loads (9-3 to 9-7 which ever is applicable).

first point: Both methods addresses same behaviour. Moment magnification is a term used when we are taking second order effects due sway of structural system while elastic second order method(Euler) addresses slenderness of individual members. ACI code directs us to reduce stiffness of members when computing forces due to second order effects so ETABS requires us to specify stiffness modifiers and then it can calculates theses effects . Second point: Code uses Stability Index or Pu/Pc as tool to estimate magnified moments and second order effects can still be there in braced frame as there might be some slender member (Euler's equation is used in that case). Third point: Yeah one must check for slender members. I think, limiting values for drift control are not there to control structural damage instead they are for non structural damage(citation needed): hence it does not mean that there are no second order effects. Cant comment on remaining material.