buckling lenght factor

huangyhg

buckling lenght factor
hi all
i have a question about buckling lenght factor for steel column.
i have building which is 9 storys height with moment resistant frame(mrf) on one direction and bracing on the other direction both are for lateral loads resistance.
there is a column which is a part of the moment resiatnce frame and on the other direction it is supported by x bracing and a steel beam at floor level(shear connection)
so i have assumed buckling lenght factor for column in direction of x bracing to be 1.
and i calculated buckling lenght factor for the column in the direction of the moment resiatnce frame using aligment charts.
the column is hinged on base. i got k=3.5!!
first question. using k factor=1 for direction of bracing is ok?
second question, i do not feel ok by using kfactor=3.5! in direction of moment resiatnce france because this will lead to extremely big section. so shall i assume k factor on mrf direction=2?
                                           thanks all
answer to first question:  yes.
answer to second question:  3.5 sounds awfully high--did you use g = 10 for the pinned base?  if so, your girder is probably not stiff enough, and the building would sway too much, anyway.  a stiffer girder would be in order.
daveatkins
the alignment charts are developed based upon "elastic" buckling. for columns with kl/r less than c'c, this is not a correct assumption. see example 3, pg 3-6 thru 3-7 and table a on pg. 3-8, 9th edition, aisc manual for the "inelastic" buckling procedure for reducing the k.
i hope this helps!
using k=2 is gives you a place to start when designing so you don't have to randomly pick sizes until you have something reasonable.  you should go back and check your assumption by computing the actual k factor with the sizes you've chosen and verify all your numbers.  you may have to do a few iterations to arrive at a satisfactory result.  if you're getting a k of 3.5 then you probably don't have enough stiffness in your frame anyway, as daveatkins suggested.
well thanks daveathinks and jike for quick reply,
yes i used g=10 for pinned base
jike you are talking about aisc-asd89 as a refrence for this k factor for inelastic buckling?
well jike i checked asd89 and found the example but there is a point not clear yet.
does the reduction factor equal to value fa/f`e?
i have diffrent grade of steel than a36 and a50 that are listed in the table, so what is the soure equation for values listed in the table?
note:- i am using steel with fy=275 mpa
the equations should be listed in the specification section. you may need to convert to ksi to make sure the equations function properly.
well jike fe is function of lenght of member and buckling lenght factor(k) and also raduis of gryation(r), so how can this table be general for any section case?