Buckling Lenght Factor

[dawn836]

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

 

[DaveAtkins]

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

 

[jike]

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!

 

[UcfSE]

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.

 

[dawn836]

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?

 

[dawn836]

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

 

[jike]

The equations should be listed in the specification section. You may need to convert to ksi to make sure the equations function properly.

Yes, if I remember correctly, the reduction factor is fa/F'e.

 

[dawn836]

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?