K-value AISC 13th edition help

[alphaxy]

I am currently using 13th edition. however i found a large values for K compared to 9th edition of AISC. this is because the moment for each beam is considered. Does this happens to all of you when designing the steel? I am doubtful that I'm at the wrong track since my K-value is large. can anybody tell me how to do this quickly and easily but correctly.


thanks in advance
alphaxy

 

[Lion06]

The method should be the same when calc'ing k for the 9th edition or the 13th edition. Are you sure you're using the same nomograph (sway frame vs. non-sway frame) when comparing values from the 9th vs. the 13th editions?
Also, I don't think the moments in the members contribute to the calculation of the k value, only the stiffness of the members.
If you have a sway frame, k will always be greater than 1.0. If you have a non-sway frame, k will always be less than or equal to 1.

 

[alphaxy]

thank you StructuralEIT..your comment about 9th edition the way it should be is entirely correct, but it seems the method for calculating k-value in 13th edition is different although it uses the same nomograph. the stiffness of a member for sway frames rely on a modified length which considers the moment in beams which is used to calculate Ga & Gb for beams = I/L then the same way for columns = I/L. Im confused with this method. should i use modified length only for beams = I/L or also for columns = I/L???

thanks in advance

 

[Lion06]

Yes, I realize exactly what you are saying now. I looked at this pretty intensively because we do a lot of Flexible Wind Connection buildings which means the leeward connection is a pin under lateral loads. For sway frames, the introduction of the Mf and Mn is an attempt to incorporate the actual fixity of the beams as opposed to called it fixed or pinned. I would definitely take this into account since anything less than 100% fixity will increase the effective length of the girders (and reduce the stiffness they contribute to teh column). This will essentially increase the k value, so it would be unconservative to ignore it.
I hope that helps. If I failed to answer your question, let me know.

 

[JoshPlumSE]

Um, why are you even using the K value? Based on Chapter C and Appendix 7 in the 13th edition you should be able to eliminate the need for the K value almost completely.

Eliminating the use of K values is actually one of the real benefits of using the 13th edition.