K-bracing v. X-bracing 3

[vincentpa]

For steel braced frames, what is the maximum aspect ratio (bay length/story height) that you would use for X-braced frames? What is the maximum aspect ratio you would use for K-braced frames? When do you typically use each one (i.e. in which situation would you choose K-bracing over X-bracing)? I am designing a building with a 40' bay and a 15.5' height. What drift limits do you typically use, 0.0025 x story height?

 

[J1D]

I don’t know if there is a maximum aspect ratio suggested by any codes. In practice, if the ratio is close or larger than 2.0 x-bracing will look odd , and you need more x-bracing pairs to reach certain height. In this case K-bracing is better. On the other hand, if the ratio is close to 0.5, x-bracing is better.

Both bracing types provide effective lateral support. They do have some differences, however:

1. x-bracing introduces extra restraint into the frame, with x-bracing, you can then design “tension only” bracing members. But if the structure is exposed to temperature, extra stress will be generated in the braces, beams and columns nearby.
2. with K-bracing the structure is close to a static state, no significant temperature load although they are exposed. But a beam will get involved into the bracing, significant axial load will go though the beam. In addition, the gravity load on the beam will cause considerable axial load into the bracing. But, on other hand, this might be beneficial sometimes, if you need vertical support help from the bracing to the beam.
3. For a K-bracing, since the unsupported length in lateral direction is much larger, and it is in the weak-axis bending direction, a lateral bracing in the mid span might be required.
4. In industrial structures, in lots of cases, the bracing type is determined by not to cause clash to pipes, equipment, etc.

For the building you’re designing, a K-bracing seems good. The drift should not be a problem (1/500 is the storey drift limit I use).

 

[Taro]

Just for clarification, I believe the configuration you are asking about is "chevron bracing" or "inverted V-bracing", not "K-bracing". K-bracing (where braces connect to the columns at mid-height) is discouraged because of the potential for collapse of the columns when the compression brace buckles.

In general, I try to select configurations where the angle of the brace is between 30 degrees and 60 degrees from horizontal. Extreme brace angles can be less efficient and may cause messy connection details.

 

[WillisV]

Another option for a wider bay is to use multi-story x-bracing concepts which are extremely efficient (e.g. chevron at the first story and inverted chevron at the second story)

 

[vincentpa]

Yes, I should've written chevron bracing. I mistakenly thought K-bracing and chevron were the same thing. Thank you for the clarification.