Questions about X bracing 2

[Leftwow]

Hello Structural Community!

Here is a background.
I have a scenario where I'm assuming existing bracing can't withstand compression and would work in a tension system only. I know that AISC is discouraging the use of tension only systems because of increased risk.

However I just have a few questions regarding bracing

1. What K-values are you guys using? I've read that you can use up to 0.7 as a maximum and most people are using 1.
2. If you have an WT section that is connected in the middle, can you set the Ly as half the brace length?
3. In the situation of 2, how can you set your Lz as half the brace length when you still have a k value of 2?

I understand if one brace goes into tension and the other in compression that the brace in tension is theoretically bracing the other brace in compression but wouldn't they have a tendancy to slide? Then put eccentric force on the tension member?

 

[JAE]

There's a great article in AISC's Engineering Journal - 3Quarter 2013.
Below is the front end summary and one of the concluding paragraphs:
If you are an AISC member it is free (or $10 if not a member)
Found HERE

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[WillisV]

If you are assuming it can't go into Compression then the buckling length is irrelevant.

 

[WillisV]

If you are actually concerned with bracing the compression diagonal with the tension diagonal, JAE's reference is one source, also AISC added the following to the Commentary of Appendix 7 in the 2016 Spec. (p. 16.1-569 available for free:

https://www.aisc.org/globalassets/aisc/publications/standards/a360-16-spec-and-commentary.pdf

PDF page 625)

 

[Leftwow]

Thanks so much guys. There's an issue with my AISC login, so hopefully that gets fixed soon. WillisV, still early in my career, just trying to have some questions resolved.

 

[JAE]

WillisV - nice reference - thanks for posting.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[KootK]

I went looking for JAE's paper and it googled right up, for free: Link. The link is to AISC's website so I assume that there are no intellectual property issues. If someone knows otherwise, do let me know.

With the AISC blurb, what do we make of the "discontinuous midpoint connections" bit? Can we safely assume that we're good to go with K=0.5 so long as one of the braces is flexurally continuous across the joint? Conversely, if the brace forces are about equal, are we really able to go with K=0.5 even when neither brace is flexurally continuous across the joint? Pretty great to have an official AISC statement on this now.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Leftwow]

Koot assuming you are using the k=0.5, instead of using it's equivalent of L/2?

 

[KootK]

Yup.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[WillisV]

Conversely, if the brace forces are about equal, are we really able to go with K=0.5 even when neither brace is flexurally continuous across the joint?

No. The base paper only had one or the other discontinuous, though I see how that could be confusing from the verbiage. 2022...