'X' Bracing/out of plane bracing Analysis

[mrpid]

To all,

I am curious how most engineers handle the analysis of 'X' bracing with regard to lateral support out of plane.

I noticed the "Tension Only" thread as well, so I thought it might be a good time to start a discussion on vertical bracing. I typically use RAMAdvanse, but have used other 3D analysis such as STADD and RISA. When building a model I typically do NOT add a node at the intersection of the 'X', otherwise I would have to manually input the effective length of the brace, as I do not consider the members braced out of plan. Now of course Tension Only members will avoid this all together; however, your reactions will be higher having only half the members doing the work (half being relative to a symmetrical structure).

I have also read articles, where a certain amount of out of plane lateral support can be provided to the compression member by the tension member, based on the connection and the continuity of the bracing.

Again, I was wondering how others view this and their approach to design.

Thanks

 

[DaveAtkins]

I consider the compression diagonal to be unbraced over its full length.

A colleague of mine believes the tension diagonal does brace the compression diagonal.

So I guess I'm not helping much, am I?

DaveAtkins

 

[WillisV]

Answer: it depends on the stiffness of the braces and the configuration of the connection. See the papers referenced here:

http://www.aisc.org/SearchTaxonomy/EngJournal.aspx?id=7806

 

[WillisV]

Err..try this one:

http://www.modernsteel.com/steelinterchange_details.php?id=386

 

[vandede427]

It's a beam-column. Assume it is not braced by the tension member. Apply in-plane MWFRS wind forces in compression. Apply C&C wind forces in bending.

 

[JAE]

If you don't use tension only members for your X brace, you have to be careful that your compression brace is modeled correctly (or eliminated if it is a tension rod).

Even if you use a rod in an X brace, a typical analysis program will not "know" that it is a flimsy rod that doesn't do anything in compression and will, within the solution matrix, solve the model assuming the compression rod takes axial stress just like a tension rod....and your results will not be realistic.

In answer to your main question, we usually model as tension only braces - for rods and angles and such. For heavier sections, where we do want to take compression, we use the intersection as a brace point in the code strength derivation.

 

[DaveAtkins]

To avoid the problem JAE brings up (here I am getting on my soapbox again!), use the analysis software to do the ANALYSIS, then take the results and DESIGN the members by hand.

DaveAtkins

 

[mrpid]

Thanks for the various inputs, as I like to hear how others handle different scenarios. I also wanted to start this thread as a common mistake I have seen with engineers I have worked with is understanding when they put a node on a beam/column/bracing within design programs seem to read it as a brace point whether it is or not. The programs are getting better and there are manual overides to avoid these conflicts, but it is something engineers should be aware of (so a post on a forum other's can read and add information is the way to go). And Dave, I will stand with you on the soapbox. I believe that it is very important and highly advisible to be able to reproduce the outputs from software applications by hand. I do not think everything needs to be designed by hand as that is the advantage of design aid software, but you need to understand the software otherwise bad input = bad output.

WillisV,
Thanks for the article very interesting read.

 

[burtonSTR]

I remember AISC recommander KL=0.8L.
A more exact solution is
KL=L1+L2(2m-1)
m=sqrt(0.523-0.428(T/C))>=0.5
T=force in tension bracing
C=force in compression bracing
I would like to use K=1 if no special reason. A little conservative but simple