X bracing buckling length 4

[saad73joss]

Hello,
(sorry for my english)
I have an X bracing system. the 2 braces are connected in the middle. the X bracing can work in compression and tension.
I attached a sketch
I would like to know the value of ky and kz for buckling length in both planes.
Lfy = ky L and Lfz = kz L.
Of course I will appreciate if you refer to a text whatever it is.
Many thanks

 

[ToadJones]

There are a few schools of thought here but I usually make ky and kz = 0.5.
The logic being that as one brace is in compression the other will be in tension and not allow the compression brace to buckle out of the plane of the page (in your sketch) or buckle in the weak direction. It wouldn't make sense either for the bracing to buckle in the strong direction.

 

[WillisV]

There are numerous papers on the subject, but as ToadJones states the conclusion after much reading will be in most instances an effective length of 0.5 may be used.

If you still care to go through all the reading and reach this same conclusion, feel free!

"Design of Diagonal Cross Bracings" Part 1, 2, and Discussion. Picard and Beaulieu. Engineering Journal, AISC 3Q 1987, 4Q 1987, and 4Q 1989,

"Effective Length Factor for the Design of X-bracing Systems" El-Tayem and Goel. Engineering Journal, AISC 1Q 1986

Or if you care to go with energy methods:

"Practical Application of Energy Methods to Structural Stability Problems". Shankar Nair. Engineering Journal, AISC 4Q 1997

 

[saad73joss]

Hi ToadJones and Willis V
Actually the issue is the buckling out of plane. I did kz=0.7 instead of 0.5 just to keep some security.
The problem is the Control Office Socotec insists in puting kfz = 1.00 !! do you imagine so i was obliged to justifie many things....
I thank you so much for the references given, really thank you.
I will look for thoses books in google...
Cheers

 

[JAE]

This is from a previous, similar post: "We just did a literature search on the concept of using the tension brace to laterally brace the compression element. It sometimes can do the job but depends on a lot of factors. What I did learn was that you cannot always say the tension brace adequatly does the job. Do a search on Modern Steel Construction and there are some Q/A responses that lead you to some articles that AISC publishes. We've scaled back to use 80% - 100% of the total compression length in our compression brace design - with a k = 1.0."

Go here for the actual thread: thread507-293728

 

[WARose]

That's the way I've always done it JAE (just disregarded any help from the tension diagonal). I remember the last time I tried that [i.e. using anything less] it just turned into a research project (for each brace) that I didn't have time for.

 

[ToadJones]

I guess you could look at this from a stability bracing standpoint of APP 6 in AISC 360-05.

 

[fancypants]

I believe ToadJones hit the nail on the head. Use app 6 and if you need lots more info, look up the papers from Yura and Helwig on stability bracing. The supporting brace, in this case, the tension brace, has to have enough stiffness out of plane to prevent the compression member from buckling.

 

[fancypants]

Thinking back, you could use the old rule of thumb that the supporting member needs to have about 2% the stiffness of the member being supported. Thus the bending stiffness of the tension brace would need to be 2% the axial stiffness of the compression member. This does not always end up being the case as has been stipulated in the mentioned papers. However, it has been done for a long time.

 

[ToadJones]

Fancy-
APP 6 maybe even easier if you use A-6-3....where Pbr = 0.01Pr.
This is only 1%.

 

[paddingtongreen]

I always used k=1 for the long member and the two short ones.

I reasoned that although the tension brace might have the strength it probably didn't have sufficient stiffness, especially when the short members were in tension.

It is all very well to say it can resist 2% or 1% but how much does it deflect? What does that deflection do to the compression member?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[fancypants]

I believe the compression member trying to buckle would cause the deflection. Thus, the bending stiffness would have to resist the buckling of the compression member.

I did not say this before, but I would not use the tension member as a brace just for feel good reasons. Looking back again at the provided jpeg, the one brace is attached by a gusset running by the continuous brace. With that in mind, you would now have to take the gusset stiffness into consideration with the brace. Not to practical in my opinion.

As a matter of fact, i would not use the pictured set up as compression tension braces because of the gusset. A better solution would be to rotate the braces 90 degrees with the strong axis facing out of plane. Then it would be an easy matter to brace them in plane.

 

[dik]

and if you get really fancy... the compression member can have post buckling strength after the 'base' has translated. the base being the tension bracing.

The Aluminum Design Manual published by ALCAN has a variety of bracing conditions and if there are two fasteners at the end of a X brace, then they use a value of 0.45 times the diagonal length.

Just looked at a recent project, and the engineer has used HSS round tube for x-bracing. The ends and the mid point have been flattened and bolts connecting the ends to the structure have been punched through the flattened part and a single bolt through the flattened mid point. I'm going to have to check that for my next project.

Dik

 

[WillisV]

No need to speculate or re-invent the wheel, the paper references I posted above cover the subject quite thoroughly and are really not that hard to follow and implement in design practice. Also since they are all EJ papers they are free to AISC memebers as well.

 

[saad73joss]

Thank you all
Really intersting to go through all your comments
Actually I did 2 calculations before I discover WillisV documents.
I took 2% Fancypants
I calculated the deflection out of plane and then I introduce a bending moment due to excentricity to calculate the brace regardeless the bukling length
But I checked the brace in compression and I found 0.7 L even conservative

By the way, you are lucky because the frensh code is not developped, they try. Actually Europe except UK are more concrete than Steel
Again many many thanks

One more question: I have AISC manual steel but I did not manage to find " APP 6 in AISC 360-05" any help to download it

 

[ToadJones]

AISC 360-05 is the 2005 edition of the Steel Manual (Black Book)
Appendix 6 is in the specification portion of the book.

 

[saad73joss]

many thanks

 

[JLNJ]

I like to use the entire length so that when you get a call from the field that they cut one leg off to get a door in you still have some strength!