Limiting Slenderness Ratio
Limiting Slenderness Ratio
(OP)
From Aisc (Ninth Edition) Section B7, it is stated that for members whose design is based on compressive force, the slenderness ratio kl/r preferably shoud not exceed 200 and for members whose design is based on tensile force, kl/r preferably shoud not exceed 300.
I am designing a frame with cross bracing, primarily intented for lateral forces having beams that span up to 31 ft. due to architectural requirements.The Kl/r value governs in the beam design.
Is it ok to disregard the kl/r limit considering that the code uses the term preferably? What exactly is its effect to the member designed and the entire structure?
thanks...
I am designing a frame with cross bracing, primarily intented for lateral forces having beams that span up to 31 ft. due to architectural requirements.The Kl/r value governs in the beam design.
Is it ok to disregard the kl/r limit considering that the code uses the term preferably? What exactly is its effect to the member designed and the entire structure?
thanks...






RE: Limiting Slenderness Ratio
DaveAtkins
RE: Limiting Slenderness Ratio
These limits seem a litle odd to me, particularly the compressive case, which seems to be well into pure elastic buckling.
200:1 is /far/ beyond what is normally seen, and any slight errors in eccentricity of loading will render such a simple analysis optimistic.
The tensile case is interesting, again far outside what I would expect to see, but I can't see much harm in that so long as we are talking about rods, rather than sections.
If anyone out there wants to correct this , be my guest.
Cheers
Greg Locock
RE: Limiting Slenderness Ratio
The recommended limit of kl/r for tension members is intended to give a size of member that will have a minimum level of stiffness to avoid unwanted "slapping" or vibration of the member.
RE: Limiting Slenderness Ratio
The AISC LRFD 3rd edition commentary states that the Kl/r ratios for compression are based on Engineering Judgement and Economics. One should look at this commentary for further information.
RE: Limiting Slenderness Ratio
In our Indian code, the conditions of l/r are binding and so I would expect from other codes too. Ignore such limits at your own risk because if the structure fails due to fall of a meteor on it, you will be held liable for crossing the line. It is preferable to ignore the word "preferably", as you cite, just to be on the defendable side.
RE: Limiting Slenderness Ratio
Have fun!
RE: Limiting Slenderness Ratio
AISC 9th Edition: B7, Sect. 5-37
"...The above limitation does not apply to rods in tension. Members which have been designed to perform as tension members in a structural system, but experience some compression loading, need not satisfy the compression slenderness ratio."
In context, the paragraph says above described members need satisfy KL/r<300, but not KL/r<200.
RE: Limiting Slenderness Ratio
Stouter member or second order analysis? Easy decision in my book.
As far as the tension member case? I've used swaged & turnbuckled wire rope for tension-only bracing... where does that fit in?
RE: Limiting Slenderness Ratio
I think that the preferable for compression is more important than the preferable for tension.
RE: Limiting Slenderness Ratio
RE: Limiting Slenderness Ratio
It is interesting to notice that from kL/r=100 to 200 (kL/r=100 is somehow corresponding to the 0.5Fy point, which is the elastic limit of AISC) Pr decreases substantially, whereas from 200 to 300 the decrease is not very significant at all. As stated by 3dboy, I guess the 200 limit is not solely from the concern of pure axial strength, it is for dealing with the additional bending moment, construction imperfection, etc
RE: Limiting Slenderness Ratio
If your "lateral forces" include earthquake forces you had better pick up a copy of AISC Seismic Provisions because you have a completely different set of kl/r provisions depending on your type of lateral system.
If your braces are "tension only", check the building height restrictions of the ASCE 7. In many cases you are required to take some degree of compression. In addition your kl/r is sprecifically restricted for the bracing members. No preferably less than language.
Do your lateral forces include earthquake?
RE: Limiting Slenderness Ratio
For your second-order analysis, make sure you add a bunch of nodes along your beam. Some programs are not smart enough to capture P-(little delta) effects.
You should read the 2005 AISC Spec. I think there might be some changes to the KL/r limitation wording.
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