Unbraced Length of Wood Beams

[phamENG]

I searched the forum, but all I can find are questions about wood framing bracing steel. I'm looking for wood framing (or even GWB) bracing wood beams. AISC provides us with the very useful Appendix 6 for calculations of required strength and stiffness of bracing members for steel columns and beams. Does anyone know of a similar resource for wood beams? Would it be appropriate to go to the commentary for AISC App. 6 and use the equations there, modifying as needed for rectangular wood rather than steel beams with flanges? My thought is to to use the entire compression region as my compression "flange" and substitute the appropriate values. h[sub]o[/sub] would be distance from compression centroid to tension centroid.

I realize a lot of people probably don't pay a lot of attention to this check, but I'd like to get a better understanding of it so I can establish my own judgement before calling it "good by inspection." Thanks.

 

[jayrod12]

You should not be using AISC for wood members at all. Their guidance for bracing, i.e. 2% may be a good idea to meet but otherwise you should be using the NDS.

Generally speaking, wood members don't have buckling issues as they are typically fairly wide when compared to the depth. Also, they typically have members framing into them at a fairly small and typical spacing, 16"/24"/32", and therefore the unbraced length doesn't come into play.

If you are checking this for a specific case, perhaps you could provide a little more information and we can help more.

 

[KootK]

Wood lags behind steel in terms of its treatment of stability bracing. That's likely because, as jayrod mentioned, typical proportions relative to allowable stresses make wood members less prone to stability issues. Still, were I to attempt to dive into the weeds on this, as you seem to want to, AISC app 6 would me my goto reference as well. It's state of the art and should be adaptable with a little judgement.

The most popular US wood text on the market still lists inflection point bracing as valid for Pete's sake. Boo.

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.

 

[phamENG]

Thanks, jayrod. I only bring up AISC as it's the only resource I'm aware of with specific design recommendations/requirements, and I'm trying to understand the fundamental theory as it applies more generally, and then specifically to wood. I know that those calculations are based on some initial assumptions specific to steel construction standards, but I couldn't come up with a better starting point. (Don't worry - I won't be designing any wood beams with Fy=50ksi, or any Fy for that matter.)

NDS tells you to consider the unbraced length, but then doesn't define the requirements to consider a point "braced" beyond saying that it must prevent lateral displacement. I'm looking for a way to quantify that. I realize that, in most cases, it's not a necessary check for the reasons you mention. I'm interested in the cases when it does become important, but if I'm unable to quantify the requirement I'm not sure how to identify those situations. Thanks again.

 

[phamENG]

Thanks, KootK. Glad to know I'm not too crazy (stress on the 'too'). I admit, I've been admonished for being too thorough at times - but I still think it better than the reverse. This came out of a house I'm working on that just made me think "what if those 3 nails aren't enough..." and down the rabbit hole I fell. I'll keep a close eye out for more obviously questionable scenarios and come back if I have a good one for more specific discussion.

 

[UKJim]

I would generally be comfortable with a point brace designed for 2% of the total compression stress resultant in the beam. The "2% rule" has been around for a long time and has served the profession well.

If the beam in question had an extraordinary amount of sweep, then I may reevaluate this approach. But for an initially straight beam a brace load of 2% seems like a conservative approach to me.

The total stress resultant could be calculated using MQ/I.

 

[jayrod12]

Keep in mind that a couple of nails easily gets you 400 lbs of load transfer in direct shear, therefore good for 20 kips of "chord" force. I'd bet you'd be hard pressed to find a wood beam working that much.

 

[WARose]

I think one of the reasons there hasn't been much demand/push on this is because even some pretty "flexible" (plywood) diaphragms can give you the stiffness you need. We are talking on the order of 10-40+ kips/inch. That is stiff enough to brace pretty much anything in wood.

 

[Tmoose]

"wood beam"

http://www.awc.org/pdf/codes-standa...-WCD1-ConventionalWoodFrame-ViewOnly-0107.pdf

http://www.survivorlibrary.com/library/light_and_heavy_timber_framing_made_easy_1909.pdf

 

[RFreund]

A couple things I dug up:

http://files.engineering.com/getfil...b4&file=Stability_Bracing_of_Wood_Members.pdf

http://files.engineering.com/getfil...4c0f9ce042&file=Stability_Factor_AWC_TR14.pdf

EIT

http://www.HowToEngineer.com

 

[phamENG]

RFreund,

Thank you! The first document is just what I was looking for. Sadly the second still states "design of lateral and/or torsional bracing for wood beams is currently beyond the scope of the document," but it is encouraging and goes on to state that further research is being considered to "quantify and develop design provisions for lateral and torsional bracing systems." It also provides an in depth review of the code provisions and the theories used to back them up, which is always valuable.

 

[Shotzie]

This thesis Link gets into the lateral torsional buckling capacity of a wood I joist. I skimmed it and didn't see a specific load that would need to be resisted, but they did report the amount of deformation recorded while the beam buckled, it could help in determining the load required to be resisted to prevent LTB.