timber frame knee brace 2

[poseiden]

Considering a simple post and beam frame, can knee braces be used to reduce the clear span between the posts? Currently, the beam is undersized for the uniform load and the distance between posts. Is this an 'acceptable' solution?

 

[DaveAtkins]

Could be. But you are putting a horizontal thrust into the post, as well as an axial force into the beam. These must be checked.

DaveAtkins

 

[poseiden]

Thanks Dave
I was hoping for some more input from others or a link to an analysis.
I can understand the the horizontal thrust applied to the post. But, how much of the load from the beam does the brace transfer. It wouldn't necessarily be all the uniform load on the beam to the midspan. The question is - is how much? Since the beam is supported on both ends with posts, seems that the brace starts taking up some load when the beam starts to deflect.
If I had a post with a horizontal member supported with a brace, it would be fairly easy to figure the load transfered into the brace(half the distance from the post to the brace, plus any cantilever from the brace outward).
Any help or links on knee braces would be appreciated. I've tried structural timber searches without much luck. I also get alot of medical "knee brace" results. thanks

 

[SlideRuleEra]

poseiden - Here are a couple of links that may help. Not quite what you are looking for, but a start...

http://www.aisc.org/MSCTemplate.cfm...tomSource/Faq/SteelInterchange.cfm&FaqID=1887

and

http://www.hnd.usace.army.mil/techinfo/ti/809-04/ch7b.pdf

see page 7-54

http://www.SlideRuleEra.net

 

[poseiden]

Thanks SlideRuleEra-
asace pg.8 says that they don't address wood braced frames; and the other also deals with steel. Thanks, but I'll keep looking. One of the Wood Associations had a pdf document with a section on knee braces. But, when I opened the pdf, all the sections just had headings with the remainder of the pages blank. When I searched the Association's web site, I could't find it. Seems there should be more on timber braced frames.

 

[apsix]

The load resisted by the brace will depend on the relative stiffnesses of the beam and columns (and also of the axial stiffness of the struts to a very minor degree). To determine this a frame analysis would have to be done. I don't think there is an easier answer.

With regard to the load taken if supporting a cantilever, I think you will find that the reaction is (w L^2)/(2 l), where;
w = udl
L = total length of beam
l = span between supports
This is not equal to half the distance from the post to the brace, plus any cantilever from the brace outward.

 

[poseiden]

Thanks Apsix-
You're probably right about the reaction for the cantilever. I was simply trying to convey that a reaction for the brace can be easily found with a known uniform load, and only 1 post/column.
But, when the beam is supported at both ends, the reaction (for the brace) is more difficult. Your suggestion with a frame analysis using the stiffness (modulus of elasticity-E) of the beam is probably necessary. I do have values(E) for southern yellow pine, as well as the uniform load,spans, and posts. Unfortunately, I'm unfamiliar with the proceedure for a wood frame analysis.
Any links would be helpful. thanks

 

[DaveAtkins]

You really need at least a 2D analysis program to do this correctly. You COULD use moment distribution, but I barely remember how to do it anymore, and it is too hard to explain in this forum.

DaveAtkins

 

[SacreBleu]

Enercalc.com has a free 2D frame analyzer download called FastFrame.

 

[AggieYank]

Analyze the brace to see if it is stiff enough to act as a brace. I would only use the live load when calculating this, but make sure they installed the brace after all the dead load was in place.

If you're looking to change your unbraced length, then a simple brace such as this isn't effective, since it won't guard against lateral torsional buckling (no resistance in the out of plane direction).

Like others have said, if you are trying to change the actual clear span length of the member being supported, a frame analysis is required.

 

[SacreBleu]

Wood knee braces are unreliable, due to cracking at connectors, etc. I would be very careful...using lag screws and steel gusset plates.

 

[poseiden]

Thanks All-
Your comments are well taken.
Yes, basically we're trying to reduce the clear span of the timber beam as to not exceed the allowable moment.

SacreBlue- Thanks for the link on the frame analysis. Yes, you're right on the concern for the connections as well( I didn't want to complicate things initially). In old timber frame structures, braces were primarily used for lateral stability, and mortice/tenon joints were used with wooden pegs. The joints were just as important as the frame. But, the contractor/timber framer now (and on pevios jobs) uses lag screws on the braces. This simlifies the mortice/tenon joint. I've seen this used on the ridge beam on a previous timber frame house he's built. The exterior skin of the house are the 6 inch foam insulated srtuctural panels, which provides lateral stability.
Stiffness of the brace isn't a major concern since they are 6x6 inch southern yellow pine (which is a stonger wood).
Posts are 6x6, and the beam is 6x12 inch deep (15' span).
Cracking at the connectors is a good point to consider. Luckily, the wood is already kiln dried, and the species is stronger than eastern white pine, which was commonly used in the past. Now, I still need to consider the lag bolt connection. 2 are used at each end of the brace.
This seems to get more complicated than just using a deeper beam( I figured we needed a 6x16 to do the job, but it's not available). thanks again