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The yin & yang of braces #4312 10/21/99 04:03 PM
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bartpop Offline OP
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I have witnessed two completely opposite methodologies when it comes to resolving the static forces acting in a timberframe through the braces. An engineer tells me that I should treat the braces as if they were posts; Essentially that a beam that joins a brace below can be divided into two separate beams; one beam between the post and brace, and one beam between the brace and the far end of the beam.

At first this made sense, but after I thought about it I realized that if this were true, braces would almost always be carrying huge loads and would be some of the largest timbers in a timber frame, and inspection tells me that this is not the case.

Fast forward to last night. I was checking out a book on "timber engineering". Primarily a book on designing glulams and metal connectors, etc., but with a brief section on "traditional joinery". Their text stated that they considered braces strictly as members for resisting lateral loads -- wind, seismic, etc. When they worked their calculations for beam sizing, joint shear etc, they sized the beams as if the braces weren't even there. They concluded that since beam shrinkage would likely increase the distance that braces would need to span, and the braces would stay the same length, considering the braces for weight bearing was dubious.

This made much more sense to me. Although, I think that the truth lies somewhere between both methods.

Anyone with more experience want to chime in?

Re: The yin & yang of braces #4313 10/24/99 12:55 PM
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Rudy R Christian Offline
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I'm not sure the "truth" lies in between these two methodologies. It may instead connect them at a diagonal.

My understanding of correct use of braces requires they exist in pairs (in general). For the most part the loading condition that requires bracing, wind for example, will likely work from more than one direction from time to time. In effect this means braces in pairs actually function as one brace. When a lateral load is applied to the system, one half of the brace pair goes into compression while the other goes on vacation.

When this condition occurs, the brace indeed can assume a tremendous load, but unlike most timbers in the frame, the load is assumed in true compression allowing a small section member to do a great deal of work. The timber which accepts that load from the brace however normally ends up dealing with bending and shear forces. For this reason I have been taught to think of braces more as pry bars than posts.

My experience with barn frames has shown that braces do indeed resist static loading as well as dynamic. Purlins in major/ minor roof system are a good example. The braces are typically placed across from one another at the purlin posts which allows them to work directly against one another. Typically the brace legs are increased with the span of the purlin. Of course this pairing doesn't work at the gable end, but often the tie beams are hewn to "camber" making them much wider than they are tall at midspan. This works well to resist thr thrust of the brace.

From a load study point of view, I do consider it practical to reduce the span of a member by one brace leg if it braced at both ends. If however it is only braced on one end, as in your example, the span cannot be reduced. Conversly, if the brace is pegged you may want to increase the loading factors for the beam since from time to time the brace may end up in tension.

Not much here to relieve the dubiousness. Now if we can just get this straw to at least turn into silver

Re: The yin & yang of braces #4314 10/25/99 02:50 AM
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bartpop Offline OP
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Perhaps I should cut directly to the chase! I'm wrestling with this question because I'm engineering the joinery for a timberframed garage/loft design I have been working on for... Oh, it seems like a really, really, long, long time now. It's a relatively simple frame: 24' x 28', principle rafter/common purlin, 12/12 pitch roof, kingpost truss bent plan with a 24" kneewall.

I've been using the TF Guild Joinery & Design Workbook as well as other texts, to educate myself on just what forces are at work within my frame, and making calculations to assure that my joinery can take the forces that my building codes require. Everything has been going great... that is until I started trying to resolve the forces being carried by the knee braces. While I can easily calculate the roof forces and their distribution through my rafters, purlins, posts and struts... I'm at a loss when I try to resolve what portion of total load is being carried by my kneebraces. Short of an computer-generated FEA analysis, is there any way for me to determine these forces on my own? The method for determining the load on the kneebraces seems to change, depending on who you ask.

I've already had an engineer stamp my frame based on my the layout and sizing of the beams, but it's up to me to make sure that the joints are up to the task. I've got most of the joints worked out and within acceptable values... with the exception of the kneebraces and attached members. I'd like to have some accurate numbers to deal with, rather than rely completely on good juju!

I'm sure that the lawyers would advise me never to rely too heavily on good juju.

Specifically, my design uses a strut between the principal rafter and tie beam to reduce the roof load at the end posts. This strut, in worst case scenario on the center bent, transfers 8721 lbs of the roof load to the 8" x 10" tie beam at a distance of 3'4" from it's end, and offset 8"OC from the knee brace below. I have been told that the entire 8721 lbs should be transfered directly to the brace... which sounds to me like it may cause some problems with bending stresses on the post, as well as requiring one honkin' big brace. I've seen this strut solution used on many larger frames without abnormally large braces so I think that this approach must be in error.

That being said, my trees have been chopped and delivered to the sawmill... (I got pretty excited when I got my engineering stamp), and I'm trying to prepare my final cut-list for the sawmill this week.

My engineer tells me that he thinks the frame will be fine, (I added the strut AFTER his analysis, to reduce the end loads) and honestly I don't think the frame will ever experience forces anywhere close to those required for engineering (the stick-framed walls and roof absorbing much of the load). Still, I'd like to design the frame by-the-book as this is my first, but definitely not my last, timber frame!

[This message has been edited by bartpop (edited 10-25-99).]

Re: The yin & yang of braces #4315 10/25/99 11:04 AM
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Brian Wormington Offline
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Don't worry.

I think the problem of how much vertical load the brace carries is more a function of the tightness of the joinery than of the actual geometry. If the brace is a little undersized, it will carry nothing until there is enough bending in the cross beam to load it and then start to share the load with the end support. (vice versa for an oversized brace).

Wood is very strong in compression parallel to the grain. (from the NDS tables, the value is 1500-2000 psi for most species). If you assume 1500 psi you'd only need 5.8 sq in. of cross section to support your entire load. A 3x5 brace would have almost 3 times this cross section. Because of the load sharing effect, I think even an undersized brace would never fail.

Re: The yin & yang of braces #4316 10/25/99 07:00 PM
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bartpop Offline OP
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Hmmm... My NDS tables show design values for compression parallel to the grain generally in the 350 - 900 psi range. For my species, Ponderosa Pine, the allowable compression parallel to the grain is 700psi for #1 posts, and 325psi for #2 post, which will place me closer to allowable limits if I use a 4" x 6" brace. (364 psi)

Am I still missing something here?

Re: The yin & yang of braces #4317 10/26/99 12:49 AM
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Brian Wormington Offline
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What I see.

Page I6-- NDS Revised Supplement to the Revised 1991 Edition.

Dry Service conditions,sawn Lumber, 2"-4" thick. Bearing design values parallel to the grain Fg --- Ponderosa Pine = 1580 psi.

Re: The yin & yang of braces #4318 10/26/99 04:00 PM
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Rudy R Christian Offline
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I'm not a big fan of juju either, but a good glob of spooge goes a long way.

No input on the numbers, but I am a little confused by the location of the bracing in your truss. When we design a king post truss, the braces join the midspan of the rafters to the base of the kingpost. The typical layout is running the barce parallel to the opposing roof pitch. The trick is to allow sufficient wood beyond the brace to resist shear.

This arrangement keeps the force in the brace from pushing down on the bottom chord. It is also what is typical in historic trusses. In my opinion it's the correct layout and allows the chord to work purely in tension (other than it's own weight).

There are even king post trusses in existance that have no connection at all between the kingpost and the bottom chord. These and other historic trusses seem to only get in trouble when a ceiling load is added. Which is why i'm a little concerned about your bracing settup.

Re: The yin & yang of braces #4319 11/09/99 09:25 PM
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JS Robinson Offline
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I think the 1580psi (Table 2A?) is the tabulated bearing strength of a species. In this case your still designing the brace, so I would use the Fcparallel value of 700 or 325, if the member will be 5x5 or greater. Larger members have higher tabulated values b/c imperfections won't effect the overall perfomance as drastically as smaller members.
As for braces, I agree with the pairs, so that there is always one in compression. And I think they're function is to resist lateral loads. ie Apply one wind load case, determine the compressive force, then check other load cases. I definitely wouldn't view a beam as two because a brace was connected to it. I hope this is helpful, good luck, Joe
PS Once you select the member, then you need to check the bearing str at the beam and the column, typically at some angle, see section 3.10 of NDS.


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