The "No Brace Knee Brace"

Many times I've run across a situation where a knee brace supporting a floor loaded bent girt or connecting girt caused more problems than it solved. For example, if the knee brace picks up loading from the beam, the compression forces in the brace can become quite high. This, in turn causes tension and prying shear in the girt to post connection. (Not to mention problems designing an adeaute house for the knee brace!) To avoid these complications, consider 'floating' your knee braces. Avoid houses of any kind. Create a regular single pegged connection in the beam. At the post, shorten the tenon, elongate the mortise and slot the peg hole in the tenon parallel to the grain. This will allow the connection to 'slip' under load and prevent the brace from doing any load-carrying. The downside of this is that the brace will offer no lateral stability to the exposed frame during raising. You'll have to make sure the frame is well guyed and temporarily braced until the shear walls are constructed.

Enjoy Timber Engineering, David

Hi Dave Hourdequin PE,

Where is the question to which the above post is the answer ?

I am having some difficulty following your argument mainly I think because of the various terms that you have employed.

A bent or more correctly a cross frame does not contain any girts or girding rails. Girts run around the perimeter of a building, generally at mid height and for this reason are sometimes called girding rails or mid rails depending on whether or not a floor is present.

I think that the terms used should therefore be beam or cross beam, and post or main post.

I have never heard of or seen anything similar to the above proposed brace configuration and therefore I find myself struggling to understand the point or need to fit a brace that cannot work ?

When I understand the question better then I should hopefully be able to make a more informed reply.

Regards

Ken Hume P.Eng.

Ken, I'll give you a call. I volunteered the info because I thought it mighht be helpful to some. After we chat, I'll craft a reply that will hopefully clear up any confusion.

Thanks for checking in.

David

Ken,

ThrowIng aside the brace configuration that David is describing, you have to realize that terminology for various components differ from your side of the pond to ours. Girts here often are short horizontal pieces that fall into bents. A bent is typically referred to as you say a 'cross frame'. Middling, girding, minor, major, underling - all the various modifiers you used are not commonly used here.

Mike

Hi David,

Thank you for that kind offer and professional courtesy but there is really no need for you to call me. It is more important for you to try and explain more clearly the content of your initial post and especially to direct this at beginners in this craft who are likely to be more confused than me.

Mike,

You have explained nothing to me about which I was not already aware.

The definitive peer reviewed work on timber framing terminology is by :-

ALCOCK, N.W., BARLEY, M.W., DIXON, P.W., and MEESON, R.A., 1989. Recording Timber-Framed Buildings. London : CBA.

Following on from this work a glossary of terms was assembled and published in America by :-

BEEMER, W. and ROWER, K., 2003. Timber Framing for Beginners VI A Glossary of Terms. Timber Framing., No 68, 12-17.

This second publication rightly makes reference to the earlier definitive work by Alcock et. al. and endeavours to provide equivalent North American terms for the longer established European terms.

The TFG later published a web based glossary of terms - The TFWiki in an attempt to peer review, refine, correct, ammend and develop the Beemer - Rower Glossary of terms but it appears this is now closed to public access preventing continuing development and correction of that resource.

Beginners can still consult the 2003 Beemer - Rower glossary of terms via the menu link provided of the TF Guild website home page though since this is still at odds in places with the earlier work by ALCOCK - BARLEY - DIXON - MEESON it cannot yet be fully relied upon to the same extent as the earlier work.

Regards

Ken Hume

I found the OP to be very confusing and hard to follow. I thought it may have been spam at first! I was glad to see Ken's first check in, and am waiting a clarification from the PE here in typed words.

Carry on then, Ken.

Proposing an un-housed connection for the knee brace at both ends, and then a arrangement with an elongated peg mortise at the brace's lower tenon, along with a deepened mortise to allows the knee brace to, well, NOT perform its function.

Firstly, making the connection un-housed weakens the connection greatly - a brace works most effectively/directly if the end grain of the brace can bear more or less upon end grain in the receiving member, and if as much of the fiber in the brace can be directed into the receiving member - and that means housings in the post and beam to encapsulate as much of the brace section as possible.

As the engineer noted, "The downside of this is that the brace will offer no lateral stability to the exposed frame during raising. You'll have to make sure the frame is well guyed and temporarily braced until the shear walls are constructed."

Which begs the question of why would one bother to mortise and tenon braces in to place if they are to have no structural function in such a configuration as the poster has suggested? Even the modest 'function' of braces to hold parts together, like a beam to a post, during raising is defeated in this arrangement. The shear walls (I presume he means SIPs or interior walls with other forms of shear components) do the work after the frame is together, which again renders the braces superfluous. Scabbing on temporary braces such as 1x4's, as the poster mentions, makes more sense in terms of the final structural arrangement where other shear components do the work. In the arrangement suggested by the engineer, the knee braces become essentially decorative.

Knee brace 'function' when loaded, as the original poster noted, does involve creating high tension and prying loads at the beam's connection atop the post (or at the beam's end joint at the side of the post if the post extends further up beyond the connecting beam). Perhaps the problem is more the knee brace than anything else? Perhaps a knee brace sized so that it bends a certain amount when loaded, to absorb some of those tension/prying loads would be a better option? Or no knee brace at all.

Finally, the relish on the end of the brace tenon is generally rather minimal and prone to failure (especially if subject to tension loads), and some expectation of the connection maintaining integrity over the years as that tenon end portion rolls/slides over the peg in the post seems rather optimistic.

The 'solution' of creating a knee brace connection which results in a brace which cannot function as a brace is supposed to, however, does not make much sense to me. Perhaps there is more to this than meets the eye - a diagram might help make the picture clearer.

In addition, the OP suggest in the first sentence that "Many times I've run across a situation where a knee brace supporting a floor loaded bent girt or connecting girt caused more problems than it solved." Was that on paper and the number world or in real life? I have never had this problem show up.

Hi,

The use of cross braces between main post and cross beam is somewhat of a rarity in traditional timber framed buildings with these braces more usually found higher up between the main post and tie beam (normal assembly). One of the reasons for this is I suspect to do with the very thing that Mr. Hourdequin P.E. raises in his initial post i.e. that significant bending can be induced in the lower part of the main post when bracing is employed at this level to help support a cross beam being subjected to heavy floor loading. The traditional cure for this is simply to omit lower level braces and increase the number of studs supporting the cross beam.

Regarding missing out pegs this phenomenon is not common but can and is found especially where a lot of connections are made at the same point or would significanly weaken or stress remaining timber after heavy morticing work has taken place. I have digi pics of the long wall of a 4 bay - 60 foot - aisled hall where it appears that only 8 pegs have been employed in that entire wall and yet the building has apparently stood with tight joints for 4 - 5 centuries.

Some things are difficult to understand or explain - I feel a Rumsfeldian moment coming on.

Regards

Ken Hume

Doesn't look like the OP is going to clear up his post. I don't get what he was going for.

From my view, a brace is simply a 2 force memeber, carring forces along it's direction. If in compression, you don't need any pegs. Tension is not preferable if it can be avoided, but can be employed, like in a kingpost bent.

He seems to be advocating what is essentially Post & Beam construction with decorative knee braces.