Well, I decided to start a new thread to address the engineering implications and potential solutions for brace tension loading. There is a very interesting thread in the general discussion area that may have gone off track because of this discussion.

I know of a few ways to mitigate brace tension, but I’ve had problems finding a balance in my frame designs that will accommodate brace tension. I’ve designed what I consider to be relatively robust and well braced frames and the one issue that gets chased around is brace tension. I’m interested to hear how others accommodate brace tension from an engineering perspective. I’m not a fan of mechanical fasteners and really want to design frames that do not require a shear wall feature to carry lateral loads. I know that SSP’s can carry ‘all’ the shear load and that some post and beam builders will not have any bracing in their homes at all. That’s fine, but I’m interested in stand alone timber frames, frames that do not need additional lateral support to carry wind loads.

Thanks,
pete

Please define your term "SSP"?

Sorry Jim,
SSP = Stress Skin Panel, which usually consists of oriented strand board/foam/oriented strand board. Others call them SIP = structural insulated panels. I believe the panel industry does prefer SIP, but I’m not sure.

pete

Allow me to play devil's advocate.

How many traditional timber frames do not rely on the walls/roof to stiffen them up? Old barn frames are stiffened greatly by the siding. Old houses by the siding/lath and plaster, etc. Older houses by the infill. Take away these skins and you'll find a frame that moves with the weight of a human -- even if there are lots of braces and the joints are tight.

Is it possible that the quest to have "stand-alone" frames that are stiff enough to handle wind loads without depending on either the skins/infill or steel is a modern idea fueled by a misunderstanding of traditional buildings.

In other words -- the incredible difficulty of designing stiff mortise and tenon braced frames may be an example of our proving history -- timber frames have always needed the help of their skins.

Any thoughts?

Gabel

I think one should consider the whole building when looking at design and engineering, If there is boarding going on the frame, then what work is the boarding/Sip/strawbale/whatever able to do to stiffen things up?
That being said, there are times when walls are not enclosed, as in a pavillion, and a frame that can stand on it's own is neccessary. So talking about brace tension or other ways to stiffen the frame is a good thing, at times a neccessary thing.
If you're looking for tension from braces, you need the same things that resist tension in tiebeams... long tenons and multiple pegs. If we can give value to the ties, we can we not give value to the braces????

Gabel,
I agree fully with your statement. I think that timber frames have always relied, to an extent, on the siding system for stiffness. I’ve tried to ‘shake’ many different buildings in various degrees of finish to get first hand exposure regarding how flexible or how stiff a frame actually is. Some times I’ll get few guys in the loft and we’ll do the old 1,2,3, shake. Siding and SSP/SIP do stiffen timber frames. Unfortunately, today’s building codes do not provide any clear path to account for this. Using SSP/SIP to augment the timber frame, or provide the full lateral load resistance system, is done. But designing a stand alone timber frame while keeping brace tension at manageable levels is proving difficult for me?

Thanks,
pete

I've considered and used a couple system not mentioned, not to go off thread , but one method not to consider is dovetails. I realize they would be out board on the post and wouldn't be visible (and they reduce the size of the post) ,but where wood only is needed for strong corner in tension they fit the bill.
What if any effect would skew pegging do? By skewwing I meen placing the peg so it just nicks the blade of the tenon so the relish can't split out , but the blade can't come out.

@MTF,

I am not certain if there are any studies on this. I saw some studies done on pegs but not tenons.
I am trying to visualize what will happen if a load is applied to a bend with three posts and four braces. Most frames are joint very tightly so the load would put immediate compression on two off the braces and would do nothing to the other two braces. And if the joinery due to shrinkage or poor craftmanship has some "play" the braces if pegged should be able to resist some tension if the tenons were made long enough. A short tenon will not be able to resist the amount of tension that a longer tenon would. I think that the thickness of the tenon and the wood species would matter also. It takes more force to shear off a 3" thick tenon rather than a 11/2" tenon.
Also if the braces were actually made longer the would probably do a better job 'bracing' the frame. Yet most frames only see 36" braces but have 8'-10' posts.

@ Gabel,

that depends on what you consider a traditional frame. Are you refering to traditional frames globally or just in the US?
I do agree though that any frame wether simple with little bracing/small braces or true stand alone does benefit from siding or roof sheathing nailed to it. If you have a frame with infill and want the frame to rely on it than in all likelyhood the infill would start falling out rather sooner than later because of the frame wrecking and breaking the infill.

Once the frame is closed in the problem wether there is tension on the braces or not is minimized to almost nill anyway. I do not believe that it is such a big concern after all. Otherwise we would not see frames that are hundreds of years old, because then they should have failed after just a few storms.

Thanks for the responses.

I may try later today to post a picture or two to help further explain my questions.

I do use ‘deep’ braces were I can, we use 4’ and 5’ braces often. It’s not uncommon for our braces to enter the head room and go below head room at times. I’ve contemplated using 6” or 7” wide brace material in certain locations to gain additional tenon capacity. I’ve thought about ‘fat’ tenons, double brace tenons, using oak braces in pine frames and I’ve thought about using steel. There I said it ‘steel’, such a dirty word! I’d like to think I can design a ‘traditional’ timber frame using ‘traditional’ bracing. I’m not against using a lot of bracing, we do, but I’ve stopped short of using ‘non-traditional’ bracing.

I’ve seen a few buildings in the NE that have used large post to sill braces. I’ve heard those braces referred to as ‘tension’ braces. These ‘tension braces’ are only on the exterior of the building, gable walls, etc. I like the idea of stiffening a gable wall when practical by use of sheathing and additional bracing, but it’s the interior bends that are problematic. Let’s say you have a 48’ long building w/ 12’ bend spacing. The gable walls only see 6’ of wind load while the interior bends see 12’ of wind load. That’s my definition of a free standing TF. Those interior bends do not benefit from the gable walls additional shear capacity and must carry their full 12’ share of the wind load acting as a ‘standalone frame’. That appears to be the most common approach to evaluating TF’s for wind load. It is this approach that generates high tensile loads in braces for many TF’s, small or large. This is the area were I’m struggling. I’d like to get a sense on how other timber framers and engineers approach this problem.

EH
“I am not certain if there are any studies on this. I saw some studies done on pegs but not tenons.” I agree that most of the research appears to be directed toward ‘pegs’. My reading indicates that there many studies that evaluate the failure mechanism in mortise and tenon joints and timber frames as a ‘whole’. I believe most of the studies indicate that many mortise and tenon joint failures are relish related as opposed to peg related. Further, when evaluating a single story or two story frame, Dick Schmidt’s work, most frames start to fail do to tensile loading of braces, actually relish failure of the brace tenons and/or brace pegs.

In closing, my conclusions to date follow.
For lateral loading (wind) brace tension is many times the major failure mechanism for timber frames and brace tension is potentially the most difficult aspect for standalone timber frame design, it is for me. I’m trying to understand how best to accommodate brace tension loading in my designs.

Thanks again for the responses.
pete

What about dovetailed free tenons on centered braces, using dry material? We used those at Rindge, which had no skin.
There are lots of traditional timber frames that don't use or rely on skins and get plenty of wind. Look at all the market halls in Europe.