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.

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’.

Well that depends on the design and use of the building.

I feel like I'm missing something here. I've always understood that braces are ALWAYS compression members. Preferably used in pairs, depending on the direction of force, one is doing the work (compression) and the other is on holiday. As has been mentioned often in this forum, it is not at all uncommon to see peg-less braces in old barns and other traditional frames.

EHC- You state that in your 3-post example, the lateral load would put two braces in tension and do nothing to the other two. I would argue that the load would do something to the other two- compress them! Eliminate the pegs to eliminate tension. The 3-post bent should be designed so that the compression of two of the braces is adequate for the lateral load from one direction and the other two are there for load from the opposite direction. This just takes advantage of wood's property of being stronger in compression than in tension.
A great example is a multiple queen post truss covered bridge. The braces/struts are all in compression, that is they run diagonally from the top of each post to the bottom of the next going toward the abutments so the two in the middle form an inverted 'V'. If you look at a steel truss you usually see the opposite since steel (for the sake of this discussion, let's assume flat stock, riveted or welded) performs better in tension than compression.

I'm curious to see exactly why braces in tension are required. I don't want to assume too much and I hope I'm not stepping on any toes!
Cheers

Dan F

did not check my post until now. My mistake. Tension was supposed to be Compression.

I'm with Dan.

what do you suppose this was for? Medieval English

Derek,

I have seen and worked on a lot of roofs that were not sheathed. They had a tyvek type mebrane across the rafters and furring strips on top, laid perpendicularly to the rafters. The purpose of the furring was to provide cross ventilation and be the installation layer for shingles/slate/reed. The furring cetainly did not add to the structual integretity of the roof or frame. So the stand alone frame is no myth.

Thanks for the replies.

I’ve tried to add some pictures of a few recent projects, but my brain has limited capacity. How do I do it??

The pictures are of engineering analysis results for two frames that were engineered for 110mph wind loads. The largest problem in each frame was brace tension. Releasing the tension in the braces, compression only, causes other problems within the frame such as very high post bending.

Thanks,
Pete

MTF,

if the picture is on your computer you would have to upload it onto say Photobucket or any other online site of your choice. The picture then gets a URL that you can copy and paste when you post or you can just add a link to the picture. To add the URL just click on the image or URL button below ( when you write a post you can select below the frame you are writing in. A window opens up and you can type in the URL code.
Hope this causes enough confusion for now.

Thanks for the thread MTF!

I think it was mentioned in the other thread on braces, but the back-issues (#62, 63, 64 I think) of the tfg newsletter have excellent articles on braces working in tension and compression. A person built (for his thesis?) real timber frames (one and two story, oak and pine) and instrumented them with load cells, and pushed on them with hydraulics to obtain stiffness #'s, and ultimately pushed them to failure. It's a very scientific analysis. The whole set of back issues is available on DVD here on this web site. I am not associated with the tfg in any way, but I was happy to buy these back issues for the wealth of information they contained, and I figured some of that $ might help support this discussion forum.

Don't know if it's already been mentioned in this thread, but the post-to-beam joint that completes the triangle formed by the "brace, post, and beam" is one that needs to be carefully looked at too. The smaller the brace, the more stress this joint will see under wind loads, etc.

These images might be an interesting sidenote to this discussion, the thought to post them was sparked by Mo’s graphic.

These are from a smallish Dutch barn ( though not a classic NWDB ) built in the Highlands of New Jersey when it was still part of New Netherland.

These are tension braces meant to augment the tieing joint. The system was meant to overcome the thrust of a simple rafter type roof , but the design was insufficient and likely began to go into failure almost immediately. The iron rods were placed as the roof continued to spread over time, no two seemed to have been forged by the same blacksmith, some are much later and have turnbuckles.

The gable bents have the drop tie, and the main ties, from there it gets progressively worse…

Only the first bay has this odd “upper loft” and only bent II has this secondary tie lapped through the standing tension brace.



The bents from thereon only have the brace to overcome the massive thrust and bending moment being imparted into the posts and the four plus feet of “kneewall” above the tie, with this resulting



An interesting aspect of this frame is that the carpenters connecting the scribe points were curiosly free to choose joinery semi-randomly, sometimes cutting a lap dovetail, sometimes useing this tensile cog. Every inboard bent with the cogs on the laps has this shear failure, which runs from the shoulder which supports the plate to the tie, made all the worse in that a shear plane was created in cutting the joinery, which runs from inside the plate tenon , through the cog and on through the peg holes in the tieing joint. They all popped relativly identically.



How it’s weathered as many years as it has is almost miraculous, that having far more to do with the succesion of stubbornly frugal farmers that maintained it than it does with good design.

Thanks, Will. Great pics and it clearly demonstrates my lack of knowledge regarding braces as tension members. I think it also demonstrates WHY we see so few braces as tension members!

Will,

Very nice pictures.

It’s tough to tell from the pictures, but it appears that the roof system is a straight gable without additional support other than the eve plates. The thrust at the plates do to gravity loads would introduce the destructive tension loading. Designing to avoid tension loads do to vertical loads by use of queen posts or canted purlin plates eliminates that problem. I’ve been successful minimizing tension loading by use of those methods. It’s the lateral loads that are presenting a problem for me.

If you peg your braces, they will carry tension loads during a wind event. If you do not peg your braces than achieving a viable frame design using ‘standard’ engineering practice can be extremely difficult, at lest it has been for me.

Derek,
What type of loading is generating a 3.1k tension in the dovetail brace in your picture?

How do you engineer your frames for wind loads??

Thanks, pete

We've a common problem in the north east, hay tracks. They would tell farmers to remove queen ties to accomadate hay tracks and roof loads went right to the plates and overloaded the tie beam/ post connections. Many smaller frames , like carriage barns had no rafter support system at all. Most agr. buildings in this area where not deep and pegged brace which agravated the design flaw. Then the iron man was called.
I have used steel and with a little artistic flare in can greatly compliment your work.

They are great pictures too! It looks as though maybe shrinkage of the brace may have started the problem, dovetails have to stay tight. I saw a dovetail like that once with a bolt in it. It loked about the same , but held. That is alot of tension on that joint , wow

MTF – Yes, I tried to accurately describe the situation and said I was posting the images only as a “sidenote” I saw them myself as only tangential to the nature of your thread, but food for thought all the same

I will try to find the time speak to your greater point. Today is spoken for and I’m not sure there’s a short answer.

thanks Will,

understand, no short answer, ever!!

pete

Great thread:

Long braces were used by the better millwrights to steady structures such as the Mulley saw mills of the 1846 vintage and earlier.
One that I am familiar with had no short braces but relied only on 4" by 6" long braces crisscrossing each other in the outer wall cavities, and repeated for each every bent, notice none were pegged either, lets make that clear here. These braces ran from the sills up to within 12" of the upper plates, and were on an even run and rise.
They were intentionally inserted to steady the building as the sawing equipment ie; the log frame carriage started forward and stopped between each stroke of the pitman during the sawing action.
The building was 2 storey, and had identical bracing on the lower floor.
The huge timbers over and under the saw blade guides (20" square by 30'long) had double braces and double supporting posts on each end, the timbers sat in dovetail sockets between each set of supporting posts with wedges to ensure that they could not move,
even with all this lateral support the building would still respond to the sawing action slightly, and uncanny as it may seem you could feel the frame respond even to a person walking around.

NH

I've got some photos of NH's mill from TTRAG '02:

This one shows the big beam that holds the top of the saw blade guides:

(That's NH in the center)

Here's an end-on view of that beam, showing the double posts that hold it up:


And here's the unpegged overlapping long braces he was talking about:


CB.

Thanks Diaku:

Those pictures tell it all especially the ones of the long braces that go with the conversation thread.

This old mill was a wonder of building technology of years gone by, and the use of the long braces a good example of historic mill wrighting, in conjunction with a good Historic Timberframer, with whom I suspect worked closely with the prospective operator or mill owner of that time.
Many things had to be understood before the frame was cut, for instance a good understanding of the waterpower to be used, and how it would be installed after the frame was up. What direction the inflow of water would be coming from, the topography of the surrounding land, and the list goes on and on.

Once again thanks for taking the time to post the pictures for the enjoyment of all.

NH
Richard