Mid Span Beam Joinery Question(s)

Hi all. I've been watching this forum for a long time as a result of my google search hits and finally decided to join and ask some of my own questions directly. I'm in the process of designing and building a barn (for myself) with a mid-span beam roof system which will support common 2x rafters. While working out my scarf joint locations I've been wondering why you never see beams at the top of a post, joined to the post faces instead of set on top, similarly to beams part way up (such as with floor support beams). I have seen a photo of such a system but I'm left wondering if the bending moment would be simply translated through the post and into the adjacent timber, which would move the problem from the eliminated scarf joint to the spline or tenons at the post (where there would conceivably be more bending stress).

This would be attractive to me because I could order and handle shorter timbers and eliminate the scarf joints altogether; all good things for me as I'm an amateur on a budget that allows for limited equipment support.

A wordy first post that could benefit from pictures I know, but I haven't yet figured out how to add pictures (working on it).

My apologies if this has been covered in a previous post, I did a history search but didn't find anything that quite addressed this.

To post pictures here, you need to have them uploaded to some other picture hosting site. Once uploaded there you can insert the photo image tag in the body of the post and it should show.

Traditional timber frame design has the plates above the post so that the rafters sit on the plates. Having one layer of timbers going from eave wall to eave wall (the tie beams), and other layer of timber going gable end to gable end (the plates) creates a "grid" of timbers that makes the frame very ridged.

If you join shorter timbers to the sides of the posts (interrupted plates) this is more of a modern design method.
As long as the joinery is strong and true it may work for you.

Jim Rogers

Thanks! I'll probably stick with the more traditional approach and simply locate my scarfs carefully. There's likely good reason it wasn't and isn't often done the other way.

There is a rather lengthy thread on interrupted plates. Don Perkins contributed to it. Yes, it is more of a modern joinery solution but has a strong historic approach in the New England area as well.

Try this. http://forums.tfguild.net/ubbthreads.php?ubb=showflat&Number=16174&an=

Though not common, there is a number of vintage timbering systems that tend and/or have eliminated an abundance of long members, as Tim as given a great example of one. So though not common, interrupted connecting timbers and related "short timbering" is very traditional in a number of timber framing cultures (e.g. Old English, Germanic, Eastern European, Asian).

From a design perspective it is a tradeoff between perhaps not wanting to work with (or having) longer timbers. As such, if this is the case, the application of a great deal more joinery then become necessary. Inherently the strength necessary for a frames durability must be acquired in one of the two critical elements...be it longer members or more joinery.

Being a "modern project" you have the luxury of pulling from whatever traditions you please to achieve both your structural and aesthetic goals. If more joinery isn't an issue, then there are a number of design parameters that could be employed to achieve your goals without long timbers. Many modern designs, within the contemporary timber frame aesthetic, would support your interest in this design system. As is the case of a project we are currently on with 8 meter plus bent spans (~35' drip edge to drip edge) and 6 meter bays (~20'.) We had no real choice but to go with the "more joinery" approach and forgo an continuous connecting timbers for the length of the structure.

Good luck and we all look forward to reading more about your progress...


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Thanks all! It looks like I have plenty of reading and considering to do while I prep my stock. I may still default to a continuous plate given time constraints and not being able to wrap my head around how the interrupted plates can be joined successfully; it seems as though the interrupted plates would still see moment reactions at the posts and mid span similar to those of a continuous system, and that would be a lot to ask of the joints. I do appreciate the input and direction, I was really getting nowhere trying to research this through other means.

One way to join interrupted plates would be with a spline. Here is a link to a view of a spline joint:

http://timberframehq.com/spline-joinery/

Jim Rogers

I just want to point out that you can indeed find long milled timbers, such as the 20 or 35 footers that Jay points out, at least in most places. And believe it or not, it's actually cheaper to hew out a 35' timber than it is to buy one.

Our next project, which is well under way, 40'x60' has 6-40'-10"x12" tie beams. The 60' plates are scarfed in the middle. A bunch of 24'6x6 joist lodged on top of the ties, covering 12'bays. I will do everything I can to avoid splices and excessive joinery. All white pine.

Hey Brad...

I agree it is....WAY LESS EXPENSIVE...to hew out big beams than to buy them...

Even a chainsaw mill is more cost effective (in most frame projects) for really long beams than buying them if a jointed system won't get the job done in meeting load requirements. We are setting up soon for a swing blade system to handle (at minimum) at least 20 meters (~65') just because more and more commercial frame projects are asking for these larger truss span assemblies...

I my personal experience, I have gone moved toward use of continuous plates on top of the posts and moved away from more modern "bent" styles with shorter interrupted connecting members. I have found that buildings are straighter and truer during and after raisings; with usually no racking needed. Using more the "Benson style" it seemed that things always had to be racked to line them up properly. I think that this also speaks to added resiliency as the frame ages and is in use.

Hi Sean...

Do you think it is the "longer timbers" that makes the turness of the frame easier to accomplish during raising...or...could it be the layout modality of the frame?

Now that we are moving into larger and larger frames..."longer timbers" (40 meters plus) is logistically much harder to achieve than shorter 6 meter timbers. As such..."line layout"...presents as creating much "tighter" joinery the first time around with zero test fitting ever necessary before raising day. We can even have several shops at once working on the same frame, even with "live edge members. These too come together well at raising time.

Further...if joints are "cut correctly" the frame should stand "square and level" without much racking in most frames...I got that "tidbit" from the Amish I learn from. Again, I think this stems from "layout" and the Asian designs motifs we tend to follow. With the addition of long splines and corbel elements designed into the frame...we see further "shortening" of frame members, yet still achieve tighter truer frames. Sense this conversation started, I have been looking back at many of the Asian designs overall, and many are comprised of "stack," corbeled, and generally much shorter timbers that we see in Western framing overall...yet have the oldest and some of the most enduring frames.

I am very curious about monitoring and observing how we all approach our "craft" of timber framing and comparing the modalities for efficacy and stiffness of frame with the varied techniques we each employ...Of course this is outside the scope of "historical work" which comes with its procedural designs and construction modality mandates of of means, method and material.

Regards,

j

Sean, to me this is the crux of our building problem, what do we know, how do we know it and will it work the first time? Before specializing on tf, I worked on old buildings. Just about every building had foundation flaws and level reference became crucial concern. So I acquired a builders transit Vernier to 5 minutes and as I became more concerned about level and angular resolution I bought used a surveying transit, Lietz-Sokkisha with a direct read vernier to 20 seconds and a Brunson dumpy level with the level vial at 10 seconds per division. Having those instruments changed the way I thought about marking, measuring and building control. I was lead by the tools to incised layout and desire to match the frame to the existing field conditions, as close as I could measure. My basic method is square rule with numerical scribe mapping, as needed. If desired, I'll go into the process.

I'm so happy with long timber and a water level.

Tim...your the man!!!!

I am on this huge project in Wisconsin, and the other contractors doing work here all have there transits, lasers, and "whatnots"...I have no issue with that...We even use a little Bosch Laser for "rough in" work...Nevertheless, for real accuracy, predictability, going around corners and (in my view) the very best method, and probably will be for the foreseeable future, is the good old tried and true...water level. It simply is the most accurate and fastest system collectively still for doing this work...

I am planning on dedicating a large section, of the book I am working on, to illustrate traditional layout systems for our craft that typically aren't well described. Part of that is "traditional foundations and their layout systems." Water levels are a huge part of that. I am opening that section with a story from the Mountains of Northern Turkey where I observed the creations of such a device over at least 12 meters long (~100') made from the intestines of a goat, and some hand blown glass vials. This method of finding level is still the most accurate and thousands of years old...

As for long timbers...in general...I do not disagree. Yet, when you are dealing with a lot of them or members over 80 feet...We have found the comparative logistics of "more joints - small timbers) fare outways the logistics of acquiring, moving/transporting, and assembly of larger timbers. At least those over 7.2 meters (~24') in length. So, unless the project site has really nice long timbers that can be hewed out, we find it is always mort cost and time effective to go with joinery and small timbers. As for strength the difference is marginal at best (comparatively) and there is gobs of research (mainly in Japanese unfortunately) that studies this. Frames with "short members" that are "made long" are just as strong in many cases and more importantly...very flexible. This is par to the reason that even the Keel of many ships are still "scarfed" and not a continuous keel or made of several laminated timbers...Both systems are more than applicable and have there strong points (no doubt) historically however, typically short out weights long in overall context of timber frame design from a global perspective in both ship and land based architecture...

Water levels are so simple. Goat intestine may put it over the edge for today, in a pinch, though. A new idea for a sassy goat.

I had a bad experience with a water level, once. The bucket was placed in a tractor bucket....... the hydraulics were slowly leaking down.

They just can't be match unless you spend many thousands of dollars for "green laser" highly calibrator survey and layout tansits...Even then...they still match the accuracy...

I got to watch the entire process of how to use a goat (not only for a festive barbecue) but also making a water level...It actually worked extremely well. You can see if there are any "air pockets" (aka bubbles) in the line...which really screw things up for many novice to this method...and...the nature of the intestine seem to aid in the water level reaching...equilibrium...much faster...

I could see that being a really issue with a tractor being part of the package...

Today, I typically use a 30 meter plus (~100' to 150') by 25mm to 30mm diameter clear levin tube and fix one end to a stationary midpoint for the project. We further "syphon fill" the tube and keep that in a larger container of water to make sure that the ambient temperature stays the same...as temperature gradient in a "water level" will also make "balancing" and leveling uneven...

Keeping the temperature even, and no bubbles and things work as smooth as glass...and pretty darn fast as well...."easy peasy - nice and easy!"

Should have a whole thread on water levels. Have not see them discussed here on the forum that I recall. I actually thought about water temps and my water level just the other day, determined that my siding job was not critical and ran with it. Could be an issue in a long day where you start out cool in the morning, 50's and hit high 80's after lunch while using the same set up.

Looks like Tim and Jay will run with water levels and that's very fine. My choice of old school optics relates to the environment I worked in during the 70's and 80's as crew leader and site manager. Among the projects I managed was a sprawling meandering house with 49 exterior corners. I was the guy who laid out the cut, the footings and foundation, so I was in need of serious optics to place the house. The surveyors transit and dumpy level were bought used for less than $500 total. To this day, such instruments can be found for similar prices. No electronics to go glitchy. All the details of operation and adjustment can be found in old surveying and engineering texts. The really cool aspect of old transits is the principle of recursion or the procedure of increasing resolution through repeated and accumulated readings.

Nineteenth century precise mechanical tech (although my gear is post WWII) will outlast any electronic device.

That's interesting. I have a long span to level and a goat to do in, (after the Fall rut of course.)

Hi Tim, et al.,

Sounds like a good deal Tim...Maybe over the weekend I'll start a thread if you don't beat me to it...which forum subject here do you think is the best place for it?? It does deserve its own thread and not clogging N. Butlers conversation...I am reviewing his CAD model of the project in question now, and I am sure he will have more to report, and/or ask questions about later on in this thread..

Hey Roger...The use of survey tools is an art in and off itself...I remember being the "stick man" in the Marines. If you have the talent for an old optical system...I say go for it...It still would act as only a "rough in" tool (like today's lasers.) Perhaps the new post should include the batter systems and other aspects of foundation work ans we have all done for timber frames...There are many tricks out there to share and getting them down in a discussion would be nice for folks trying to learn...

I also (for some applications) like long span levels as well. We have a 24' (5' section -expansion bolted) aluminum level that is really nice for some applications and large wood 3,4,5, (6mx8mx10m) that is also a must for aspects of this work.

I think some of us here also do like to "eat goat," as it is one of my favorite meat dishes...I think perhaps the "goat method" and others would be something to do a Journal Article about...

It's a two year old ram with its testicles intact. I'm not eating that but if the intestine will be of service it would be nice using it. Looking forward to your details Jay.

I loved Goat!!!...So weather 2 or 10...male or female...If one is accustomed to many of the Middle Eastern dishes...it's all good...Wish I was closer...I would love to help you out with him D.W....He would be great curried or pit barbecued...I will try to outline a "step by step" in the posting...would love feedback from any that try it out...

Regards,

j

OK; I'll "bite" on this one.
Goat is the most widely eaten meat in the world. We get one every year for the freezer.
On a side note, Jack Sobon claims the animal intestines were also used for chalk lines...

Will, that was frog gut chalk lines.

Jay, post it up in the "tool" section.

Goat is one of my favorite critters.

"Tool Section" it is then...Outstanding!!

I too love Goats...I use to have a beautiful herd of Nubian, and some really big black "Boer" that had some "Bengal" in him (freaking monster Ram at over 529k (~240lb)...Thank goodness he was a "good boy" and even playing he could (did!!) hurt me...

I'm sure that given time and connections I could source timbers of the appropriate length. Perhaps I could even find them on my lot and hew them, but time is precious and my connections are few. I'll give it some thought though, I do have some nice pine and red oak fit for harvest, but that's still quite a task to put in place!

N Butler,
What length of timber are you needing for your plates? I don't think there is anything wrong with a good scarf joint when needed.

Jay,
You are probably on to something with the layout and execution having a lot to do with the buildings coming together straight without additional help. The arrangement of plate above posts with rafters joined to those plates still seems preferable to me than interrupting the cross members at the plate. I could not put my finger on a quantifiable reason at first but I after some thought I think I have one to field.

Lets consider this joint intersection under racking pressure. We will also assume that there is knee brace from the post to the rafter carrying horizontal member and there is a rafter placed directly over the post.
With the horizontal member broken up by the posts the only thing resisting the racking pressure (including any levering action imposed by the brace) is the joint itself, which by definition is a potential pivot point. I'm however not implying that the joint can't be designed to properly resist this force.
With the plate passing over the post it gains two critical advantages. Gravity, (always a force to worked WITH when possible) is on the joint's side, augmented by whatever load is transmitted through the rafter. Also, the string of post to plate joints is going to working more in unison to resist the destructive pressure. If interrupted by the posts, it would seem that they are working concurrently but not really "together" if you catch my meaning.
Any scarf joints necessary to the arrangement would have to treated as a separate variable entirely, as they are all over the place structurally in how well they mimic an un-scarfed beam. But I think that we can agree that it is possible to produce scarf joints that perform highly enough to not detract form my argument.
This is just my own thought experiment based on my own notion of structural "rightness". It just seems more "right" in my mind that the plate should pass over the posts, and I don't even think for purely "historical" reasons. I imagine that an engineer could point out some fallacies in my thinking, which I would love to hear! Has any comparative study been done that anyone knows of? Could be an interesting joint busting demo at the net conference!....

Running the plate over the post is preferable; bringing the ends of two plates into the top end of the post creates 3 joints at the ends of timbers - something to avoid if possible since it can induce splitting.

Hyland,
My plates are 29 feet in length, not un-doable, but somewhat daunting to work with as I'm primarily a one man operation with the exception of the raising (when I get to that point).

As to the mechanical behavior of the interrupted plates, I'm uncomfortable with this given the application. I am an engineer by day, and though I don't design timber frames I do have an understanding of continuous beam systems, and I just can't get behind interrupted plates for a load bearing application like this without some secondary support mechanism such as a truss or the corbelled bracketing from the Asian traditions. I'm sure it's done successfully, but the loads we design for aren't always necessarily seen and I'm not going to make a science experiment out of my barn.

I've read that scarfs can attain 25 - 30% of the beams original strength characteristics, but I haven't ready a whole lot on the testing done to support that. I'm sure someone has done it, but I would like to see full scale, or even small scale testing of traditional scarf joints at skewed angles of attack to mimic the vertical load and thrust combinations that they really have to support. I can place my scarf where the beam is at equilibrium under the maximum loading, but as load cases change, so does this location, and understanding the relative capacity of these joints, as it changes across their profile would be crucial in properly engineering their locations.

I've eaten goat once, it was much better than I expected, given the critter's temperament.

Sassy goats are the best:)

Hah! No different then hateful roosters; they make very satisfying meals.

All the more reason to stick with pig, if you ask me, Just fine companions and so delicious after that. One for next year's hung up.

We have 3 little pigs, too. Huff and puff the do. They love goat milk! sorry so far off topic.

LOOK!...Squirrel!!...wait...what were we talking about...??

We all wonder...me more than most!...yet if the topic is good...oh well...

Yes, it starts to get out of hand and someone should put a stop to it. A few weeks ago I had some tasty rabbit and that's got me thinking but it's a question of strong family resistance.

Long as I'm here... As for more non-edible use of animal production, if I can commodify a fellow sentient being so crassly, I made up some very nice, if somewhat smelly in the initial stages, blue paint from the extra goat's milk I'm getting just now.