I've learned that the half-dovetail-shouldered-wedged-through-mortise and tenon joint is a strong one. And I am thinking about building a small barn following the "high posted cape" design shown in one of the popular timber framing books.

Looking at the tie beam joint for this "tie below plate" design which involves a half dovetail tenon joint, I noticed that the wall plates require a mortise that is cut into the half dovetail tie beam joint. I am concerned that it will weaken this strong tie beam joint. It also looks that there really isn't that much room to peg the wall plates.

Do you think that the wall plate mortise (and peg holes) cut into the half dovetail joint weaken it?

Here is a pic of the joinery in question. Note that this pic shows a smaller wall plate (almost a "girt"?), but the design requires a larger 7x7 or 8x8 "wall plate" (which would require a larger mortise).



Thanks for your time.

George, first of all, for what reason do you intermittent plates need to be so big? I assume you are not talking about the top plates, which tie the bents together. Are you using this plate to support your floor joists assembly? Or are the ties supporting your floor?

Otherwise I would tend not to worry about the wall plates weakening the timber so much. Yes it is a vastly weakened point on the timber, but if your timbers are suitably large enough that should be no trouble. And remember, our timbers are often sized to accommodate the joinery, and as such are typically quite larger than is needed for most of the timber's length. So just size your timber so that this joint will work for you.

Also, I am not convinced that pegging the wall tie is even necessary at all. it should never be under tension anyhow, if you have a solid top plate just a bit above it anyway. So I wouldn't worry about not having a whole lot of room to peg, I highly doubt tearout will be an issue here.

As always, take what I say with a grain of salt. I don't know exactly what your frame looks like, so I am doing an aweful lot of assuming here.

Hope this helps some
DLB

The high posted cape design that I am remembering is on page 48 of the book by Steve Chappell: "A Timber Framer's Workshop: Joinery, Design...".

Below is a pic of a similar frame design. Yes, I am not talking about the top plate. The design shows the braces attached to the intermittent wall plates which doesn't make sense to me. Wouldn't it be better/stronger to join the braces to the top plate and move the wall plates down or eliminate them?

If the braces are joined as the pic shows, then shouldn't the intermittent wall plates be pegged? Even though the top plate would do a good job holding the bents together,it seems that if the wall plates are not pegged they could slip out over time (which would weaken the important 45 degree brace)?

If they are double braced like shown, I don't think they will have much a chance of moving. This is especially so once you wrap the frame in siding, SIPs, or what have you.
However, my thought process is thoroughly influenced by the German methods, which often times involve no pegging or pinning at all but rely on loading forces to hold things together.

I would tend to say that it would in fact be good to do as you say and eliminate the wall plates and instead brace to the top plate. I can think of a number of reasons why this would be a better setup than that pictured.

http://www.tfguild.org/joinery/part1.pdf
fig 1. at that link shows the setup that I think of when thinking of a frame such as this. fairly straightforward, and proven.

It makes tremendous more sense to me to brace to a primary structural member such as the plate, rather than a redundant, secondary member like the wall plates.

Me too, get rid of the wall plate and brace to the top plate with longer braces. It saves material and time and works better. Or lower the girt to the window height, above and below, stiffening the frame. This would make your barn more Barney with out the purple color. While you at it lodge the joist on top of the tie beams. Steve C. is so new school.

Tim, I have often wondered that in reference to Mr. Chappell. That said, I do respect his work.

But I'm all for lodging timber on top of others, and not so cracked up about everyone's obsession with making things join at a single level. I think you can preserve a lot more structural integrity by staggering your joints, allowing you to get by with smaller timbers. Also, lodging timbers allows for simplified joints, saving on material removed and definitely saving on time and labor involved. I know this comes from my German obsession. They use gravity for everything, and it seems to work well for them. The French seem to do so too. Is this uni-level framing hooey an English invention?

DLB

Hi George, I try, as much as possible, to avoid some of the framing conditions in this plan, which is an example of the timber frame revivial style of the 70's and 80's.

1. I offset the top of the tie beams to the bottom of the floor joist and wall girts, which avoids over-crowded joinery in the wall posts and all the notching in the tie beams.

2. I avoid 20 foot spans on carrying beams.

3. I avoid unsupported roofs that load the plates with high thrust loads. Instead I utilize, alone or in combination, structural ridges, purlins or trusses to carry and resolve forces. Additionally the scheme you presented will magnify and concentrate the forces on the tie-post connection.

4. Keep the wall girts, which I view as joists, in plane with the joist and halflap the brace into the girt and run the brace into the plate.

5. Create overhang with rafter tails, buildings need shelter also.

I agree with Tim and Roger.

Thanks for your replys and wisdom. I always felt that the designs in the newer timber framing books didn't really match the old barn designs I have seen. I like "old school" rather than "new school".

After eliminating the wall girts in the 1st design picture, I think the design is now beginning to match the popular "Shaker Toolshed Design" from the Jack Sobon/Roger Schroeder book. I am now thinking about taking this Shaker toolshed design and increasing it's size for a barn. Not sure of the size yet (want to add a bent and increase length of tie beam).

In Sobon's book he talks about increasing the size of the Shaker toolshed and what has to be added to support the size increase. Possibly adding a post in the middle of each tie-beam and also more rafter support (to stop outward thrust?). I'll read up on this.

Thanks

It seems like a similar High-Posted post comes up here every few months on the forum, this drawing is why it is so important to drive home how dangerous this design is as drawn. (the first drawing, though collars in the gables alone do not resolve the problem)

That said I tapped this off before I saw your most recent post late this morning, but I'll put it up anyway because I see you are taking advice and evolving your design...

The thrust imparted to the Plates, and the bending moment this introduces to those Posts is massive. And yes, the planning of a three way connection and the removal of more section at the same location as the Tie and its Dovetail Tenon is poor planning and design.

I'm with DLB, it is not necessary or even architecturally interesting to insist that everything be in the same plane, if you raise the joists up so they override the Ties all or in part, this will also in part spread the moment over a greater section of the Post , and shift the flooring and the tieing action the flooring and the diaphragm it provides higher and away from the large amount of section removed for the Dovetail Tenon and its Wedge,. ( it also allows the Wedge to be driven from the inside, and provides access to it to allow it to be snubbed up as the frame seasons)

There is also no imperative to tenon the Girt, though moving it up to a plane higher than the Tie allows for longer tenons which might better handle the tension the braces and wind load will cyclically put them into.

A better solution would be to tack un-tenoned Girts (in reality simply a joist and one that carries half the load of any of the others) into shallow housings, and to pass the braces through them an on up into the Plates.

I'm also with Roger, you must introduce a system which will resolve or redirect thrust before it gets to the Plates. Simple rafter pairs will not work