I was wondering if it is a good idea to offset the joists' joint location into a tie beam that also has joists joining into it from its other side as well. It seems it may weaken the tie beam more if the joists both join the beam in the same location. It would not be that difficult to layout the joist mortises on one side of the tie beam offset from the opposing joists pockets. tb

Yes, if they were alternated evenly, that would weaken the tie beam less. How will it look, though? CB.

It's my understanding that reguardless of where the pockets are placed, you have compromised the strength of the timber. So alternating the pocket layout shouldn't help in the strength department. If the timber is sized correctly one could place any number of pockets in any location. How about placing the members on top of the tie/girt and using no pockets. Tim

I agree with just sizing the carrying beam accordingly. However if you did offset joists you could create maybe some aesthetic interest. Maybe with the help of some partitions and the right spacing between On Centers of each bay you might be able to make the whole "room" offset from the previous when in fact it is not.

You may get some slightly stronger areas locally, but you cannot calculate it or rely on it for sizing your tie beam (not significant). I wouldn't. You should assume that the area where it's notched is doing nothing in bending(bending stress). I speak only from a mechanical engineering standpoint and not necessarily from timberframing experience.

This might be a question for the engineering section. I'll try a specific example for clarity. 8x10 Tie. Span=X if it too long we will just put another post in the middle. 1" Housings. 4x6 Joists on both sides of Tie. After the tie housings are cut you still have roughly 4" below the joist on the tie that has not been removed. This wood is in tension. On the top of the tie you would have compression wood. Since the joists would fill these housings in the compression wood (4.99" anyway) wouldn't you have only a 1.01x1x4" section that is sacrificing the strength of the tie.

If you housed joists only in the compression wood of the tie-beam would it only take away a very minimum of its original stability (no housings at all).

Any elaboration on this example would be appreciated.

Mo

thanks all. Daiku , you are right to ask how it would look? I am not sure as I have never seen this. I thought maybe you might have though. Its probably not woth finding out going by these responses though. perhaps better that I use the extra energy to frame the joists using soffit tenons, as they seem to be the superior joint in this location. Thanks again everyone for the critique. Always helpful. tb

Yes, soffit tenons are very nice. They handle the structual issue and are easy to cut however, they bring up an assembly problem, if you are willing to deal with that they are a great way to go. I put in a couple soffit tennons, to hold things stright etc., with the rest, drop in pockets. As for a one inch deep housing that seems a bit on the short side, although I know a lot of folks use that set up. Maybe I am being a bit to traditional using two inch plus deep pockets. If its a barn/out building I still have no problem simply putting the joist on top of the ties, some times no lay out just knock around until they look well spaced, the joist are usually sawn only on two sides, with tha bark pealed. Tim




AA

Mo:

I don't think this analysis is valid. You can't count on the joist filling that housing. For one thing, the joist may shrink in width, and not fill the pocket. The joist may even be cut slightly undersized. And the wood fibers in the joist are running in the opposite direction, so you would be compressing the side grain. You have to reduce the section in the tie beam by the full size of the pocket before doing your load calcs.

I took an engineering course at Heartwood school where we discussed this subject. They gave us some guide lines to go by.
I created a drawing based on these guide lines.
Here is a drawing of drop in floor joist to tie beam connection "rules of thumb."

See if this is helpful to you:




The dimensions are just for this example.
If you change the tie beam to a deeper beam you can adjust the floor joist size and the depth of the pocket.

The purpose of the scoop at the bottom of the floor joist is to "eliminate the point of fracture," which would be created if you just cut out a 90° notch.
This type of scoop or curved cut can be seen in many old structures.

Any comments are welcome.


Jim Rogers

Daiku, thanks for the correction. That makes good sense.
Jim, Great drawing! Thanks. I will defintely file that one away.

In Jims picture, the 5/8 rule, works great. In one of Sobans books he shows placing the joist in up side down and adzing the bottom to the surface of the timber than flipping the joist right side up, no lay out once agin and there is that beautiful sweep. Tim

Jim, those are really helpful. Are there such general guidelines for soffit tenoned joists or other joinery style such as dovetail, or tusk tenons? tb

Isn't a soffit tenon and a tusk tenon the same thing?

Jim, I think they are nearly the same. Soffit tenons are located on the bottom side of the joist/beam while tusk tenons are located up from the bottom, more to the center some times with a housed soffit shoulder below it. My question of terminology with this is if you had a 4"x8" joist with a tenon at 4 and 6 inches down with the remaining 2" adzed off, is this a soffit or tusk tenon? Maybe this is a knit-picky question, but when yoour communicating on the shop floor it might make a difference. Tim

I always think of soffit tenons having the angled cut from the top of its tenon getting slightly larger at the tenons base, much like the diminished haunch cut seen on tiebeams. Also like Timbeal said of the tusk tenon , which has its tenon more centered and perhaps still shouldered on its bottom. I see the soffit also being superior in the shear value as its tenon is angled on top and full bearing dimension on the bottom, which also allows the beam it joins to, to not lose any wood on its edges like the drop in joists. tb

I have had a question as to how many framers are lapping floor joists the 1" and using a timberlock type screw? Daiku and Moe have referenced this in posts. I have heard that some engineers prefer this to a deeper lap or dovetail. Any truth to this? Whats the most common connection here?
I chose to go with a 3x4 lap on a 8x10 beam with a 4x6 joist and a 3/4 inch locust peg driven from the top. This would have left 2 inches where joist pockets were adjacent each other. I did alternate spacing so this wouldnt happen, but from a engineering standpoint I think you still only have two inches of relish there. I just felt better about it when spacing them.
I have also wondered if the 3/4 northcott peg is comparable in shear strength to the timberlock screw?
As for how the spacing looks I will let you know the frame isnt totally up yet. I think it will look great! Thanks for any insight. BB

I typically use 1.25 inch joist pockets. Here's the rationale: A dovetail, or some other joinery will weaken the carrying member, which will then have to be up-sized to compensate. The joint will be hidden by the flooring. You can't tell by looking at it whether it was dovetailed or not. Also, the screws will actually hold it down tighter, reducing squeaks. A peg won't do that. I'm just balancing what's traditional with what's practical. If old barn builders had access to these screws, you can bet they would have used them. CB.

Thanks for the response. Never thought about the squeak, much easier to cut also. It makes perfect sense to me! BB

Pictures people, please! They are worth a thousand words and I have a hard time understanding some of these descriptions, given some slightly different terminology etc..

So what's a video worth? How about two?

Joist Pockets Part I

Joist Pockets Part II

Yea! 89 seconds, done. Nice template, too.

Daiku. I am not sure what you mean by 1.25 inch joist pockets. A 5x7 joist adzed down to 4" at its ends would travel only 1.25 inches into the beam? And then screwed vertically through this stub tenon and the beam? 1.25 inches doesnt seem like a lot of room to do this, what screws do you use for this?? tb

tb, like to hear from others but does why do the joists have to be adzed down. if you keep them rectangular, it is less work and as long as there is a housing you don't worry. please share with us your thinking that joists have to be adzed.

One reason mo is to keep within the "rules of thumb" that I posted at the bottom of the first page.
And to have enough wood under the joist end to support it.

The end of the joist is cut square (I typically use a 6x8 tapered down to 6x6). There is no tenon. A square 6x6 pocket, 1-1/4" deep is cut into the carrying beam. You drop the joist in from the top, making for (much) easier assembly. The 10" timberlok screws are driven from the top of the joist at an angle into the carrying beam. The relatively small amount of wood removed from the carrying beam keeps it stronger. Note the large amount of wood below the bottom of the pocket. The carrying beam in this drawing is a 10x12, so there's 6" left below the pocket. CB.

mo:

Tapering the joists make for a smaller housing in the carrying beam, so you weaken it less. The joist typically doesn't need extra strength there at the end, it needs it in the middle where the bending moment is greatest. If you do the math, you may well find that it's optional, but of course it also looks great! In a production shop, we use a power adze. CB.

Mo, I think Jim and Daiku pretty much explained it, but in my own words: To have deep enough joist to carry the floor load in bending and deflection, but at the end where shear is the primary concern , to adze down helping with shear concentration and to reduce the amount cut into beam and to keep the tenon at neutral axis and to not need an extra deep tiebeam , like 14" deep or so. I do also think NH was referring to a step lap. One could find an example of a cog in Jack Sobon's "Historic American joinery" book, which ,though it is a very small book, is probably my favorite tf book in my collection. Daiku, what exactly is a power adze?Daiku , how far in from the joist end do you start the screw and how much angle do you put it in at? Doesnt the the top of the screw stick out above the joist top surface? Sorry about all of the questions , but I never heard of this before and seems considerable option for my own designing. tb

Here's the "Power Adze":



We start the screws maybe 3 inches from the end, at about 30 deg off plumb. We aim for the bottom back of the housing, maybe a little above that. The screws are self-counter sinking (at least in pine). Just drive them down into the top of the joist.

Thanks Daiku, good post! BB

Thanks for sharing Daiku. This has turned into good post. By the way, in terms of the original question of offsetting the joist pockets, which I do believe is helpful to the strength of the beam, I have drawn some pictures in sketchup and they look really cool. We were wondering if it might look weird but I think it generally looks pretty cool as it tends to define the bays . Anyway thanks for all the input. tb