Joist question... drop-in or dovetail

[Aikenback]

Hey JC,
I recently have downloaded Thomas Tannert's thesis on RDC from U of B.C. 2008. 256 pages of engineering testing, data, etc. Just for my own peace of mind. Talk about a dry read! One of the strongest recommendations is that the material be fully dry, and the joint created carefully with minimal gapping, and Definitely reinforced with a GRK or other self taping screw installed at 55 deg. Coming from a conventional carpentry background, I found there were things with timberframing that can be counter intuitive to anyone uneducated in timberframe and other historical building techniques. (as some of your other posts have shown). And I was wondering if my aversion to mortising deeply into a supporting beam midspan for a tenoned joist was justified.  The original reason for using the RDC method was for ease of use, but the main reason now for me is that it seems to be lest intrusive to the supporting beam, therefore weakening the supporting timber less than a row of tusk mortises through the middle of the beam. Especially where there are joists on both sides. Considering my short time (so far) in this type of construction, I would like to ask what has your (and other long timers') experience been with engineering beams for tusks over RDC's. Does the supporting beam  ( in general) need to be enlarged significantly due to the tenon? I'm already using some big stuff, don't want to go too crazy. I suppose joists dont HAVE to be mortised, they could sit atop the beam. I do like the look though, and on exterior work,  barn swallows love those joist spaces on the beam. However they do eat mosquitoes.

[Jim_Rogers]

I would like to ask what has your (and other long timers') experience been with engineering beams for tusks over RDC's?

I guess I may have missed it. What is/are RDC's?

Jim Rogers

[Aikenback]

Sorry jim, didn't mean to leave you out. The engineering thesis I was reading (trying to read) calls these new joints Rounded Dovetail Connections. RDC's.

[Jay C. White Cloud]

Hello Aikenback,

I can't believe you got through the read of Tannart's thesis, (can you say nose bleed!)  great info, but wow that is hard to all chew at once.  I haven't gotten through all of it, but have a copy I "gnaw at," when ever this subject comes up.  I think now you understand my concerns with this joint and have addressed them well in your frames.

To answer your question about "aversion to mortising deeply into a supporting beam mid-span for a tenoned joist," each case is different, as you well know. Perhaps though, your concerns aren't as justified as you may believe.  The primary evolution of the RDC is ease of execution of joint and assembly of frame.  Your concern for weakening the supporting beam can be alleviated by placing the "tusk" in the correct location within the receiving member.

Don't worry about your short time doing this kind of work, for it is refreshing to see how thorough your approach to the craft is.  If there is to be "modernization," in approach to cutting and executing joinery, it should be done with the kind of care and forethought you have put into it.  To address you question about the receiving beam I present the following for your consideration and questions:


For the most part, (but there are cases,) the supporting member does not have to be enlarged, or enlarged very little.  It is the placement of the "tusk," that is critical.  You need it to penetrate the neutral axis of the receiving member above or near the vertex of the "hour glass," center.  In this region of the timber you are between the "compression" and "tension" zones, thereby not "weakening" it, or very little.  Housing into the supporting member is another issue, where the "rules of 2s and 3s - 3s and 4s," should be applied.  I will explain this as the Amish "barn wrights," taught me.   On top of your receiving member, a housing should not get closer than 50 mm (2") preferably 75 mm (3") across to the other side.  On the side of the supporting member, there should be 75 mm (3") preferably 100 mm (4") of wood beneath the inserting timber. They also said this was more a guide than a "rule," but it has stuck with me all these years and has served me well.  There is, of course, exceptions and should you have concern consult with others.

I tend work in "folk styles," where receiving members don't receive but support, because the other member rest on top or is only partially housed.  This all depends on the style you work in and/or your own individual style/preference.  I like barn swallows (and other birds;)  giving them a place to nest and rest in architecture can be a good thing. 

[timberwrestler]

I tend to do tenoned joists a little differently, like this:



I think that's generally called a soffit tenon, but I think that may be backwards (the tenon's not on the soffit after all).  I like it because it's a little less meat out of the receiving beam.  That's a 5x8 joist, reduced to 5.5" going into an 8x10.  The 8x10 has a nominal 1/2" housing.  With a lot of those dovetailed joist details, when the receiving timber shrinks, the joint looks like crap because the joist isn't housed.

I like the look of the smooth reductions, and engineers love em (even the NDS throws them some love).  Here's a row of drop in joists to give you look:

 

[Jim_Rogers]

I like that design a lot and I use that as a tying joist to hold the sills and tie beam to each other.

Jim Rogers

[Jay C. White Cloud]

Hi timberwrestler,

You are correct, it is called a soffit tenon, (soffit meaning the underside of an architectural structure such as an arch, a balcony, or overhanging eaves, etc.)  It is a wonderful joint if you are doing a "member reduction."  The tusk is the first one in my example and can even be done using a spline that would connect both inserting members coming into the receiving member from opposite sides.  These are a wonderful family of joints that can really tie and strengthen a frame.

 

[Aikenback]

Hi everyone,
Yes I am aware of neutral axis but also was thinking of the shear area down the center between the compression and tension areas (obviously, lol). I always considered the shear area to be as wide as the beam is, but if the forces are hourglass shaped, I can see there is less concern.

Some new LVL beam material in conventional framing is marked in that area as an area drilling is allowed to some degree. It was more a full row of mortises I was leery about. Especially from both sides.

I can always run numbers, and I guess advice is a click away. usually the beams I am mortising into are oversized just for aesthetics anyway. It could be an extra advantage. As far as the beam/joist reductions, that is one of those counter intuitive things.

We were taught never to notch the bottom of a supporting member without supporting the full thickness of the member in front of the notch. Otherwise the member is only considered as deep as the notch.

Then an engineer told me how much I could reduce a fairly major beam one time and I was amazed. I was reading a pdf I downloaded from the tfg, and they actually gave a simple formula for the radius they would like to see based on the amount of reduction. Not only a great look, but spreads the shear from the reduction over a larger area. Like you guys didn't know.

I probably read it recently in one of Jim Rogers posts from years ago. Lol.

P.S. I said I downloaded the thesis, didn't say I got thru it, lol.

[Aikenback]

By the way, very informitive discussion for me. One of these days I will start compiling reference materials from the tfg etc. I do have several recommended books. People really have no idea how educated true carpenters really are. Its a shame.

[Jay C. White Cloud]

Hi Aikenback,

Check out this link: http://blog.ftet.biz/2009/01/keyed-and-key-laminated-beams.html. on "key-laminated beams."  There has been several "keyed" and "tooth" lamination techniques through the ages to make small beams into big beams.  When you consider the dynamic at play when designing one of these "traditional laminated" beams, you begin to understand the brilliance behind "tusk" and "soffit" joinery.

I can't remember where on here I shared the "rules of 2s and 3s -3s and 4s" but the day I was taught it, they also taught me "less is more," when reducing a the end of a beam to avoid "concentrated shear" at a notch or to save from taking more out of a receiving member.

Regards,

jay

[Aikenback]

Yes I have seen the keyed beams, they are pretty cool. And totally make sense. I notice, after thinking a bit, the amount of material between tenons in a row of soffit/tusk mortises would control a significant amount of shear anyway. Thanks for the link.

Sorry Dan we hijacked your thread, I guess your going to have to decide yourself, this could go on for ever! Lol

[Jay C. White Cloud]

I'm sorry too, I have only been here a short time, and now think maybe I should make suggestions about moving certain conversation, (tangents,) to a new post.  Maybe once I'm more familiar with the forums culture, I will suggest that when we begin to "hijack" a thread.

Hi Aikenback,

I knew once you thought about the two together you would make the connection!

Regards,

jay.