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2 joists and a beam above a post joinery

Started by barasingha, October 03, 2016, 06:59:03 PM

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barasingha

I have a need to support a 8x12 summer beam's end from an 8x8 post.  But there is also a 6x12 joist leaving both sides of the summer beam above the post.  There a two spots where this is warranted: one of them has the post continuing up above the summer beam, the other does not.

Joinery recommendations?


barasingha

Here is my current plan for floor joists, sills, and summer beam? Girder?.  The total span is 22' counting the sills.  The sills sit on rock walls so the free span is 21'.  The girder is supported by 8x8 posts on 9' centers and splits the span.  The joists are on 3' centers and span 10' 2".  The girder stops at a stairwell and fireplace and then continues on the other side of it.  At these two points I would like to have the post support the girder and two joists.  Ideally, the post would continue above the floor for the one by the staircase.

I have read since my original post that 3 and 4 way connections are to be avoided.  I guess this is why top plates are usually above the tie beams?  Are there any options?  Maybe move the post in and cantilever the girder?  Continuous 22' joist over the post with the girder tenoned into it?  I guess I could always use steel brackets.



Here is the floor plan with holes for fireplace and stairwell.


ChugiakTinkerer

Some questions from a complete newbie...

Are you constrained by height for the placement of the joists with respect to the girder?  I'm new to the realm of joinery so I tend to think in terms of butted square ends, but it seems you could lower the girder quite a bit to where the joists are practically sitting on top.  Much like the top plate sitting above the tie beams.

For the post that you want to extend updward at the staircase, is it feasible to use two posts rather than one continuous one?
Woodland Mills HM130

barasingha

Not constrained per se... more of a desire to have headroom under the house.  I will have a basement with a height between 6.75' and 7.5'.  In the future I'd like to be able to finish it out.

It is certainly feasible to use two posts in place of the continuous one.  It is also feasible to use a bigger post in that location to provide more meat for joinery.  Or two posts below the floor side by side--one for the girder and one for the joists.

Thanks for the questions.  I am probably more of a newbie than you.

barasingha

Would a shouldered mortise/tenon for the girder leave enough meat for the floor joists?

How is what I want to accomplish different from the examples of a post, tie beam, wall plate joinery?

If the girder used a 2" vertical tenon and shouldered into the post an inch, that would leave 3" of post on each side for the joists to tenon into (also shouldered 1").  If the post was increased to a 8x10 then the joists would have 4" of post to tenon into.  This make sense to me for the post that continues up the stairwell. 

The post that ends at the first floor however twists my brain into a knot.  It would need to tenon into the girder from below and the joists would then tusk into the girder like all the other joists.  But now the girder is open ended because of the joist mortises.  It makes more sense (in my head) to run the joist across the post the entire span and tusk the girder into the joist but the girder is set 2" below the joists and it would defy all the rules of mortise depth and tenon depth unless I got crazy with something like this:



I have never seen or heard of this and surely someone with experience has come across situations like this.  Where's the experience at?

barasingha

Here is a better illustration:



What are the weak points of a joint like this?  Are there any strong points?  What happens if the joist were cut in two?


ChugiakTinkerer

My understanding is limited, but I think putting pegs in this joint will be a challenge.  Are you looking at traditional pegs or something like Timberlinx?
Woodland Mills HM130

Jim_Rogers

Thanks for that drawing. I has helped me to see what you're talking about.
First off the post tenon doesn't need to be that wide. I can be only a 2" tenon. It should be parallel to timber going left and right, and should be offset 2" from the outside face, not the face where the reduction shelf is. Then the tenon for the beam can be offset two inches down from the top of the beam and make it a full through tenon going over the top of the post tenon.
Weakness here is a short beam tenon. Post tenons just hold the post to the beam and gravity will hold it down. If there isn't enough relish for a peg then use several timberlok screws.

Assuming a 6x12 sill/beam and an 8x8 post and a 8x10 summer beam?



 

Exploded view



 

Jim Rogers

Just my ideas
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

barasingha

Nice drawings Mr. Rogers.  It's an 8x12 summer beam. 

If I moved the post tenon to the inside of the sill wouldn't more meat would be available for pegging the summer beam's 3" thick tenon over the top?  This drawing has the thru-tenon on the summer beam, 6" long with a 2" shoulder on the post to sit on.  A 5" tall tenon from the post into the sill which could be pegged thru the sill into the end of the girder.

And lastly, any thoughts on splitting the sill above the post (shown as the dotted line.)




Jim_Rogers

One thing you need to do and understand is some general frame rules of layout and cutting. One layout rule is the tenon size and offset. Normally with an 8" thick timber the tenon is 1/4 of the width thereby a 2" tenon. And it is normally offset off the reference edge or reference face 2" so that you can easily lay it out with a framing square.

The next one is the size of the peg. Normally a peg is 1/2 the tenon thickness. So for a two inch tenon the peg is 1" in diameter.

Next is the standard peg hole location. It is normally 1 1/2" off the shoulder of the timber that has the tenon. And normally centered on the tenon width.

I have added pegs and pegs holes to my drawing to show you.
However in your case the pegs may intersect inside the timbers so you should off set one, like the post peg to one side.



 

 

 

The peg hole has to be so many peg diameters between the hole and the end of the tenon in order to have enough relish between the peg hole and the end of the tenon for strength.

If you move your post tenon in and put the peg from the tie beam/summer beam on the other side you are moving the peg hole closer to the end of the tenon which will make it weaker.

Just my point of view.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jim_Rogers

If you split the split the sill above the post, what you are doing is asking the sill be be as strong as it can be at that point and then cutting half of it away. Not the best solution.

Again if you do split it then you're going to have to peg it and again there won't be enough relish at the end of the sill for strength.

I can't say for sure if this is true or not until I see the entire frame/system.

Three way joints aren't the best and making it a four way is even tougher.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

barasingha

Thanks for the lesson.  I was sizing the tenon based on depth instead of width.  I.e. 8x12 beam has a 3" tenon.

Questions:

If the summer beam were entering into a post and the tenon were vertical would it now be 3" or is it always based on width of timber?  (I understand there are a lot of variables, just asking how to properly apply the rule of thumb.)

Is the peg hole centered 1.5" from the shoulder or is the edge of the hole 1.5" from the shoulder?

The reason I moved the post tenon was because it looks like the summer beam peg, in your drawing, would cut through the sill joist/post joinery.  If the summer beam shoulders into the sill joist 1/2", then 1.5" is left between the shoulder and post mortise.  Regardless of peg placement (1.5" center or edge from shoulder), the peg will be driven between the mortise and tenon of the post.  Is that a concern?  When I moved the post tenon to the inside (my last post above), the peg would still split the post tenon joint, but would have the post tenon inside the peg instead of the lower part of sill joist. 

What if the post/sill joinery was moved to the outside edge?  Can't draw it now; basically the sill joist sat between a 2" external tenon (name?) and the summer beam.  Summer beam still tenons through the sill and above the post tenon.  The peg would now not enter the post tenon joinery.

What about shrinkage concerns with a joint like this?  I'm using green syp.  Would shrinkage of the summer beam height cause more loading on its tenon than its shoulder thereby splitting the summer beam?  If so, would shorting the post tenon to allow the summer beam tenon to bear only on the sill joist help by allowing the shrinkage of the sill joist to match the summer beam shrinkage?

I believe that the tension of this joint will only come from shrinkage as there is only floor load above.

I am also starting to believe that it is harder describing joinery in words than it is to cut it, and definitely harder than drawing it.

barasingha

I would split the sill by way of 3" half laping (2" around the post's 2" tenon.). The summer beam would still tenon through the two half laps and above the post tenon.

Should stay within the rules of thumbs if I understand them correctly.

Brian_Weekley

This is a very good reference to study...

http://www.ncptt.nps.gov/wp-content/uploads/2004-08.pdf

You should be able to build nearly anything with the joints that are shown in this publication.
e aho laula

barasingha

Thanks.  I will read thru this tonight.

On lunch I drew this up.  What do y'all think?


barasingha

Page 15 of the PDF that Mr. Weekley provided shows figures 25 and 26.  Looks good to me.  What are your thoughts?

I almost got there with my last drawing but already forgot Mr. Roger's earlier advice that the post doesn't need a big tenon.

*Edited to fix the spelling of Mr. Weekley's name.*

barasingha

The latest drawing based on the pdf Mr. Weekley posted above.



Anybody ever do/see a joint like this?

ChugiakTinkerer

Your top tenon appears to be a through tenon.  In the HATJ example it is a blind tenon with the mortise only 6" deep, leaving about  2" of the back side of the plate.  In the drawing above I would be concerned about there not being enough support for the joist.  One option is to make the tenon and mortise shorter so that at least 1/4 of the joist remains.  Or if you make the top tenon narrower then there would be a shoulder for the joist to rest on.
Woodland Mills HM130

barasingha

Thanks for the feedback Mr. Tinkerer.

I am using a 2d software for iso drawings and as a result cannot draw dimensions (or they'd be wrong) and cannot draw them to scale because I am not geek enough--meant as a compliment to geeks.

The sill is a 6x12 beveled to 6x9 at the post.  The post is 8x8 with 2x5x6" tenon and 2" summer beam shoulder.  The summer beam is 8x12 with a 7x2" tenon into the sill.  The sill mortise is 4" down from the top and 3" up from the bottom.  The sill's bottom sits on the summer beam and post joinery for the full 8".

The joinery in question goes here where the floor ends for a fireplace opening:


Because I am not proficient with timber frame terminology I am not sure which "joist" you are referring to.  Are you concerned about the girder (summer beam) that ends at the post, or the joist (sill plate) that continues past the post?

ChugiakTinkerer

I am referring to the joist (sill plate) member.  The 7" x 2" summer beam tenon appears to be 6" long.  Is that the case?

My earlier concern related to the amount of sill plate that is taken out for the mortise, and what I perceived to be a lack of shoulder for the sill plate to rest on the summer beam.  This crude sketch shows the areas where under sufficient load the sill plate could fail.



 

Your explanation made me realize there is a 1/2" shoulder on either side supporting the full thickness of the tapered sill plate, shown in green on the sketch.  I don't know enough about timber framing and your specific loads to know whether that is sufficient.  From the standpoint of supporting a beam, in my experience 1.5 inches is the minimum amount of end support required.  So I would want to increase that shoulder to at least 1 inch, if not 1.5 inches.  That means the summer beam tenon would be narrower by 1 or 2 inches.
Woodland Mills HM130

barasingha

I understand your concern now.

My joist/girder joinery for the rest of the floor is posted above in a picture.  They use a 1" diminished? shoulder.  Because of your concern, I just realized how similar my proposed post tie in is:



I agree a 1" shoulder would be better as it is what I used in the rest of the joists.  That leads me to the idea of moving the post and joist in 3 or 4" and cantilevering the 2x6 decking.  This allows the girder to continue past the post 3-4" and allows me to split the joists into 2 like the rest.

This is a simple floor with mainly gravity loads.  There will be shrinkage, which should be minimal end to end, and tighten up the floor.  The floor will also be a diaphragm for the rock wall perimeter.

It would be so easy to place the girder on the post with a post cap and hang the joists from hangers, but I'm trying to build different to fulfill a craving in my soul.  I guess I take for granted how convenient modern framing is.  Never stop learning.

Side note: As I write this I have a 2 and 4 year old coloring next to me and they make our drawings look terrible!

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