What type of BENT would work here?

[ShimodaLife]

Hi. I'm hoping someone can offer an idea for a bent that makes sense, with my 18 x 36 house design. I've got a few pics below to help you understand my request. I'm building a TF home on a steep, rocky slope, working with an architect who doesn't know TF'ing but is willing to try, and I have to give him a mockup of the frame itself, so he can start understanding the loads, structural implications, etc.


 
Here's a bent design I've come up with. The ground is steep, and very little of it is level, so I've got a front post that is about 15' tall, a center post about 16', and a back post about 5',... and each of them sits on a separate ground level. I cannot change the ground or better level the area; it's mainly solid rock. I'm not showing the foundation or stem walls, but it's a lot of concrete and rebar. This is what I've got to work with.

My first issue is: it doesn't look like any bent I've ever seen. So I'm wondering if anyone has a better idea, given the ground requirements. A pencil sketch would be welcome,... something that looks and acts like a bent. Or tell me that mine will work.

To help orient you around the house, the man is standing at the entrance in Bay 1, looking down the mountain. In the upper part of Bay 1 above the entrance will be the master bedroom. Bay 2 and 3 on the main floor will be living room, and the space above will be open, cathedral ceiling. The 2nd floor (2nd level, back of house) will be toilet/bath (Bay 1), dining (Bay 2) and kitchen (Bay 3). Above the toilet/bath in Bay 1 is just enough room for a loft with ladder access.

As you can see, headspace in Bay 1 will be about 8' ceilings in the entranceway, with 16'-18' ceilings in the Bay 2-3 living room. Headspace in the second level will be about 8' in the Bay 1 toilet/bath, and about 16' in Bay 2-3 dining/kitchen. Master bdrm ceiling height is about 7', not ideal, and in the loft, just 5' near the eave.


 

(if you've been around, you may recall me posting a year ago or so, getting feedback from the experts. I had to fire my first architect, took a year to find a second, and now trying to proceed with my dream...)

I've got plenty of detail drawn into this frame, and will post the pics below. But really, the thing that is bogging me down, is: "can I make this bent work, or do I have to restart?"



 


 

 

Thanks for reading.
JT

[Jim_Rogers]

The first issue that I see is that the tie beam meeting the post, and the plate is just above that area.
there should be at least 8" between the top of the tie beam and the bottom of the plate. so that any rafter thrust does not push out on the post, causing it to split:


 

if you're using two pegs stagger them so that they are not in the same grain line vertically in the post. 
As the tie beam shrinks with drying the beam could be held up by a high peg in the tenon. So, we use two pegs, but both are lower in the tenon to allow the tie beam to shrink down from the top and not split from a high peg holding it up. A split tenon could weaken the joint.





The solution is to raise the front plate up a little. But that may mean all plates have to go up the same amount which also means longer posts.

Placement of the scarf joint could be improved by moving it so that the lower half is supported by the brace, if your timber lengths can afford it.

Other than these, I would think that this bent design would work.

Jim Rogers

[Don P]

There is another way I've seen an engineer look at that extended post. It is a lever. The rafter is providing the push. Do you really want to make that lever longer?

[Hilltop366]

Is that a ridge beam?  Would rafter thrust still be a issue?

[Don P]

Well now you just went back and looked at the real subject of discussion  :D

[ShimodaLife]

The first issue that I see is that the tie beam meeting the post, and the plate is just above that area.
there should be at least 8" between the top of the tie beam...

The solution is to raise the front plate up a little. But that may mean all plates have to go up the same amount which also means longer posts.

Placement of the scarf joint could be improved by moving it so that the lower half is supported by the brace, if your timber lengths can afford it.

Thanks Jim. Rather than raising the plates (posts are starting to get mighty tall), perhaps lowering the tie beams would be an easy fix. Exception would be Bent#2 (between Bay1&2), as there will be a door to the master right there, which needs more headroom. I wonder if I could adjust or leave out just that one tie beam?
Good point on the scarf joints. Timber lengths won't be an issue.
So the bent design might work? That's music to my ears.
JT

[ShimodaLife]

Well now you just went back and looked at the real subject of discussion  :D

What's that, Don? Your insight into a problem would be very welcome. Those three plates, with the center one acting as a ridge beam... is that all wonky? a disaster waiting to happen? 

[Jim_Rogers]

The first issue that I see is that the tie beam meeting the post, and the plate is just above that area.
there should be at least 8" between the top of the tie beam...

The solution is to raise the front plate up a little. But that may mean all plates have to go up the same amount which also means longer posts.

Placement of the scarf joint could be improved by moving it so that the lower half is supported by the brace, if your timber lengths can afford it.

Thanks Jim. Rather than raising the plates (posts are starting to get mighty tall), perhaps lowering the tie beams would be an easy fix. Exception would be Bent#2 (between Bay1&2), as there will be a door to the master right there, which needs more headroom. I wonder if I could adjust or leave out just that one tie beam?
Good point on the scarf joints. Timber lengths won't be an issue.
So the bent design might work? That's music to my ears.
JT

One tie beam close to the plate won't be an issue, unless DonP says you have some rafter thrust.
Jim Rogers

[Don P]

I'm agreeing with Hilltop, as long as the ridge doesn't sag the rafters are simply hanging from the ridge and setting on the plates, there is no outward push.

Jim, what happens if the post tenon is inboard of the peg holes in that diagram?

[Jim_Rogers]

Jim, what happens if the post tenon is inboard of the peg holes in that diagram?

DonP: which diagram? I'll assume you mean the first one that shows the post split? 
Then most likely the post would/should not split. However, when a frame is designed, the designer sets some "general frame rules" or notes. One rule is the tenon size and spacing. In the first drawing the rule was 2" of layout face and then 2" thick. This is a standard for timbers 8" thick. With a 1" peg.
You can center tenons and or some other location but that's up to you to remember these rules and follow them with every joint.
Jim Rogers

[Ljohnsaw]

Couple of thoughts:
Pretty narrow master bedroom.

That loft tie beam at the center post with the knee brace coming up.  Looks like the brace butts right to the tie beam tenon.  To the experts, is that kosher?

[Hilltop366]

More of a design question than a timber frame one but did you look at running the ridge line the other way?

I'm thinking that you could get more headroom / useable space in the upper levels..... or I could be all wrong.

[ShimodaLife]

Couple of thoughts:
Pretty narrow master bedroom.

That loft tie beam at the center post with the knee brace coming up.  Looks like the brace butts right to the tie beam tenon.  To the experts, is that kosher?

I agree it's all pretty cozy. Wish I had more space overall.
Here's a bigger image of that center post. The tie beam has a 4" tenon and the brace a 3" tenon, so there's only 1" of wood left between them. Per Jim's note, maybe I should be using 2" tenons everywhere, and leave myself more wood in the post...?
 


 

[ShimodaLife]

More of a design question than a timber frame one but did you look at running the ridge line the other way?

I'm thinking that you could get more headroom / useable space in the upper levels..... or I could be all wrong.

Running the ridge beam the other way, just so I understand: right now it's an east-west orientation, so what if I ran it north-south, is my understanding correct? So if I kept the house the same dimensions, that would shorten the ridge beam from current 36' to 18'. Interesting idea. Never even considered that as an option. Being an real noob with TF'ing, I'm not sure the design and construction implications. Thanks for the comment.

[Hilltop366]

Yes thats it, so instead of the entire 36' back having a low ceiling it would only be on each 8½' end. 

One step further would be to have two full levels by raising the lowest floor up to the same height as the other and a second floor above that and skip the loft all together, this would also make the basement more useable for utilities and storage.

[Jim_Rogers]

Couple of thoughts:
Pretty narrow master bedroom.

That loft tie beam at the center post with the knee brace coming up.  Looks like the brace butts right to the tie beam tenon.  To the experts, is that kosher?

I agree it's all pretty cozy. Wish I had more space overall.
Here's a bigger image of that center post. The tie beam has a 4" tenon and the brace a 3" tenon, so there's only 1" of wood left between them. Per Jim's note, maybe I should be using 2" tenons everywhere, and leave myself more wood in the post...?

That tenon on the beam would be stronger if it was lower. I don't understand why you would shape a tenon like that. Normally a tenon is full depth of the timber. If this timber is supporting any weight, it is likely to split at the bottom of the tenon. The peg hole should be 1 1/2" off the shoulder of the beam. And centered on the tenon, if only using one peg.
The peg hole in the brace tenon should be 1 1/2" off the shoulder and 2" off the bearing end of the tenon. These locations are the standard/traditional peg hole locations, on brace tenons.
Jim Rogers

[rjwoelk]

I don't see the tie beam housed, my engineer (Firetower), wanted min of 1 inch of bearing in the post for the tiebeam to rest on.

[Don P]

I agree with the past few posts. Do be aware rules of thumb are kind of like the pirate's code, they are more guidelines. Think through the stresses involved on each joint and why you are doing what you are doing. This is one. Usually peg low, closer to the bearing so the member doesn't end up supported on the peg when the beam shrinks. Spread out joinery, you should be checking the net section after joinery has damaged the timber. Those things are described in the NDS, good to review. But mainly, it looks like Hilltop's idea might also solve a roof water issue on the uphill side. 

[ShimodaLife]

Don, RJ, Jim, Hilltop,
I really appreciate your comments, and am taking them all to heart. My only excuse for not doing things properly (peg hole placement, tenon style, tenon length, etc) is ignorance. I've read 5 books, read countless posts here, and built one TF tiny house,... and that's the extent of my TF knowledge. My goal is to get better, and your comments are my best path in that direction.

On that note, Hilltop's idea for re-orienting the roof sounds like a good one. Shedding water would definitely be an issue behind the house, where it would all start gathering between a concrete foundation and solid rock. I've done a quick and dirty draft, and I like the look of the bent, first of all (see first image below). Second, I think I can incorporate a king post design, which is different and looks nice (second image) 

I haven't really investigated *how* to incorporate the king post design. On top of the two middle plates, or something more involved. Need to do that now...


 


 


 

[Ljohnsaw]

Depending on your snow load, just a matter of sizing that tie beam to support the weight on the king post.  Or, you can continue the king post to the ground like I did.


 

[ShimodaLife]

Depending on your snow load, just a matter of sizing that tie beam to support the weight on the king post.  Or, you can continue the king post to the ground like I did.

Lucky for this Canadian: no snow in Shimoda. No snow load. ;D
I like the look of your build, but I don't think I'll continue down straight. I've four bents; putting in a fifth would crowd things too much.

[ShimodaLife]

I think I'm moving into unorthodox territory with this new bent structure. You'll recall the suggestion to reorient the roof, which I've done and I'm happy with. But now the redesigned bent is looking, well... You see there's a beam along the top, essentially acting like a plate (see detailed xray view for mortise-tenon design). The bent looks strong (braces are hidden), and the top beam seems necessary to seat the rafters, so I'm wondering how to connect each of the four bents together. Is it just purlins at this point (posts are all 8x8, so a 3x5 "typical" purlin seems weak), or a more robust summer beam in the 6" to 8" diameter size...?



 


 

[ShimodaLife]

Answering my own question here, but sharing also: what I've come up with is a 5x8 purlin between the bents, with a 1-1/2" x 4" housed tenon. On the 2 center posts, that'll mean a through mortise to accommodate both tenons from the purlin on each side. I'll peg these, but don't show peg holes in the drawing. Because of the varying heights of the front, middle and back posts, I double the purlins (top and middle) only on the front posts; on the middle and back, I just have a top purlin (height of the middle post is 12'...). 

Any feedback is very welcome. Cheers.



 


 

[Jim_Rogers]

Well, with this roof layout your outside bents will have some thrust, unless you've hidden the ridge support timbers. Having a ridge support will reduce the thrust.

When I have two timbers joining a center post there usually isn't enough relish in the tenon from the peg holes.
That is, the distance from the end of the tenon to the peg hole. Because of this I usually use a spline joint/timber. This 2by timber is long enough to go through the post and into each timber. And each timber is sitting on a shelf edge on the post.



 

Jim Rogers

[Don P]

All depending, raising those middle "bents" to support rafters that spring from the walls, balance and overhang at the H frame and clasp at the peak... I do not believe would produce thrust.

That's structure, we've spun this thing's head around, is the space inside working?