Previous Thread
Next Thread
Print Thread
Rate Thread
Page 1 of 3 1 2 3
??s about joist height and brace placement #25569 02/21/11 09:03 PM
Joined: Jun 2010
Posts: 49
D
danfink Offline OP
Member
OP Offline
Member
D
Joined: Jun 2010
Posts: 49
I’m modifying the “garden toolshed” from Jack Sobon’s first book in a number of ways. One of the things I’m doing is extending the post both “below” and “above” the crossbeam, meaning more headroom on the first floor, and a larger kneewall in the loft up above. I ran into the problem he warns against, and put the brace pocket in the same area as the mortise for the crossbeam. There are two solutions I can think of and I’m definitely leaning towards the first, but would really appreciate any feedback, thoughts, or concerns any of you might have.

Solution #1: Extend kneewall height even further, putting the brace pocket up above the crossbeam mortise. I’ve drawn this out, and that means the kneewall height will be extended a total of 1’8”. This leaves me with 3’8” between the top of the plate and the top of the crossbeam. Jack says not to allow more than 4’ between the top of the plate and the top of the crossbeam because the spreading force of the roof is magnified at the post/crossbeam connection as the distance is increased. So, I’m (barely) within this rule. Do people think 3’8” between the tops of these two members is alright?

Solution #2: Instead of using 3’ braces here, use 4’ or maybe a 30-60 will work. My hesitations are this is my first frame, so I want to keep it simple. It seems like there is lots of information out on how to layout/cut 3’ braces. A 4’ brace wouldn’t be too different, so maybe that could be in the cards. I’ve never even come close to working on a 30-60 brace, know very little about their layout, and am not even sure if they would solve this problem. Also, the other braces in the frame would still be 3’, so this means there would be two different sized braces. I like the idea of that, but in the effort of keeping what feels like a dauntingly unfamiliar project as simple as possible with the goals I have for it, this seems like it could maybe mess me up. I also was going to make a brace layout jig, which then wouldn’t work for a longer brace.

Any ideas? Like I said, I’m leaning towards Solution #1, and have already drawn that out, but want to know if anyone thinks the distance between the plate and the crossbeam is too much, despite being within the 4’ rule. Or, if anyone disagrees with my hesitations about implementing a 4’ of 30-60 brace instead.

That’s it, but it may be relevant to say the posts are 8x8s. Solution #1 would give them a shoulder to shoulder length of 10’11”. On top of them sits the 8x8 plates where the rafters are step-lapped into. The top of the plate with Solution #1 comes in at 11’6”. Difference between top of crossbeam and bottom of plate would be 3’ (a lot?).

Re: ??s about joist height and brace placement #25587 02/22/11 03:08 PM
Joined: Mar 2002
Posts: 1,685
J
Jim Rogers Online Confused
Member
Online Confused
Member
J
Joined: Mar 2002
Posts: 1,685
Solution #3: make it a shorter brace so the pocket is above the cross beam (tie beam)....

A lot depends on your snow load for your area....where are you located?


Whatever you do, have fun doing it!
Re: ??s about joist height and brace placement [Re: Jim Rogers] #25589 02/22/11 05:08 PM
Joined: Jun 2010
Posts: 49
D
danfink Offline OP
Member
OP Offline
Member
D
Joined: Jun 2010
Posts: 49
Im in Western Massachusetts, which isnt quite northern New England, but years like this one remind us what winters can have in store for us. At Heartwood I was advised to use 50 psf for Massachusetts snow loads.

Re: ??s about joist height and brace placement #25622 02/24/11 12:14 AM
Joined: Nov 2005
Posts: 305
T
timberwrestler Offline
Member
Offline
Member
T
Joined: Nov 2005
Posts: 305
Dan,

I live in Becket, and I sometimes teach at Heartwood as well. Not sure if we've met. Let me know if you have any questions or need a hand.

At 12' wide it's a pretty small building, so I suspect that you're kneewall height is OK. Especially if Jack OKs it. 30" braces would work fine (42 7/16 length) as Jim mentioned.

Another consideration, that I can't remember whether Jack addresses, is that you'll need some kind of girt or nailer to at the level of the joists to catch the end of your flooring (assuming there is a loft space). That adds even more joinery at the post/TB location. There are few ways to improve that situation.

Re: ??s about joist height and brace placement #25624 02/24/11 01:44 AM
Joined: Feb 2006
Posts: 718
Dave Shepard Offline
Member
Offline
Member
Joined: Feb 2006
Posts: 718
In discussing this design with Jack last year, I believe he suggested that the snow loads for this building might need to be checked against current codes. I'll ask him tomorrow when he comes to the shop, if I remember.

Cutting a 4' brace is the same as a 3', you just have to calculate the hypotenuse.

I've cut 3-4-5 braces. Layout is with the square. I like to leave the degrees out of the thought process. They are also easy to layout and cut.

Last edited by Dave Shepard; 02/24/11 01:50 AM.

Member, Timber Framers Guild
Re: ??s about joist height and brace placement #25634 02/24/11 05:10 PM
Joined: Oct 2002
Posts: 209
Will B Offline
Member
Offline
Member
Joined: Oct 2002
Posts: 209
Dan,
Can you please cite where Jack says that 4' max is OK between the tie beam and the plate on a kneewall? I'm not saying he's wrong and I don't have a copy of the book with me, but I'm surprised.

A quick calculation shows that an 7x7 with a 3' cantilever above the tie beam to the plate MIGHT be OK if there was no joinery, but after taking out the tie beam housing, mortise and peg hole it wouldn't (assuming an allowable fb of 1000lbs/sq.in, 65lb.sq.ft. combined load, 14' width and 30" rafter spacing). There's about 1500 pounds of roof thrust at the center post top and over 50,000 inch-pounds of moment at the tie beam joint.

In our workshop where we build this frame often, we always limit the kneewall to 2' max.

I'm not an engineer, and I could be missing something. God knows I hesitate to say Jack is mistaken, but he may have assumed something we don't know. But I do know it's dangerous to rely solely on rules of thumb without understanding the underlying principles. These types of calculations we cover in our Timber Frame Design course at Heartwood and I believe can be found in the Guild's two design workbooks. But it's always best to consult an engineer if in doubt, and by doubt I mean not knowing how to figure this out yourself. I realize that's why your asking here.

I remember one time I heard Jack say something different from what he said the year before, and when I asked him he said "Well, last year I was wrong!" And we didn't catch it then, did we? Gotta love the guy.

Re: ??s about joist height and brace placement #25642 02/25/11 01:13 AM
Joined: Feb 2006
Posts: 718
Dave Shepard Offline
Member
Offline
Member
Joined: Feb 2006
Posts: 718
I talked to Jack today about this briefly, and he did say that moving stuff around presented the problems you ran into. He did not elaborate further. I also asked if he felt the design needed to be reviewed to see if it meets current codes, and he said yes.


Member, Timber Framers Guild
Re: ??s about joist height and brace placement #25644 02/25/11 01:32 AM
Joined: Dec 2007
Posts: 1,882
T
TIMBEAL Offline
Member
Offline
Member
T
Joined: Dec 2007
Posts: 1,882
Will, page 101,first paragraph, "the distance from the top of the crossbeam to the top of the plate should not be increased beyond 4 feet because of the spreading force". I wonder is this is part of the re-do?

The post in the red book are 8x8,btw.

Not a bad idea to increase the post size, maybe to 9x9. Also, if you had two braces coming together at the same point on a post, one on one side and one on the other, and using 1" reduction/housings on a 7x7 the braces conflict, and on an 8x8 the touch, the 9x9 leaves you with some grace. Just another reason to use a bigger post.

Re: ??s about joist height and brace placement #25660 02/25/11 02:52 PM
Joined: Oct 2002
Posts: 209
Will B Offline
Member
Offline
Member
Joined: Oct 2002
Posts: 209
Thanks, Tim,
An 8x8 with a 1" peg, 1.5" thru mortise and 1" housing and a 3' kneewall still doesn't quite make it with an actual f of 1029lbs/sq.in. and an allowable of 1000 (note this is higher than any grade of Eastern White Pine, but hemlock might get you over the hump). One thing that Jack might have considered with the 4' limit was the load range; if there was no snow load then the cantilever can be longer. As with any design, local conditions need to be factored in. There are safety factors built in to these calcs, as well.

Re: ??s about joist height and brace placement #25706 02/28/11 05:40 PM
Joined: Jun 2010
Posts: 49
D
danfink Offline OP
Member
OP Offline
Member
D
Joined: Jun 2010
Posts: 49
Thanks so much guys for your input.

So, here are some options as I see them, either individually or in combination with each other:
1. I can use hemlock and red oak for all my posts. Looking at Will’s quick calculations this could get me over the hump of the high kneewall. This seems like a good idea no matter what, as there are certainly some harvestable oaks and hemlocks that could be used for this project.
2. I can lower the post height and kneewall by 6”, and then use 4’ braces along the long wall. This would leave me with a kneewall height of 3’2” (top of crossbeam to top of plate).
3. This problem is addressed in a couple of books I’m using, and one of the solutions shown throughout is using diagonal struts joining from the top of the tie to the upper ends of the posts. I could do that on the two end bents (wouldn’t really want for the middle bent for loft accessibility). I don’t know how this extra removal of wood in the post would subsequently reduce its strength, but the brace pocket would only be a few inches below where it joins with the plate.

The last thing that’s running thru my head, and I understand this forum won’t help me calculate this, is that the walls/insulation for this building will be straw bales with a earth/lime plaster. In a house this size (small single story), the bales could be structural in themselves. I am going to try and see if they can be relied upon to help prevent this spreading force at the tops of the posts.

timberwrestler, I’m not sure we’ve met, but I’d love to be lent a hand sometime. At least four braces will be cut from curved timber, and that will be a first for me. Also, some timbers may be hand hewn, and I’ve never worked snap-line layout before, and I’m sure I’ll have questions about that.

Thanks for all your input gentlemen; I am invigorated by this process. Makes me want to design my next structure completely (with some more practice and training of course),
Dan

Page 1 of 3 1 2 3

Moderated by  Jim Rogers, mdfinc, Paul Freeman 

Newest Members
HFT, Wrongthinker, kaymaxi, RLTJohn, fendrishi
5134 Registered Users
Powered by UBB.threads™ PHP Forum Software 7.7.3
(Release build 20190728)
PHP: 5.4.45 Page Time: 0.023s Queries: 16 (0.006s) Memory: 3.2215 MB (Peak: 3.3984 MB) Data Comp: Off Server Time: 2024-03-28 20:13:24 UTC
Valid HTML 5 and Valid CSS