Good afternoon folks,
I've gotten some excellent advice from folks from this forum so I thought I'd reach out for your advice about a frame I'm design. It's a 24' x 36' workshop. The timbers will be hemlock and pine with red oak braces. The frame is a modified version of one in Steve Chappell's book, "A Timber Framer's Workshop". The frame in the book is a 16' x 22' saltbox with two bents. The joinery for the tie beam is a wedged half dovetail in the front and an English tying joint in the back. The roof pitch is 10/12 but 9/12 for my frame. The tie beams are 7" x 12", posts are 7" x 10", rafters are 7" x 10", braces are 3" x 5". I've put the bents 12' on center, the 16' makes me nervous. I've built two timber frame sheds and a 28' x 36' barn with a 12' x 36' shed addition so I'm not a complete novice. I'll be using square rule. Below is my design, any comments/suggestions? I'm wondering if I have too many braces (36" run). Any comments/suggestions? Thanks! Joe
(https://forestryforum.com/gallery/albums/userpics/14285/Side_view~3.jpeg?easyrotate_cache=1675636610)
(https://forestryforum.com/gallery/albums/userpics/14285/Front_view~2.jpeg?easyrotate_cache=1675636645)
The standing braces (the ones going from the top of the tie beam to the inside of the tall posts) should have 5" tenon at least. Those will be in tension.
Jim Rogers
Thanks Jim! Excellent insight. I was wondering about those braces, I don't believe he gave any explanation in the book as to why they were there but I may have missed it? I guess I was thinking braces work primarily in compression, and function best that way. However, I haven't seen any frames where the braces were functioning primarily in tension. The front posts are handling a lot of roof thrust given the roof slope and width of the building. The short posts supporting the longer rafters on the back do mitigate some of the thrust, but the front posts do extend 4+ feet over the top of tie beam in the front. Lots to consider.
I would send that to an engineer but I also wonder, why this design?
The left side lower rafter notch is concentrating stress. The extended post tops are going to see significant thrust from the untied rafters. I would expect the post mortise at the tie to fail before the tension braces can arrest movement. Quantify the thrust, then design restraint. I seriously doubt a pegged M&T is capable.
As the posts cant or even as the rafters deflect, the ridge lowers, the short floor posts lift the right hand rafter heel out of its connection at the tie and the roof pitches left. I'm not in love with it :D.
Thanks for the input Don. Not sure what you mean by "I would expect the post mortise at the tie to fail"? It's a wedged half dovetail which is very good at resisting tension. Do you mean that you think the post would fail at the mortise (i.e. break at that point)? From what I read in the book, it sounds like this is a frame they've built a number of times, my frame is somewhat wider (24' versus 22'), but my bents are 12' on center versus 16' in the book.
Yes, I would not trust the post itself. Nothing succeeds like success but not a design I'd be comfortable building.
Should there be a plate across the top?
i.e. A timber for the rafters to sit on?
Quote from: Dan_Shade on February 06, 2023, 05:53:35 PM
Should there be a plate across the top?
i.e. A timber for the rafters to sit on?
Dan:
That design is one of the "somewhat modern" styles with interrupted plates.
The roof system is a principal rafter with purlins roof system. In this system the roof boards run from eave to ridge over the purlins.
Some traditional principal rafter and purlin roof systems do have continuous plates that the rafters sit on.
With the common rafter roof system, like we built at the last workshop the roof boards (if used) go from gable to gable.
Jim Rogers
I'm looking at my book now.. it really just seems like you're adding an extra two feet on the tie beams and then lengthened all the rafters to make up for it..so 24' vs the 22' in his book. Seems reasonable right?
If you can why not add some collar ties?
I have also read many times that braces act in compression only..but I see Jim's point that the knee wall is like a lever. So if that's the case why not skip the mortise and lap dovetail them to both posts and tie beams..that way if they do act in tension you won't be relying on the peg. It's no mortise but you could get pretty fancy with those dovetail details.
I'm by no means a know it all but judging by the books and old plans I've studied this frame doesn't seem like anything out of the ordinary.
Well I stand corrected..in Steve's book he states this frame is good for this size and smaller. This being said..I'd feel comfortable adding substantial collar ties and doing those lap dovetails. If it was for a paying client I'd say timber engineer would be wise..for yourself..if it fails you'll learn right?
jmaine:
I know nothing about timber framing so I won't go there.
It is my opinion that the first floor ceiling height is too low, the width is too narrow, and the length is too short, but only at one end, for a work shop. My sugggestion is if you build it do it in such a way that you can expand at least on one end.
GAB