Architect's TF design - safe??

[ShimodaLife]

Hi All, I'm continuing my TF house project here in Japan. I've popped into the forum a couple times and got guidance from members, for which I'm grateful. I completed my Will Beemer-designed and self-built TF tiny house in June, which was my practice session for the "real" TF house next year. For the real TF house, I've got a local architect working with his structural engineer to design the house, based on my design inputs. I have to use this team because they, not me, can get the designs submitted and approved by the authorities. Now, these two have never designed or built a western-style TF, and it shows every time we talk or look at a new version, but they are very experienced in "normal" stick-frame design and build. Which brings me to my dilemma:

The latest structural draft that they've presented is seen, in part, below. I don't know how to react to this. It's definitely not a western-style TF which I requested, and my challenge is to turn it into a TF with known joinery and girts and tie beams etc. I have to go back to them a coach them on the changes they have to make to make the design *my* design. I wonder how more experienced members of this board would react, or what would be your next step?

Note:
- lack of any diagonal bracing
- lack of most tie beams
- no detail in the joinery at all, but particularly at the main rafter-post-plate connections

Any and all questions/comments are welcome.
Thanks,
JT


 


 


 

[scsmith42]

JT, I would consider hiring a western firm that is experienced in TF design and have them design the structure to your specs, and then have your local folks review the design and walk it through the permitting.  The western firm should be able to provide the engineering calcs to assist with the review.

The main thing that the western firm would need to know is any special earthquake requirements, etc.

Jim Rogers here on the forum can either do the design or refer you towards a good firm.

Best of success to you with your project!

[ShimodaLife]

Best of success to you with your project!

Thanks for the guidance. I'm afraid we're well beyond that, mainly due to the contract and how much I've paid these guys already. I've got another idea, to be shared soon...

[ShimodaLife]

Perhaps my original post was too vague, or whiney. My apologies.  ;D

What I've decided to do is to take the architect's design and make it into a TF design myself. Then give it back to the guys and make them understand the joinery. I'm not quite a rank amateur, but pretty darn close. So if any pros on the board see anything that I should be aware of, I welcome your comments.

Here's my first cut of feedback for them. I've exploded all the joints so they can see clearly what is going on. We'll have a review next week.

JT


 



 



 



 



 

[canopy]

I like very much the step lap rafter seats and half wedged dove tails in the tie beams. Areas I would scrutinize are:

-verify the post to plate joinery is adequate for the wind load in your area. this could require more relish, more and bigger pegs.
-i don't see any bracing in the frame like knee braces.
-the scarf joints in the rafter (butt joint) and plates seem weak. The longer the scarf joint, the stronger it is. Have a look at "Historic American Timber Joinery". Part 6 discusses scarf joints.
-the plates are laying horizontal. beams are usually cut rectangular and placed long side vertical to prevent sag.

[Don P]

What is sheathing this? Is there adequate support for that. Is that sheathing plane your bracing also?

[ShimodaLife]

I like very much the step lap rafter seats and half wedged dove tails in the tie beams. Areas I would scrutinize are:

-verify the post to plate joinery is adequate for the wind load in your area. this could require more relish, more and bigger pegs.
-i don't see any bracing in the frame like knee braces.
-the scarf joints in the rafter (butt joint) and plates seem weak. The longer the scarf joint, the stronger it is. Have a look at "Historic American Timber Joinery". Part 6 discusses scarf joints.
-the plates are laying horizontal. beams are usually cut rectangular and placed long side vertical to prevent sag.

Thanks Canopy!
- post to plate joinery --> When you say "bigger" pegs, I think you mean bottom tenons, ie, longer, but don't understand "more." Where would I put or make more bottom tenons?
- no knee braces --> yes, I'm following their lead, but this is wrong, so I should include knee braces. Thanks for the prompt.
- current scarf joints are insufficient --> agreed, and will correct.
- plates lying horizontal --> didn't even think of this. Very good feedback!
Thanks for the guidance.

[ShimodaLife]

What is sheathing this? Is there adequate support for that. Is that sheathing plane your bracing also?

It's going to be plywood or SIPs of some sort. The sheathing plane should be the frame itself (if I've understood your question properly...), the posts, sills, and plates.

[Ianab]

What is sheathing this? Is there adequate support for that. Is that sheathing plane your bracing also?

It's going to be plywood or SIPs of some sort. The sheathing plane should be the frame itself (if I've understood your question properly...), the posts, sills, and plates.

The structural members like posts and plates aren't "bracing" as such. Sure they fold the roof up, but they have to be braced in some way to stop the structure folding up like a house of cards. 
Imagine a solid wood bookcase with only shelves and uprights. Now push on the top corner from the side. It will flex and maybe fold flat.  Now screw a piece of ply to the back in multiple places. It will take about 100X the force to fold it up now.  You could also add some angle braces at the corners of your shelf, it would achieve a similar thing. This is like the corner braces that are often used in timber framing, 
So this design without bracing would be vulnerable to high winds, pushing at the top corner. Cladding it with structural grade ply is an option, even just a few that are strategically placed to do the most good, simply stiffens up the whole structure against wind and earthquake loads. 
Triangles are your friend when it comes to structural strength. Be it a couple of bracing members, or a sheet of ply which basically works as a triangle from every screw or nail to every other screw and nail. But the bookcase example makes it easy to visualise. 

[ShimodaLife]

Now screw a piece of ply to the back in multiple places. It will take about 100X the force to fold it up now. \

Right. thanks for confirming that, Ianab. That's what I thought was being asked, but I misunderstood "sheathing plane," thinking instead he meant nailing surface *for* the sheathing. As I said, I'll have plywood or SIPs (or OSB) as the sheathing plane, and it will be affixed to the frame itself.

[Don P]

The sheathing does the bracing work in most buildings. The failure mode for those members is buckling out of plane. If the bookcase back is only nailed around the perimeter as you push on it the plywood will buckle and pop outward. Each will have maximum span distances between framing members that the sheet goods are attached to. That will dictate the spacing of more framing. SIPs can span further than ply between supports.

[canopy]

When you say "bigger" pegs, I think you mean bottom tenons, ie, longer, but don't understand "more."

More as in pegs. Standard joinery for timbers of that size is a 3 inch long tenon with a single 3/4 inch peg. For holding the roof down this may not be sufficient. On my house frame the tenon of the post to plate needed extended to over 5 inches long for extra relish, the size of peg upped to 1 inch, and 2 pegs used instead of one. For peace of mind have an engineer consider the wind load in your area and make sure the joint is made to suit.

[ShimodaLife]

When you say "bigger" pegs, I think you mean bottom tenons, ie, longer, but don't understand "more."

More as in pegs. Standard joinery for timbers of that size is a 3 inch long tenon with a single 3/4 inch peg. For holding the roof down this may not be sufficient. On my house frame the tenon of the post to plate needed extended to over 5 inches long for extra relish, the size of peg upped to 1 inch, and 2 pegs used instead of one. For peace of mind have an engineer consider the wind load in your area and make sure the joint is made to suit.

Ah, okay, I've got it now. Yes, I didn't show any pegs in above drawings, so will correct that. I understand the suggestion of 1" instead of 3/4", but I've not seen any books or drawings that rely on 2 pegs at joints rather than one; I will start looking for that now. Great feedback again!!

[Joe Hillmann]

I personally think you should look for a different engineer.

The fact that they don't have a clue what they are designing and you are coming to strangers on the internet to ask "is this safe" suggests to me that they aren't keeping up with their end of the contract and if they aren't willing to bow out of the contract on their own you would have a good legal case if it came to it. 

And continuing to throw away good money because you already spent x amount, and that x amount will be wasted if you switch to a better designer is a good way to continue wasting more money and not getting a good design.

The only way I personally would stick with a designer that wasn't keeping up their end is if the only reason I hired them was to get a rubber stamp on designs I already had, and the amount I was paying reflected how little work the engineer had to do.  And that doesn't appear to be the case with your current situation.

[ShimodaLife]

I personally think you should look for a different engineer.

The fact that they don't have a clue what they are designing and you are coming to strangers on the internet to ask "is this safe" suggests to me that they aren't keeping up with their end of the contract and if they aren't willing to bow out of the contract on their own you would have a good legal case if it came to it.  

And continuing to throw away good money because you already spent x amount, and that x amount will be wasted if you switch to a better designer is a good way to continue wasting more money and not getting a good design.

The only way I personally would stick with a designer that wasn't keeping up their end is if the only reason I hired them was to get a rubber stamp on designs I already had, and the amount I was paying reflected how little work the engineer had to do.  And that doesn't appear to be the case with your current situation.

All fair points, and arguments I've had with myself many times over already. But don't discount the advise I receive on this board as merely opinions of strangers. While "strangers on the internet" might be good copy, these strangers may help me solve this silly, expensive problem in the most effective manner available. Also, this board is likely the single, most-concentrated location of the internet's specialized knowledge of all things Timber Frame. The feedback one gets here (after sorting out the dross) is the best information you could ever want about a TF issue.

[bannerd]

Girts will also prevent it from folding up like a book shelf without paneling. Working on a exposed beam house now and trying to figure out how to make the girts disappear in a LSC in lay.  Since it's rough cut... hard to tell they're even there.  Food for thought.