Cabin plans - feedback welcome

[TimFromNB]

Hello fine folks,

Below are a set of plans I drew up for my cabin based on a few books. I would greatly appreciate it if anyone could provide feedback, especially on member sizing and scarf location. I will be doing a stop splayed wedged scarf. I do have capacity in my logs to add some dimension to beams if necessary. I have tried my hand at beam calculations, but I must be doing something wrong because even the book designs I am test calculating are failing.

Snow load here is 52.2 psf.
Using Eastern White Pine.

Any info would be appreciated.

Thanks!
Tim




 


 

 

 

 

 

 

[Roger Nair]

I would take a close look at the order of raising, the loft floor joist, loft joist to beam joint and the tie beams supporting the loft. 

Order of raising is not clear.  Plates on opposite side are placed in opposite order.  I believe it would be better to set both plates first on the open bay to form a secure box.  Use the box to tie off the far bent to control the outward lean with come-alongs and set the floor joist with soffit or tusk tenons closing as the joist are set.  I would also extend the underscarfed plate to capture the brace rather than have the a spreading force act on the scarf by bedding the brace into the overscarfed plate.

I would spec 4 x 7 floor joist and 7 x 12 ties that support the floor system and AVOID drop-in joist.  That is my habit for a start in calculation given the spec.  There is a beam calculator in the forum tool box that could prove useful, I have not used it and only refer to my suggestion as a start point.

[scsmith42]

From a design perspective, I would consider adding about an 8' shed overhang off of one eave of the cabin.  Split it in two with a bathroom on one side and a kitchen area on the other.  The table can be under the loft with the front part of the cabin reserved for living room type seating.

[Grizzly]

I can't help with the technical stuff but your plan reminds me of a friends place. Just him and his wife. They talked of just living in it till they built something bigger but they've kinda got used to the low cost of living. Very cozy and easy to heat. Theirs might be 20 x 30 though.

[Dave Shepard]

Is there any way you can get a full length plate? The ideal scarf location is to have the lower section land over a brace. Where you have them, should there be racking, the brace would force the scarf apart.

[Don P]

Not an engineer but I'd check the shear on the tail of the middle tie tenon against the thrust from the roof.

I do like the single piece plate idea and the advice to use soffit or tusk tenons on the joists.

Eric Sloane wrote one time about a mistake he had made in a book. As the years went by he saw it repeated over and over and he knew he was the "authority" at the root of it all. It happens, the best we can do after that is try to get back to the real facts rather than repeating the error blindly.

I'm guessing you were looking in Benson or something that came from it and comparing it to what my toolbox calc was coming up with. I wrote Ben Brungraber asking about that table and sent links to my calcs. I just dug that series of emails up, it was from back in 2010. I was coming up with what seemed to be some very optimistic design values associated with tables there and then in several other books. He had written Jim Gruber, Benson's cousin who wrote that chapter with pretty much the same question. He is unaware of any failures that have occurred from using those tables but reiterated that it would be better and code compliant to stick to the NDS design values.

To the best of my knowledge the calcs here and at timbertoolbox.com are putting out good numbers if used correctly. One way to check is against the spancalc on the awc.org website, they are the folks who publish the NDS and write the span tables in the codebook. If anything I have tried to err conservatively. If you do see a problem please do send me a pm!

[Roger Nair]

To follow on Don's point on thrust, on the open bay, place plate to plate ceiling joist to tie the thrust and drop the collar on the gables to the plate.  Inward sloping posts can run from inside the central posts to near the collar to rafter joint.

I have not made it a practice to follow or recommend Benson or Stewart Elliot books from the early revival of timber framing.  I have many reservations.  The most consistently great writing with solid reasoning is by the late Ed Levin, research the back issues of Timber Framing from the TFG, the DVD of the collected issues is the best source.

Remember, I am just spitballing. 

[TimFromNB]

Thanks for all your feedback guys. I have mostly read the Sobon books and this member sizing comes from Beemer's recent "Learn to Timber Frame" book, which is a variation of the Sobon design. I also have a few from Benson but his style is quite different. I don't have access to a full 20' plate but can get a 16' to at least have the longer piece of the scarf sit on the center bent post and next brace as suggested by some of you. I did not think of the effect shear would have on that joint.

[Heartwood]

Tim,
As said on page 132 of my book, if the loft joist span goes to 10' the size should go to 5x6 or decrease the spacing.
The scarf joint (instead of a solid timber) would add some stress to the center tie beam joint, but the wedged half-dovetail and a 1" pin can handle the shear fine. I would suggest a bridled scarf for a plate, as shown on page 144, instead of a splayed wedged scarf, which you would use if there was tension and/or shear in the plate, which there isn't. The blade helps resist the outward bending.

[classicadirondack]

Longer rafter tails for overhang?

[TimFromNB]

Will - Thank you for the suggestion for the scarf joint. I have also reduced the spacing for the loft joists. Your book was a great read, very thorough explanations.

Don - I found out what I was doing wrong when calculating: I had not adjusted snow load for roof pitch or "slipperyness" and was basically calculating for flat roof. The 52.2 psf comes down to approx. 29 psf. Also, when specifying the span for the rafters in the calculator for bending moments, should I put in the horizontal run (i.e. 8ft) or the actual length of the rafter (11.32ft)? I have increased rafters to 5x7, posts to 8x8, plates to 8x10 and ties to 8x10. 

Classicadirondack - Overhangs are great, I will build them with strapping on top of rigid insulation.

Revised scarf joint (30" overlap, 2"x5"x6" centered tenons, 5" out from brace):

[Jim_Rogers]

Rafter snow load is the run (horizontal), rafter dead load including decking and roof materials is rafter length. You need to combine these to get the total load in lbs per sqft.
Jim Rogers