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Rafter-tiebeam distance

Started by Rougespear, June 09, 2016, 10:58:42 AM

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Rougespear

I am wondering about typical length constraints for distance between the rafter plate and top of the tiebeam using an 8x8 post?  Is 3' alright in a common rafter setup 18' wide?
Custom built Cook's-style hydraulic bandmill.

Jim_Rogers

Depends on snow load and thrust.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Rougespear

Snow load is 25psf, and dead load is 15psf... any thoughts/help on calculating thrust at the post?
Custom built Cook's-style hydraulic bandmill.

Jim_Rogers

That's what my engineer does for me.
I don't have a formula for that, yet.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

timberwrestler

The TFG's red book (first joinery book) covers it I believe.  As the kneewall gets bigger, the forces on the tie to post joint become quite high--higher than what can be handled by pegs. 
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Rougespear

The tiebeam to post joinery will be a wedged half dovetail, so that should be more appropriate (compared to a pegged mortise and tenon) for dealing with the increased roof thrust from the higher knee wall, yes?

Jim: do you have gut reactions to knee wall sizing above the tiebeam to help get the dimension in an appropriate ball park before the engineer reviews it?
Custom built Cook's-style hydraulic bandmill.

Jim_Rogers

I think that anything above 3' will require either a bigger post in the plane of the bent or a tension brace to the top of the tie. Not the best due to taking up floor space on the second floor level.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Ljohnsaw

Is there any way to have a supported ridge beam?  That would negate the outward thrust (to a large extent).
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

timberwrestler

The only close point of reference that I have is a 20' wide frame with a 4' kneewall, common rafters over a continuous plate (the same form as your design).  There was 5.6 kips of tension on the inner bents at the post to tie connection.  That called for threaded rod with big washers.  That can be hidden, so it really wasn't a big deal.  The client however, was convinced to put in a structural ridge, along with posts.  The number doesn't really matter, you still need an engineer.  Email the Timber Frame Engineering Council, and they can recommend a PE for BC (their 'Find an Engineer' section is getting modified).  Fire Tower Engineering is licensed in BC, and that is who I use.
www.uncarvedblockinc.com
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