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Author Topic: Timber frame deck  (Read 800 times)

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Offline wbrent

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Timber frame deck
« on: March 21, 2020, 04:24:35 PM »
Building my first timber frame. 12x16 shed. Rather than a concrete slab Iím thinking of having 6 screw posts placed and building a proper timber frame floor. My questions are around size of timbers. The brackets on top of the screwposts hold a 6Ē timber. So Iím hoping to use 6x8Ē timbers running the 16í lengths. Made up of two 8 footers supported by the screwposts. What would I need to cross the 12í span?  More 6x8ís. Every two feet?  What if I wanted to squeeze a 8000lb tractor in there? Would that change things back to concrete?  

Offline Don P

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Re: Timber frame deck
« Reply #1 on: March 21, 2020, 07:18:32 PM »
I doubt that foundation could handle the tractor.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline wbrent

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Re: Timber frame deck
« Reply #2 on: March 25, 2020, 06:02:40 PM »
Ok. Leave the tractor out. Do my timbers sound ok for the base of a building that size. 

Offline Don P

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Re: Timber frame deck
« Reply #3 on: March 25, 2020, 09:11:41 PM »
You'll need more info to check the 6x8 sills, aside from the floor they are possibly supporting what is above, need a sketch and snow loads. Also joinery if the sills are notched. Species and grade of material.

To check the joists you need to decide on floor load, I'm assuming the screw piles are going to be set up for residential loading of 40psf unless you specify heavier.

So start with a sketch, species, and loads and click on the toolbox, bottom of the left hand column, hit the beam and column calcs. Post all that if you get stuck and I'll try to walk you through how to figure it out.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline wbrent

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Re: Timber frame deck
« Reply #4 on: April 02, 2020, 03:15:13 PM »
Ok Iíve made some progress re my floor.  Screw piles will be set up residential. So yes 40psf. But Iím going to fabricate my own brackets to accept an 8Ē beam. So now I can use 8x10 for sills. They will have a mortise and tenon joint at each corner and pinned. My floor joists measure in around 6x7. Just under 2 feet apart. My question is now about the pockets for the joists. Iíve seen a variety. One plan Iím looking at has the pocket recessed only an inch into the sills and about four inches deep. Others Iíve seen semi more like 3 or 4 inches into the sill and the same in depth. Any preference for these pockets. 

Offline Jim_Rogers

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Re: Timber frame deck
« Reply #5 on: April 02, 2020, 03:33:46 PM »
The depth into the pocket depends on the load on the beam. And the depth down into the beam as well. if the load is heavy you need a lot of wood under the pocket to support the load.
There are two area to consider load perpendicular to the grain. One is on the joist and one is on the pocket in the sill.

You need to consider the rules of thumb on the reduction of the joist based on the size of the joist and sill.



 
this shows the amounts you can remove from the sill and joist.

Hope this helps.

Jim Rogers
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Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline Don P

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Re: Timber frame deck
« Reply #6 on: April 02, 2020, 03:58:31 PM »
The 8x8 beam in Jim's drawing is then a 5x8 when you check the beam.

The minimum bearing depth for the joist is 1-1/2" but you should check the bearing area against the load and maximum allowable load perpendicular to grain for the species.

Max notch depth on the joist is 1/4 of the joist depth.

2 more options are a soffit tenon or sitting the joists on top of an unnotched sill.
The future is a foreign country, they will do things differently there - Simon Winchester

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Re: Timber frame deck
« Reply #7 on: April 03, 2020, 03:05:56 PM »
Whatís a soffit tenon?  
Think Iíll make it a bit easy on myself and make it 2 3/8Ē into the sill x 5Ē deep. That way I can rough it out with my skill saw at maximum depth of 2 3/8Ē. 

Offline Don P

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Re: Timber frame deck
« Reply #8 on: April 03, 2020, 08:17:06 PM »
A soffit tenon preserves the upper, compression, edge of the beam and the bottom edge of the joist is in the area of the neutral axis so there is not a reduction in bending strength of the beam. Think of it sort of like an I beam, it is preserving the flanges and mainly removing material in the lower stress area that would be the web.

In any case do not remove more than 1/4 of the joist depth from the bottom edge, the less the better there and make that transition gradual, a sharp cornered reduction on that bottom, tension, edge really promotes splitting.



The future is a foreign country, they will do things differently there - Simon Winchester

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Re: Timber frame deck
« Reply #9 on: April 09, 2020, 07:15:44 AM »
Ok. I am cutting my joists and sills pretty close to that diagram above by Jim. The diagram shows a scoop on the bottom of the joist at a 1:4 ratio. Is there a structural reason for this?  Why canít it just be notched out?  

Offline Don P

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Re: Timber frame deck
« Reply #10 on: April 09, 2020, 08:50:15 AM »
Quote
 a sharp cornered reduction on that bottom, tension, edge really promotes splitting.
From the TF Standard;
Quote
2.3.3 Coped Tension Face Notches

A gradual change in cross section compared with a square notch decreases the actual
shear stress parallel to the grain nearly to that computed for an unnotched bending member with
the same net depth dn. Such a gradual change shall be achieved by providing smooth transitions
between surfaces with no overcuts at reentrant corners.
This comes from the NDS which says basically the same thing.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline Jim_Rogers

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Re: Timber frame deck
« Reply #11 on: April 09, 2020, 09:03:32 AM »
What he said.
Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension


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