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Timber frame deck

Started by wbrent, March 21, 2020, 04:24:35 PM

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wbrent

Building my first timber frame. 12x16 shed. Rather than a concrete slab I'm 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 6" timber. So I'm hoping to use 6x8" timbers running the 16' lengths. Made up of two 8 footers supported by the screwposts. What would I need to cross the 12' span?  More 6x8's. Every two feet?  What if I wanted to squeeze a 8000lb tractor in there? Would that change things back to concrete?  

Don P

I doubt that foundation could handle the tractor.

wbrent

Ok. Leave the tractor out. Do my timbers sound ok for the base of a building that size. 

Don P

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.

wbrent

Ok I've made some progress re my floor.  Screw piles will be set up residential. So yes 40psf. But I'm going to fabricate my own brackets to accept an 8" 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. I've seen a variety. One plan I'm looking at has the pocket recessed only an inch into the sills and about four inches deep. Others I've seen semi more like 3 or 4 inches into the sill and the same in depth. Any preference for these pockets. 

Jim_Rogers

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

Don P

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.

wbrent

What's a soffit tenon?  
Think I'll make it a bit easy on myself and make it 2 3/8" into the sill x 5" deep. That way I can rough it out with my skill saw at maximum depth of 2 3/8". 

Don P

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.




wbrent

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 can't it just be notched out?  

Don P

Quotea 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.

Jim_Rogers

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

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