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Floor joist spans fo cabin?

Started by Aeneas61, April 20, 2016, 05:59:58 AM

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Aeneas61

Building a small timber frame house, 16X21...for the floor joists, can I span 16 feet between the sills unsupported or do I need something like a summer beam down the middle? Planning on 8X8 sills, if I can run the joists across unsupported, what size do they need to be? 6x6 or 4x8 et.

Thanks
Josh

Aeneas61

guess I should mention the joists most likely will be yellow pine (shortleaf, virginia)

Aeneas61

Just thought I should also mention I need about 24" between joists, too early in the morning! ::)

Jim_Rogers

Is the first floor of the cabin?

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

Aeneas61

Jim,

Yes, its the first floor, going across from oak 8x8 sill to oak 8x8 sill, the loft will be a much smaller span, not worried about that one, just the 16 footer downstairs...


Josh

Jim_Rogers

Are the 8x8 oak sills going to be continuously supported by concrete or stone foundation or will it be supported on point loaded piers?

Jim Rogers

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

Aeneas61

they are supported by point loaded piers, 6 of them, one in each corner and two midspan of long side...

Jim_Rogers

I ran some numbers through the beam calculator in the red tool box here on this site and a 6x8 beam will pass at 24" on center spanning 14'8" between to 8" sill timbers. But it will deflect almost 3/8" in the middle of the floor. I would go with something heavier to reduce the deflection.

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

barbender

  I would put another beam midspan myself.
Too many irons in the fire

witterbound

I'd add a bem too.  Not that hard to do, and will cur down on an otherwise bouncy floor.

sbishop

If you look in my album you would see what I did, 22 ft span, bean down the middle with a pier in the middle. I ended up using 2x8 joist in hangers.

This would save on material

Sbishop

Magicman

QuoteIf you look in my album you would see........
How are we supposed to know which picture to look at?   ???   smiley_headscratch
Knothole Sawmill, LLC     '98 Wood-Mizer LT40SuperHydraulic   WM Million BF Club Member   WM Pro Sawyer Network

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Raider Bill

Quote from: Magicman on April 21, 2016, 09:10:03 PM
QuoteIf you look in my album you would see........
How are we supposed to know which picture to look at?   ???   smiley_headscratch

His gallery is laid out nice. Pix are labeled in groups, wall, floor,roof etc.
The First 70 years of childhood is always the hardest.

Roger Nair

If it was my project, I am biased towards deeper section floor joist.  For example a 4 x 10 or maybe even a 3 x 10 could handle the span compared to a 6 x 8 with a 48 sq. in cross section a 30 or 40 sq. in. cross section produces a considerable savings in board footage hence lower cost.  10 inchers will be stiffer and offers the option of no central beam and additional cost and labor savings.  I have not referred to any formula or tables, it's just an experience based suggestion.  As always design should follow specs on species and grade, so get calculating or hire it.
An optimist believes this is the best of all possible worlds, the pessimist fears that the optimist is correct.--James Branch Cabell

Jim_Rogers

Quote from: Roger Nair on April 22, 2016, 11:33:21 AM
If it was my project, I am biased towards deeper section floor joist.  For example a 4 x 10 or maybe even a 3 x 10 could handle the span compared to a 6 x 8 with a 48 sq. in cross section a 30 or 40 sq. in. cross section produces a considerable savings in board footage hence lower cost.  10 inchers will be stiffer and offers the option of no central beam and additional cost and labor savings.  I have not referred to any formula or tables, it's just an experience based suggestion.  As always design should follow specs on species and grade, so get calculating or hire it.

I completely agree. Here we have a case where timbers are already cut and then the user says: "make me a frame using these."

This isn't the best way to design a building. Because it is difficult to attach a 10" deep timber to an 8" deep sill.

The rules of thumb for sizing fall into play here:



 

I haven't closely studied this design in order to figure what is right or best, but there should be a solution somehow.

Jim Rogers

PS. normally we size the floor joist first to support the load and then size the sill to support the floor joists. When you do it the other way around your spacing of the floor joist must be altered to fit the size joist you need to use.
Here you have limited the spacing as well. Not the best method to design a floor system. I'm not bashing anyone, I'm just explaining that this isn't the best method to get a good answer.
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Roger Nair

To support 10 inch stock with an 8 inch beam, one could notch the beam as per Jim's illustration and have the remaining upper portion of the joist extend over the beam to meet a band or apply a ledger to the beam and again extend the top portion of the joist to a band or go ultra simple and bear the unnotched joist over the beam and lock the joist with housings in the band.  In all cases, use bridging.
An optimist believes this is the best of all possible worlds, the pessimist fears that the optimist is correct.--James Branch Cabell

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