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#25072 - 01/08/11 04:02 AM Joist Sizing
Andre L Offline
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Registered: 01/08/11
Posts: 3
Loc: Ontario
I have a question regarding joist sizing and spacing. I see there is already and old thread on this topic but it doesn't seem to work when I click on it. I'm going to build the shed out of Jack Sobon and Roger Schroeder's book 'Timber Frame Construction' and use it as a cabin. I want to increase the width of the cabin to 14'. The book recommends that you increase the size of the joists to do this but doesn't specify the size needed. Am I looking at 4"x8" joists? I'm assuming that spacing the 4"x6"'s at 16" o.c. won't solve the problem as it's joist sag that is the issue. Thanks.

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#25077 - 01/08/11 01:32 PM Re: Joist Sizing
Jim Rogers Online   confused

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What type of wood would you like to use for the floor joists?

As a cabin, the floor load could be considered a "first floor" combined load of 50 lbs per sqft(40 lbs live load, 10 lbs dead load).

Jim Rogers
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#25078 - 01/08/11 03:29 PM Re: Joist Sizing
Paul Freeman Offline


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Registered: 02/24/99
Posts: 137
Loc: Lyndeborough, NH
I haven't looked at the cabin in a long time but I seem to remember the joists running over the tops of the beams? If not then the longer spans can create problems in any notching of the underside of the ends of the joists. There are some "relatively" simple calcs that can be made to determine the allowable notch size or you can "scoop" the material out and gradually diminsh the timber size to your beam pocket's depth... within reason.

By the way, if you're using pine and it's a first floor load as Jim defined above, the 4x8's would work, but they're bouncy with a 1/2" of deflection fully loaded.
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#25084 - 01/08/11 05:22 PM Re: Joist Sizing
Andre L Offline
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Registered: 01/08/11
Posts: 3
Loc: Ontario
The wood would be Red Pine.

The plans in the book call for 4x6 joists cut down to 4x4 where they bear on the sills to fit into 4x4x3 inch pockets. Can I fix the problem of the bouncy floor by adding another plate across the length of the cabin to cut the span of the joists down to 7'?

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#25092 - 01/08/11 11:39 PM Re: Joist Sizing [Re: Paul Freeman]
Jim Rogers Online   confused

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Originally Posted By: Paul Freeman
I seem to remember the joists running over the tops of the beams?


The first floor joists are drop in joists into the two long sills, with a tying joist in the middle. As mentioned they are dropped into 4x4x3" pockets.

If the flooring is something line 2" tongue and groove you will spread the floor load out over several beams and hopefully reduce the bounce factor. My calcs show a 7/16" deflection in the middle of the floor (not considering spreading the load).

Red pine would be grade #2, size 4x8 at 16" oc.

You can add a center long sill if you wish but it will need to be supported at both ends and probably the middle as well to reduce the bounce.

Good luck with your project.

Jim Rogers
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#25093 - 01/09/11 12:58 AM Re: Joist Sizing
Paul Freeman Offline


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Posts: 137
Loc: Lyndeborough, NH
Hmmm, I wouldn't recommend a 4" notch in an 8" joist without checking horizontal shear. FWIW, in white pine, a 6x8, at 48" o.c. with a 40# total load doesn't make horizontal shear.


Edited by Paul Freeman (01/09/11 12:59 AM)
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#25099 - 01/09/11 03:14 PM Re: Joist Sizing
Jim Rogers Online   confused

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Posts: 1661
Loc: Georgetown, MA, USA
Here is the standard detail we use for the long sill drop in floor joist pocket:



Edited by Jim Rogers (01/09/11 03:15 PM)
Edit Reason: wrong label
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#25104 - 01/10/11 12:54 AM Re: Joist Sizing
Paul Freeman Offline


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Registered: 02/24/99
Posts: 137
Loc: Lyndeborough, NH
Jim, I'm not worried about the sill but rather the split that occurs in the joist at the underside of the notch, I refer to it as a horizontal shear failure, but my engineering school training only lasted long enough to make me dangerous.
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#25116 - 01/10/11 01:56 PM Re: Joist Sizing
Jim Rogers Online   confused

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Loc: Georgetown, MA, USA
Yes, Paul, I see your point, now. And I agree that there is reason for concern.

Of course the joist could be full length (no center sill) and 4x6 if supported underneath the middle with a hidden sleeper timber, that is supported by some foundation pier at each end and in the middle.

And the proper joist end should be cut to prevent the splitting of the joist.

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#25119 - 01/10/11 06:04 PM Re: Joist Sizing
TIMBEAL Offline
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Registered: 12/10/07
Posts: 1882
Loc: Maine
Why not cut the pocket deeper? With a 8x8 sill you would still have 2" below a 6" pocket. If I recall, Brungraber once said 2" under a pocket would work. Anyone else heard of this? What is the rule for pocket depth? 5/8 the depth of the sill is one choice so you could safely go to 5" deep pockets on 8x8's. How about 9" deep sills?

I see a ratio of 1:4, I also see a steep curve leading to the reduction. At least this is the case with my adze, I could not cut that curve with it. It would have to be extended for the adze to work properly. When ever I get the chance I put them in upside down and adze to the top of the sill then flip them over. In general I use the length of the adze head plus the width of the blade back from the end to gauge where to start hacking. This is adjusted to how deep I have to go.

I just noticed the "or more" note. Well, maybe it would be close, the curve. At one time I measured out the curve, silly me.

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#25126 - 01/11/11 02:34 PM Re: Joist Sizing [Re: TIMBEAL]
Jim Rogers Online   confused

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Registered: 03/14/02
Posts: 1661
Loc: Georgetown, MA, USA
Originally Posted By: TIMBEAL
What is the rule for pocket depth?


At the class I took at Heartwood school where they taught us beam sizing and other engineering stuff, they gave us this rule of thumb for floor joist to sill beam situation:



We also learned that you should size the joist first and then size the timbers to hold up the joist next. And work you way down to the foundation, making timbers large enough to hold everything up properly.

If that is the case, then maybe the sills in this extended frame will have to be deeper to be stronger.

As I mentioned just adding a supporting sleeper under the middle may solve the problem without changing a lot of joints and beam sizes.

Like this:



He would just have to make it so his foundation would support the ends and add a pier to the middle.

Jim Rogers
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#25127 - 01/11/11 02:43 PM Re: Joist Sizing
Paul Freeman Offline


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Registered: 02/24/99
Posts: 137
Loc: Lyndeborough, NH
That's perfect! Notice the "scooped" out notch, that's exactly what we do to avoid the whole horizontal shear issue. If you run the calcs for square joist notches there's no way that would work on your longer spans, but the gradual transition works great.

But, if you add the midgirt like you show in teh picture it won't be an issue anyway, unless the floor load was very high because you're bringing in a tractor something.

Nice examples, thanks Jim (and Tim!)
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#25132 - 01/12/11 02:02 AM Re: Joist Sizing
Andre L Offline
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Registered: 01/08/11
Posts: 3
Loc: Ontario
I like the sleeper idea, I'm gonna run with that one. I won't be bringing in any tractors!

Thanks for all the help!

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#25134 - 01/12/11 02:15 AM Re: Joist Sizing
Paul Freeman Offline


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Registered: 02/24/99
Posts: 137
Loc: Lyndeborough, NH
Make sure you size it right, a support in the middle would help significantly.
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#25135 - 01/12/11 04:32 AM Re: Joist Sizing
Dave Shepard Offline
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Registered: 02/19/06
Posts: 718
Loc: Alford, MA
I'll be using two sleepers to support the threshing floor in my current project, as the span is almost 10'. The sleepers in turn will be supported by three Samson beams, as they are about 45' long.
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