Shed pier spacing


For my 10x12' shed, I'm using two 12' 4x6 for beams on piers to support the 2x8 joists (rim and interior joists rest on the 4" top of the 4x6-- no hangers are used). Can I use a 6' span between piers (three piers)? I typically see 12' sheds on four piers. I'm using 10" Sonotubes with rebar, so the concrete strength will not be an issue. At issue is a buried pipe that would be right under a pier in a 3'-on-center pier arraingment.
Thanks for any help and opinion. Chuck
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C Miller wrote: > For my 10x12' shed, I'm using two 12' 4x6 for beams on piers to support > the 2x8 joists (rim and interior joists rest on the 4" top of the 4x6-- > no hangers are used). Can I use a 6' span between piers (three piers)? > I typically see 12' sheds on four piers. I'm using 10" Sonotubes with > rebar, so the concrete strength will not be an issue. At issue is a > buried pipe that would be right under a pier in a 3'-on-center pier > arraingment.
If you are going to use only 3 piers on 12 ft (ie: 4 ft spacing), I like 4x10 and 2x10 lumber, based on my gut and too many strength of materials classes a long time ago.
But then again, I'm sometimes accused of building brick out houses, but nothing has fallen down yet.
Lew
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Is there any reason you can't use 2 piers, straddling the pipe, in place of the 1 problem pier and stay with the 3' spacing otherwise?
Art

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Maybe I'm missing something. You're using two 4x6s on the outside of the long edges to support the ends of the joists. I take it you want to have just one support in between. Will you have another 4x6 on the middle pier to support the joists? If so, then the question becomes, will your 2x8 joists (at some unknown spacing) support your design load while spanning a 6 foot span? To help answer that question, let's refer to the following. I am sitting next to an in-progress addition to my house that was designed by an actual architect. The second floor uses 2x10s 12" O.C. to span 16 feet. Architects design around maximum deflections per code. For a uniform distributed load on a beam supported at both ends, max deflection varies as to the 4th power of the length (i.e. for a particular load, if you doubled the length, the deflection would increase by a factor of 2^4 or 16). In your case, this means that scaling the span length down from 16 feet to 6 feet reduces the deflection by a factor of about 50. The max deflection is also inversely proportional to the moment of inertia. Moment of inertia is proportional to the cube of the beam height, which means max deflection is inversely proportional to the cube of the beam height (i.e. doubling beam height reduces deflection by a factor of 2^3 or 8). In your case, scaling the beam height down from 10" to 8" doubles the max deflection. So, roughly speaking, using the same design loads as in my addition, your deflections would be about 25 times less. Now, that's for 12" OC. Even if you were to double the spacing to 24" O.C, your deflections would still be 12 times less than in my addition.
So, if the initial scenario is as I described, I wouldn't lose any sleep with the design loads.
todd
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todd wrote:

No, just two 4x6 on the rims, with 12' 2x8 sitting on top of the 4x6's. Length of the stringer and interior joists are 10':
+---------+---------+ <-- 12' 2x8 rim joist on top of 4x6 | | | | <-- 10' 2x8 interior and stringer joists | | +---------+---------+ <-- 12' 2x8 rim joist on top of 4x6
"+" indicates piers.
I was thinking that the 12' 2x8 rim joists would tolerate a 6' span. I'm not enough of a materials engineer to determine if the beam and 2x8 rim combined constitute an adequate support. This is my primary concern with the pier placing.
As far as the interior joists: Norm's 8' by 12' shed this season used 2x8's; I'm not sure what two extra feet would do. Load tables I've looked at for floors say a 2x8 is adequate up to 12', however.
I like Art's suggestion--even if it results in "uneven pier spacing". And I'm with you Lew--build it like it a brick house! I've got to keep those cold Colorado winds from huffing and puffing... :-)
Thanks all.
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Looks like a good place for a cantilever, both directions.
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Even modifying my earlier remarks to account for a 12-foot spacing, you're still below the design loads for my addition.
todd
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