Floor Joist Support beam question

[MachineMan]

I'm biulding a 320sq-ft storage shed. It will be 20'x16' with 2x8 joists 24OC and 2x8 ends. On either end and in the middle of the 16ft span the Joists will be supported by 4x4 beams with concrete tube peirs. My plan has an excessive amount of concrete peirs and I was wondering if I change the support beams to 4x6 if I can then reduce it to 3 or 4 peirs per each 20' beam. Assuming I can find an off the shelf 20' tread 4x6 beam....

See foundation picture:

http://i5.photobucket.com/albums/y168/ctkamp/Foundation.jpg

[Under_Miner]

You might have some difficulty finding a direct answer to that. Most span tables that tradesman keep with them only cover S-P-F, Hem-Fir or Douglas Fir lumber, not your 4x6 timbers. You'll have to ask for some literature from your lumber supplier, or simply go with what they're supplying and trust their judgement.

[MachineMan]

Isn't the treated lumber supplied in Doug-Fir? I used the span tables to make sure I was well under the limits for the 2x8 joists. But was not able to locate any info that would help me determine how many piers I need under the floor grid.

I'm planning to get everything at home depot.

Another question... Will 8" diameter tube forms be wide enough our should I use 10" ?

Thanks.

[suemarkp]

First question is are your stipulations on the drawing really what you need - 50 PSF live, 20 PSF dead? That is rather substantial for a shed.

The way you calculate the required post diameter is to determine how much weight is being carried by each post, calculating its support area (concrete pad) in square feet, and then comparing it to your soil's bearing capacity in PSF. The weight being carried by a post must be less than the PSI rating of the soil. I believe you can use a value of 1500 PSF for soil strength without doing a soil test (unless this is being built on a sand dune). As a quickie calculation, if you take 320 square feet * 70 pounds of load and divide by 12 posts, you'll get 1866 pounds per post. I'd round up since some will carry more than others (depends where the roof bearing walls are). So you'll need about 1.3 square feet of concrete under each post. Use the 18 posts in your design, and a square foot each would be plenty. So round sonotubes less than 14" probably aren't going to cut it. If that's an issue, then see what value can be used locally for soil strength and keep the huge number of posts.

The beams are non trivial too. There aren't good span tables for those, but there are some general tables with a host of assumptions. First, it seems like the 4x4's on the ends aren't doing much. You could span much more if you put 2x10's or 2x12's on the ends and did away with the 4x4's. With preferably doubled 2x12's on the edges, I think you can easily do 7 to 8' post spacings on the perimeter.

Sizing the middle beam is kind of like sizing a window header. This span calculator will get you close, but its not perfect because your building is smaller than the choices they provide. http://www.cwc.ca/design/tools/calcs/SpanCalc_2002/pdf/can_6.pdf?span_type=1&member_type=999

This indicates for a 20' shed (yours is only 16), two 2x6's can span 4' 11". I don't know the PSF assumptions in this table or the deflection assumptions. Your 4x6 is a little stronger than two 2x6's, and the 16' span is shorter so the beam supports can be farther apart. You could probably go to 6' post spacings for the middle beam.

Here's a link to the first page of this span calculator and it takews you to different sections depending if you are doing rafters, joists, or headers/beams. http://www.cwc.ca/design/tools/calcs/SpanCalc_2002/

[MachineMan]

I used a high load as the worst case. I might be storing heavy items in there. I put that 4x4 under the edge as a ledge for the joists to sit on avoiding the need for hangers, maybe its not a good idea. The dirt is clay like with rocks.

Using that span calc the grid could work like this:

If I switch to 2x12 (Doug fir #2) joists at 16 OC I can span the entire distance without the middle beam at 50PSF+10PSF dead. Max span is 16' 6" and mine would be 15'6" with doubled 2x12x20' ends. With double 2x12 ends my max end cap span would be around 8'11", meaning I would need 4 piers on each side. If did triple 2x12x20' I could use 3 piers on each side. Then get very large diamter tubes.
This would be a cleaner set-up but cost substantially more.