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bwalak
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# Posted: 28 Nov 2012 08:48am
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Hello I am new to this fourm, I am building a 20x24 1.5 story camp in northern/central maine. I plan on using a pier foundation with 3 rows of 4 piers set 8 feet apart (4 down each side, 4 down the middle) the span from the outside piers to the middle would be 10 feet. When building the floor do I need to first lay girders down along the piers then build the joists and floor on top? or can I just build the joists/floor right ontop of the piers and double up the headers along the outsid and middle piers? I plan on using pressure treated 2x8's or 2x10's for the joists. Thanks
Bill Walak
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toyota_mdt_tech
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# Posted: 28 Nov 2012 09:12am - Edited by: toyota_mdt_tech
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Sounds like you need girders, then lay your joist across it. Not sure how big the girders need to be, but floor joist at a 10 foot span, the 2X8 should work fine. You can splice the joist across a girder by overlapping them by several feet. Just use blocking between the joist over the center girder. Mt Don will have the girder size for you I suspect. 
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MtnDon
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# Posted: 28 Nov 2012 10:24am - Edited by: MtnDon
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You do need girders. In order to safely size the main girders we need to know your areas snow load. Parts of Maine get into the 90 PSF and some even higher. Use the document as the end of this LINK to find your town and report back with the ground snow load figure. If the data indicates "case study" then you need to ask somebody in the local building department. Case study areas are common and occur mostly in mountainous areas where the climate is very variable, hence each case can be different.
This will also be needed to properly size rafters and headers above windows and doors.
Also will the upper floor have any central supports or will the upper floor be a clear span from one side to the other?
Also what will the height above grade for the piers? You will need a method of bracing the piers to the girders and to the floor joists.
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bwalak
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# Posted: 28 Nov 2012 07:53pm
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Thanks guys I really appreciate it.
Mt Don, The snow load for the town in CASE STUDY but the towns around it and close by are 90 PSF. The upper floor will have some supports, I plan on building a 12x12 room under part of the loft. I was planning on building the piers 12 inches above grade and using all PT wood for the girders, joists ...etc.
Also I had mentioned before that I was planning on doing 3 rows of 4 piers, do I really need the middle row if its only going to be 20 feet wide? or is that overkill?
Thanks,
Bill
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toyota_mdt_tech
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# Posted: 28 Nov 2012 09:00pm
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Quoting: bwalak do I really need the middle row if its only going to be 20 feet wide? or is that overkill?
I would think you most certainly would need it.
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MtnDon
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# Posted: 28 Nov 2012 09:32pm - Edited by: MtnDon
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Quoting: bwalak 3 rows of 4 piers, do I really need the middle row if its only going to be 20 feet wide?
It can be done on a 20 foot wide but may be pushing the limits. Douglas fir grade #2, 2x12, can span that IF placed 12" OC. Or if you can find 2x12 select structural grade DF or SYP can work on 16" centers.
IF you go with two rows we call the floor joists a clear span floor. If you do the main floor as a clear span, then the upper floor should also be done as a clear span. If you used main floor interior load bearing walls to assist carrying the upper floor loads they will cause increased point loads on the main floor joists. That will likely push them beyond the safety margins.
IF you did no center beam with #2 2x12 DF 12" OC foor the main floor the upper floor could be the same 2x12 on 16" centers as we usually set the upper floor loading at 30 PSF instead of the 40 PSF for the main floor where heavy items are more likely. And more people.
If the surrounding towns are 90 PSF I guess I'd go for that although it would be great to know what the CS would be rated. t might be higher, but it could also be lower. I would not bet on lower without having official confirmation though.
In that case the main beams come out as follows: for 20 foot wide building, 90 PSF ground snow, upper floor...
... (4) 2x12, grade #2 (Douglas fir-larch, hem-fir, southern pine and spruce-pine-fir); maximum pier spacing 8 feet.
If you used a center beam under the floor it doesn't really change the sizes of the main beams as the potential heavy snow load transfers to the beams under the two long walls. A beam down the center would make it easier to make a solid floor though with smaller joists. #2 2x8 would work. That would mean a beam and properly spaced posts down the main floor, above the centrak main floor beam, to carry the upstairs... OR the 2x12 clear span for upstairs....
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bwalak
Member
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# Posted: 29 Nov 2012 06:45am
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Thanks Mt Don, This really helped me!
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rayyy
Member
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# Posted: 29 Nov 2012 05:48pm
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I went with a support down the middle with 9 1/2 inch wood I
- beams for joists.20 foot wide factory built roof trusses 8 in 1 pitch.They span the 20 foot just fine with out support.I don't have a second story loft though.My cabins 20 x 30,free standing.
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toyota_mdt_tech
Member
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# Posted: 29 Nov 2012 06:04pm
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Trusses, unlike rafters are designed to span with no support except from themselves.
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beachman
Member
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# Posted: 30 Nov 2012 08:47am
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I agree with most above. Sounds very similar to our place in NB - just accross the border from you. We used 3 - 2x10' boards nailed together for each girder - both sides and in the middle to rest on the support posts. Then laid the joists on top of these. It made for a good strong floor with no weak areas. We aslo have a 12 x 12 loft with a dormer - worked out well. One thing we changed was to blow out a 4x12 section on the side to accomodate the kithcen area - not regretted.
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