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Liamey
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# Posted: 24 Jul 2014 03:31pm
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Quick question if I may. Building a 16x24 cabin. I have 3 rows of 6x6 posts in place, 5 posts in each row making 15 posts in total.
I am ready for placing the laminated girders, but my dilemma. Should I use 2x10 3 ply or 2x12 2 ply i.e number of girders laminated?
The cabin will be a storey and a half and the floor joists will cantilever over the girders by 6 inches each side.
Thank you.
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bldginsp
Member
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# Posted: 24 Jul 2014 09:09pm
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A dimensioned drawing would help
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Don_P
Member
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# Posted: 24 Jul 2014 10:16pm - Edited by: Don_P
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Agreed, with snow load info.
Assume 6' span between posts in a row, 8' to line of center posts on main floor, second floor clearspan 16', unknown snow load, roof bearing on outer walls only? Too many unknowns to answer what is needed for that situation.
For background general knowledge, comparing the two girders' basec section properties in bending strength and stiffness;
Section modulus of a triple 2x10 is 64.17, Moment of inertia = 296.79
Section modulus of a double 2x12 is 63.28, Moment of inertia = 356
A double 2x12 is slightly weaker in bending but considerably stiffer...
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Liamey
Member
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# Posted: 25 Jul 2014 08:57am
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Hi guys I appreciate the replies so far. I shall try my best to get a dimensioned drawing up, but tbh most is in my head.
The cabin floor plan will be 16x24' however due to wanting a cantilever joist for strength, the posts are in a 15x23 pattern. Three rows of five posts i.e two outer side rows and a middle row. The girders would sit right on top of the 6x6 posts. Later, 16' floor joists would span all three girders being cantilevered. From the top of my head each row has a post 69" OC. And between each row 7' OC. Basically there would be a little bit less than 6" overhang on the joists as the post measurements are on centre. I hope this is making sense so far as my eyes are beginning to spin!
My land is remote so carting materials in can be hard. I have finally reached the stage where I can start placing the girders on top of the posts. The girders will be 24' and there will be 3 of them for each of the rows. I am wondering which will give the better strength, we do have snow load as I am in Canada. I am aiming for one and a half storeys, but not a full sleeping area on top - more like a little storage area. What would be best to go with 3 2x10 nailed and laminated for each girder or 2 laminated 2x12 for each row. I am thinking about the amount of material having to be lugged in, maybe the laminated 2x12 would be best? I guess if I went this route the girder would be placed on the outer edge of the posts rather than OC?
Appreciated guys - thank you
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Don_P
Member
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# Posted: 25 Jul 2014 06:47pm
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we do have snow load as I am in Canada.... need snow load in pounds per square foot to be able to size a beam.
I assume the loft joists bear only on the outer walls.
One thing people often forget is that rafters need a tie across their bottoms to restrain the horizontal spreading force from the load above trying to "toggle" them (typically a ceiling joist performs this function) that tie can be raised no more than 1/3 of total roof height. If it is still in the head banger zone either the walls need to be taller or a ridgeBeam is used. If a structural ridgebeam is used it needs to be sized for the load, supported on each end and probably in the middle and it will take half the roof load off of the eave girders. So that decision directly affects the girder load and sizing as well.
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Liamey
Member
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# Posted: 25 Jul 2014 07:13pm
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Hi Don P
I'm not sure but we might be talking different things. I am still very early on in my cabin build - wish I was further along though lol. At the moment I am just talking the post and beam. I can either laminate 3 2x10 or 2 2x12. I'm just wondering what would make more sense out of the two sizes. The posts are in the ground awaiting the beams. Once I have this done I will be ready for the floor joists, probably 2x8 or so. The floor joists will cantilever my 3 beams.
Regards
Liamey
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Don_P
Member
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# Posted: 25 Jul 2014 11:00pm
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You're not realizing what those girders are supporting.
The main floor,
the loft floor,
the roof,
the snow on the roof
From your verbal description all these things are bearing on those girders...
You cannot size a beam, correctly, until you understand and quantify the load the beam is carrying.
Best luck with your project  .
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