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mleman
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# Posted: 23 Aug 2022 03:23pm
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Hi everyone - first time posting! I'm building my first small cabin, and could use some advice on constructing my beams. I have a lot of building experience in a DIY/homeowner capacity, but tend to overbuild to compensate for a lack of experience. I'm going to try not to do that this time, but want to make sure I'm not under-building it now. The cabin will be a 16' x 32' shed roof/lean-to structure constructed in post and beam style. It will be placed on 15 sonotubes, arranged in three rows of five, spaced 8' apart. I plan to construct my three 32' beams using (2) 2x10 PT. Assuming that's sufficient (is it?), where should I place my joints for maximum strength? My options as I see them, are to: 1) have two 16' in front and 8' - 16' - 8' in back in order to stagger the joints, or 2) butt two 16' beams together and tie with a mending plate. These beams will sit on 6x6 PT posts, and the snow load here is 60 psf. Any input or advice will be greatly appreciated!
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travellerw
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# Posted: 23 Aug 2022 04:52pm
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The advice I got was to stagger the joints at least 4ft apart, even if it meant throwing out material.
I would also suggest you use new foam based construction adhesive (e.g. Great Stuff Pro) instead of polyurethane bases ones (like PL). The foam adhesives outperform the old brown poly ones in every aspect.
Finally, you need to look at span tables to ensure your beams are up to the task. Using pressure treated lumber reduces the span it can carry. This is your build, I suggest you find some span tables to make sure. However, from my research a triple 2X10 PT laminated beam would be sufficient, especially if you are using 3 of them.
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Brettny
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# Posted: 23 Aug 2022 08:39pm - Edited by: Brettny
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Breaks in the beam lumber should be done over a post and no post should have 2 breaks over it.
So for a 32' beam you would do the following.
8'-16'-8'
16'-16'
That should be 8' sono tube spacing, no two breaks over a single post and 5 posts per run.
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mleman
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# Posted: 24 Aug 2022 07:54am
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Quoting: travellerw Finally, you need to look at span tables to ensure your beams are up to the task. Using pressure treated lumber reduces the span it can carry. This is your build, I suggest you find some span tables to make sure. However, from my research a triple 2X10 PT laminated beam would be sufficient, especially if you are using 3 of them.
Interesting... see, that's why I should be consulting the hive mind here. I always assumed treated lumber was stronger than non-treated, mainly because it's so much harder to cut and seems to dull the heck out of my tools. Thanks for the info!
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mleman
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# Posted: 24 Aug 2022 07:58am
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Quoting: Brettny Breaks in the beam lumber should be done over a post and no post should have 2 breaks over it. So for a 32' beam you would do the following. 8'-16'-8' 16'-16' That should be 8' sono tube spacing, no two breaks over a single post and 5 posts per run.
That's what I was thinking - even though this has more joints than (4) 16', they'd be staggered and placed over supports. I'm starting to second-guess if a double 2x10 would be sufficient, or if I should go with a triple based on the previous comment. Maybe a double with plywood between? Again, I don't want to over-build, but I don't want to skimp either. Regret is a terrible thing.
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gcrank1
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# Posted: 24 Aug 2022 10:16am
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Rule of Thumb and Good Sense:
Dont cut corners or costs on the bottom end. It holds up the top end.
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Brettny
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# Posted: 24 Aug 2022 10:59am
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Quoting: mleman That's what I was thinking - even though this has more joints than (4) 16', they'd be staggered and placed over supports. I'm starting to second-guess if a double 2x10 would be sufficient, or if I should go with a triple based on the previous comment. Maybe a double with plywood between? Again, I don't want to over-build, but I don't want to skimp either. Regret is a terrible thing.
I would check the cost/strength on useing 2x12 also.
I'm planning a 20x32 and will be useing 6' spacing on sono tubes. We are going to pull in the last tube on each end by 12in thus making the sonotube total length 30'. You may also want to bring in each wnd tube a bit. It will keep rain off the cement and post. Have you thought about adding another sono tube to cut back on the 8' spacing?
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ICC
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# Posted: 24 Aug 2022 03:50pm
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Quoting: mleman The cabin will be a 16' x 32' shed roof/lean-to structure constructed in post and beam style.
Questions: does 'post and bean' mean a pier and beam foundation or an actual wall/floor/roof post and beam frame construction?
And, with a 60 psf snow load are you sure a shed type roof is the best idea?
Also, with a shed-type roof spec'd it is obvious the center beam is not to support the roof loads. Roof loads would be transferred down to the two outer rows of piers. The center beam would then be included to support a floor-only load. Is that correct? Or is the soil bearing capacity so low that the extra footings are required to spread out the load?
A center floor beam can reduce the required size for the floor joists, but it is entirely possible to build a stiff, non-bouncy floor suitable for installing ceramic tile without any cracking, for a 16 foot wide building without using a center beam. Hem-fir, doug-for and SYPine can all span that. Hem-fir 17'7", doug-fir 18'1" and SYP at 16'6" makes it too. 2x12, #2 grade, 16" OC. Have a look at the AWC just and rafter calculator. https://awc.org/calculators/span-options-calculator-for-wood-joists-and-rafters/ it is a great tool. But it does not do beams.
Sometimes a center beam makes more sense to me when coupled with a typical gable roof that uses a structural ridge beam which needs foundation support under the ridge line.
Beam sizing usually needs an engineer or some extrapolating of data tables in the IRC, chapter 6. With a 60 psf snow load your initial choice of doubled 2x10's for beams is insufficient. Four 2x10 would need a pier spacing of about 6'5" if not using a center beam. If a center beam is included, the pier spacing with 4 layer 2x10 beams increases by only 7" to 7 feet, because most of the outer beam load is coming from the roof and down the exterior wall framing. That is figuring that this is a single story structure.
If building on beams the floor stiffness can be increased if as Brettny suggests, the floor joists cantilever, or hang over the beams. This can be done safely by an amount equal to the joist height. That is a 2x12 floor joist (actual height 11-1/4") can overhang the beam by 11-1/4 inches. That shrinks the actual joist span.
Not sure if you are aware but span refers to the distance between supports...for example, a 16 foot joist supported by a 4 layer beam (6" actual width) on each end is actually spanning 15 feet.
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ICC
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# Posted: 24 Aug 2022 03:55pm
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Oh, one other detail.... what kind and size of footing is being planned for under those piers?
One more, ... Sonotubes are mentioned, but no mention of whether that is to be a solid poured tube of concrete with the post sitting on top or if you are envisioning the wood post running down into the concrete. Both have potential problems from an engineering design viewpoint.
The best piers would be solid concrete or masonry units that extend upwards to the level required to place the beams. Which gets one to the place where a typical poured concrete or masonry foundation wall on a concrete footing is easier and much better.
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ICC
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# Posted: 24 Aug 2022 05:18pm
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Regarding PT wood vs non-treated wood of the same species, grade, spacing, etc..
There is virtually no mechanical difference to the woods. The biggest difference can occur if the pressure-treated lumber has been incised. This is sometimes necessary as some species, such as doug-fir do not accept sufficient treatment chemicals unless it is incised. (The surface looks like a sprocket has been run over it)..
There are some differences but for non-incised wood the difference is so small that of the actual difference becomes important the structural limits were probably being pushed to begin with.
For the curious have a look at this spec table from the southern pine association. https://www.culpeperwood.com/wp-content/uploads/pdf/Treated_Span_Chart.pdf
There is only a 4 inch difference (9'4" vs 9'0") for a 2x6 joist under the same conditions.
Wood used in wet conditions is also subject to a re-rating from the standard tables which assume kiln dried wood is used. PT may be wet when purchased but once air dried down to a similar MC as non-treated it is no longer considered wet. The reduction is about the same.
IIRC there is a two foot reduction in span rating for a 2x12 doug-fir floor joist IF the d-f is incised. It is the incising that causes the reduction for that example, not the PT.
The southern span association has numerous tables available for download... https://www.southernpine.com/span-tables/joists-rafters/ For some reason I am having a problem with the following link... maybe it will work, maybe not.... it is for their PT products... https://members.southernpine.com/publications/download/55116f96b4bc78145137a0a5/205_T reated%20Pocket%20Span%20Card_0315L.pdf
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Aklogcabin
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# Posted: 27 Aug 2022 11:32am - Edited by: Aklogcabin
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I don't have a bunch of different links but I do have some experience. I would use 2x12 because you can. Stronger is better. If you put a piece of plywood in the mix somewhere in the sandwich it would help considerably. And give you a straight edge to help keep things straight. Lots of good construction adhesive out there. I like titebond 3 for projects like this. And use galvanized nails or treated screws. Good ol hot dip 16d nails are good. I also like to use a C clamp to squeeze the sandwich together. Get better compression I believe. Hard to get straight 16' lumber. Look at the end grain of the board. Grain running the short distance will stay straighter because it is cross cut. And use the lumber against itself. Using the bends n twist to work against each other. This is where a good clamp comes in handy.
I would use 3 rows of support beams under the floor joists. They will have a bounce otherwise. I believe there is a fellow cabin guy in wi dealing with that now. If it is done to keep moisture away from the foundation beams you shouldn't have to worry about frost.
I prefer and I believe that most construction books recommend putting the walls directly over the foundation. Cantilevered works, but again if you can keep the weight of your building directly over the foundation, I believe it is better. And the siding should go down far enough to cover everything.
How far off the ground are you planning ? It's good to have plenty of air movement. Maybe consider putting a plastic vapor barrier down somewhere in the plan.
Hey just some other ways of doing things. I used cedar utility poles for the foundation posts and they have worked great.
Good luck n have fun
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