Sister 2×6 on top of 2×6 – load calculations
I am having trouble finding PSF load calculations on sistered boards for my theoretics.
Using Yellow pine or douglas Fir at #2 or above, I see potential for a 16 inch offset in beams, for a 2×12 can span 18.5 feet for 30PSF live, 10PSF dead at 360 deflection. source: (http://www.southernpine.com/media/SPtable1_060113.pdf)
I have been looking through http://www.awc.org/pdf/STJR_2012.pdf as well for a few better standing measures, but don’t see calculations for a sister 2×6 to on top of 2×6, putting a binding agent between the boards, and then reinforcing with plywood 6×12 inch panels (gussets) on the 5 1/2 foot points, then reinforcing further with solid 2×12 bridging at the inner edging of the reinforcment supprt panels.
I am curious if there is a reliable way to calculate a sister joist pairing or if anyone has further thoughts on better strength application. I am unable to do side by side sister due to one end being held up by joist hangers (would not fit flush) and that I need the 2×12 solid bridging to aid in deflection and sway.
Theoretically, if paired properly, including nubbins, the deflection on live load at 16 foot span would be equal to that of a 2×12.
Replies
I had to do this once under a hip rafter. The engineer signed off on it so I don't know how to do the calcs. They have to take the gussets into account and there's no easy way to do that. I do think you will need continuous plywood and not just the small pieces you show. Lots and lots of nails. Why do you want 30# live load? That's not enough for a floor. For that you'll need at least 40#.
30 pound minimum is due to it being a second level, non-bedroom, non plain storage, not first level. 40 pound would be great, but the requirement on the second level non-main living is 30 minimum.
2x12 would be great and calculations would be done using a guide sheet, if that were started with, but the assumption is that the 2x6 would be pre-existing to a cieling. 2x12s could be added in between, but then the weight adds up quickly. I will see what lateral sheer can get for calculations if I can find a resource. Perhaps a substituion on a 1x6x12 piece instead of plywood will make the calculation easier. Weight comes in question as the 2x6 pre-done assumed to be attached to a large header using joist hangers, and a 2x4 load bearing wall on the other side. Less weight equals better concept, but needs more calculations that are a bit harder to complete without the right source.
When the architect signed off on the box beams, were they matched with contiguous plywood along the beam, or gusset sections?
Plywood gussets.
In my case the plywood was done continuous on both sides in 8' pieces on each side with the joints staggered. I don't remember the nailing schedule but it called for 16d joist hanger nails i.e. a 1.5" nail the same gague as a 16d. The hip jacks were already in place so the plywood only got nailed to the bottom 2" or so of the top hip rafter. The building department had already accepted the hips, but I had to have something else engineered that I screwed up and the engineer wouldn't sign off unless I fixed the hips.
Stronger and stiffer than a 2x12 as shown IF
IF - you account for lateral shear at the ends. This means you need more gusset at the ends, you can leave the gussest out of the middle of the span.
Search for 'lateral shear' when you search for beam calculation formula.
If you do not account for lateral shear (parallel to length of beam) you only have the equivalent of sistering side by side.
I aint going to do the lateral shear calcs for you, but off top of head you need a couple feet of gusset at the end, and a foot or so (all glued) at 1/3 the way toward the center. Anything extra is margin.
It seems to me that the way you're doing it, with a gap between the two pieces and relatively narrow gussets, you lose a lot of the advantage of a single 12" beam. This is because the gussets will not provide much lateral resistance, so when the boards bow they will "slide" relative to each other.
But you should get at least the strength of a 2x6 twice.
vertical sistered joist
As junkhound said, latereral / horizontal shear is a major factor in what you propose doing. I might mention, when calulating the shear of plywood, you only consider thoes plies with the grain running perpendicular to the force being considered. At least that was the case back when we were doing nail & glued box beams and stress skined floor and roof panels.
I'm not sure I agree with him as to needing only to be concerned with the gussets at the ends of the joist. If you want to get the load carrying (bending) capacity of both joinst, they need to be tied together so as to act as one. It's not likly that any vertical (shear) load you would incur would exceed the value of a single 2x6.
I may have missunderstood him.
Just stack them up and use structural screws to attach them, no gussets needed.
It's not clear why you need the "plate" between the pieces. It certainly adds no strength.
In theory, if you made the gussets each several feet long, you would be able to approach the strength of a 2x12.