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I have a span of 16½ feet. The floor joists are doubled-up 2×10 Douglas fir 16″ o/c. The flooring is 3/4″ T&G plywood. I am planning to finish the basement, but I would like to take some of the bounce out of the floor first.
A friend of mine claims I can increase the strength of each set of doubled-up joists by screwing and glueing another 2×10 horizontally to the bottom of each set of joists. He claims a strength increase of 40%. Can someone confirm this?
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If I read this right, your friend suggests attaching another 2x10 lying flat to the bottom side of each of the floor joists. This would in effect create a "T" shaped floor joist. If that is indeed what he proposed, then yes that would stiffen up the old floor. I don't think you would need a 2x10, to get the effect. a 2x6 or even a 2x4 would suffice. Lots of glue and screws would be needed though. Attaching sheets of of 3/8" CDX plywood or OSB to the bottom side of the floor joist would also help. The stiffening effect would not be nearly as much as the 2x stuff, mostly because it is hard to get 16 foot sheets of plywood or OSB. Even the drywall will stiffen the floor, as I discovered when I finished the basement in our previous house. The floor in our living room was fairly bouncy and became noiticeably less so when I installed the drywall. I think the room had 14 foot spans of TJ on 24" centers. Floor was plenty strong but really springy. Even though the 12 foot 1/2 dry wall didn't cover the entire span, it still made a difference.
*I would start by installing blocking. Don't put it down the middle of the span, though, the idea being to alliviate any harmonics. I blocked my I-joists and was impressed with the results. It won't increase the load capacity of the floor, but blocking will take out a lot of the bounce.
*Do I have this right? A sheet of drywall or other sheet material glued and screwed to the bottom of the joists would lessen the bounce in a floor? Wouldn't the sheet material be akin to a shear panel in a horizontal plane? If so, how would a shear panel resist compressional forces perpendicular to the plane of the panel? The only thing I can think of is that such a panel would prohibit the joists from wracking. But, I don't think this accurately describes the movement of joists when subjected to compressional forces from above the floor plane.Please help me out of the dark.
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"Wouldn't the sheet material be akin to a shear panel in a horizontal plane?"
I think in this case it would be more akin to a flange on an "I" beam (though I suspect to a rather limited extent with drywall) also, the added mass will reduced resonant frequency of the floor, possibly making the "bounce" less noticable.
Jay
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I don't think the drywall would do a lot but plywood - and I suppose osb - if properly attached and joined can do a lot because on the bottom they are in tension, not compression. The APA has a nifty design guide for calculating this.
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I have the same problem. I first tried sistering 3/8" plywood to either side of the joist cut to the joist width. This worked but not to the degree that I wanted. I am going to use sheets of 3/8" plywood glued and screwed to the bottoms of the joists. This will create a stressed skin and will greatly stiffen the floor.
I believe the drywall described above did not stiffen the floor as much as it dampened the vibrations. The net effect is the same under normal conditions.
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You may want to look at the APA info on the glueing. Construction adhesive won't work well. Also, it explains that you should start with a full sheet midspan (parallel to joists of course.) Joints in the ply should be made with a piece of the ply 2' by joist space wide, glued and clamped or screwed. I always uesed Titebond and ring shanked nails, but was working in a shop on tables. You can probably get as much benefit from 1/4" as 3/8" if joists are 16" o.c.
*The bottom of your floor joists are not in compression, they are in tension. The sheet material works great for this.-Rob
*Classic case of the "China syndrome". Long span joists, in a lightweight floor. When you walk by the china in the cabinet rattles.
*When gluing and fastening my subfloor, I automatically ran the 4x8 sheets perpendicular to the joists, never thinking the directional strength of the ply could be used to strengthen the joist span, rather than bridge the load to the adjacent joists. How do the rest of you run your ply subfloors? David
*David - I do same as you say but mentioned above if putting on ply on the bottom for additional strength - where its in tension - that the APA documents show using it parallel. When I've built what are more or less free standing "planks" of ply and 2 by and where long spans were needed, I follow the APA docs and both faces are parallel - but thtas a very special situation.
*I'd consider jacking the joists slightly in the center and then sistering with 2x12's or deeper. As you are trying to make them stiffer, and they are otherwise strong enough per code, they don't need to extend all the way to the bearing points on either end. 3/4 of the distance should be enough. I'd glue and screw. The material added should not be green.For stiffness you need depth. For added material to add strength, it has to be under stress. If you just add plywood to the joists already under load sitting there, the plywood only begins to add strength after the joist is severely stressed, and that doesn't do much good.If you have the ceiling height and too many pipes and wires are not in the way, I'd consider sistering with deeper material, even if you only put them 32" on center and it doesn't go all the way to the ends it should have a noticable effect.
*In TOH 2 issues ago there was an article about how Tom Silva used a glue lam to span a good distance. In this same article he used three completely different nailers within 12" of his face without eyewear of any kind, but I digress. There is a shot of the floor above and it appears to have something like 1 x 10 boards sistered on the side of the floor joists. There is a ton of headroom, so their height is not an issue. They had been there a long time, probably original. I don't know why they were there, but they must make a hell of a stiffening rib! If you want the formula to calculate deflections, I can fax it to you. I also have a spredsheet that will do the bending calculations for you, just punch in the numbers.-Rob
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I have a span of 16½ feet. The floor joists are doubled-up 2x10 Douglas fir 16" o/c. The flooring is 3/4" T&G plywood. I am planning to finish the basement, but I would like to take some of the bounce out of the floor first.
A friend of mine claims I can increase the strength of each set of doubled-up joists by screwing and glueing another 2x10 horizontally to the bottom of each set of joists. He claims a strength increase of 40%. Can someone confirm this?