My house was built in the 1970s when apparently there was a huge shortage of either lumber or smarts.
2x8 joists, 12" O.C.,14' span
1x3 X bracing in the center of the span
1/2" plywood subfloor nailed with spiral nails - no glue Oak hardwoord floors nailed perpindicular to the joists - the flooring is short with most pieces less than 3' Easy access from below in full basement
Floor bounces noticeably from normal walking, knick knacks rattle, etc,
I tried on the other side of the beam with 16" O.C. 2x8 joists solid blocking with little success.
I've seen 3 suggestions and want to know if anyone has tried them and what the result was
1 Metal X bracing installed on either side of the current bracing (splitting the difference between the center and the ends of the joists)
2 Sistering the joists with glued & screwed plywood on both sides (All plumbing & electric are either below the joists or parallel to them)
3 Installing a plywood 'ceiling' in the basement glued & screwed to the bottom of each joist. (I guess the idea here is to make the whole joist sytem a giant torsion box)
The best option I could think of would be for you could sister the existing 2x8 joists with a 2x10 joist screwed and glued to the original. A 2x10 on 16" centers is good for a 14'-6" span. You wouldn't have to go the full span with the 2x10's just make sure you do at least the middle 2/3 of the span. You would loose a couple inches of headroom but you would get the strength you need.
If you had access from the top by removing the subfloor (I'm not sure how deep you're getting into this) you could go the full distance by just notching each end of the 2x10 to the height of the 2x8 and dropping the 2x10's in. Piece of cake! Hehe. On television it could be done in less than an hour...
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Your floor system is a bit a "skimpy" side.....1/2" plywood & 2x8 joists.
If oyu do decide to sister the joists.....I would highly recommend propping the current joists with a series of jack stands to relieve a bit of the load and to make sure the new wood take some of the dead load.
I would glue & brad nail (or staple, M2 type) the hell out of the sister. Adding 3/4" plywood sisters to each side of the joists would pretty much double the floor stifness. Adding a single 2x10 per joist would nearly triple the floor stiffness.
If you want to get fancy & minimize wood cost....you could add a 2x4 (flat) to the bottom the existing 2x8's. Thus making a T-beam.
I "doubled" some 2x4 ceiling joists (very old house) by slathering up the top edge of the 2x4 with a SIKA wood repair epoxy. I mushed the new 2x4 onto the bed of epoxy, stapled a couple side gussets to the assembly.
I tested a prototype (mushed / not clamped) assembly to failure....failed at a shear stress of 175 psi but failed in bending. One the glued up members had a nasty knot.
You could use a wood workers glue instead but pre-drill the 2x4 and screw (6" o/c?) it to the 2x8 in lieu of clamping. And then shoot in a couple nails between the screws. Again pre-jack the old joists / floor to relieve some load and keep the jacks or struts in place until the glue cures (24hrs?)
I'd go with the above and definitely *jacking the existing joists while sistering them*! I'd run a 4x4 perpendicular to the joists and jack under that so they are all even. If you go with the 2x8's you will need
2x4 spacers between the 4x4 and the existing joists to allow room for the 2x8. Tack the spacers in place to make things easy. I did something similar to my front porch and let it go weeks raising it a little more at a time. For your job I might jack them so they actually start to bow upwards (not much, just a little) then sister them, again taking your time to jack it up letting things bend back slowly.
I have done this to a joist that the HVAC people cut most of the strength away to fit a duct in a difficult and tight area. I ran a 1 1/2" wide by
1/8" deep dado down the length of the 2 x 4 so it would easily fit on the bottom of the joist as I was working alone. I added a good quantity of glue into the dado and drove a screw about every 12 inches through the flange into the bottom of the joist. Worked great, added a lot of strength and stiffness to the joist.
As you know the problem stems from the fact that the 12' span is on the long side and 8" is on the short side for depth of joists. You are on the wrong sides of both dimensions for any solidness of the structure so guess work by non-professionals will not work at all.
I am a civil engineer but not a structural one. I took the structural courses and work with structural guys all the time. For them, it is a simple calculation to determine the degree of stiffness you would get with either #2 or #3 approaches. Try a structural engineering consulting firm and pay them to do the calcs for you - might be a few hundred bucks but it would save you a lot of money over making a mistake with the wrong guess. Note - you need to ask the right question to get the right answer. You want to know the amount of deflection in the new situation versus the deflection in the old case. Normally, they design for a structure that won't collapse or flex too much. You already know it won't collapse, you just want to improve the deflection which relates directly to the amount of bounce. Bounce will be twice or more the deflection so if you want minimal bounce you need very low deflection.
I don't think #1 will help at all and I think that #3 by itself will not work because it would place too much stress on the bottom plate. There is no top plate solidly fastened to the joists. The floor wood is not parallel to the joists and, even if it were, the pieces are short and thus not contiguous along the joist line. Thus there is no 'box' - no stresses in a top plate to balance the stress in the bottom plate. Need a structural engineer to confirm. You may need to do both #2 and #3 - this would be a 3- sided channel.
Also, ask how many screws are required to transfer the stresses - there are tables of allowable stress per screw.
You probably shouldn't use plywood. If you use 1" by 8" boards you save a lot of cutting and may get a better final product. Wood strength differs considerably so ask about the type of wood you should use - the calcs would be based on an assumed type of wood.
Just a guess but a structural engineer may well tell you that deflection (bouncing)will be too large for anything that is only 8" deep and spans 12 feet. They may suggest that the depth has to be increased. Prepare yourself to answer that question - can you go to 12" depths? Would that create problems of headroom or conflict with plumbing or wiring?
The reinforcing boards should be one-piece for best strength, thus 12' long. Can you get such long pieces in place? Shorter lengths can be done but require proper design or you won't get the improvement you need. One idea is 6' pieces with 6' overlaps at the seam, all glued and screwed.
The existing joists are already sagging due to overloading. Ask the structural engineer if they would carry their share of the load in a new reinforced structure. Also, ask if the joists should be jacked up to be straight while the reinforcing boards are screwed and glued in place. If you just add new boards to the existing amount of defection you may not get what you want. Or you may have to use stringer reinforcing. This question may depend on the condition of the walls and floors above these joists. Are the walls OK? Are there cracks, etc, that you would fix after the main structural problems were solved?
I should also point out that glue is only as good as the dryness and cleanliness of the surfaces glued and a proper gluing procedure. Unless you do a lot of very good cleaning the glue won't hold. If the wood is damp for some reason glue won't hold at all. Also, for glue to hold the surfaces must be flat so there will be no significant gaps. This is probably achievable with thin plywood but not with thicker plywood. You need thicker for strength. Maybe a lot of screws will pull the sheets close enough together, maybe not. Use lots of glue so it will soak into the wood and span gaps.
Do both glue and screw but use enough screws to transfer the stresses in case the glue doesn't hold. If the glue does hold it will be an extra bonus.
I'd jack up each one and add a 2nd 2x8. Are both ends on top of support? If one end is held by joist hangers then you ought to be able get the full length. The most strength is required at the ends. If you can't get the full length then use two pieces and overlap at the center with a strip of 4 ft 1" osb cut the long way. You can get a ton of them from one sheet. Glue and screw whatever you do.
I have a closet off a landing that I spanned 8' using doubled 2x6's. Needed to maximize the space under it. It's pretty solid.
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