Bouncy floor. New beam didn't fix?

We just completed the construction of our new house (did all work ourselves).
All floors are constructed with 2x10 joists at 16" OC, with 3/4" OSB subfloor, and solid blocking every 8' of the span.
The floors feel quite solid throughout the house, except in our living room where the floor is a bit bouncy. The span there is about 15' from sill to sill, just about the max for 2x10 Doug Fir joists.
We have the same 15' span in our master bedroom and the floor there is quite solid, so I suspect the problem is due to our large entertainment center (a few hundred pounds I'm sure) that sits about mid-span in the living room.
Since the house is single story over a crawlspace, I decided to add a beam mid-span to stiffen up the living room floor under the entertainment center.
I used two 2x8 x 10' boards screwed together for the built-up beam. I supported the beam on three posts, spaced about 4' apart, which rest on top of concrete deck piers. I would have preferred a poured footing, but getting the materials back to where I need them in the crawlspace was rather difficult. The deck blocks seemed like the easiest approach, and I spaced the piers four feet apart to minimize the load on each pier.
The floor had sagged about 1/2" where we have the entertainment center, so I used an automotive floor jack to jack the beam up level and tight against the floor joists.
Everything went rather smoothly, and should have effectively reduced the span to about 8' on each side of the beam.
Unfortunately, when I climbed out from under the house and came in expecting the bounce to be gone, I discovered the floor still bounces... Not as bad, but it is still significant.
So, now I'm wondering what went wrong and what I do now.
The beam should have made the floor quite solid, and yet it still bounces? The ground in the crawlspace is quite firm, so I don't think there is any "give" to the piers.
This weekend I'll probably have my wife bounce the floor from above while I climb underneath to see if I can discover anything. But, I'm interested in hearing comments from others with similar experiences.
Thanks,
Anthony
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I'd not use any joist near its max span limits. Be conservative and switch to the next bigger. Undersized joists are a surefire way to get excess deflections and vibrations. Not undersized according to code, but undersized compared to that nice, solid feeling folks expect in a house.

There can be a couple of reasons why these are different. Distribution of weight can be one, since the heavy entertainment center in the middle of the room could result in a different, lower, resonant frequency and that can be more noticible.

If the new support beam doesn't give, you would expect a big change - the reduced span would have a much higher resonant frequency (about 4 times higher due to halving the span). This makes me suspect that the support is actually not that solid. Even if the initial installation is solid, the concrete piers will settle under load. Some settlement will happen quickly and some will occur over a longer period of time. This is dependent on the soil characteristics.
Let us know what happens when you watch from underneath.
Mike
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Michael,

Yeah, it was a weak design decision on my part... The majority of the house has spans that are around 12', well within the capability of the 2x10 @ 16" joists. Only the living and master bedroom have the 15' span. I approached the span limits in those two rooms to keep things consistant. In hindsight, I should have added another support or switched to larger joists in those two rooms, but that does little good now.

Yeah, that's what I'm thinking too.
I'm gonna give it a couple of days to settle in a bit, then I'll check things out better.
Anthony
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SNIP
I agree with all Mike's comments, except

Shouldn't that be the square root of 8 (2.8) not 4?
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On 21-Oct-2004, snipped-for-privacy@aol.com (Bob K 207) wrote:

I'm working from memory and may be wrong (time to review my old dynamics stuff). At any rate, the frequency of the halved span will be higher and by more than double.
Mike
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What kind of flooring in each room? Is, perhaps,the bedroom a wood floor and the living room just carpet over the OSB? That could make a pretty big difference.
3/4" OSB doesn't exactly sound "overbuilt" to me. If the Living room is just carpet, you might consider pulling it up and screwing down a layer of 1/4" or, better yet, 1/2" ply over what's already there, using a very tight pattern with drywall screws no more than 6 inches from each other, and I don't mean just on the seams. That will tighten it up tremendously.
I had a kitchen floor that flexed a bit. The tile guy said he needed to add an inch of plywood to make it suitable for tile. That would have raised it an inch above the floors it met in other rooms. There was already a layer of 3/4 inch boards, covered with 3/4 inch plywood! I simply screwed the existing layers together as described above and then laid 12 inch tiles myself. ZERO cracks or grout problems in 10 years. My wife does jumping excercises on it all the time while cooking. If it had been new construction, I might have used adhesive betweeen the layers and slightly fewer screws.
BB
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BB,

Identical floor structure in both rooms.
Carpet Padding 3/4" T&G "Edgegold" OSB subfloor (nailed every 6"). "Subfloor" construction adhesive between OSB and joists. 2x10 joists @ 16" OC (15' free span between supports). Solid 2x10 blocking in middle of span (8').
Other than a non-bearing partition wall, the two rooms are basically one.

Nope, it's not overbuilt, but the OSB seems sturdy enough everywhere else in the house.
Anthony
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Then I would do exactly what I outlined.
BB
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I've investigated the bouncy floor quite a bit. Often it is the furniture in the room that gives the feeling of bonciness. A tall unit will rock more than a dresser, thus giving the impression that the "floor is bouncy".
Depending on how the unit is situated you might have the front legs over one joist & the back legs over another. This will allow the unit to rock back & forth.
Also i would suggest just putting piers with jack screws (you can buy or make yourself from 1 1/2 pipe & 1" bolts) under the set of three or four joists rather than one long beam. With the jack screws you can tighten them up as needed.
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HerHusband wrote:

I would have preferred a poured footing, but

Can you see that the new beam really is "tight" against *all* the joists? Maybe you'll need to tie each joist to it in case there's enough variation it the 10" dimension of the joist boards and some of them aren't really sitting hard on the new beam.
Just a WAG.
Jeff
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against
Many years ago had similar problem; our kitchen floor sagged. maybe a half inch or more due to the weight of the stove and sink counter which sat longitudinally above approx. three joists. Also a ten foot run of hall closets over several adjacent joists backed onto the kitchen wall, so there was a lot of weight up there! As a solution we jacked up that part of the floor a little bit at a time, using a small hydraulic car jack on top of a wooden post, sighting across the kitchen and hall floors from the top of the basement stairs; jacking gradually until that part of the floor was 'just a little bit too high'! Then built a load bearing, double header, single floor plate 2x4 stud wall underneath, in our full height unfinished basement resting on the concrete basement floor; which spread the weight nicely. Wedging the stud wall firmly in place we then released the jacking. IIRC after a couple of days and one/two minor adjustments it settled into place nicely level. And has been that way for over 30 years. Wedges and/or shims of thin plywood were used for 'fine' adjustment and 'packing' the gaps to get everything level. That wall later became one side of a 10 by 13 foot room directly underneath our kitchen and houses an electronic repair bench. The wall also contains a storage closet the lower part of which stores unused luggage above which are hung occasionally used oilskins, sea boots etc. Although added later and not necessary the sides of the closet also support adjacent joists and buttress or stiffen the stud wall itself. Come to think of it the secondary breaker panel for the kitchen end of the house is also in that closet wall above and the added basement stud wall provided means of later routing down some wiring for added basement circuits. Overcoming the problem actually created new possibilities.
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room
A little advice I discovered by accident while renovating my house:
Use pressure treated joists--they are made from a much harder, denser, stronger grade of wood than typical doug fir. I did some ceiling joists recently with a 12' span and walking up there (flat roof), it's as if the frame were sitting on concrete on the ground.
Short of replacing or adding additional PT joists, I'd recommend a carrying beam perpendicular to the existing joists, supported by lally columns on cement footings. Lots of extra work, but ultimately, it solves the problem.
Normally, I do 2x12 joists for floors. And I love PT joists. They are almost like steel "I" beams in terms of rigidity.
-- Take care,
Mark & Mary Ann Weiss
VIDEO PRODUCTION FILM SCANNING DVD MASTERING AUDIO RESTORATION Hear my Kurzweil Creations at: http://www.dv-clips.com/theater.htm Business sites at: www.dv-clips.com www.mwcomms.com www.adventuresinanimemusic.com -
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<< Use pressure treated joists--they are made from a much harder, denser, stronger grade of wood than typical doug fir. >>
True often but not always. The really tough PT is yellow pine, heavy and dense. But I got some PT at my local lumber yard a while back that was some kind of softwood from Nebraska IIRC. Not at all in the same league as the Southern yellow pine. HTH
Joe
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PT around here is all Doug Fir or hemlock it isnt any harder at all. Often times its much weaker.
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Replacing joists is probably going to be an enormous job; what's he going to do - lift up the whole house and replace all the joists to make them the same size?
I suggest converting the first floor / basement ceiling into a torsion box to stiffen it. Toss up a 4x8 foot sheet of plywood under the area that's bouncing and screw it in with drywall screws on six inch centers. If this seems to help the problem, take it back down and put glue on every joist before putting it back again. You're not getting this back off, so be sure you don't cover up anything important (like a cleanout). You can extend the plywood to cover the entire basement ceiling.
Look up torsion box on Google - the technique is used in furniture, in interior doors and in cardboard boxes. It's probably more expensive than using the correct sized joists in the first place, but it's a possible fix for the problem. Use thin plywood to avoid having to lift a heavy weight to the ceiling of the basement. Quarter inch should be more than enough.
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