Hello All,
I have a question for you on a problem with the sub floor on the house I just built (my house). I framed the subfloor according to plans, but it is not nearly as stiff as I expected it to be. When my 7 year old son runs on it it bounces like a trampolene. It doesn’t seem right, and I’d like to fix it.
I framed it the subfloor using 14″ BCI 60 series I-joists, covered with 3/4″ T&G OSB screwed and glued. The maximum span for these joists is 15′. The engineer that drew up the plans said this would give a floor with very little deflection. This was important as we planned to install 18″ floor tile and my tile guy said a very stiff floor was needed to avoud cracking the tiles. The engineer says this floor should have a maximum deflection better than L/960 and can’t explain why it seems bouncy.
One thing I noticed when I framed the floor was that there didn’t seem too be much blocking betwen the I-joists. There was end blocking 16″ on center for the 2 end bays on each end of the house, and blocking 16″ oc under a shear wall in the middle of the house. That was it for blocking, and it seemed kind of light to me, but that was what the plans called for and the building inspecter signed off on.
The question I have is what can be done to remove the “bounce” from the floor? Would adding more blocking between the I-joists make the floor stiffer? Could I nail some 2×4’s to the bottom of the joists on a diagonal to make the floor stiffer. I have a big crawl space, and easy access, so can do whatever I need to from below.
What do you recommend?
Replies
I did a thread on floor vibration a while back that might give you some insight on floor vibration:
http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=21010.1
But honestly - A floor of 14" I-joists spanning 15' should be pretty danged stiff. Guessing at a few values in the spreadsheet I have, you floor should vibrate at somwhere around 24 cycles per second.(CPS) If you read the thread above, you'll see that anything above 15 CPS should be acceptable.
A few thoughts come to mind. First - I have heard that floors that vibrate at DOUBLE a specific frequency can feel to some degree like they're vibrating ar HALF that frequency. In other words, if your floor is vibrating at 24 CPS it might feel kinda like it was vibrating at 12 CPS.
I assume one end of the I-joists is resting on a foundation, and the other end on a beam ort wall? Beams can change vibration frequency. If one end is on a beam, can you give us the details on the beam?
How far along in the construction process are you? Furniture, interior non load bearing partitions, drywall, and flooring can change the feel of a floor a heck of a bunch. If you aren't living in it yet some patience might be in order.
If pictures are an option that might give us some clues. And if you have the framing plan in PDF format I'd sure like to look it over.
I don't see many houses 15' wide, so I am wondeing if it is 30' long I-joists over a center beam, and that can introiduce strange vibrations. Also if the ernds are in top hanger hardware instead of seated directly atop a foundation wall, that can induce extra vibrations.Gotta go read the restt of thread to catch up now
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Id say its for your engineer to explain and correct, assuming he knew you were planning on tile. However it could be as simple as installing a mid-span beam on the underside of the I joists, that should cut the deflection to something like 1/4 of what it is now.
Thanks for the replies guys.
Boss Hog, The I joists are actually 30' long on a 16" center. They are supported on each end by a 2x8 sill plate sitting on top of an 8' poured concrete foundation wall. There is a 3" gluelam that runs down the center of the house supporting the I-joints at mid span giving an effective span of 15' for the I-joists. The Glu-lam is hung on hangers at each end and has a couple of posts that support it down to footers in the middle of the crawler. The sub floor is covered with 1-1/2" of gypcrete for in-floor heat. The load of the gypcrete was factored in to the engineers calculations.
The house is still under construction, 99% finished. All framing, electrical, plumbing and drywall done. The gypcrete has been poured, and hasn't seemed to make much difference to the perceved bounce. We are still painting and tiling, so no carpets down yet or furniture. The kids love running around in the house while I work on it, hence I noticed the floor "bounce". Just walking, framing the walls on the subfloor, etc I have never noticed the bounce. It may be a vibration problem as you suggest, but you are talking above my head when the subject of vibrations and harmonics comes up.
Bluejae, I talked to the engineer, he says the if the floor is built to the plans, it is fine, and there is nothing he can do about it. Not a lot of help actually. They got paid, and I don't think they have too much more interest in it.
I ran a few numbers and it appears your joists are adequate. What size is the 3" glulam? It may well be that this major support beam is not sized appropriately. Is there any give (noticeable movement) at the supports of the glulam?
Also, if you are planning on tile it is my opinion that 3/4" substrate is not enough, even for ceramic tile (definitely not enough for stone). Particularly since you plan to use large format tile (18"). Adding another 1/2" or 3/4" (use full glue coverage) would also stiffen up the floor between joists.
" if you are planning on tile it is my opinion that 3/4" substrate is not enough,"Did you miss the part about the gypcrete?
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I did miss that. I don't like placing tile on gypcrete though it can be done.
If it's only noticeable when your kids are running, but feels OK when you just walk across it, the floor will probably be fine. Although I'm still curious about the details on the glulam - Like depth and post spacing.
Before borrowing money from a friend, decide which you need more.
As I suspected, you have full length joists which can cause this. I have built houses with a cut specified in the top chord directly over the beam. When I asked why, the explanation given was to prevent vibrations. I don'tunderstand the theory why so I won't atempt to explain it, but that could be the problem.Are all the joists snug to the beam?An i-joist floor can be made stronger by making it a sandwhich, runing strapping at 16" OC perpendicular to the joists on the bottoms. I would thiunk it best to have done so before the gypcrete pour, but doing it now can't hurt a thing.
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"I have built houses with a cut specified in the top chord directly over the beam. When I asked why, the explanation given was to prevent vibrations. I don'tunderstand the theory why so I won't atempt to explain it..."
I've specified that on floor trusses, but have never seen an I-joist manufacturer recommend or allow it.
The theory is that if you have a "loaded" room on one side of a house, (such as a kitchen) that when the floor members deflect, it will sort of "pick up" on the part of the floor member on the other side of the center beam.
If the other side is an empty toom, then it becomes sort of "tight", like the top of a drum. That means that the floor can sometimes vibrate (or at least seem to) when it normally wouldn't.
If that doesn't make sense let me know and I'll try to explain it another way.
Bumpersticker: Honk if you love peace and quiet.
intuitively, that makes sense edit - you question whether I joist makers would approve it.I mentioned it as a possible solution, rather than a definite suggrestion. He should run it by the engineer in charge and the manufacturer before inplementing that one.
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Edited 8/12/2009 5:10 pm ET by Piffin
It would seem to make sense that cutting the top flange of I-joists would help in some situations. I think it's an idea worth considering. But - With I-joists you have other issues, like vertical shear at bearings. That isn't a problem with floor trusses of 2X joists. So I think it would need to be run by the manufacturer. I'm honestly more concerned with what they think than an engineer on a project.
If you lend someone $20, and never see that person again, it was probably worth it.
A lot of engineeres only know deflection and have not beenaware of the problems of vibration
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It is up to the engineer to explain why his specification did not seem to produce the desired results. That's why they are licensed and you pay professional fees. I'd press him again, humbly and politely, to keep digging until he has an answer.
Having said that, such a long span can feel bouncy even when it is within the deflection spec. A spec of L/960 means the middle of the span will deflect 1/960th of the span when fully loaded. That works out to 3/16". I realize that running around on the floor isn't fully loading it, but the actual deflection is probably quite a bit less than it feels like. You could measure it from underneath. Put some blocks on the ground and measure how the gap changes when people stand in the center vs. staying off. It might be that the deflection only feels bad.
When there is more mass on the floor it will have a greater constant deflection but less bounciness as the load changes. So it will feel less bouncy when the tile is installed.
Load and deflection tables are accurate, so if the engineer did not make a simple dumb mistake, the floor is probably within spec.
Blocking can reduce bounciness a little bit. It distributes the load slightly to the adjoining joists.
Greg,
I think I would run a strongback on the bottom of the joist at mid span. 2x4 on the flat ,2x8 upright. You could even post down from this if you wanted more support.
KK
Keep in mind that what is needed for structural integrity is far less, in many cases, than what is needed to limit "bounce". Standard span tables size things to limit "bounce" to what is comfortable for normal adults walking, eg, but don't really account for kids being rambunctious.
In the case of tile, the important thing is not the total amount of "bounce" in the middle of the floor, but the amount of bending that is encountered in any relatively small section of floor (across, say, a 4-foot distance). One could in theory have several inches of "bounce" (with, say, a heavy-set adult walking across) on a large floor while still staying below the tile-suitable limits for "flex" across a small distance.
Which is to say, it may just be a perception thing, and once your kid gets used to the house maybe he'll confine himself to bouncing off the walls like a normal kid.
"Standard span tables size things to limit "bounce" to what is comfortable for normal adults walking, eg,"That is bull. They don't adress bounce at all. They deal with loading and deflection.Bounce and vibration are separate issues from anything codes deal with
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Deflection and bounce are related.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
"Standard span tables size things to limit "bounce" to what is comfortable for normal adults walking..."
I missed that the first time through. But Piffin is right - Span tables do NOT take vibration into account - Only bending and deflection.
The history of liberty is a history of resistance. The history of liberty is a history of limitations of governmental power, not the increase of it. [ Woodrow Wilson, 1912]
I wasn't talking about vibration -- "bounce" is deflection.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Nope - They're completely different animals. Deflection is a prediction of how much something will sag with a given load on it. Vibration is the frequency it will vibrate at after an impact load. They are LOOSELY related. But they're not the same thing.
The strong take from the weak, but the smart take from the strong. [Emerson]
I DIDN'T SAY "VIBRATION". I SAID "BOUNCE". TWO ENTIRELY DIFFERENT THINGS.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Vibration and bounce are the same thing in terms of floor design. Deflection is something else entirely.
Do not walk behind me, for I may not lead. Do not walk ahead of me, for I may not follow. Do not walk beside me, either. Just leave me the heck alone.
Well, we disagree.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Well, if you will get it right, you won't be in disagreement any more
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I dealt with this as part of my work for 20+ years. I also did quite a bit of reading on the issue of floor vibration. So I don't agree that it's a difference of opinion. You're terminology is simply wrong.
Committee - a group that keeps minutes and wastes hours.
Adding weight to the floor causes deflection. Jumping up and down causes more deflection, because acceleration "looks" like weight. Yes, resonance is also a factor, but generally not so much at "jumping up and down" frequencies -- resonance will tend to be at higher frequencies.Also, sudden deflection is more disconcerting to the person standing on the floor than is gradual deflection (which generally isn't detected at all).My point is that "usual practice" is to stiffen a floor sufficiently to avoid that disconcerting feeling (under "normal circumstances"), and this "usual practice" is embodied in the code.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
My point is that the terms I'm using are the industry standards, and they make sense.
Do not regret growing older. It is a privilege denied to many.
How does the industry define "bounce"?
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Is there any x bracing? I always like to add it at mid span on anything more than 16'. I know you are less, but it may help. Better to put it down before the sub floor, but no impossible.
Greg - we hear a lot about this with deeper I-joists.
The manufacturers say you don't need midspan bracing ... but it helps and I always specify it beyond a clear span of 12 feet.
You want to be careful with nailing into I-joists if they have laminated top and bottom chords - you can split/delaminate the chords with side-applied fasteners, so check with BCI for appropriate fastening points. The web is generally quite nailable.
Suggest you do the following: at midspan pack out flush to the chord with plywood (if necessary) and block solidly (not X pattern) between joists. Then run a 2' width of 3/4" plywood across the joist bottoms, glue/nailed or glue/screwed into the blocking and joist chords.
Good luck with the gypcrete/tile combination ... I've heard horror stories (and don't use Gypcrete any more).
Jeff
Gents,
Thanks all for the replies. It makes sense that it is possible to have a floor within designed spec, but still deflect when you have long spans. I haven't tried to measure the deflection in the floor, but I'd bet it is within the L/960 that the engineer used to design it. It just seems bad with my son running on it. In this sense the engineer is correct.
I think I will try the strongback/bracing approach at mid span. It sound doable without too much difficulty. As long as I don't spit the flanges on the I-joists, or nail up thru the OSb into the radiant heating tubes inside the gypcrete. Thank you for all the replies and suggestions.
Cheers,
Greg
Greg,Strongback goes on the bottom of the joist. Nails should be nowhere near any tubes or gypcrete. 2x4 nails on the bottom of the joist on the flat. 2x8 nails on the edge of the 2x4. Long lengths, no cuts like for blocking. Wish I knew how to draw a pic.KK
Coonass,
Thanks for the clarification email. I guess I garbled what my thoughts were in my last post. I hadn't really decided whether to use blocking between the joint bays or a strong back. I'll do one of the two. I do get what you are talking about when recommending a strongback. It has the advantage of no side nailing on the I-joint flanges, or near to the heat tubes like blocking would require. Also, I coud try the strongback and see if it makes a difference. If no benefit, I could still try blocking. That is probably the most straightforward way to go. Thanks for the clarification, and I think I have a plan forward now.
Cheers,
Greg
Are you sure the engineer used L/960 for his calcs? Standard is L/360, which would give you 1/2" of deflection in the joist--enough to feel quite bouncy. I usually use L/480. BossHog is really the expert on this though, and like him I am curious about the size and post spacing of your center carrying beam. There is usually a LOT more weight on them than people realize, and could even be a safety hazard if undersized.
Mike,
I can't recall what the depth of the glu lam was and am not near the house right now. I will check it later today and get back to you with lam size and support post spacing. I am not a structural engineer, but to my uncalibrated eye ball it looks pretty stout.
Cheers,
Greg
I understood him to say that it ended up being about L/960 - Not that it was specifically designed for that.
If at first you don't succeed, destroy all the evidence.
I have been following this thread and out of curiosity i looked up the span tables on line. In the tables a BCI 60 14 inches deep can span 21 feet with a deflection criteria of L/480. it appears that the engineer over designed this floor and conventional wisdom is a stronger floor is less bouncy.
i am wondering if this is a "Vibration" problem as posed by some one earlier in this thread? 15 foot span for the joists is not that long. It also sound strange that is bounces when the is gypcrete on it. You would think that the dead load of the gypcrete would deaden the bounce?
Keep in mind that the calculated deflections that have been discussed are at maximum floor load ie the entire floor loaded to 50 pounds a square foot. in an empty house there cannot be anywhere near that load on the floor. No way that the floor should be deflecting at all with no load.
Sorry to say i have no idea what is causing this but it does not appear to be under engineered.
I AGREE - IT IS DEFEINITELY NOT UNDER-ENGINEEREDsorry the capsit is some sort of unique situation to this house. Like everyone else, I want to know the beam situation, but I also suspect the overbeam extension thing I mentioned
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Good points. L480 with a 20# live load or with a 50# live load are going to be two different results too
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I use 40# live load plus 10# dead load in most situations, but the gypcrete might add more weight.
You might be onto something with the continuous span idea. The floor system is already stiff, as another poster mentioned, and a continuous span makes it even stiffer. Think of it like a cantilever; the additional stiffness created by the continuous span is calculated the same way.
One of the things Boss Hog talks about in his vibration thread (IIRC) is that vibration is more of a concern with engineered materials than with natural materials. Every material has its own frequency, that you can activate like finding a harmonic note on a guitar. With sawn lumber, one joist's frequency will be something like 40htz, the next one is 50htz, then a dense one will be 100htz. With engineered lumber, the hertz will all be very similar. That's why the little kid running across the floor will activate the harmonic. If the kid ran more slowly, or weighed a bit more, he wouldn't "sing" with the floor system.
"One of the things Boss Hog talks about in his vibration thread (IIRC) is that vibration is more of a concern with engineered materials than with natural materials. "
Not exactly.
I do think that vibration is more of a problem with engineered materials. But only because people seem to think that floor trusses and I-joists can do a lot more than they are designed to do. So they tend to push the limits and end up having problems.
I am not crazy. I have just been in a very bad mood most of my life.
For this type floor, I'd be figuring 15#dead and 50# live. L480
That what I used for a game room with pool table 28'span. No complaints. That was opemn web trusses - I think they were about 16-18" deep. Must've jumped the track in my long term memoryI normally use 15/40 most of the timeI know some people get away with 10/20 in bedrooms, but that light scares me
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The prescriptive span tables I usually use give the option of 10# or 20# dead load, and 30# or 40# live load. I like the idea of going even heavier with the live load though. Nothing like walking by cupboards and having teacups rattle. I don't like 30# for bedroom either.
Let me ask this silly question. With the house empty and no carpet or drapes, does it have a cave like echo when your boy runs around? Could that give you the impression of "bouncing" when it fact it is stiff? Keep in mind, I'm just asking. I don't mean to affront since you're there and I'm not, but I've noticed myself in unfurnished and unfinished houses that they seem less solid.
MG,
You are right, the empty house is noticeably more echo'y than it would be with carpet, drapes, furnature, etc, but there is a definate bounce to the floor. It is not the echo factor from screaming kids that is the problem. There is a noticable deflection to the floor when my kid runs on it. It seemed plenty solid when I was laying out and framing the exterior and partition walls on the sub floor, so I was kind of surprised that my son could make it bounce when he runs on it.
Since it only bounces when my son runs on it. (Little kid, maybe 50 lbs) I am begining to think it might be the vibration issue that some have suggested. Maybe when he runs, it is just the right vibration to make the floor bounce and carpet and furnature will make the problem go away.
I am right in the middle of installing some cultured stone on the exterior, but just to be safe, when I get that project wrapped up will try to firm up the floor with one of the suggestions presented here.
Thanks,
Greg
strongback or strapping solution either one can be done below the i-joists so no worries about hitting your radiant tubing
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Hopefully, a china cabinet will not become part of this discussion.
Work for the greatest vital intensity - the greatest solidity and aesthetic reality. Finally, eleminate everything non-essential. Reduce to the absolute essence. ~ F.C. Trucksess
GreginAlaska:
I'll be the contrarian here. You've said "...It doesn't seem right...".
You're comparing a seat-of-the-pants feeling from the rompings of a seven-year-old to a scientifically proven and verifiable span table. If I were your engineer, I would tell you to chill then I would start getting irritated.
You haven't lost anything yet. Beef up your floor some more, it will make you feel better. But let's not get feelings mixed up with the fact that your floor is over-engineered and will easily perform flawlessly.
Have a housewarming party and we'll try to fill your floor with drunken Breaktimers. Let your engineer calculate that.
Kowboy
It all depends on whether he asked the engineer to design for strength or for a floor system that doesn't vibrate.Sure, it may have more than enough strength to hold the load, but that doesn't mean it performs well.I've never fallen through a trampoline, but I wouldn't want to tile one...
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Greg, This is not my area of expertise but the experts here are missing the obvious.
There is no doubt that a dose of "cialis" would stiffen it right up. :< ] Good luck!
I dont believe there is anything wrong with your floor design. The vibration you are concerned about will go away once the gypcrete and interior framing/flooring/funiture.. are in place. These items effectively dampen the vibration that you are seeing right now. Unless you are planning to have a fitness gym or a dance parlor in this area (repetitve motion in a large open space), I cannot forsee any problems. If you want, replace the gypcrete with concrete. It is margninally heavier (25psf vs 19psf if 1.5" thick) and will damp the assembly a bit better. Given the engineered design employed in the floor I highly doubt there will be a problem with any other parts to support the added weight, but please check.
The only way I could see that this is "bounce" (aka deflection) is if the joists are not seated properly on one of the supporting elements. The design loads required by code are normally very conservative. Lets say for example your home is 1500sq ft. That means you should not see much in the way of deflection until there is 60,000lb of live load plus another 22,500lb of dead load. That is close to a loaded tandem gravel truck. Do you think you have that much in there now?
Since continuous joists (30' rather than two 15') were used, you might want to double check with the engineer that you dont need any bracing near the glulam. The compression force in the chord (I joist flange) moves from top to bottom near a mid support in a continuous span. If the chord is not large enough, it may need some bracing there, but I would hope the engineer took this into account.
Brad