We’ve talked here several times about floor vibration, deflection, and such. I generally comment on any discussions about floors, but have bever really explained why my thinking has evolved as it has.
Since I started in the truss business in 1984, I’ve been involved in designing a lot of floor systems. While most went well during those first few years, we occasionally had complaints about “bouncy” floors. I always followed conventional wisdom about floor design – L/360 max deflection and such. Anyone having problems was basically told “tough luck”, as no one knew better.
In the 1990s, Virginia Tech did some research on floor vibration. I ended up with a copy of their research paper, and slowly began to understand more about floor vibration and how it works.
Their research attempted to define what kind of floor vibration was annoying and what to do to prevent it. They came to the conclusion that a floor vibrating at 15 cycles per second (Hz)or faster was generally percieved to be acceptable. A flooor vibrating between 12 and 15 Hz was marginal. Anything below 12 Hz was unacceptable.
Then they came up with formulas to predict the vibration speed of different floor systems – Conventional joists, I-joists, and floor trusses. They more or less show that changing spacing does little to change the vibration charachteristsics of a floor, while changing the depth makes a lot of difference.
Maybe it would help to try an example. Let’s say a guy comes in and wants a 28′ clear span floor. He wants to keep the height down, though, and wants floor trusses as shallow as possible. We start out with a 18″ deep floor truss 19.2″ O.C., which works according to the span charts. I check it with the vibration formula and it comes up at 11.3 Hz – in the marginal range. The customer then asks how about 16″ O.C.? This only increases the Hz to 11.8. What about 12″ O.C.? That changes it to 12.4 Hz. I suggest increasing the depth to 22″ deep This will change the floor to 14.1Hz – A dramatic increase, and pretty close to acceptable.
It might help to think of strength and stiffness as 2 separate issues. For instance – A 1″ thick layer of ice on a pond might be very stiff, but it isn’t very strong. A tightrope walker’s cable might be very strong, but isn’t very stiff. Strength and stiffness in floor design are only loosely related.
I’ve never had a customer who followed my advice who was unhappy with their floor performance since I started paying attention to this.
If you don’t have the math skills to crunch these numbers, (I cheat and use a spreadsheet)there’s a simple way to insure performance. Any floor member that designs out at 24″ O.C. will have a fairly high Hz rating. Take an 11 7/8″ I-joist – According to the span charts, they will clear span 16′ 3″ at 24″ O.C. But use them at 16″ O.C., and you can clear span 20′, which will almost guarantee you an unccceptable floor. Use them at 12″ O.C. over multiple spans, and they’ll go 23′. (That’s absolutely nuts) So I’d suggest using a depth that designs out at 24″ O.C., then set them on whatever spacing you want.
I still have a copy of the vibration research that I could fax if any of you are particularly interested. Lotsa pages, though, and I can’t guarantee it would be readable. Email me at [email protected] if you’re interested. I could also email you the spreadsheet I use for wood webbed floors if any of you want to play with it.
Vote Democrat it’s easier than working
Replies
Thanks, Good little article,BH
Hope you don't mind if I innoculate it with a little levity (pun intended).
How do they measurer the Hz? Something practicle for on the job grunts like counting the ripples in a glass of wine while someone walks or dances across the floor? O rmaybe see if the flickering of light from a pole lamp matches thatr of a candle on a breezy day?
back to serious - does the paper adress whether loading from furniture and appliances dampens the vibrations measurably?
I really like your Ice/tightrope illustration. I'll try to remember it for clients. When remodeling, some of them are looking for so much space that doesn't exist that I've got to use floor systems that are minus 2" in thickness. ;>)
I have no idea how they measured the Hz, or how they came up with the math to predict it. Anyone out there know?
As far as I know they went to houses that were under construction and took the measurements. So there wouldn't have been any furniture to dampen things.
Today I dialed a wrong number... The other person said, "Hello?"
I said, "Hello, could I speak to Joey?"...
They said, "Uh... I don't think so... he's only 2 months old."
I said, "I'll hold."
Boss
They would use an accelerometer to measure the actual movement. The could have used a frequency counter to get the rate. But they probably used an oscilloscope (scope) at least at first to see the type of signals that they where getting.
And they should have had some kind of test protocol such as standing still in the center of the room and dropping a fixed weight from a given height.
Was there any discussion about the use of thicker sub-flooring glued/screwed and/or REAL STRUCTURAL CROSS BRACING.
<i>"Was there any discussion about the use of thicker sub-flooring glued/screwed and/or REAL STRUCTURAL CROSS BRACING."</i>
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I don't think they took any of the above into account, but I'm not sure. I've asked a few questions about the research from time to time but haven't gotten a lot of answers. I suppose the students who did the thesis have moved on, and may be flipping hamburgers at McDonald's.........(-:
A skeleton walked into a bar. "What'll it be?", asked the bartender. "I'll have a beer and a mop,"
Boss,
Thanks for the tutorial. It makes a lot of sense once somebody explains it.
Is DESIGNING TO REDUCE FLOOR VIBRATIONS IN WOOD FLOORS by J. D. Dolan
the paper you're referring to? It's available in PDF at http://www.alpeng.com/pdf/dolan.pdf
Yup, that's exactly what I was talking about. But the original paper had a lot more detail and more examples. Apparently this is a summary of the research, I guess.
Those who live by the sword get shot by those who don't.
Could you possibly post the title, author(s), date, etc. on that original paper?
Location of the citation to "Designing to Reduce Floor Vibrations in Wood Floors" by J. D. Dolan has changed. The new URL, as of 23 Feb 2006, is: http://www.alpeng.com/upload/19272/dolan.pdf
Edited 2/24/2006 12:49 am ET by RickG
Very interesting -- Back in the days of silent movies and big theaters with big pipe organs, some of them put in extra big pipes under the floor that were sized for 8 Hz, well below the nominal 20 Hz bottom end of human hearing. These were only used for horror movies, to punch up the scary parts.
-- J.S.
Very interesting indeed. The vibrational frequency
will depend on the mass per unit area and the stiffness.
When you reduce the number of joists, the additional
depth required is minor. There is an overall weight
reduction because you have fewer, but only slightly heavier,
joists. The vibrational frequency goes up, which is
desirable. If you simply add joists of the same depth,
you add both mass AND stiffness, which tends to keep the
vibration frequency the same. You get little benefit from
increasing the number of joists.
This is not obvious, but if you think about it makes a lot
of sense. I always thought that construction with joists
24" OC was sort of a cheap way out. Looks to me like its
actually the better approach. I don't think the deck flexes
much more versus 16 OC. Saves materials, saves time, makes
plumbing and electrical that much easier.
Now, I wonder why they make engineered joists symmetrical?
The top chord has the decking to reinforce it. If you made
the bottom chord wider and the top thinner, it would be much
more stiff with the same total amount of material and the same depth.
> Now, I wonder why they make engineered joists symmetrical?
Perhaps it's because anything that has a right side that should go up can also be put in upside down.... ;-)
-- J.S.
Well, real lumber joists have a crown.
Trusses have an up and a down side.
Nobody ever installs those upside down.
"Trusses have an up and a down side.
Nobody ever installs those upside down."
Wanna bet? It's happened to me at least 3 times that come to mind. There were probably many others I don't know about.
Maybe ignorance really IS bliss..............
Edited 6/14/2002 6:56:47 PM ET by Boss Hog
The I-joists are symmetrical because it is the most efficient shape for bending moment capacity, which is the primary function. When the bending capacity is not pushed to the limit, and the subfloor is properly fastened, deflection and bounce are reduced to acceptable levels....that's not a mistake, it's rustic
Thanks a lot for the information. I fully understand every word,i.e. when ordring floor systems call the expert and hope that person is as professional and as knowlologable as Boss Hog.
P.S. I vote Republican. That will assure me high cost for materials and low pay.
Again thanks for the information.
Charlie
You measure the vibration with a digital FFT analyzer. The accelerometer generates a complex waveform in response to the vibration present. The analyzer performs an FFT (fast fourier transform) on the waveform and produces a "spectrum" which is simply a plot (graph) of vibration amplitude versus frequency. For a floor, you do an impact test. You attach the accelerometer at the desired location (usually mid-span) and set up the meter to "trigger" on a certain vibration amplitude. Then you bump the floor and the meter records the vibration.
The floor will vibrate at its natural frequency, much like a tuning fork. This natural frequency is mostly a function of stiffness. To increase stiffness you increase depth, not mass or width. I'm pretty sure a 1x6 (on edge) is stiffer than a 2x4, even though it has 21% less mass (never tested this though).
Everything has a natural ("resonant") frequency, including your internal organs. Your organs are really just spring/mass systems where the springs are the connective tissue. I suspect that the low frequencies are bothersome because they coincide with the resonant frequencies of our internal organs.
Also, as the frequency of vibration increases, the amplitude decreases and it dies out more quickly, so it's not as noticeable.
You may have guessed I do this for a living..... and yes, I am an engineer (mechanical)..... but not really a "typical" engineer I don't think.
You've aroused my curiosity, so I guess I'll have to test the large expanse of unfinished living room floor I can see from where I'm sitting. I'll post some plots of the results. You'll find it interesting I think. Never be afraid to buy the best -- you'll always be happy with it!
Boss,
I have two questions for you regarding the spreadsheet that I have from you to determine the amount of vibration in a floor. I don't know if you emailed it to me or posted it somewhere, but I have it saved.
My floor is according to the specs in the attached .xls file. I have some vibration that I'm trying to deal with. My strategy is to apply 2x4 furring strips at the 1/3 and 2/3 span points, glued and nailed to the bottom flange of the I joists. I'm pretty sure this will work, but discussion with our very own David M. over at JLC brought a few questions to my mind.
1) In the attached worksheet, should I include the weight of the drywall on the underside of the joists (assuming it's finished)? There is a place for the weight of the subfloor, but nothing for ceiling finish below.
2) Are the outcomes of the spreadsheet predicated on having a solid drywall ceiling attached to the underside of the joists? I know that torsional vibration can make stiff joists seem "bouncy", so I thought that maybe one of the assumptions of the spreadsheet is that the joists are tied together at top and bottom.
Thanks for your time.
Jon Blakemore
RappahannockINC.com Fredericksburg, VA
Several thoughts come to mind. I assume your I-joists are 11 7/8" deep? I don't like to span them over 18' in general. What do the I-joists sit on? If they sit on a beam, the beam(s) can contribute to vibration. You asked about including the weight of the drywall in the vibration formula. My take on the situation is that you don't do that. As best as I can remember, the research was all done in unoccupied homes that were under construction. So there was no carpet, furniture' or drywall in the houses. I'm not sure I understand something - You ask about drywall weight on the underside, but you're talking about adding 2X4 strapping?If the I-joists have drywall on them, strapping won't do anything. If there's no drywall and you DO put strapping on, I would just screw it on. That way you can always take it off easily if it doesn't work. Do you know the brand name and series of I-joists that you have?That's all I can think of at the moment...
Avoid cliches like the plague.
"I assume your I-joists are 11 7/8" deep?"
Yes, 11-7/8" Georgia Pacific GPI40 series.
"What do the I-joists sit on?"
2x6 stud walls on concrete at both end points.
"I'm not sure I understand something - You ask about drywall weight on the underside, but you're talking about adding 2X4 strapping?"
I'm planning on adding stapping to my joists but asking about the drywall to see if I'm using the calculator correctly.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
If the I-hoists sit on stud walls, the strapping is probably your best bet. I wouldn't guarantee that it will work, but it's your best shot. Would you let me know how it turns out, just for the sake of my own curiousity?
If we do not succeed, we run the risk of failure
Will do.
Jon Blakemore RappahannockINC.com Fredericksburg, VA