Hello, even though it is my first time posting a message on this forum, I have been regularly reading postings and would like to thank all of the people who take the time to generously give invaluable advice to those who need it.
I have noticed for some time something that really bothers me with my family room floor and was hoping that someone on this forum may have an answer for me.
My house is about ten years old and I have build it myself and by subcontracting those things that I could not do. A couple of years later, while sitting on a couch in the middle of the family room floor, I noticed that I would feel the vibrations every time someone walked across the room. The floor consists of carpet over 3/4 in. plywood nailed and glued to 2x12s douglas fir joists spaced 12 o.c. The span is 22 ft.
Recently I became more and more annoyed by the problem and thought I may have a problem with the deflection of the joists, so I measured it. Tied a string to the bottom of the joist streched over two nails placed at each end of the joist. The distance between the string and the bottom of the joist at each end was 1/2 in. With no additional weight, except for the furniture that's in the room, the distance between the bottom of the joist and the string in the center of the span was 5/16. Then I asked some friends to stand close together right on the center of the joist. Their combined weight was 510 lbs. With my friends on the joist the distance between the bottom of it and the string at the center was 3/16.
This would indicate that deflection is within acceptable limits, so I cannot explain why I would experience the vibration mentioned above. Does anybody have an explanation as to what the problem may be?
Even at 12 inches on center and with using #2 Douglas Fir, the span exceeds the acceptable live load capacity for this species and grade of lumber. Maximum span for 2x12 #2 Doug Fir is 20' 7" presuming a dead load of 10psf and only 18'10" presuming a dead load of 20psf. Your joists exceed your maximum spans by 2-4 feet presuming #2 grade lumber.
Any framing table or building code will confirm this. See here:
(Use 40PSF Live load/10PSF Dead Load for living room floors in the tool above.)
Unless you used select grade Doug Fir (which is unlikely because of the bounce and your wallet) the size of lumber and grade is likely too small for the span.
Even #1 Doug Fir can only barely meet the 22' span when the dead load is 10psf or less.
Not even sure why this passed framing inspection because the floor is undersized.
Should have used engineered I joists or decreased the span with center beams.
You could have also reduced the span ability of the joists if you bored or notched the joist anywhere in the middle 1/3 of the span or notched or bored the joists larger than allowable in any other portion of the span.
To correct the problem you could either install a beam midway under the span, sister another joist to the existing ones, add additional joists at 6" on center or all of the above.
Bottom line is the joist are too small for the span.
Modulus of Elasticity is the amount a material will bend under pressure or weight. Most materials have a consistent modulus of elasticity in every direction but some, like wood, because of its grain, do not.
The amount of this modulus of elssticty will vary with the physical characteristics of each species of lumber and even vary with grades of lumber.
The modulus is established by scientific experiment and fixed for grades and species of lumber.
Deflection is generally expressed as a ratio of the length of the span divided by a constant such as L/Delta where L=length or span and Delta=400 or 600.
For purposes of Building Code Tables, the deflection is measured not only by known moduli of elasticity for known species, but also their known dead loads, or the weight of the material itself.
To that is added the calculated live load or the weight of all people, furniture, and objects on the material.
For Building Code Purposes, the deflection is measured as L/360.
At 22 feet, or 22/360, the deflection is measured at .0611 feet or .7333 inches or about 5/8ths of an inch.
Using L/600, however, the deflection becomes 7/16ths inch or closer to the 5/16ths inch you observed.
Although your unscientific 'test' may have only resulted in an observed deflection of a little over 1/4", this does not replicate the scientifically established and allowable code spans and deflections by species and grade.
Your measurements may have been off simply from the wood being "crowned" or the fact that the plywood connects it to other joists..
The bottom line is your spans are too greate for the lumber and grade and species.
Hope this helps. All errors are my own.
This message has been edited. Last edited by: homebild,
Thanks, Homebild. I thought about installing a beam in the middle of the span. Could 3 2x12 laminated beams be used to cover the length of the room (24 ft.) without any support in the middle? I would like to avoid having any columns in the room below.
No. You will need at least 2 columns and each column must be set onto a footer under your concrete basement slab at least 30"x30"x30".
This means you will need to cut out or jackhammer up and dig out these areas for the new concrete footers.
You will also need to cut pockets into your block wall to fit the beams and will need to install rebar and grout into the webs down to the wall footer to support the beam ends in most cases.
You should consult your local building code office before you do anything so you can do everything properly this time and avoid additional costly framing and structural problems.
In light of the work involved with installing a beam and of the columns that I would need to use, I think that it would make more sense to sister the joists.
The problem with sistering, as I see it, is that it would be difficult to install sister joists of the same size as the original joists unless I were to lift up the floor, which I would want to avoid for obvious reasons. Would it work if the sister joists were one size smaller than the original and nailed to the top part of the originals? For example, I could use 2x10 or 2x8 covering the whole span, placed right underneath the plywood making up the subfloor and nailed every so often to the face of the original joists.
No. You because joist of smaller dimension will have even less ability to span the distance and help with carrying the loads.
You will either need to install joist of similar size and material, or install the center beam (your best option) or install additional joists at 6" on center.
You really need to consult your code office here for an approved repair and/or a structural engineer or architect who can properly size your repair in light of your loads.
If you have a second story in the house or even a ranch home with a center wall bearing on the first floor, your only solution is the center beam in basement.
Nino0- You are getting some seriously bad advice here. I haven't run the numbers to check on the strength of the existing joists (though 22' does seem awfully far for a 2x12 to span), but just ensuring proper strength will not ensure satisfactory vibrational performance of your floor system. This is a COMPLETELY separate and unique serviceability check in which the deflection of the joists enter into the calculations, but are not even close to the sole contributor. The idea to control vibrations is two-fold. First, you want to ensure that the system will not resonate with any forcing frequency (e.g. someone walking or running across the room). This can result in larger than expected deflections from static forces and cause strength problems. This is done by making sure the natural frequency of the members is far greater than the forcing frequency. Second, you need to ensure that the acceleration is below a certain level. As the floor deflects under that footfall (someone walking on the floor) it springs back to shape when the force is removed. That springing back into shape is what causes the acceleration of the member. This is what you are experiencing. There is no strength check that can mitigate this. What I would suggest is sistering the existing joists with small steel channels. This will greatly increase the stiffness of your floor system and reduce (if not eliminate) the vibrations you are feeling, while allowing you to keep your space open below.
Sorry I can't give better advice, but without laying eyes on it, it's kind of hard to. Just to be clear, though, any advice that recommends mitigating your vibration issues through strength means alone is errant (in a big way).
Structural Engineer
Posts: 31 | Location: Philadelphia, PA | Registered: 20 July 2008
