Bouncy Floors
Bouncy Floors
(OP)
I have a small problem. I've been involved with a project that has a small vibration problem with floor. The floor consists of a 3" normal weight concrete (total deck thickness) w/1.0c 26 Ga. deck over 28k7 joists spaced 2'-0" o.c. spanning 38'-0". Half of the building has been built and in some areas the floor is bouncier than others (obviously to do with product location, interior partitions and so forth). We are in construction of the second phase that this point. Is there anything at this point in time that can be done to reduce the vibration? If something is done is it going to be worth it for what money will be stuck into it? I personally don't think the floors are bad at all but I can't convince the owner of this.
P.S. The floors were checked for vibrations using the AISC design guide 11 and found to be satisfactory.
Thanks for your help
P.S. The floors were checked for vibrations using the AISC design guide 11 and found to be satisfactory.
Thanks for your help






RE: Bouncy Floors
There is not much you can do at this point other than to re-evaluate it after all the finish material is installed. Make sure that your assumption of finishing and damping matches what was actually done in the field.
I have noticed small things affecting vibration, like sprinkler piping running parallel and hung from a joist or sawcuts that the GC arbitarily decided to put in the structural floor or cracks (this can reduce the floor stiffness). Other people on the floor also can act as shock asborbers which may affect the vibration (although I wouldn't count on that).
RE: Bouncy Floors
I totally agree with everything you said. I'm going to double check my calcs and make sure my damping assumptions actually match.
AISC design guide gives some options of stiffening the system by possibly adding additional steel to the bar joist. I just don't the performance will be there thus creating even a more upset owner.
You do bring up a good point. During construction it was brought to my attention that some cracking had occured in the concrete. Probably due to the fact that the wire mesh was put in the wrong spot. The original design was a non-composite deck so cracking wasn't an issue. The deck could support the loads alone. But the vibration check assumes they are compsite. So this would definitely have an impact on my floor system.
Is my thinking right?
RE: Bouncy Floors
I am generally not shy about telling this to the Owner or Contractor.
RE: Bouncy Floors
Also for joists you are supposed to reduce the moment of inertia per section 3.6 of the design guide due to the effect of the joist seats. Was this done in the original analysis?
Typically when we do an all joist floor such as this we double up the joists at the column lines. This helps mitigate the propogation of vibrations throughout the system and is very effective.
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Thanks for the help everyone.
RE: Bouncy Floors
What spans are the joists? I would place some dead load in the area such as filing cabinets. This will help dampen the vibrations in some cases. There are several things that can be used to dampen floor vibrations. Some are very expensive!
I always use 4 inch slab with 1.0 floor deck.
Regards,
Lutfi
RE: Bouncy Floors
RE: Bouncy Floors
I did a quick calculation using SJI vibration guide with the following parameters:
--Slab total depth is 3.0 inches
--Concrete strength = 3.0 ksi
--Deck depth = 0.5625 inches
--Joist span = 38 feet
--Joist size 28K7
--Joist spacing = 24 inches on center (assumed based on 9/16 deck)
--Design live load = 60 psf (assumed)
--Additional dead load = 10 psf
--Percent live load acting = 10% (this one is a wild guess)
Here are the results based on SJI digest #5:
Frequency = 5.26 cps
Heel drop results in amplitude of 0.0063 inches that falls within slightly perceptible range/curve
Impactor amplitude computes 0.0036 inches that falls just below the slightly perceptible range/curve.
Of course if any of the above parameters change, then the results understandably will as well.
Regards,
Lutfi
RE: Bouncy Floors
Regards,
Lutfi
RE: Bouncy Floors
I guess I make this a learning experience. From now on I'm not going to hesitate to use a 4" slab.
I guess I've alwaysed designed using a non-composite deck and bar joists for 4 story buildings and less. Should I be using girders and going to a composite deck? Say spacing my girders at 4 or 5 feet. What are the advantages? For office space isn't it hard to get the deck to work for a 2000# load that has to be checked if the girders are spaced at 4 or 5 feet o.c.?
RE: Bouncy Floors
I heard that placing “balloons”, or similar containers, with water have good effect on dampening he vibrations because it emulates human body, which is full of water! I think this was mentioned at one of Dr. Tom Murray’s lectures.
I used to disregard user complaints till one of the CAD drafters asked me to site inform of their computer screen. The darn thing made me sea sick in less than five minutes due to vibrations that I could not feel standing up!
Good luck.
Regards,
Lutfi