Bouncy Floors

[wale01]

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

 

[jike]

If you have checked it by the AISC guide, then you have met the standard of care. The problem is that vibration perception is subject to personal sensitivities and therefore one person may say it is acceptable and another may say the opposite.

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).

 

[wale01]

Thanks for the help jike.

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?

 

[jike]

Unanticipated cracks and ill conceived sawcuts can and do affect the floor stiffness and therefore vibration. If I remember correctly, I read this in the AISC design guide.

I am generally not shy about telling this to the Owner or Contractor.

 

[WillisV]

wale01 - Note that for vibrations the definition of composite is different than what we as structural engineers typically think of as "composite" (where different materials are tied together strongly enough to resist design loads, deflections etc.) For vibrations even the friction between the concrete and non-composite metal deck surfaces is enough to be considered "composite" for checking vibrations. So a little slab cracking would not affect that assumption.

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.

 

[wale01]

WillisV, I did use the reduced moment of inertia per section 3.6. I would tend to agree with jike that cracking or control joints would have an effect on the system. I look at it like this; If the joist and deck are assumed to act compositely (if thats a real word) then the concrete deck would be in compression. In order to develop the concrete in compression those cracks would have to close. Thus creating excess deflection before the concrete can fully develop in compression.

 

[wale01]

One other thing I might do is go to a 9/16" deck versus a 1" deck to gain more concrete and stiffen my system a bit.

 

[sundale]

As a retrofit repair for such issues, one of my old employers used to weld additional small hot rolled angles or A706 rebar x-bracing between adjacent bar joists to act as bridging rows. To reduce perceived vibrations, he said that the "best" solution was to install these discontinous bridging x-braces in a random manner...

 

[WillisV]

wale01 - I am familiar with the research and assumptions that went into the creation of the criteria presented in Design Guide 11 and can assure you that some cracking in the slab - even fairly severe cracking - has no noticeable affect on the overall vibration characteristics of the slab. I would be very reluctant to pin the problem on the composite versus non-composite assumption.

 

[archeng59]

I always use a 4" conc slab on 1" deck. the extra inch appears to have helped eliminate the vibration problems I witnessed in floor systems using a 3" slab on 1" deck. Similar idea to using a 9/16" deck.

 

[wale01]

WillisV-I guess I'm still not seeing your point. So your saying severe cracking has virtually no noticeable effects on vibration characteristics? So basically your saying if you pieces together concrete chunks (like a puzzle) that thats going to act the same as a slab that's properly reinforced with very little cracking. Your basically neglecting the effects of the concrete. Thats how I see it. Maybe I'm wrong. Sorry for venting. I've really been put under the gun and looking for any sort of help.

 

[WillisV]

wale01 - based on actual testing of slabs that have exibited cracking, yes I am saying that a "reasonable" amount of cracking has no noticeable effect. Cracks do cause the system frequency to decrease however as odd as it may seem they also slightly increase the damping of the system (the cracks inhibit vibrations from propagating through the system). These behaviours taken together tend to negate each other.

 

[wale01]

WillisV-thanks, I finally got that through my thick skull. I think my best bet at this time is to go to a 9/16" deck vs. 1" deck. I'm also going to add an additional joist at each column line to help mitigate the vibration through the floor as you pointed out.

Thanks for the help everyone.

 

[Lutfi]

I agree with willisV. I have taken several seminars with Dr. Tom Murray. He is key author of AISC design guide. Some cracking has no impact on the vibrations.

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

 

[wale01]

Lutfi- I have 28K7 joists spanning 38'-0" with a 3 inch slab with a 1.0 deck. I think by going to a 9/16" deck will help matters a little bit. I personally don't think the vibration is bad at all. The owner's rep. was a truss designer and thinks he knows everything about vibration and deflection. So he's convinced alot of people that the floors were never designed right. Its really frustrating, so any help would be greatly appreciated.

 

[Lutfi]

You have one of the worst cases. This situation depends on human sensitivity. It all varies from person to person. Did you compute the vibrations?

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

 

[Lutfi]

by the way, 9/16 deck will help some becasue it increases the dead load of slab.

Regards,
Lutfi

 

[wale01]

Lutfi-I did calc the floor vibrations and pretty much come up with what you did. Basically at the acceptable range. Usually if this were an office building I would have increased the stiffness. But this is a retail building with a few offices. So I was a little more lenient on the design. I've designed numberious buildings and ended up with results in the same range and hand virtually no problems. So I didn't hesitate to stick with my original design. I really think more rigid boundaries have a huge affect on vibration. I probably assumed too much live load since I used 15 psf. Of course where the problem is at there are no partition walls (open space) and probably the least amount of product. Which obviously makes sense.

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.?

 

[Lutfi]

Welcome to the age of paperless offices and partition free open spaces.

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