huangyhg

bouncy floors
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
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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).
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?
unanticipated cracks and ill conceived sawcuts can and do affect the floor stiffness and therefore vibration. if i re
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.
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.
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.
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...
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.
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.
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.

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