floated concrete slab

[dsmith]

hi - i'm building a new studio on the 3rd floor of a 'mill-construction' building. the existing floor consists of hardwood on top of 2x6's on edge. the floor as it is is quite boomy and to keep sound out of the control room i had planned on floating a concrete slab that would be roughly 19' x 25'.
what would be the best way to do this? i had planned on 1/2" neoprene-2x4 on there sides-rockwool-1 layer osb- tri-ply roofing material-2" concrete slab. am i missing a layer? is concrete be the best option?

thanks,
david

[sharward]

Welcome to the forum, David!
The answer depends on whether or not the existing floor structure will support that many TONS of concrete, and I'm placing all my chips on the "not" square.

If you're NOT already on the bottom floor, with earth under concrete, it's also a good chance you can NOT put enough mass to make the floated room thing work, without causing a cave-in and possible injury or death. (source)

By the way, although probably a moot point, floating concrete on Neoprene isn't as good as floating it on EPDM rubber. The lifespan of Neoprene under such load is estimated at 10 years, versus about 25 on EPDM.

[dsmith]

hey sharward, thanks for the welcome -

the building is industrial and is/was meant to hold some heavy machinery. i've attached a gif which shows the floor construction. (it's missing the fact that at 20' wide and at 25' long intervals there are steel i beams.)
they were apparently built to have a slower burn thru rate... something to do with insurance.
good point about the weight... i think that given the steel i-beams which are about 6"x12" and that the slab would span the distance between four i-beams that the weight would distribute to the structural beams anyway. (if that makes sense) ... it used to be a clothing manufacturer and was filled with hundreds of heavy compressor driven sewing machines.
not sure how to estimate the weight of the slab.

if the floor can hold the weight, i'm still wondering if i'm on the right track or if it's possibly a case of over-kill?

thanks again,
david smith

[Aaronw]

Just found this link to calculate concrete:

http://www.hi-lite-systems.com/pages/se ... o/ex3.html

[sharward]

the building is industrial and is/was meant to hold some heavy machinery. i've attached a gif which shows the floor construction.

The .GIF didn't come across too well -- it's hard to see the details.

. . . it's missing the fact that at 20' wide and at 25' long intervals there are steel i beams. . . i think that given the steel i-beams which are about 6"x12" and that the slab would span the distance between four i-beams that the weight would distribute to the structural beams anyway.

That's a critical "life or death" point. It may take an engineer -- ideally the person who engineered the building in the first place, if that's an option -- to answer these questions with authority and certainty.

. . . not sure how to estimate the weight of the slab. . .

I found numerous references to the weight of concrete just by Googling. There are some references to it here as well. Note that there are many kinds of concrete, and each has different weight characteristics.

Anyone else have any thoughts on any of this? I'm just getting the ball rolling; I don't know how to steer it!

[knightfly]

Just off the top of my head, a 2" concrete slab by itself should add about 22-25 PSF to the floor - I'm off to work so no time for anything more... Steve

[dsmith]

thanks guys -

[sorry about the gif quality - those are meant to be 2x6's pressed together.]

i checked the link Aaron posted. If i calculated it correctly it seems like it would equal approx. 31 psf. I wasn't initially concerned about it because the construction people I had come in to consult didn't seem to think it was an issue at all. I think I will need to speak with an engineer or architect as sharward suggested. I'll also try to dig up some info. about that type of floor construction and will post it if anyone is interested.
Still unclear as to which layers are necessary or unecessary if i do end up pouring a slab. OR whether i need to do it at all...
-david

[knightfly]

Here's the general idea -

http://www.johnlsayers.com/phpBB2/download.php?id=3096

and

http://www.johnlsayers.com/phpBB2/download.php?id=4001

[dsmith]

thanks a lot for those links knightfly. i've been reading through the other threads regarding floated floors, which are enormously helpful btw, and i saw that you were able to determine the mass-air-mass resonance of a floated floor, i think it was on sharwards 'partial garage conversion' thread. I can't seem to find any reference to an equation towards finding this (mam resonance) on this forum... i'm hoping someone might be able to point me to a link or a site or program where i could figure this out. That way, i'll be able to do a better cost analysis and compare the performance of a floated wood floor vs. a floated concrete slab...
the main reason i'm thinking about the concrete to begin with is the fact that the existing floor vibrates and resonates at low frequencies, and it's especially resonant when you stomp on it, lasting about 3 seconds... a nightmare for tracking kick drum and svt bass amps especially, which i'll be wrestling with later.

Using those materials for a floated floor would give a resonance of around 31 hZ, which means that you'd have no isolation below at least an octave and a half ABOVE resonance, or around 45-50 hZ - best you could do in that case is to make SURE you have enough rockwool in the cavities to be compressed by about 1/2" when the floor goes on the joists - that would help damp the floor and keep ringing down.

Better, if you can, would be to put ONE of those layers of OSB down and then plastic, and pour a 3" thick concrete slab on top - this would lower the mass-air-mass resonance to around 15 hZ, which would drop even further when you added the weight of the walls and ceiling, to around 10 hZ - this would put all audible frequencies above your resonance, ensuring good isolation throughout the audio range.

hopefully i will be determining later today whether or not the floor construction will support a concrete slab. below is a photo of the ceiling which shows the underside of the 2x6 timber on edge, nailed one to the next, creating an extremely solid foundation for the floating floor. (for those who may be unfamiliar with 'mill construction', there are no joists, so to speak, resting on the metal I-beams, rather it is a solid mass of on-edge 2x6's covered in hardwood). the second photo shows the location of the I-beams on the ceiling which are also mirrored in the floor's construction. pillars are at 20 foot centers wide and 25 foot centers long and the ceiling height is 11 feet floor to 2x6 timber. *planning on boxing in the roof drain and sprinkler drain (in red), and the sprinklers will HOPEfully end up in the lowered ceiling.


Also, if anyone needs this in the future, using Aaron's link for estimating the weight of JUST THE CONCRETE SLAB, i used this equation:
thickness of the slab divided by 12 multiplied by 150. So, in my case, i estimated (2"/12)*150 = 25 pounds per sq.ft., which ended up being exactly what the pro's estimates were during my morning of phone calls. Again, this is useful only as an estimate, and doesn't include the weight of the forms etc.
Here in montreal, of the 7 or 8 places i called, the typical cost including pump, labour, concrete, and finishing is $1500 (canadian) incl. tax(15%) for a 2 1/2 inch thick slab covering 450 sq.ft. on the third floor of a building.

[knightfly]

The tool I used for rough calculation of m-a-m is an older demo version of Insul, a really expensive prediction program (around $1000) - the newer demo has been changed so that it randomizes materials, making it pretty much useless for anything other than drooling over. If you're interested in learning more, go here

http://www.insul.co.nz/index.html

I talked to Keith several months ago about again offering a "personal" version for around $200, but at that time he didn't think they would be doing that; there's no info on pricing without emailing them, you might check it out to see if they've changed their mind about the cheaper version... Steve