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Old 03-20-2016, 10:56 AM   #1
Executive Flooring
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Deflection with increased dead load

How would you go about calculating deflection with an increased dead load (say SLU or deck mud)? Or should I just contact an engineer/architect
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Old 03-20-2016, 12:02 PM   #2
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I think mud is about 12.5 lbs per foot. SLU would be less if thinner. get you in the ballpark..
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Old 03-20-2016, 12:22 PM   #3
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Ray, i believe the deflecto tool uses a 50 lbs per sqft factor for the calculation of the deflection rating. I'd run the numbers using the tool first to see where you are. If you need more information, PM me and I'll run the numbers for you.
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Old 03-20-2016, 02:38 PM   #4
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Very good question. As said, Defelcto is a good place to start. Here's another online calculator, which is also listed in the Liberry.

the top one is a free span calculator.
http://www.awc.org/codes-standards/c...tors-software/

And here's a list of common building materials & floor systems we're likely to see with tile floors. This is also in the Liberry.

see last link on page 2. And if you haven't recently, might be time to have a look at the other articles.
http://www.johnbridge.com/vbulletin/...=114739&page=2

We typically don't get engineers involved for these simpler floors - those with simple span joists & uniformly distributed loads. Once the joists get more complex, or some unusual material is used, or big point loads are involved, then we get the engineers to weigh in.

Another source of material weights is the manufacturer. Here's Custom's Levelite. The weight is right there at the top of the page
http://www.custombuildingproducts.co...levellite.aspx
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Last edited by dhagin; 03-20-2016 at 02:52 PM. Reason: +last bit
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Old 03-20-2016, 04:36 PM   #5
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Wood tends to be plastic, and will continue to bend under a constant load for up to 100-years, with most of it occurring in the first couple, and while it will continue after that 100-years, it creeps very slowly and does eventually reach an equilibrium. This article is interesting on this subject... http://scholar.lib.vt.edu/theses/pub...097520/etd.pdf
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Old 03-20-2016, 05:10 PM   #6
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if this helps... for dead loads..10psf is standard with carpet and 15psf is standard for tile so i would assume at least around 20psf for mud.. im sure somebody better versed might know more exactly..
.keep in mind this number includes drywall ceilings etc..so if basement unfinished it will be less...etc

Then you add to that number the live load which is standardized at
all dwellings will be 40psf and bedrooms 30psf live load.

So if your doing a downstairs room with tile you would add 15psf dead load + 40psf live load..hence 55psf

Also keep in mind real heavy objects like islands will add a point load that needs factored in.

What lumbar and span you dealing with?
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Old 03-20-2016, 07:02 PM   #7
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To answer the OP's question, deflection is proportional to load. So if you have a deflection value for, say, 50 psf, but want the deflection for, say, 70 psf, just multiply by (70/50).

For purposes of determining whether your tile will crack, there's no need to consider the elastic deformation due to the dead load of any materials installed before the tile. That deformation will be present at the time of tile installation and won't stress the tile.

As Jim mentioned, wood does creep. Creep means slow deformation after the initial elastic deformation due to the load. I read somewhere that the long term creep for wood joists is about 50% of the initial elastic deformation. Since creep can occur after your tile is installed, it is a good idea to consider it.

The deflectolator gives you a conservative estimate for deflection based on 50 psf total load. That could be 40 psf live load plus a 10 psf creep contribution from a 20 psf dead load.

Cheers, Wayne
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Old 03-20-2016, 07:34 PM   #8
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Great info and thanks for everyone's input.

I don't have the exact numbers right in front of me at this point but basically the situation I have is a wood floor that has quite a dip in it. Just shy of 2" at its lowest point to the highest point. The overall area that would need to be leveled is about 12'wide (with that 2" dip right about the middle) by about 13' long (if I recall correctly).

2" at its deepest point adds a whole lot of weight whether I use SLU or mud, and that extra weight is what concerns me. If the job wasn't for a friend I probably wouldn't even be getting involved but since it is, I want to make sure I'm going about it the right way and not just throwing in some extra floor joist thinking it's going to work.

I will try to get the exact measurement on everything for you guys soon.
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Old 03-20-2016, 11:50 PM   #9
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If the floor you describe was one of my jobs, I'd wanna know why it dips 2" before doing anything else. 2" in the middle of some joists is huge, must be way undersized joists or broken, and/or some big load is pushing them down that wasn't accounted for.
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Old 03-21-2016, 12:23 AM   #10
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Quote:
Originally Posted by Wayne
For purposes of determining whether your tile will crack, there's no need to consider the elastic deformation due to the dead load of any materials installed before the tile. That deformation will be present at the time of tile installation and won't stress the tile.
Wayne.
This is inaccurate and unhelpful for us here. Please study Dr. Woeste's article in Wood Bits from 2007 linked above - he explains it quite well. Dynamic deflection has some bearing on whether tile systems crack or not, but that's not really what *we* need to know when we analyze these things. We need to know if the complete joist assembly is adequate to support our tile installation. The only way we can know this is if we consider all the applied or anticipated loads when determining deflection.

So, when determining deflection, use *all* known loads when you're figuring these things out. Read Dr. Woeste's article I linked to above.
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Old 03-21-2016, 07:05 AM   #11
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If you were gonna tile an upstairs area would you need to calculate the new tile weight then somehow ? Just asking
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Old 03-21-2016, 07:55 AM   #12
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Quote:
Originally Posted by Dana
This is inaccurate and unhelpful for us here. Please study Dr. Woeste's article in Wood Bits from 2007 linked above - he explains it quite well.
OK, I did and I don't see the conflict. Perhaps you could point me to the section you feel is in disagreement with what I wrote?

Quote:
Originally Posted by Dana
Dynamic deflection has some bearing on whether tile systems crack or not, but that's not really what *we* need to know when we analyze these things. We need to know if the complete joist assembly is adequate to support our tile installation. The only way we can know this is if we consider all the applied or anticipated loads when determining deflection.
Yes, but deflection from what state? My understanding is that it is deflection from the state at which your tile is installed that matters.

For example, consider a wood joist floor system supporting tile above and a custom very heavy ceiling below. The joists have to be selected to be strong enough for the total dead load and for the design live load, that's a given.

But the deflection related stress on the tile is going to depend on the construction sequence. If the tile is installed before the heavy ceiling, then the elastic deformation due to the weight of the heavy ceiling is going to stress the tile. The weight of the heavy ceiling should be 100% included in the load for a deflection check on the tile. Plus likely another 50% or so for future creep.

If instead the tile is installed after the heavy ceiling is installed, then the only way the heavy ceiling is going to stress the tile is through its contribution to future creep.

BTW, the AWC calculator linked to in post 4 uses only the live load for its deflection calculation. Adding a contribution for creep is beyond usual engineering practice (to my knowledge) but seems prudent for a deflection sensitive material like tile.

Cheers, Wayne
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Old 03-21-2016, 01:58 PM   #13
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Wayne.

last page, col 1, p 5
Quote:
This question is easily answered by recognizing that the
design dead load and live load are additive and that the sum
of the two components determines the maximum span.
This is not a theoretical discussion, in which case I may even agree with some of your points. This is a discussion by tile installers where we're trying to figure out if a joist system is adequate to support a tile installation, i.e., is the joist over-spanned?

Secondly, creep is based on long term deflection under load. Normally, the long term load is a dead load, which is typically the smaller of the live/dead load sum. Creep, in our discussion, based on long term dead loads for most floors is relatively small compared to design for live loads. So, creep deflection will be something less than 1.5x total deflection. We can argue about what it is, but again, it's not really helpful here.

One more time. Our typical case involves a guy showing up on an existing job where we're told to install tile. We need to know if the *joist system* is adequate to support our install. To do that we need all dead and live loads to determine if the joists are over-spanned.
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Old 03-21-2016, 02:33 PM   #14
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Ray, I sure agree with Dana that you need to find the problem first. All deflection calculations are irrelevant in your case, since something is broken, defective, or missing. None of the usual assumptions built into those calculations will apply to you.
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Old 03-21-2016, 02:46 PM   #15
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Quote:
Originally Posted by Chad
If you were gonna tile an upstairs area would you need to calculate the new tile weight then somehow ? Just asking
Yes.
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