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Unread 02-20-2008, 04:48 PM   #1
Frank Woeste
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New Deflection Rules for Ceramic Tile

In the past, many engineers/architects/tile contractors were faced with the question--what are the appropriate design criteria for wood-floor framing systems supporting ceramic tile? For over a decade, the primary reference documents for the selection and installation of ceramic tile on wood joist floors (2006 TCA HANDBOOK for Ceramic Tile Installation) stated the following under Requirements:

• design floor areas over which tile is to be applied to have a deflection not greater than 1/360 of the span when measured under a 300 lb. concentrated load (See ASTM C627).

This specification related to a 4'x4' laboratory test specimen with 2"x2" (nominal) simulated joists bearing solidly on a concrete pad. The above specification had no rational translation for the joist system designer. The issue has been resolved by new language added to ceramic tile industry standards and the 2007 TCA Handbook (www.tileusa.com/publication_main.htm). The attached article was published in the Building Safety Journal of the ICC (December 2007) and it explains the new rules for floor sytem design. It also addresses dead loads for typical tile installations. Comments and discussion are welcome.
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File Type: pdf Wood Bits Ceramic Tile on Wood Floors Dec. 2007.pdf (143.4 KB, 306 views)

Last edited by bbcamp; 02-20-2008 at 05:30 PM.
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Unread 02-20-2008, 05:45 PM   #2
bbcamp
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I read the attached article with an eye on how to improve our own Deflectolator, which uses a 50 psf uniform live load to determine deflection as compared to L/360 and L/720 (ceramic and stone floors, respectively). We make no provision for dead loads. Since most of our members are re-modeling an existing structure, we believe that dead loads have already been added to the structure prior to tiling (note that the tile and setting materials would be live loads in our scheme). Thoughts?

BTW, I tweaked you link to the TCA publications page.
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Unread 02-20-2008, 07:36 PM   #3
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Yeah that's just what i was thinkin Bob.
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Unread 02-21-2008, 12:17 AM   #4
cx
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You keep writin' articles with that Nielsen fella, Frank, and we're gonna hafta open a new wing in our whirl-famous Liberry for y'all.

Looks like you are suggesting we should not use anything less than a 20-pound dead load in calculations for any new construction design that calls for a tile installation?

As Injineer Bob points out, we deal more with remodel situations here than with new construction.

For new construction we generally tell our visitors that the architect or builder should have already made provisions in the design for the tile or stone installation if it was included in the plans. If the tile or stone is an afterthought, we'd point out that the builder should either make provisions for it after the fact or they should settle for some other floor covering.

In the remodel situations, we use our deflection calculator that Bob designed for us, with the rather conservative 50-pound load factor.

Most of our visitors are using CBU or membranes as their tiling substrates (F144, 147, 148). Occasionally we'll have folks installing the thin mud bed (F145) and very rarely anything heavier than that. So, while we're likely over the ten pound dead load, Bob's calculator still keeps us pretty safe, I would think.

What would you recommend we do differently?
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Unread 02-21-2008, 07:01 AM   #5
John Bridge
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Hi Frank,

Great article as usual.

It seems the new language seeks to insulate the tile contractor from all responsibility for determining whether a floor structure is suitable for tile. While that end is certainly welcome in this camp, I doubt the idea would hold water in court of law. The question will always arise as to whether a person "should have known" whether conditions were correct for a particular installation.

Charging the "owner" with certain responsibilities at the onset of the design process is not feasible, in my estimation, when the project is a remodel -- or even for new residential. The owner might not have decided at that point whether he/she wants tile or laminate flooring, for example. It seems to me that certain areas of the future home should be designed to carry tile or stone as a matter of course.

I'm not being critical here, but I would hate for any tile contractor to read the article and come away with a sigh of relief in the sure knowledge that he's no longer responsible.
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Unread 02-21-2008, 07:32 AM   #6
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The issue of dead load specifications for joist floors is important to the engineer (because it relates to floor strength) but may not be of special concern to the floor tiler regarding a mortar bed (because it is floor stiffness not strength that relates to tiled floor integrity). That is, as long as the floor is adequately strong.

For example: If a 2" thick mortar bed is laid on top of the subfloor, it is placed in a "fluid" state and then cures only after the floor sags (i.e. because of the extra mortar weight). Therefore, no matter what the dead loads are due to the mortar bed, there would be no subsequent cracking of the tiled floor - or the the mortar bed itself - due to the dead load from the mortar bed. Of course, this consideration is valid as long as the floor is strong enough. So, it would seem that the issue of the actual weight of the mortar bed (20 psf or 10 psf) is not a major issue for the floor tiler?
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Unread 02-21-2008, 08:13 AM   #7
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There's still the matter of creep, Michael, not an insignificant factor in your 2" mud example there.

My opinion; worth price charged.
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Unread 02-21-2008, 09:46 AM   #8
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Good point, CX. The floor may indeed creep (e.g. continue to sag with time) somewhat more with a 20 psf mortar load than, for example, a 10 psf mortar load. This ongoing creep would place a greater stress on the floor tiles over time. But, it would seem that the initial deflection/sag of the floor due to the motor load itself would be much, much greater than the subsequent sag due to creep from this load? It is therefore good news that the initial floor deflection due to the mortar load is not, in itself, a factor affecting tiled floor integrity.

Now, I am not knowledgeable about this creep factor for wood (presumably dried wood on a previoiusly-built floor). Perhaps others can elaborate?
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Unread 02-24-2008, 03:42 PM   #9
Frank Woeste
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Creep Deflection

You can learn about creep deflection from the attached one page. Note that creep deflection is a function of lumber moisture content as well as the (sustained) load level.
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File Type: pdf Creep Deflection from Hoyle and Woeste 1989.pdf (82.5 KB, 460 views)
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Unread 02-24-2008, 04:12 PM   #10
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Frank:
Thanks for your excellent information on the creep deflection of wood under stress. As the actual "dead" load of a ceramic tile mortar underlayment is a small fraction of the ultimate stress loading capacity of the wood joists (and is also a small fraction of the actual total joist loads - dead plus live), it would seem clear that the (long-term) creep deflection of wood joists under a 20 psf floor loading would not normally be a threat to tile floor integrity (i.e. regarding tile cracking, debonding, etc).
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