I’ve searched these archives as well as the John Bridge forum but can’t seem to get clarification on this question. I want to determine the deflection (l/xxx) for a flooring system. With the span tables I know what is good for l/480 or l/360. I want to put natural stone (travertine) in a bath and know it is recommended to achieve l/720. The choice for natural stone came after framing was complete and covered by drywall.
I have 11 7/8 I-Joists with a 2 1/2 flange. They are GPI 65 (similar to TJI 560 I believe). They are spaced 16″ oc and span almost 16′. I have 3/4″ plywood subfloor on top. I have an old-school tile guy who prefers to do 1″ mud-jobs with metal lath.
My problem is I cannot find documention on how cement on top of subfloor will affect the deflection properties of the floor system though I have to think it does. The calculators I’ve seen don’t take this into account.
I’m sure I’m missing something obvious and for this I apologize. Any pointers to where I might go to determine my deflection would be much appreciated.
-Jonathan
Replies
there was a post here about three weeks ago that talk of this, the post was about plywood and floor tile.
Adding the mudbed to the floor will cause permenant deflection. As in a slight "sag". the deflection to be concerned over is the flexing of the floor system as you walk over it or add loads and then remove them (filling a large tub then emptying) will cause a sag then spring back to where it was. With a good mud job you should be fine.
Thank you for the quick responses. I understand about the permanent deflection from the dead load of the cement. As you pointed out, I'm concerned about the deflection from walking on it. My gut tells me that the joist system + mud job should be satisfactory but I want to have confirmation prior to putting in the tiles. I understand how to calculate PLF or PSF but don't know how to use that to calc the live load deflection. Ultimately, I can call an Engineer at the joist supplier but I want to have an idea before I call.
I did see the previous post regarding deflection and plywood but I guess I'm trying to see how to factor in hardened cement into the calc for deflection.
Thanks again for the responses.
Adding dead load from concrete + tile certainly will affect the deflection of a floor system. But it only affects the DEAD and TOTAL load deflection, not the LIVE load deflection.
The only way to know your deflection for sure is to run the I-joists in the software that the manufacturer has written for that purpose. If you talk to your supplier they should easily be able to get that done for you.
You'll have to tell them that you're using concrete and tile, and let them know what dead load you want added to the floor system to account for that. But it's easy to run, if you can get the request to the right person.
TJI 560, 11-7/8", 16" OC, 40/20 loading... that's good for l/480 at 23'-8" span and l/360 at 26'3 span. I'm guessing wildly but I'd say you're going to be l/720 at a span of 19' or 20', and that at 16' you're totally covered. The mortar bed is icing on the cake.
Those max spans sound awfully long for 11 7/8" I-joists. And keeo in mind he has a lot more dead load than the 20 PSF shown in typical span charts.
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Boss,When I read David's post I thought the same thing. Maybe he was looking at 16" tall joist accidentally?Well, I consulted my TJI span book and David's right. TJI 560 (3-1/2" flange) will span 23'8" with 40/20 loading and deflection criteria of L/480.The original poster said he has 2-1/2" flanges which is not comparable to TJI 560, but even TJI 110 (1-3/4" flange) will span 17'3" at the given criteria.Since the OP has a span of "almost 16'" I'm sure he will be okay. The mud bed will add dead load but not too much to matter, I wouldn't think.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
> The mud bed will add dead load but not too much to matter, I wouldn't think.
An inch of mud is about 12.5 pounds per square foot.
-- J.S.
Two weeks ago when I was looking in the truss joist floor span tables I thought the same thing. I looked at it for a while thinking it couldn't be true. So if I am reading into this correctly, If trussjoist is telling me that I can take a 560 series 11 7/8" deep joist and span 23'-8" and still meet l/480 deflection criteria...
23'-8" / 480 = 9/16"
(is that right or am I missing something here)To me, although l/480 sounds good on paper, I'm inclined to think your gonna feel that 9/16" if your standing in the middle of that span. As I run into cases as described above more and more lately, I wonder if I should be asking engineers to design for l/600 Also, I noticed that if the floor panels are nailed not screwed you are supposed to reduce all spans by 6"
Edited 12/15/2005 3:32 pm ET by xosder11
I think spanning 23' plus with 11 7/8" I-joists is nuts - I don't care what the span charts say. One thing to keep in mind here is when they talk about L/480 in span charts, they're referring to LIVE load deflection, not total. If they used L/480 for LIVE load, they probably used L/360 for TOTAL load. So in the example you mentioned - A 23' 8" span - The allowable TOTAL deflection is about .8", or 13/16". I'm gonna have to do a thread devoted to that subject one of these days....Have you seen the thread on floor vibration? It's worth a read, if you're concerned about floor performance:http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=21010.1
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Thats a great discussion thread. Thanks for the link.
"I'm gonna have to do a thread devoted to that subject one of these days..."
I got a better idea... how about a FHB article on it?!
I hate to ask, but how nuts would it be to run those I-joists at 23'-8"? For my upcoming shop, I have plans to do exactly that, or something just like it. The building is 24' wide. The designer specified a huge glulam down the middle and 2x10s hanging from it. I'd rather skip the glulam and have a flat ceiling. An alternative (in 12" of height) is a steel tube. Also expensive and a PITA. The I-joists aren't exactly cheap but the lack of a beam is a major plus.
Deeper I-joists are a possibility, I guess.
"...how about a FHB article on it?!"
Actually - The subject is in the outline I sent to Taunton. But if they write an article on it, they'll probably want an engineer to write about it.
"...how nuts would it be to run those I-joists at 23'-8"?"
Depends on how deep the I-joists are. I wouldn't personally recommend anyhting shallower than 16" deep for that span.
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Boss,Could you post the spreadsheet that you reference in the "floor vibration thread"? The one that you use to determine the Hz of the vibration.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
"Could you post the spreadsheet that you reference in the "floor vibration thread"?"
I'd be glad to post 'em - But they're at work, and I'm at home right now.
I sent myself an email at work to remind me to post 'em tomorrow.
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Here ya go.I attached 2 different spreadsheets - One for wood webbed floor trusses, and one for I-joists.
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Thanks, very interesting. I tried putting another layer of plywood on in the I-joist version, and it went from marginal (14.4) to annoying (11.8). So, as is often the case, more weight is a bad thing. It looks like inertia doesn't help.
-- J.S.
I don't think the formulas are perfect. They're generalizations based on an I-joist floor with one layer of plywood. Adding a 2nd layer might not affect it in exactly the same way as having a heavier I-joist. Someday years in the future I expect some sort of vibration formula will be part of codes and span charts. But until then I'd consider this just a rough guidline for floor performance.
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I ran the numbers on the TJIs I might use. The chart on the Trus-Joist website gives EI (joist only) as a three digit number, so I added 6 zeros. Looks like with 11-7/8" series 560 at 23.66' span the result is 12.7 (marginal), but if I upgrade to the 14" joists then I'm at 15.3
Good info, thanks for the help.
Personally, I wouldn't use I-joists 14" deep spanning more than 22' or so. But that's just personal preference.The 14" ones would sure as heck be better than the 12" ones.
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mud job with lath is sweet but a LOT of dead load on structure not built with it in mind look into 1/2" hardi backer applied on light coat of thinset and screwed into subfloor tape joints as you tile great adhesion, strong & light in comparison Tek makes a newer thinset designed not to transmit cracks from substrate look at stone enhancer and a good sealer after install
I cant think of the full name of it ,
The handbook printed by the Tile Council gives all recommendations.
Tim
Handbook For Ceramic Tile Installation by Tile Council of America.
There ya go! <G>
Thanks .
Tim