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Unread 11-24-2020, 01:32 PM   #1
Demonic
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Addressing I-joist vibration before marble install

Hey all,
I've researched as much as I can about this but would still like to create a thread. I bought a house last year and am renovating a room that will serve as a formal dining room. The house was a custom build in 2007, and the room was spec'd for large format porcelain tile. It currently has 20x20 porcelain tiles, which are in great condition, no cracks even in the grout. Unfortunately I can't stand the look. I'm planning on replacing them with either 18x18 or 12x24 marble. The current tiles are on thinset on 1/2" ply, on top of 3/4" ply, on top of Boise AJS-20 I joists, spanning 16'4". The bottom 3/4" ply is glued to the I joists, and the top 1/2" ply is screwed into the lower ply.

Going by the Boise span charts for the AJS-20 series, which are 11 7/8" by 2 1/2", my deflection is over 720, and close to 960 for the 16'4" foot span. My concern is there's definite vibration when you stomp on the floor, and I've read this is a common complaint of I joist spans when they reach 16 feet. I'd really like this floor to feel solid when people walk across it. The basement area under the floor is completely unfinished and open, with access to the joists. I don't plan on finishing the basement, but would like the try to keep the area relatively open as I'd like to later use it as a basic shop space. The long dimension of the dining room is 20ft, but the open length from foundation wall to next foundation wall under is 26ft. I'm considering running a double 9.25 x 1.75 LVL beam across the 26ft length (green line), cutting the span of the 16.3ft joists in half. I'd support the beam with lally columns on either end resting on the foundation footers, and a 3rd lally column in the middle with a new footer placed in the basement slab. I spoke to a custom home builder I'm close with, and he said if I already meet the deflection reqs, then adding the beam is probably a waste of money and effort and he would instead do blocking and strapping.

I'd like to minimize transition height to the adjacent wood floor. So I was planning on keeping the existing plywood layers after removing the thinset, then covering with Nobleseal CIS using EXT, then the marble tile.
What are you opinions? Would you proceed with the effort of installing a midspan beam and the new footer in the middle to support the lally column, or would you just block and strap? Or something else?

Thanks,
Austin

Ps. I'm removing that insulation, not adding it in.
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Unread 11-24-2020, 01:59 PM   #2
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Maybe a small steel I beam could save you the middle basement column, leaving the basement fully open.

You can infill the tgi web with solid lumber that fits in the web height wise. SSI essentially each"beam" would be a sandwich with two lengths of solid lumber on the outsides and the tgi in the middle. Then you could add cross blocking in between.

You can also use steel strapping used to strengthen plates that get cut out for ductwork. About 1"width by 20"long, but with 16" screws holes for attaching to 16"centers. You could run a row of these perpendicular to the tgis, crossing the whole span screwed up to the underside to lock tgis together.

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Unread 11-24-2020, 08:14 PM   #3
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Welcome back, Austin.

While cross bracing is sometimes specified for those engineered joists, they're frequently designed with a full ceiling below that might help the "feel" of the floor. My first recommendation would be to contact the manufacturer with your concern and see what advice they have. You can be sure yours would not be the first such call.

And while your intended beam might certainly dampen the vibration you don't like, I think it might be overkill. Were I to experiment without contacting the manufacturer, the first thing I'd try would be to install some long 1x4s screwed into the bottoms of and perpendicular to the joists. I'd run two rows at the 1/3rd points of the joist span and see if it made any difference.

The manufacturers of those joist systems sometimes specify some cross bracing, which would be better than the struts I suggested, but not as easy and quick to try. Again, talking to the manufacturer would be a better first step.

I would not recommend the sistering recommended by Mike above. What you'd be doing, if I understand his recommendation, is double sistering engineered josits with dimension lumber, which is not gonna do anything significant for the existing joists. While it might have some dampening effect, it's a hellofa difficult and expensive method to experiment with, 'specially since you don't need the additional structure for your deflection requirements.

The metal strapping across the bottom of the joists is the same principle as the 1x4 "stuts" I recommended.

My opinion; worth price charged.
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Unread 11-24-2020, 09:24 PM   #4
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Demonic cx is incorrect in stating sistering your tgis is ineffective, it doubles their strength. Yes there is a cost associated with purchasing that much lumber, but it is one option to easily diy double the strength of the 16' floor. You can also RIP osb and attach to the sides as a stiffener, which is usually the choice of manufacturers when cantilevering.

2×10×16=$28
26' width is about 20 tgis
If you installed 1 per joist it's about$500 in lumber. 2 per joist$1000.
Ripping lengths of osb would be half that, or less.

Different options, different cost, different results.

Metal strapping is my preference over wood strapping, because you can add sheetrock right over it in the future. Which you may find installing plywood or gypsum to the ceiling to be attractive one day, to separate your workshop noise/ dust from your dining room. That load should be factored into the equation before choosing a solution.

Blocking/ strapping allows the joists to somewhat share the load, however is more effective in dimensional lumber framing, than with tgis.

You probably have 1/16" deflection over 16' , osb rips could probably bring that down to 1/10,000" or basically nothing.

Any architect/ engineer with the manufacturer's tables could tell you how to best get to your desired deflection. Or of course the manufacturer can guide you directly.

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Unread 11-24-2020, 11:00 PM   #5
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The core or rib of a I-joist is mostly to help hold the top and bottom pieces in their proper orientation. Blocking can help limit twist when a load is applied, that helps maintain the vertical deflection and share the load between adjacent runs. You can get similar results by adding strapping, along the bottoms, tying them together. Gluing and properly attaching the subflooring on top of the joists keeps each of them aligned and working in conjunction with each other.
Adding drywall to the bottoms of the I-joists can perform a similar action, but may be better done with the strapping first, as the screws will just eventually elongate the holes through the drywall slightly.

The I-joists work by the top cord being compressed, and the bottom one being stretched. The middle just holds them in position, so sistering doesn't do all that much. That's one reason why you can cut holes in the webbing more easily in an I-joist...it's not providing all that much strength to the assembly.
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Unread 11-24-2020, 11:52 PM   #6
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Sistering properly, doubles the strength of the tgi. Been there, done that.

The web absolutely contributes to the overall stiffness/ strength of the joist, otherwise hole size wouldn't be limited. It helps resist movement, and distributes forces in addition to aligning the cords.

Sistering can be performed several ways. Adding plywood or other sheathing material is called for by manufacturers specifically in cantilevered applications. Doubling of I joists, joined by solid filler in the web with glue and screws is another option. Doubling solid lumber to sandwich the web and glue and screw/ through bolting, is yet another option.

Each assembly has its own rating which also considers span. Either the manufacturer, or an engineer, would have easy access to these rated assemblies charts.

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Unread 11-25-2020, 07:59 AM   #7
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Thank you everyone, for the taking the time to weigh in. I should have also importantly asked - what are the chances the perceived vibration would increase the chance of the marble cracking? That was actually a bigger concern to me than something like wine glasses being able to rattle.

Regarding the beam and adding the center column - I don't mind having the column in that space, but having to cut the slab, then have a building inspector come to inspect the footing after digging, and then pouring the footing is kind of a pain that would be nice to avoid. I had thought about the possibility of a steel I-beam without the center column, but I think the cost and weight for 26ft would be prohibitive.

Boise does give details for nailing OSB or ply against the webbing to act as web stiffeners. They only mention nailing with 8d nails though - I would think I'd also want construction adhesive against the webbing, no?
And likewise wouldn't the strapping be more effective if screwed 'and' glued against the bottom flanges? Perhaps the web stiffening combined with 1x4 strapping would get me there.

I'll definitely contact Boise after Thanksgiving, and will report back with what they say after I pick their brains.
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Unread 11-25-2020, 08:50 AM   #8
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Definitely talk to bc rep or architect. Steel is a commodity, I'm in nj and last year I purchased a 24' steel w beam 12"height with drilling for bolt holes for about$3,000.

Since it isn't needed to be loaded bearing you could use something smaller, but ask an architect to spec it. You could possibly get away with a simple flitch plate and double lumber, through bolted, as small as 4 or 6"

Glue and structural screws are superior to glue and nail, which is superior to nail alone.

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Unread 11-25-2020, 08:57 AM   #9
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Quote:
Doubling of I joists, joined by solid filler in the web with glue and screws
This is the one proposed method that would double the stiffness of an I joist (plus a bit more for the solid filler). The other proposed methods are going to increase stiffness less than 100%.

Halving the span with a beam would increase the deflection resistance by at least a factor of 8.

If the bottom of the joists are clear and you didn't mind closing off the joist bays, adding a layer of plywood as a ceiling would certainly improve to the composite action of the joists to resist point loads. It could also create something like a box beam. I'm unclear on whether a single strip of plywood 4' wide or 8' down the middle would give you most of the benefit of a full ceiling.

Cheers, Wayne
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Unread 09-20-2021, 05:08 PM   #10
Demonic
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I'm still alive. I can't believe it's been nearly a year since I first posted, but I had some other house projects and have put far more time and effort into this than I imagined. I don't know how you guys who are pro's can do this for a living! I admire it. I did call BC and ended up talking in detail with one of their structural engineers. Ironically she was a bit angry with me at first until I realized she thought I was the home builder and called to ask these questions in the middle of the house construction. Once she understood I was the homeowner she became very friendly and helpful. I'll detail some things she said that I realize may be obvious to some of the experienced members here but will still detail them for reference to anyone who may be reading the thread. She stated that my perceivable vibration from jumping on the subfloor was not the vertical deflection in the I joists, but horizontal vibration that becomes noticeable once the I joists exceed a 16:1 span to depth ratio, which is right where I was at. She stated for that reason the floor assembly would still meet the specs for natural stone even if I could perceive vibration. She said adding the LVL beam at the midspan would definitely create a very strong floor but as CX said probably be overkill. I asked about sistering or web stiffening. She said that's most beneficial when you have increased point loads to address, but aren't as effective for overall vibration. I did try strapping the beams with 1x4's, using construction adhesive and framing screws, but I didn't notice a difference. So I decided I would just add the LVL and columns even if it was overkill. How hard could it be?

Oh my stupidity. It's done but it was a ton of work to cut the concrete slab, dig the footer, setup the rebar and pour the footer. Then I had the LVL's delivered and got them mounted with some friends. On the lally columns I used a code compliant adjustable plate that gives 3 inches of adjustment that allows you to preload the columns. In my head the LVL beam would then sit flush against the bottoms of all the joists, but in the reality there was slight variations of up to 3/16", so with the beam loaded I has to use composite shims to create an even load across the joist bottoms.

I then used a hammer drill to remove all the existing tiles, which had been placed in thinset directly against the subfloor. With so much thinset remaining after removing the tiles, I ended up renting one of those big walk behind Esco concrete grinders to grind all the thinset down to the bare subfloor. The original subfloor was in great condition. How do you guys remove all that thinset from a large room without one of those grinders?

Flatness across the room was excellent when I checked everywhere with a long straight edge, except where you enter one end of the room there was a roll off of about 3/16". I used a couple bags of NXT skim to level the couple feet on the one end of the room. I had the Laticrete Stratamat Heat delivered, and am now starting to cut it to fit.

Since I'm not a pro and work slowly, I'll have to thinset down the Stratamat in sections. Is there anything I should be aware of when finishing one section and going back to continue the next section? I was just going to remove any excess thinset beyond the edge of the mat before it hardens, before continuing with the next section. I'm planning on using Laticrete 254 to set the mat, which I read on here people had good results with.

For setting the actual tiles, I see a lot of people liking the 4XLT. I'm setting 12x14 marble. Ironically the 4XLT is the cheaper product compared to 254, which Laticrete pushes as their high end thinset. Would you guys still agree that the 4XLT would be better for setting the tiles on the mat?

Thanks as always!
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Unread 09-20-2021, 06:04 PM   #11
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Originally Posted by Austin
Then I had the LVL's delivered and got them mounted with some friends.
You're allowed to mount your friends in your residential framing in your area, Austin? Wow, and I thought copper shower pans were the scariest thing in the Massachusetts code!

Glad you got a floor you're happy with.
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Unread 09-20-2021, 06:53 PM   #12
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So there's one more person out there now who won't ever complain about high labor costs on a tile demo and installation.

What color is that marble you're planning to install?
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Unread 09-21-2021, 02:36 AM   #13
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For marble tile, you have the appropriate double layer of plywood? Proper thickness, grade, orientation and properly gapped, offset from subfloor and with correct fastener type and length?

I’d go with laticrete tri-lite or multimax lite over the 4xlt. My preference based on how it mixes, spreads, etc.

You’re correct in how to install stratamat in sections. Just clean up any thinset beyond the edges of what you’ve just set.

I like to put down scraps of ply over stratamat for walking / working on, to spread the load out.

I also like to prefill stratamat and let dry before tiling. As you install each day’s sections, any leftover 254 can be used to prefill. Likewise as you set, use up any leftover thinset to prefill.
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Unread 09-22-2021, 07:15 PM   #14
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Quote:
You're allowed to mount your friends in your residential framing in your area, Austin? Wow, and I thought copper shower pans were the scariest thing in the Massachusetts code!
CX I love how you'll always capitalize off a grammatical ambiguity in a post haha. Yes friends can be mounted 16" OC but per MA code if they come within 6" of a plumbing fixture you have to call a licensed plumber or the police show up at your door.

Quote:
So there's one more person out there now who won't ever complain about high labor costs on a tile demo and installation.

What color is that marble you're planning to install?
Seriously. I'm putting in a white statuarietto 12x24 marble.

Quote:
For marble tile, you have the appropriate double layer of plywood? Proper thickness, grade, orientation and properly gapped, offset from subfloor and with correct fastener type and length?

I’d go with laticrete tri-lite or multimax lite over the 4xlt. My preference based on how it mixes, spreads, etc.

You’re correct in how to install stratamat in sections. Just clean up any thinset beyond the edges of what you’ve just set.

I like to put down scraps of ply over stratamat for walking / working on, to spread the load out.

I also like to prefill stratamat and let dry before tiling. As you install each day’s sections, any leftover 254 can be used to prefill. Likewise as you set, use up any leftover thinset to prefill.
Yes subfloor is 3/4" exterior subfloor ply glued to the I joists with any parallel seams directly on the I joists, with staggered 1/2" on top, screwed through just into the bottom layer, gapped. I'm a long time lurker of this forum and with how much I had read about subfloors not being done properly I was paranoid looking at details.

I had read that some prefer to prefill the mat, and if it allows me to focus on separating the steps during the lay I'll happily do it.

I always see people on the forum gushing over 4XLT for large format tiles and it's available at my local supplier, but if the consensus is to go with something else I can hunt it down. Cost per bag isn't an issue.
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Unread 09-22-2021, 11:14 PM   #15
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Ask ten installers what mortar they like, and you'll get ten different answers. I'll tell you that it's hard to find anything bad about the Laticrete mortars.

But considering you're using a light colored stone, be sure to use a white mortar.

Nothing at all wrong with pre-filling Stratamat, you'll just need to be mindful of all the crumbs and loose sand that comes up as you work over it. So keep a broom and dustpan, or a shop vac nearby to keep it cleaned up, or you could have a bonding issue when you set the tile.
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