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Unread 11-24-2020, 01:32 PM   #1
Demonic
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Join Date: May 2019
Location: Barnstable, MA
Posts: 12
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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