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Unread 05-08-2019, 01:05 PM   #1
DonS
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Adding Subfloor Support

I am planning for a new project of installing 5/8" thick basalt tile (~16x24 but irregular shape in a pattern) on an area that is currently carpet. (Outlined in red in pic)

Originally the area was a screened porch, but is now enclosed. The flooring is subfloor ply over T&G decking that sits on 4x13 joists spaced over 4' apart.

I know I need to add joists here, but what would you suggest? Two joists in between each? Would I need a header beam across the middle of the 12' span? Can I even get 2x's this tall (13")? This is over crawl space access.

These joists cantilever beyond the foundation wall 6' and barely support a deck but that is another project for another day!

Any advice on getting this deflection correct for a stone floor is appreciated.
Thank you.
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Unread 05-08-2019, 01:24 PM   #2
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Hi Don,

You can get custom sawn lumber any size you want if you don't mind spending money, but I would use standard 2X12s (11-1/4 high). Since it's a crawl space underneath you don't need to worry about evening it up. You will not need a header in the middle for a 12 foot run.
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Unread 05-11-2019, 03:10 PM   #3
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Thank you John. I appreciate the input.
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Unread 05-11-2019, 07:17 PM   #4
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Hi Don,

That is one strange joist configuration. What is the age of the home?

I’m concerned about the load on the cantilevered deck transferring a significant uplift to the interior floor.

Do you feel strong vibration on the interior floor when someone is walking (or bouncing) on the outside deck? What is the thickness of the T&G planking. Is the planking installed at a angle?

If the planks are at a angle:
Given the huge overhang, I recommend installing a minimum of 2x12 @ 16” spacing between the 4x13 beams. Even at that spacing, I’m not sure that will do the trick for natural stone due the loads being transferred from the the cantilevered sections.

If perpendicular to the beams, I’d install the 2x12 parallel to the beams at 16” spacing (e.g. 3) between each beam.

Last edited by PC7060; 05-11-2019 at 07:29 PM.
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Unread 05-11-2019, 08:16 PM   #5
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Don, Since you want to do a Natural stone floor, I’d consider installing a strong back beam between the large 4x13 beams to counteract the uplift force imposed by the cantilevered sections.

Pete1972 did a post a number of years ago that did a great job describing the implementation of a strong backbeam.

https://www.johnbridge.com/vbulletin...3&postcount=16


Fine homebuilding also had a good article on the subject which Pete also referenced.

https://www.finehomebuilding.com/200...-i-joist-floor

With a properly sized strongback to address the cantilever forces you may be able to reduce the numbers of 2x12.
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Unread 05-11-2019, 09:00 PM   #6
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The deflectolator says:

"For joists that are SYP or Douglas Fir, in good condition, 13 inches tall, 3.5 inches wide, 24 inches on center, and 12 feet long between supports, the deflection calculated is 0.076 inches. This translates to a deflection of L / 1886."

Than means with the same joists every 48" o.c. the deflection would be L/943. Are the joists SYP or Douglas Fir in good condition?

Also required is to check the decking between joists, and to figure out the impact of the 6' cantilever.

Cheers, Wayne
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Unread 05-12-2019, 04:41 AM   #7
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Hi Wayne,

While I agree the beams are very stiff, I think it’s risky to run the numbers for the beams alone in the situation in case one think “well that’s good!” and move out thinking they got the answer they want.

When considering floor systems of with large widely space beams and TG planking between the beams, I would ignore the large beams and focus on the defections of the planking between the beam sections.

In my opinion that doesn’t apply in this situation due both to the longer spans of the beams inside the area of interest and the long cantilever.

The OP will need to take some action to tie the beams together at the midpoint in order to at least somewhat mitigate the cantilever under normal deck loads.

Even with something like a strong-back beam applied, I imagine the lift caused by 10 large teenagers or adults () out there enjoying the bounce of the floor and feel

When I think about that type of situation, I would install a 4”x12” LVL or similar beam at the mid span of the floor section beams making sure:

1. Beam is anchored to end wall using bracket that handle both up and down movements

2. Use to pipe clamp to tighten each beam to beam connection clamp before applying steel plates using structurally rated wood screws.

I think you would also need at least one concrete support under the new beams given the width of the floor and that structure would have to be designed to resist uplift too.

Last edited by PC7060; 05-12-2019 at 05:04 AM.
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Unread 05-12-2019, 09:41 AM   #8
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I agree the cantilever is a big issue that requires some analysis. I guess the thing to do is to check deflection of the main span due to full live load on the cantilever only.

As to the subfloor decking, if it is perpendicular and a good 2x material (1.5" thick), then it should be just stiff enough:

"For joists that are SYP or Douglas Fir, in good condition, 6 inches tall, 4 inches wide, 12 inches on center, and 17 feet long between supports, the deflection calculated is 0.831 inches. This translates to a deflection of L / 245."

Here I've quadrupled the height and the span, which taken together have no effect on calculated deflection. And since the decking is effectively 12" wide at 12" o.c., rather than 4" wide, the deflection will be 1/3 as much as calculated, or L/735.

Cheers, Wayne
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Unread 05-12-2019, 10:27 AM   #9
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Any chance of adding a support beam at the outside end of the cantilevered joists, Don?
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Unread 05-13-2019, 01:16 PM   #10
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Quote:
Originally Posted by Wayne
I guess the thing to do is to check deflection of the main span due to full live load on the cantilever only.
OK, I did the math, and for a cantilever of length 1/2 of the main span, the upwards maximum deflection of the main span from uniformly loading just the cantilever is 62% of the downwards maximum deflection from uniformly loading just the main span. [I was also able to check my calculation against standard formulas, you can use the deflection formula for a simply supported beam with a moment applied at one end, namely the moment generated by the cantilever loading.]

So how should this information be used in the structural design for a natural stone installation on the main span? I see two possible approaches:

1) The L/720 criterion covers deflection both up and down. So as the upward deflection is at most 62% of the downward deflection, it suffices to consider the main span deflection from the usual loading.

2) The L/720 criterion is actually for the range of deflection. In the usual case of a simple span with no cantilever, the deflection is from 0 to the downward maximum, and the range = the downward maximum. But with a cantilever, it can now deflect upwards, and so in this case the range is 162% of the downward maximum deflection. That means the system should be 162% the usual stiffness, so the main span should be designed for L/1170 under the usual loading.

I don't know which approach is proper, but obviously (2) is more conservative.

Cheers, Wayne
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Unread 05-13-2019, 05:54 PM   #11
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Nice work, Wayne! I think Option 2 is a very good statement of the situation. Although I think the OP has left the building.
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Unread 03-19-2020, 07:08 PM   #12
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Thanks for all the input!

I've been sidetracked to another project and hadn't realized there were multiple responses to my post. Thanks for the help.

Plans have changed somewhat. The deck is a 50 year old modified/repaired situation that I plan to replace with a freestanding deck. There is some decay outside the foundation in the joists and it is time for renewal that will allow me to connect the deck around the corner to my outdoor kitchen in progress.

That said, large joist cut off outside the foundation wall and sealed, with no issue with the cantilevered joists' and deflection, does the 2x12s 16" on center still stand without the strongback? Also the large joist now rest on the foundation wall (because they cantilever through) with blocking between enclosing the wall. Would the new joists hang on the blocking between the existing large joists? (the lighter colored joists in pic)

May be asking the wrong question.
Thank you!
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Unread 03-19-2020, 11:05 PM   #13
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1. If the joists are of good species and grade, yes.

2. Yes, so long as the "blocking" (segmented rim joist) is of suitable size, species and grade, and suitably fastened in place.

My opinion; worth price charged.
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