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Unread 04-21-2019, 06:25 PM   #16
wwhitney
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The Deflecto can be made to model the car decking deflection with a little manual post processing. One should also check the 4x12 deflection, which is a separate matter.

For the car decking, we'd like to run the Deflecto with a joist height of 1.5", a width of, say, 5", a spacing of every 5.5", and a length of 61.5" (based on 4x12s that are 65" o.c. and 3.5" wide). Since we can't use those inputs, we need to know that the deflection ratio (L/XXX) is unchanged when multiplying the joist height and the length by the same factor, and that the deflection ratio denominator (the XXX) is proportional to (joist width/joist spacing). [Both of these statements can be verified by playing around with different inputs and looking at the results.]

So if we run the Deflecto with a joist height of 4.5" (triple the actual), we should increase the joist length to 15.5' (triple the actual, rounded up). We can use the maximum joist width and minimum spacing (4.5" / 10"), and we get a Deflecto output of L/189. This needs to be corrected for the actual joist width and spacing (5" / 5.5"), which is 2.02 times as much as we input. Thus the corrected Deflecto output is L/382.

Cheers, Wayne
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Unread 04-21-2019, 06:30 PM   #17
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Also, for the 4x12s, we can enter everything except the spacing of 65" o.c. If we instead enter a spacing of 10" o.c., and use the worst case of a 9.5' span, the Deflecto gives an output of L/5447. The actual spacing is 1/6.5 times what we input, so the corrected Deflecto output is L/838.

Cheers, Wayne
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Unread 04-21-2019, 06:39 PM   #18
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Wow, lots of very helpful information. Can't thank you guys enough.

The face of the boards are in fact 5" wide, with a 1/4" tongue. So overall width is 5 1/4".

On the foundation sides of the house, where the beam runs parallel to the foundation wall. Can I use joist hangers fastened to the foundation to hang the 2x12 cross beam? I think I have seen Simpson strong ties for masonry. Or would I need to install a ledger?

So I shouldnt use CDX grade plywood? Out of curiosity why not? Not even with the C side facing up?
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Unread 04-21-2019, 06:58 PM   #19
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Andy - no problem anchoring the joist hanger to the masonry wall. I’ll let CX address the whys and why nots of the grades of plywood. My experience just tell me that it never hurts to go up a size and to make sure to get it oriented properly!

Wayne - I’d be happy to share the excel worksheet I use to run the calculations. It includes links to the sites I use to determine the “I” for the beams based on the size and shape.
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Unread 04-21-2019, 08:10 PM   #20
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Andy, CD grade plywood permits of a great many, and very large voids in the interior plies. You can get some serious soft spots in the panels, making it less than optimal for ceramic tile installations.

The X (not sure which cheapo home center started adding that to the description) is not part of the grade, but it is intended (I suppose) to indicate the exposure rating of the plywood. Any plywood with both faces of grade C or better with an exposure rating of Exterior or Exposure 1 is suitable for your application. Your local home center or lumber dealer will have AC or BC exterior glue panels. Lumber yard may have CC.

My opinion; worth price charged.
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Unread 04-21-2019, 08:30 PM   #21
wwhitney
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PC, I don't doubt your calculations. I'm just pointing out how to leverage the Deflecto tool that is already available. The Deflecto uses a deflection formula with an explicit shear contribution, along with conservative assumptions on G (the shear modulus) and E' (the shear-free elastic modulus). So it is expected that the answers I derived show higher deflection that your computations, which are more standard.

Cheers, Wayne
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Unread 04-22-2019, 01:39 PM   #22
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No offense taken Wayne, I was just trying the share the spreadsheet joy!

Thanks for the insight on the using the tool “out past the edge”.
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Unread 04-22-2019, 10:05 PM   #23
Kman
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Quote:
Originally Posted by Andy
On the foundation sides of the house, where the beam runs parallel to the foundation wall. Can I use joist hangers fastened to the foundation to hang the 2x12 cross beam? I think I have seen Simpson strong ties for masonry. Or would I need to install a ledger?
I don't know how many of those Simpson Strong Ties you'd have to install, but they're pretty pricey, from what I've seen. I would guess that a ledger would require fewer anchors for the masonry.

If you have a small number of them to do, then it might be easier to go that route. If I had ten or more, I'd consider the ledger for less money and less work. Plus, trying to get those ties in just the right place for the joists to hang properly might be frustrating. It's likely easier to do so with a joist hanger over a ledger.

Just something to consider....
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