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Joist to Sheathing Connection - Seismic Upgrade

Joist to Sheathing Connection - Seismic Upgrade

Joist to Sheathing Connection - Seismic Upgrade

(OP)
I'm working through doing "voluntary" seismic upgrades on a local school. The architect is mostly replacing windows and a few other "aesthetic" things. In the process, the district has asked to do some seismic upgrades because there isn't a lateral system to speak of. Attached is a pdf of a section I'm working with. Does anyone have any good ideas on how to connect the roof joist to the roof sheathing? I'm assuming that the nailing to the roof joist isn't adequate...the district is missing some drawings that show some of the detail information on the original building.

Thanks

RE: Joist to Sheathing Connection - Seismic Upgrade

Well, first, the member you are nailing to is not a roof joist, but technically a rim joist (blocking to some, hopefully continuous, between the roof beams. If you are unsure of the existing nailing, perhaps a stud finder would help, trying to find any nailing into the rim joist from below the sheathing, as the top is roofed over. I would assume any nails as 8d minimum and go with that. Short of that, remove a strip of roofing, install the nails you need into the rim joist, and re-roof the area. You can always add extra LPT4's from the rim joist to the wall double top p0late from the inside or outside as needed.

I really see no easy and good solution to attach the roof diaphragm to the rim joist if the nailing is insufficient.

Mike McCann, PE, SE (WA)


RE: Joist to Sheathing Connection - Seismic Upgrade

(OP)
Here's an alternate solution that may work as good. At least this way, I'm hooking into (2) members and transferring load to the shear wall. There is literally nothing but windows between the columns supporting the beams.

I'm not entirely sure if the joist is intended to be a rim or not...and I don't want to go down the "ASSumption" route too terribly bad.

RE: Joist to Sheathing Connection - Seismic Upgrade

Ideally, removing roof strip is bueno as per MSquared. Could you use an A35 horizontally nailed to rim and up-screwed to sheathing? And that could take care of sheathing to rim; and, then, there is still all the other sheathing nailing.

RE: Joist to Sheathing Connection - Seismic Upgrade

I was thinking the same thing initially Triangled, including the general concern for the rest of the diaphragm fasteners. I held back on pitching it because I was concerned about finding a suitable fastener that could be installed from the underside without messing up membrane. Do you know of a particular screw that would be suitable?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Joist to Sheathing Connection - Seismic Upgrade

(OP)
KootK:

That is where I'm stuck, I don't know of a fastener that is small enough and provides the capacity. It's not much, 220plf, but i don't know anything, and I don't think glue is really a good idea...maybe it is though???

RE: Joist to Sheathing Connection - Seismic Upgrade

Great minds think alike. Or maybe it's just confused minds. I thought about adhesive too. I sat on that because, If it were my project, I'm not sure that I'd ever really have confidence in the installation. When I've used adhesives, I've always used screws too for a belt and suspenders approach.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Joist to Sheathing Connection - Seismic Upgrade

I'd look at something like a #8 screw, 1/2" or 5/8" long. I'd suspect the failure mechanism would be the kind of rolling-pullout you'd get under load; and I don't know if you can find values for screws with the point end in plywood, so I'd go ridiculously conservative. Looks like you're seeking only 3 LPTs worth of shear, so maybe use 5 A35s for each of 3 LPTs. At 10 - 20 pounds shear per screw, I'd think you're fine, and the structure definitely has more capacity than at present. Get the district buy-in as it's a bit novel, and voluntary, and you're saving them money.
Alternatively, they can of course, remove the roofing and renail the diaphragm, or, they can install a new diaphragm by upnailing sheathing to the underside of the rafters.

RE: Joist to Sheathing Connection - Seismic Upgrade

If you are going to attach from the underside, I would think glue and screw. I do not trust screws much in this situation, especially into what may be only 7/16" thick.

Mike McCann, PE, SE (WA)


RE: Joist to Sheathing Connection - Seismic Upgrade

Gotta say I'm inclined more to msquared.... how about gluing and screwing 1/2" sheathing to bottom side diaphragm with 7/8" screws and then attaching A35s

RE: Joist to Sheathing Connection - Seismic Upgrade

(OP)
I had the district open up the roof and found that the roof diaphragm is 1x6 DF T&G sheathing. Not the best thing to find out, but I can make it work with the code allowable values.

I have went with msquared's recommendation on the gluing, and now that I know it is T&G sheathing, I'm placing a new "Flat" block on the underside of the T&G with 2" Simp. SDS screws from the blocking to the sheathing...see attached. I think this will work pretty well. It's a lot of work, but I don't see any other option short of taking the roof off, and they have no interest in doing that.

RE: Joist to Sheathing Connection - Seismic Upgrade

Looking good, but, I may be misunderstanding your drawing, but I suspect in the perpendicular to 1x6 direction - parallel to rafter direction, you're wanting a continuous 2x8, otherwise your flat 2x8 block may be engaging only 1 or 2 1x6s and 2 or 4 diaphragm nails....

RE: Joist to Sheathing Connection - Seismic Upgrade

The detail looks good to me. The only things I would add is to make sure the glue is waterproof, the roof sheathing dry and in good shape, and how many 1/8" beads of glue you want laid on the flat blocking before screwing it to the underside of the sheathing. If you can, specifically specify a good construction adhesive that is easily available. Not Elmer's Glue though...

Mike McCann, PE, SE (WA)


RE: Joist to Sheathing Connection - Seismic Upgrade

Might consider drying your lumber too, to get it sort of comparable to the existing 1x6s

RE: Joist to Sheathing Connection - Seismic Upgrade

(OP)
Great comments! Thanks!

I think I have a good path forward for this now. Thank you all for the advice.

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