I-Joist Rafter Details Question.

[XR250]

As much as I dislike I-joists, I am stuck with them on this job. Looking for some feedback on this detail...

1) Does the connection between the double rafters seem reasonable? Not a lot of ply to ply load here, just a monster 28' span.

2) For the connection to the top plate, does anyone know how much uplift I can get out of the i-joist before the bottom flange fails?

3) For the perimeter band, does it seem reasonable to nail into the end of the flanges or should I use a filler block?

I have not shown the blocking between the joists over the wall.

Thanks!

 

[phamENG]

Given the short backspan and total reliance on the connection, I'd feel better with a filler block - even if not absolutely required by the numbers. Same with a web stiffener in addition to the blocking over the support.

 

[jdgengineer]

I'm not a huge fan of cantilevers with I-joists. I'd be tempted to frame the cantilever with LSL or LVL material and replace the doubler with a double or triple LVL. That way you can put in a hurricane tie to the top plates rather than the SDS screws through the flange. The labor of putting in all of the web blocking may not be worth the cost vs. using a solid member. I'd look closely at your deflections on the free end as the 1:1 backspan means any uplift of the doubler from the cantilever leads to additional sag in the overhang.

What happens at the corner? Do you have a similar overhang on both sides? If so, really look at that corner deflection. We typically would model that small area in RISA 3D to have a sense as to the deflection caused by both the cantilever member and any doublers / outriggers.

 

[XR250]

Thanks for the feedback. At the corners, I am framing the main members with LVL's and infilling with i-joists.
I did not use LVL as the double because the span is 28'. The BCI90's are actually about 50% stiffer than the LVL's so I would need a triple. Might still do that. Budget does not seem to be an issue here!

 

[BAretired]

I would not rely on a top plate end nailed into the studs, to resist any significant uplift. I would prefer steel strapping or similar tension tie between stud and blocking, with blocking tight against the TJI chord.

BA

 

[XR250]

I would not rely on a top plate end nailed into the studs, to resist any significant uplift

It will have OSB sheathing on the exterior face.

 

[BAretired]

And drywall on the interior face?

BA

 

[XR250]

Yes

 

[phamENG]

Not to sidetrack the thread but....uplift with wall sheathing. Does anyone here use the SDPWS details for it with the sheathing splice with the blocking? I started using it...let's just say none of the 16 heads I seem to have sprouted will be hired again by those who received those details...

 

[XR250]

@pham - can you post the detail?

FYI, according to Boise, I have a 500lb uplift capacity fastening my bottom flange to the top of the wall as shown - which is sufficient in my case.

https://bcconnect.widen.net/s/kwjrrcrtkz/ij-34_flange_screw_provisions_2021_02

See the last footnote on page 2 (this was also confirmed by their tech rep.)

 

[phamENG]

XR - See below for a snip from the 2021 SDPWS (the oldest version I have is 2008 and it's been in there since then).

As long as you have a competent load path from the top plate to the stud and down, then you should be fine for uplift, though I would still prefer a hurricane tie. Like I said above, I always put a web stiffener over cantilever supports for TJIs, so it's easy to add a strap to it.

 

[XR250]

yea, I can see why you are getting that kind of response. I assume the 2x4 alone does not have enough cross-grain shear or tension capacity?

 

[phamENG]

Correct. It's one thing if you can control the splice locations, but if you have 9' ceilings, a slab on grade, and vertical sheathing you'll have a joint less than 12" below the top plate. All of the tension in that sheet will be taken by the 2 nails in the middle studs and 8 nails in the edge of the edge studs (assuming that the cross-grain tension capacity is negligible as the code and research tells us). That might be okay on a small house in a neighborhood well inland, but larger structures with little to no dead load on the coast it becomes a serious concern. And many of the builders around here ignore the straps because "the sheathing can resist the uplift."

 

[XR250]

On some jobs, I specify they start the sheathing at the top to avoid this issue. Still have an issue at the bottom bu a bit easier to deal with.

 

[phamENG]

Do they actually do that? Do you specify temporary bracing for lower levels during construction since the walls aren't braced by the sheathing? I don't try to dictate sheathing arrangement because, on houses, the odds of the framer seeing the structural drawings before I show up in response to an inspector's complaint are already low enough.

 

[XR250]

I have no idea if it gets done. Much easier on 1 story houses. Around here they don't sheath the walls until they are already stood up. Most framers cannot read plans though (in my experience)

 

[BAretired]

As long as you have a competent load path from the top plate to the stud and down, then you should be fine for uplift, though I would still prefer a hurricane tie.

So would I.

BA

 

[XR250]

I'm not against hurricane ties but am curious why y'all are suggesting them in addition to the screws?
The flange on this joist is 3 1/2" wide.

 

[jayrod12]

Just more confidence. I've seen way too many split chords from screws. Hurricane ties mount to the web (or filler blocking as may be the case) and all fasteners are in direct shear.

 

[phamENG]

What jayrod said. If they screw up (sorry) the installation of the screws, there's no good way to fix it without ripping it out and starting over. Screw up a hurricane tie...the damage isn't usually catastrophic (I know there's a framer out there waiting to prove me wrong) and there's typically room to add another one to fix the mistake.

Besides that, it's a more direct load path. The hurricane tie is going to deliver the uplift either directly to the sheathing or at least in the sheathing to stud interface. Granted, any cross grain tension coming from "bending" in your detail is probably more of an academic issue, but if it can be eliminated easily I like to do it.