Using I-joists as rafter ties

[XR250]

I have a 24x36 garage where I specified roof trusses. The contractor wants to stick frame it instead. He suggested using I-joists for the ceiling joists due to the 24 ft. span. He does not want to sheath the floor of the attic. I have never seen a detail from a manufacturer for attaching a rafter to the side of an I-joist to resist the thrust. Is there a reason the side of the joist could not be padded out so it can function in this manner? Probably need to block between them at 8 ft. centers as well. Seems like roof trusses would be a hell of a lot easier!

 

[HouseBoy]

Roof trusses WOULD be easier AND FASTER but... my guess is that the contractor would prefer pay "himself" to stick frame the thing, rather than pay the truss maker.
Maybe the price to YOU is about the same. (I don't know).

You do get the possibility of some "storage space" if you want it but the joist design needs to accommodate that and DEF the joists will need to be braced at top and bottom flanges. Even if you don't want that space now, it will be tempting.....

I THINK that padding the webs should be fine BUT.... most of the details I see for that indicate that the "padding" must not contact the flanges. In this case that seems a bit weird since I would think that we want the tension force to go into the flanges. I bet you can call the product maker tech support for a more reliable answer on that.

 

[XR250]

My guess is by the time he pays for the i-joists and the extra labor, he is losing money. 24 ft. span trusses can be set by hand and are pretty darn cheap.

 

[HouseBoy]

I don't disagree but.... how do you explain his preference to stick frame it?
Not saying I'm right (I'm probably not right). I just don't understand it when they want to stick frame something like this.

I agree that trusses should be WAY cheaper. Question is - what is the side benefit?

I'll be curious (if you talk to the joist maker)to hear their recommendations for making the connection you described. Let us know?

 

[KootK]

I agree with taking the tension directly to the flanges and think that doing that without beating the flanges up terribly would be important. Maybe 4' same depth LSL blocking one side of each joist.

 

[KootK]

Another option might be to still use spliced 2x for the tie but use the joists to hold it up vertically. Not sure the joists could really be shielded from taking axial in this scenario though.

 

[XR250]

I agree with taking the tension directly to the flanges and think that doing that without beating the flanges up terribly would be important. Maybe 4' same depth LSL blocking one side of each joist.
Another option might be to still use spliced 2x for the tie but use the joists to hold it up vertically. Not sure the joists could really be shielded from taking axial in this scenario though.

I thought about the spliced 2x idea. None of these make a whole lot of sense compared to roof trusses.
Did I mention I hate I-joists?

Thanks for everyone's responses.

 

[XR250]

Contractor decided to go with trusses!
Found this detail....

https://www.weyerhaeuser.com/application/files/8615/4817/5414/TB-805.pdf

My suspicion is that it is not designed to take rafter thrust - just gravity loads. They make is vague enough that it will likely fool many engineers and architects, however.

 

[txeng91]

XR250, I think this is the one you’re looking for.

https://www.weyerhaeuser.com/application/files/6615/4101/1560/TB-824.pdf

I’ve seen framers on houses that are not engineered use these all the time to clear span over garages. Of course, they don’t use they detail in the link, they’ll just brace the rafters down to the joists periodically. Open-web trusses are the standard for floor framing in my area so none of the framers have a clue about the various limitations of I-joists. Congrats on getting them to use roof trusses though. It’s always nice to have a nice engineered roof structure with clear load paths as opposed to site built stick framing, even if they refuse to actually look at the truss drawings and install the specified web bracing.

 

[XR250]

Thanks txeng91.
Seems like a lot of work to add the blocking and clinch the nails.
It limits the slope to 6:12 min. so it would not have worked in my situation anyway.
I'll keep this one in my back pocket for when needed.

 

[KootK]

One thing that I find notable about the Weyerhauser document is that they make the attachment to the joist web rather than the flanges as some of us recommended. Having noodled on it:

1) I still wouldn't want the tension in the web along the whole length of the joist.

2) I bet the glue joints can transfer the tension out of the web and into the flanges with relative ease. The shear flow demand probably dwarfs that which would arise from direct tension.

3) It is pretty great not having to worry about splitting the flanges with a bunch of nails.

 

[txeng91]

It looks like the thrust connection isn’t specified and is in addition to the web reinforcement. They reference you to the IRC tables and it’s pretty vague on if that should be to the web or the flange, although the section cut suggests it’s to the web. The reinforcement detail, in terms of length and connection, is beefier than the one for bearing only that XR250 posted. It may have to do with achieving a certain “development length” where the web is transferring the axial load into the flanges.