What is proper bracing for joists bearing on LVL

[mfstructural]

I have a 5-1/4" (3 ply) x 14" LVL that has joists running over it in a remodel of a residence. For some reason they installed shimming in between the bottom of the joists and LVL on which they sit in the form of plywood. I'm going through the options on what proper bracing is for this detail. The manufacturer's tables say that bracing shall be provided every 24" (this is a general requirement for most of the manuals I've read). There are two options I'm thinking of. The first is to use a strap around the LVL and attach it to the sides of each joist (spaced at 16"). The option is to use 2x4 or 2x6 laid flat at in between the joists as blocking and toe nail into the joists and LVL. Boise wood products has a technical publication that talks about how this is an ok method of bracing. I'm not convinced that blocking installed like this will prevent the rotation of the LVL.

What are other methods you have used in bracing an LVL or and girder for that matter with joists running over it? I know a 3 ply LVL won't be as susceptible to rotating as a single ply.

Thanks,

 

[jayrod12]

If the joists run above the beam, such that the load is delivered through the centroid of the beam, the top flange is restrained from rotating, and at the LVL supports rotation is restrained, I don't see the need for additional restraint.

 

[KootK]

I think I'm okay with the Boise Idaho business. Can you post that publication for us to review? If the shims don't project beyond the joist flanges, or can be trimmed, an angle clip fastened to the top of the beam and the joist bottom flange would do the trick. Kinda depends on the thickness of the shims with respect to whether or not something like a simpson A35 would work geometrically.

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.

 

[KootK]

I guess a better picture of the existing connection would be helpful. Of the shims are thin and cleanly installed, perhaps their introduction into the connection does not compromise the original toe nailed connection significantly?

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.

 

[JAE]

I think just using solid 2x blocking between the joists - on top of the LVL, and nailing it together is what a typical good carpenter would do.
The blocking would be of the same height as the joists but simply perpendicular to the joists between them with toenails.
The difference in height between shimmed joists and blocking?....I'd perhaps shim the blocking too to allow the floor sheathing to also fasten into the blocking.

The blocking could be placed every other joist space and still achieve essentially the same goal.

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[EngineeringEric]

Bracing aside, what type of bracing are you looking for? Compression Flange of the LVL or lateral bracing for the full depth of the LVL? if just top flange, then toe nail your joists and blocking into the top of the LVL

Another concern, plywood shims sound like a bad idea. I feel like they will shrink, degrade, and cause issues in the future. usually we want to see hard wood shims if required. May be worth looking at.

 

[KootK]

The blocking would be of the same height as the joists but simply perpendicular to the joists between them with toenails.

@JAE: unless I misunderstand, I really don't see that kind of blocking restraining lateral movement of the top of the LVL girder. And I think that's OP's concern. Maybe if you've got toenails from the ends of the blocking into the joist flange at the bottom. That kind of blocking benefits rollover of the joist rather than rollover of the girder in my opinion.

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.

 

[JAE]

KootK - yep - correct...rollover is taken care of definitely.

But almost 100% of framing like this, historically, simply uses toenails, blocking, etc. to create a connection between the "joist system" above and the supporting beam below - with minimal strength capacity needed to brace an LVL beam, this usually provides adequate bracing.

Using all sorts of angles, clips, etc. gives folks a discreet system to pin their bracing needs on - and it gives Simpson Strong-Tie and others something to cheer about too I guess.

If there's uplift, or significant horizontal forces to be delivered into the LVL, then clips, angles...yes of course.



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[KootK]

But almost 100% of framing like this, historically, simply uses toenails, blocking, etc. to create a connection between the "joist system" above and the supporting beam below - with minimal strength capacity needed to brace an LVL beam, this usually provides adequate bracing.

In conventional framing, would it not be the toe nailed connection between joist and supporting girder that that would provide bracing to the girder? I'd have to rely on OP to confirm but I thought that the issues here was the shimming somehow compromising that particular connection. I'm sure you get something from load induced friction, of course, but we'll not be relying on that for much.

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.

 

[JAE]

I guess re-reading the OP there is the issue that the plywood shims set the joists somewhat above the top of the LVL such that a typical toenail through the side of the joist, down at an angle and through the plywood, then into the LVL isn't a valid toenail.

If 1/4" plywood I'm not sure I'd even worry about it. If 3/4" or more - perhaps then the toenail is compromised in capacity.

I don't believe there is any direct NDS guidance on toenails through multiple member interfaces.

So trim the shim and simply add the blocking and nailing between all three (LVL, Joist, Blocking) is probably adequate for a small force required to brace.
I just find myself minimizing the concern in my own mind.

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