Truss Bearing Points

[StrEng007]

I have an interior bearing wall (running North/South) that supports two individual pre-engineered roof truss spans. Common truss 1 spans due west, common truss 2 spans due east.

Is it better to allow these trusses to share the top plate (they would each achieve 2 3/4" bearing at their respective ends on the 2x6 top plate)?

Or better to offset the trusses to allow each to sit side by side at the bearing point?

How do you achieve this?

 

[KootK]

I'd normally let them butt so that the sheathing joint can stay consistent across the trusses. This can also been a boon if there needs to be diaphragm shear blocking between trusses. Obviously, for straight bearing purposes, lapping would be best. And, sometimes, that is required. As usual, it's a balance of competing interests.

 

[phamENG]

Sitting next to each other would also create an interesting top chord condition - they'd have to turn it back down in the thickness of your bearing wall, which may complicate that connection. Butting up against each other they each just come to a point at the peak of the roof.

 

[StrEng007]

And, sometimes, that is required.

For minimum bearing purposes? The only thing I've seen in the code (2021 IBC, Section 2308.4.2.2) if the prescriptive measure to use 1 1/2" min. This was the idea behind my original question. I can't seem to find anything else that mentions required bearing.

they'd have to turn it back down in the thickness of your bearing wall, which may complicate that connection

Not quite sure I follow, the top chords of the two systems would act alone. The "splice" of these two truss spans is not near any ridge.

 

[RontheRedneck]

In situations like this I always lap them if at all possible.

Sharing a bearing might look good on paper. But in reality, everything varies. The interior bearing wall won't be perfectly straight. The ends of the trusses won't be perfectly plumb. The truss spans won't be perfect. The east and west walls that the trusses sit on won't be prefect.

There are just too many variables. If the trusses get built and out to the field and there's a problem, fixing it can be difficult. Better to have the trusses lap and leave some margin for error.

 

[StrEng007]

If lapping, what's the best way to maintain the continuous panel joists for the diaphragm nailing? These diaphragms require high shear capacity.

 

[SWComposites]

If the joists need to be aligned for the diaphragm, could you double the width of the bearing wall, to give allowance for tolerances mentioned above?

 

[phamENG]

could you double the width of the bearing wall, to give allowance for tolerances mentioned above?

Show me the architect that goes for that and I'll show you the love child of a pig, a unicorn, and a Pegasus. (Yes, that's a flying pig unicorn if you didn't get it.)

That said, a 2x6 interior wall is usually doable, and I have a few architects who just assume that all bearing walls need to be 2x6. Tolerance for wall placement should be within 1/4". So if a 2x4 wall is off by that much, you get exactly 1.5" (1.75"-0.25"), which is shaving it a little close. A 2x6 wall at the extreme edge of that tolerance is going to give you 2.5" which probably going to be plenty.

The trick is ensuring the tolerances are adequately specified and enforced since I'm not aware of a code mandated tolerance like there is using the Code of Standard Practice for steel.

 

[SWComposites]

Maybe the artist ( er, Archi) doesn’t need to be informed of this small change.

 

[RontheRedneck]

StrEng007 said: "If lapping, what's the best way to maintain the continuous panel joists for the diaphragm nailing? These diaphragms require high shear capacity."

If it were necessary, it would be possible to put an overhang on both the east and west truss. Then the lap in the plywood could just jump over 1.5".

Or the lap could be planned out. Most framers start laying the plywood out from the low end of a truss. Wouldn't be difficult to figure out where the seam would land.


PhamENG - I don't agree that you could just specify a 1/4" tolerance and it would magically happen. With all the variables in play it's just not reasonable.

 

[KootK]

For minimum bearing purposes?

Yes. You've got a 1.5" bottom chord feeling end condition bearing stress perpendicular to grain. One wants the numbers on that to check out regardless of codes stuff. And it's prudent to run those numbers giving some acknowledgement of the tolerance involved. For a two sided, butted bearing condition, I like to have a 2x6 bearing wall if at all possible.

If lapping, what's the best way to maintain the continuous panel joists for the diaphragm nailing? These diaphragms require high shear capacity.

What's your situation? Blocked? Unblocked? Ridge / peak location?

It might be as simple as:

1) Using 1'-9" blocking instead of 1'-10.5" blocking.

2) Using two lines of blocking, one right before the lap and one right after it.

3) Sticking with one line of blocking that does not land at the lap location.

 

[phamENG]

I don't agree that you could just specify a 1/4" tolerance and it would magically happen. With all the variables in play it's just not reasonable.

I agree 100%. That's why a) I said that enforcing it is also part of the trick and b) 2x6 is preferable. Though if I had to go with a 2x4, I would make sure it's very clear on the drawing what needs to be achieved. Around here, if I do that and show up on site and find that they screwed up, a quick conversation with the City inspector and they don't get anything else done until that wall is in the right place. I realize that's not the case everywhere, and if I were designing for a rural area with lax or nonexistent inspections and permitting I'd design it a little differently.