Out-of-plane anchorage for wood truss to CMU wall 2

[jsfrazier]

I have a building with a wood roof bearing on a CMU wall. Conventional construction in the area connects a wood top plate to the top of the CMU wall with the pre-fabricated trusses bearing on the top plate.

ASCE 7 12.11.2 requires a "direct connection" capable of resisting an out-of-plane wall force for the connection of the truss to the wall.

Some engineers have suggested that the interpretation of "direct connection" would prohibit the use of a top plate, and that the pre-fabricated wood truss would need to connect directly to the CMU wall.

I'm just curious about what others are doing for this scenario.

 

[BAretired]

Top plate is standard in Canada. Must prevent rollover, though.

BA

 

[woodman88]

I use a 2x top plate. Only one building department have asked to see the complete calculations for the top plate, which I do not include with my standard calculation package.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[JAE]

A direct connection is what a top plate/bolt/truss clip/truss connection is....vs. a non-direct connection which would be the truss just sitting on the wall with nothing attaching it.

 

[jsfrazier]

JAE - In this case, the truss would be connected to the CMU wall using a simpson LTA2 clip. This would be a direct connection of the truss to the cmu wall. Others have argued that truss to top plate to cmu wall is not direct because the top plate acts as an intermediate connection.

I appreciate your response because I agree that the top plate should be considered a direct connection, and I see that others feel the same way.

 

[MiketheEngineer]

Might blocking between trusses be appropriate in this situation??

 

[DaveAtkins]

I contend that you absolutely do NOT want the truss to sit directly on the CMU--because the truss could absorb moisture from the CMU, causing deterioration.

I think you are dealing with someone who is reading the code too literally.

DaveAtkins

 

[msquared48]

You are talking cross-grain bending here in the wall top plate for resisting tension forces to be resisted by the trusses. Technically, this is not allowed by the code.

That's what Simpson embedded masonry anchors are for, with the trusses sitting on a wood or steel ledger and the anchor nailed to the truss and embedded into the CMU wall.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[a2mfk]

I agree with Mike, though often ignored, I think the whole top plate issue boils down to cross-grain bending. A piece of building paper or connection with a "bucket" for the truss to sit on solves the direct contact with CMU issue.

 

[Cadair]

I've usually seen a 2x plate. I'm very interested in how it would be taking cross-grain bending. It seems that the connection between the plate and the truss would determine if it happens, and for uplift more than out-of-plane wall forces. Could someone provide a sketch or explain it for my slowness? Thanks!

 

[RacingAZ]

Maybe I'm also slow but I don't see any cross-grain bending for the out-of-plane anchorage. The force is perpendicular to the grain for trusses bearing on the top plate. Maybe for uplift, if there is any.

I don't see the LTA2 clip being sufficient to satisfy the out-of plane anchorage requirement. I don't have a Simpson catalog with me right now but per IBC, isn't the connection subject to a 280plf minimum load? Depending on your truss spacing, I don't think you can get there with just the LTA2 clip. If I'm not mistaken, there are Simpson connectors available for top of wall condition.

Also, if you're relying on the top plate instead of direct connection to the CMU, the top plate needs to be checked just like a ledger including the bolt spacing.

How much do you need to design the connection for? The required anchorage force would really dictate how you're going to detail the connection as trusses bearing on top plate would have a lower capacity compared to putting a ledger to the side of the wall.

 

[CTSeng]

This is a regional preference in my experience. In Florida (high uplift) they set the trusses on the CMU exclusively and are relatively good at placing embedded straps in the right location. In Michigan the builders scream if you dont use a wood plate. However, moderate lateral or any uplift load is difficult to transfer through a 2x6 flat with 1/2" j-bolts spaced 4 to 6 feet on center. I've used simpson HM9KT with some success. I do use a thin gage bearing plate stapled to the truss for better bearing and to eliminate the moisture concern.