Bracing detail for wood floor trusses parallel to concrete wall

[JStructsteel]

Whats a good detail for bracing top of a basement wall parallel to wood floor trusses? I was thinking 2x bracing inside top and bottom chord for a few joist spaces along the length of the wall.

Anyone have a good detail?

 

[XR250]

Depends on the load. Usually hard to get the numbers to work out when you actually look at the uplift on the last truss. Best to extend the wall up to the floor sheathing.

 

[JStructsteel]

Im not expecting uplift on the trusses, these are floor trusses over basement. See detail. Do you feel the sheathing is enough to brace the wall? Just check the diaphragm strength for the reaction?

 

[XR250]

Do a free body diagram on the blocking and you will see that you get uplift at the last bay (it is not from wind). It is a actually pretty significant with deep trusses and a lot of backfill.
Yes, check the diaphragm.

 

[JStructsteel]

Ok, thanks. I have never thought about uplifht from the top of wall reaction, i will look into it.

If the diaphram works, then I can use that.

Thanks

 

[Enable]

I think this is what XR250 was driving at. If you extend blocking into additional cavities you should get migration to the sheathing level in its entirety though I'm not sure at what rate. My thought there would be you'd have some continue through the bottom flange to flange path, and some go through the compression strut into the sheathing (still causing some uplift) in each cavity. At most the force in the compression strut would be equal to that continuing on through the bottom flange since if the blocking is not continuous it needs to get kicked back up somewhere, and presumably that path has roughly the same stiffness.

 

[Celt83]

You won’t be able to develop the connection reaction at the diaphragm directly so you need to drag it into the diaphragm with a couple bays of blocking. Ultimately the diaphragm does need to gather that load and distribute it to your lateral system.

Grab a copy of this book: Link

It contains a diaphragm and shearwall chapter written by Diekmann with a specific sub diaphragm example for top of wall bracing. The book example is geared towards a tension condition but the same principles can be applied for a compressive reaction.

 

[XR250]

I think this is what XR250 was driving at. If you extend blocking into additional cavities you should get migration to the sheathing level in its entirety though I'm not sure at what rate. My thought there would be you'd have some continue through the bottom flange to flange path, and some go through the compression strut into the sheathing (still causing some uplift) in each cavity. At most the force in the compression strut would be equal to that continuing on through the bottom flange since if the blocking is not continuous it needs to get kicked back up somewhere, and presumably that path has roughly the same stiffness.

Actually, my picture would show the wall only extending to the bottom of the trusses. This is what the OP's original question addressed. The uplift comes from the couple produced by the load coming into the system "truss depth" below the actual diaphragm. Uplift = (P x truss depth)/length of blocking system.

As to Celt83's point about the sub-diaphragm, that is valid. What if, though, you checked the buckling in the sheathing? Could potentially tighten the spacing of the first few trusses to help.

 

[JStructsteel]

Thanks guys, once I calc out my reactions force, I might post my final detail to see if it can be improved. This is a commercial building, so the wall height is a bit taller than a normal basement and I dont want to ignore that reaction, or assume the sheathing can take it.

 

[Celt83]

XR: the diaphragm can likely handle the reaction, was more focused on the attachment between the wall and sheathing. I’m used to seeing these top reactions in the neighborhood of 900 plf which is difficult to pass through a single attachment point to 15/32 sheathing.

Some snips from the book:

 

[XR250]

Thanks Celt.

Interesting thread from years past which discusses these issues...

https://www.eng-tips.com/viewthread.cfm?qid=166096

 

[XR250]

I’m used to seeing these top reactions in the neighborhood of 900 plf which is difficult to pass through a single attachment point to 15/32 sheathing.

Hopefully no one is using 15/32 sheathing for floors anymore! Have been in a few that did and it felt very springy. 900 plf is def. up there and is a challenge and should likely be addressed in other ways.
I think the IBC only requires the wall to be braced every 8 ft. so I'm not sure how that math works out!

 

[Celt83]

Nice find, I’ve been grabbing up some older books lately and finding a lot of back up calculations for some typical details that have been lost to time. The older books tend to explain most topics more thoroughly as well.

 

[Celt83]

Tend to see the 15/32 in houses a lot, but your right it is springy I don’t like to go less than 3/4 on floors.

 

[JStructsteel]

Im using 3/4" nominal floor sheathing.

 

[jerseyshore]

It's very rare around here to see a typical house basement wall going up to the sheathing. Nearly all have rim joists. For new construction (or basement wall repair jobs) I usually specify blocking for the first 2 or 3 joist bays at 4' o/c.

For taller or commercial projects I can understand running the wall up, but I'd probably still use the same blocking layout.