CFS Strap Brace Track Assumptions

[EngStuff]

Designing a multistory Strap brace. When a track is in compression and the web of the track is connected to the diaphragm with studs spaced @ 24” o.c. does the track have to be designed for compression for the full length or can it at a shorter length? Example, in the attached picture, if he Strap Brace is 20 feet, and there is 10,000 lbs of compression loading, does the Track have to be designed based on a 20-foot-long chord? If connected at 12” on center at the diaphragm w/ screws and studs are spaced at 24” o.c., can we essentially drop the length when checking compression to 2’-0” or would this be only considered as Flexural Bracing and Axial KyLy, KtLt.

Also, would the design be similar when the force is from the roof diaphragm compared to a floor diaphragm? Since for the track at the floor levels, the joists/studs are compressing the track web on both sides @ 24” o.c.

 

[phamENG]

I think the better question is: what does it mean to be braced? After all, those lengths are unbraced lengths. So you need to look into what is required to consider that track braced at some length less than the physical member length. For general stability bracing Yura is a great source. Light gauge has some other unique qualities that allow it to buckle in ways structural steel doesn't, so you need to look at that. This isn't exactly tracks, but it's close - and should help a bit.

Be careful with one sided strap braces. They put the track and studs at the connection point into some pretty serious torsion. I've never actually seen a multistory example work when this is considered. Not to say it can't, but I haven't seen it done.

 

[DaveAtkins]

In my opinion, the track is braced vertically by the GWB or plywood, and the track is braced horizontally by the floor or roof structure. I have never checked for buckling in this situation.

DaveAtkins

 

[EngStuff]

Be careful with one sided strap braces. They put the track and studs at the connection point into some pretty serious torsion. I've never actually seen a multistory example work when this is considered. Not to say it can't, but I haven't seen it done.

I will be doing 2-sided strap brace because of the issue you just mentioned. Much cleaner with transfer of forces(less calculations) plus it helps with limiting the FL to FL drift. The sketch I attached was just a one standard detail I marked up.

I think the better question is: what does it mean to be braced? After all, those lengths are unbraced lengths. So you need to look into what is required to consider that track braced at some length less than the physical member length. For general stability bracing Yura is a great source. Light gauge has some other unique qualities that allow it to buckle in ways structural steel doesn't, so you need to look at that. This isn't exactly tracks, but it's close - and should help a bit.

I have previously read the document you linked, and had previously gone through it, in addition to the 5th addition cold formed steel design book, also plenty of tech notes with strap braces and shear walls and manuals, but no where do we have an example of the track situation besides a splice design. The situation i have always seen is the typical stud/chord bracing where the assumptions are a bit different in my opinion. Unless there is a section or document I have overlooked.

In my opinion, the track is braced vertically by the GWB or plywood, and the track is braced horizontally by the floor or roof structure. I have never checked for buckling in this situation.

That's what I am thinking too, especially for the floors, the track Web is being compressed by the top/bottom walls. However, the flanges are braced by the studs.

I can see the assumption of the bridging being spaced 12" o.c. and analyzing the track that way. However, in my opinion bridging doesn't work like a diaphragm would. Bridging is just "bracing" (kyly, ktlt) the member and will drive an Elastic buckling, but a diaphragm is restraining the web of the member and pushing it to inelastic buckling. The tricky part; the flange would still be assumed braced instead of restrained. Like phamENG mentioned, "what does it mean to be braced?".

I hope I am making sense with what I am trying to get across.

 

[XR250]

Lots of things to check here besides the bracing. Is the gusset plate stiff enough to prevent weak axis bending of the track and jamb studs? This is my issue with bracing of this type. I typically show a deep leg track and attach the strap to that coincident withe the jamb stud. This remove the weak axis bending from the equation.