Diaphragm chord design with cold-formed steel (CFS)

[Robbiee]

Hello all,
Looking to see what works better in terms of the design of a diaphragm chord in a building constructed with hollow-core slabs, CFS stud walls as load bearing walls and cast-in-place concrete shear walls. The two options I am thinking of are:
1- CFS straps at the edges of the slabs.
2- Use the tracks of the CFS studs.
In both options, the chord needs to be continuous. How best to make CFS straps or tracks continuous? is welding a common practices with CFS? If welding is an option, what type of welding or notes are needed to be included on the drawings? I never specified welding for CFS before.
If we use splicing plate/strap to make the chord continuous, we could end up with so many screws to connect the two sides. What did you do?
Thanks for the input.

 

[DaveAtkins]

I would use the track as your chord, and put a splice plate on each side of the track.

 

[XR250]

How much is your chord demand?

 

[Robbiee]

How much is your chord demand?

66 kN (about 15kips)

 

[bones206]

How about a continuous HSS bearing distribution member below the planks? Could also be used as a chord.

 

[Robbiee]

How about a continuous HSS bearing distribution member below the planks? Could also be used as a chord.

It could. But is it a good idea cost wise? I don't think so.

 

[XR250]

Might be a great idea with a 15k demand. Makes a lot of things easier.

 

[Aesur]

It could. But is it a good idea cost wise? I don't think so.

What are the hollow core planks currently sitting on, was a track checked as beam to see if it could support the loading and transfer to the studs, it may make more sense than originally thought when considering this. It is common to use a HSS on top of the top track for vertical loading in CFS with heavier buildings and if doing so why not make it the chord as well.

 

[bones206]

The continuous HSS is also useful for tying everything together when you have shop-fabbed wall panel segments (i.e., discontinuous top tracks).

 

[Robbiee]

What are the hollow core planks currently sitting on, was a track checked as beam to see if it could support the loading and transfer to the studs, it may make more sense than originally thought when considering this. It is common to use a HSS on top of the top track for vertical loading in CFS with heavier buildings and if doing so why not make it the chord as well.

Thanks. For the track to work as a beam in this building, the hollow core slab needs to bend between the 16" spaced studs. The track doesn't work as a beam here. The proper analysis, in my opinion, is that the edge of the slab is supported on point loads, being the studs. The composite behavior, I think, is that of a strut and tie model consisting of concrete struts in the slab and steel tie of the track at the bottom. I can use the use of HSS if the floor is more flexible or floor joists placed in-between the studs.

 

[KootK]

Track feels a little light duty to me for a precast floor deck chord.

I've seen this done two ways in the past:

1) continuous HSS distribution member on the supporting studs.

2) cast a chord in place along with the topping if there is one. On CFM, you'll be more spatially confined than the detail below but I've seen it done / attempted none the less.

One thing to keep in mind is that, if there are any jogs in the exerior bearing wall, you'll have those same jogs in either of the chord systems mentioned above. It's an annoying feature of these systems.

 

[KootK]

The proper analysis, in my opinion, is that the edge of the slab is supported on point loads, being the studs.

Agreed. The planks need to span transversely if they bear right on the track below.

The composite behavior, I think, is that of a strut and tie model consisting of concrete struts in the slab and steel tie of the track at the bottom.

Precasters handle this more simply. You can estimate the punching capacity of a stud landing right under a void. The capacity works out to be very high for circular voids as the concrete above and below them forms kind of a kick-ass plain concrete arch bridge.

But is it a good idea cost wise? I don't think so.

I wouldn't rule out the HSS based on the cost of it. This is kind of a mission critical aspect of your building where you transition from heavy, stiff stuff (precast) to flimsy, buckly stuff ( CFM). It's kind of like a foundation element in that sense. You want to have some robust answer for the transition or you'll be tearing your hair out worrying about it for the next thirty years.

All money is real money but the cost of the HSS isn't going to make or break the budget on this project.

For erection safety, one really wants to weld the plank down to something as it's set. Here that wants to be an overhead weld to an embed in the bottom of the plank. It will take a bit of skill to overhead weld embeds to CFM track in the field without just burning through it.

 

[Robbiee]

Agreed. The planks need to span transversely if they bear right on the track below.

Precasters handle this more simply. You can estimate the punching capacity of a stud landing right under a void. The capacity works out to be very high for circular voids as the concrete above and below them forms kind of a kick-ass plain concrete arch bridge.

I wouldn't rule out the HSS based on the cost of it. This is kind of a mission critical aspect of your building where you transition from heavy, stiff stuff (precast) to flimsy, buckly stuff ( CFM). It's kind of like a foundation element in that sense. You want to have some robust answer for the transition or you'll be tearing your hair out worrying about it for the next thirty years.

All money is real money but the cost of the HSS isn't going to make or break the budget on this project.

For erection safety, one really wants to weld the plank down to something as it's set. Here that wants to be an overhead weld to an embed in the bottom of the plank. It will take a bit of skill to overhead weld embeds to CFM track in the field without just burning through it.

Thanks KootK,

On the last point, I ruled out the HSS as an option for this building because it can be done with the tracks. It sounds like you understood that I considered money over the "proper and safe design". That is definitely not the case.

 

[bones206]

Erection safety shouldn't be overlooked. These plank buildings on light-framed walls collapse during construction way too often in my opinion.

 

[KootK]

It sounds like you understood that I considered money over the "proper and safe design".

Hell no. That was not my understanding and, to my knowledge not what I said. I chose my words very carefully and stand by them. In summary:

1) This is an important condition, and not just for safety.

2) The HSS dollars are small dollars, particularly if they confer any benefits with regards to erection, prefabrication, etc.

3) Don't let the cost of the HSS steer your towards any solution that you don't feel cozy about here.

I absolutely did not intend to accuse you of any ethical misbehavior.

 

[KootK]

These plank buildings on light-framed walls collapse during construction way too often in my opinion.

In part, I feel as I do about this topic precisely because I was involved in such a case forensically / legally. I also used to be a part time precast engineer. In my opinion, many of the better fabricators will likely insist on the HSS or something like it as a condition of their involvement in the project. Heck, one could probably just include it in their scope and practically get it for free.