Cladding fixing to lightweight steel stud 1

[phuduhudu]

I am having a few issues working out the capacity of fixings for a heavy cast iron cladding system to some lightweight steel stud framing. The studs are 150x50x1.2mm lipped channel sections. They are clad in 12mm of gypsum board. Then we fix a stainless steel Z-rail through this to carry the cladding panels - see image

I have assumed the offset creates a moment which is taken by the top screw in tension with the bottom of the Z rail in compression against the plasterboard so then I would need to check the plasterboard for capacity in compression assuming a compression block of some height.

Then I can get published design capacities for self drilling screws in tension and shear but not with an intervening layer of plasterboard. Also even if the screw has the capacity to hold in the 1.2mm stud would I not need to check for local capacity of the sheet metal over the 50mm width between the web and lip of the stud.

The cladding guys assure me this is normal but I am a bit nervous about justifying this. I get a max vertical shear of 640N and a moment of 190Nm at one of these fixing points which gives some big forces with a lever arm of only 75mm between the top screw and bottom of the rail.

 

[jjl317]

When we face these conditions, we generally run a continuous horizontal member (hat channel shown, or potentially a track/channel) outside of the sheathing - see attached.

Then, the outer anchor can be located wherever needed (independent of stud locations) and have as many screws as required to support the load. And the continuous member can be sized appropriately to span between studs and be attached with (1) or (2) screws top and bottom, to address the eccentricity. These screws are generally limited by pullout/bearing on the studs, and we would generally consider bending in these screws over the thickness of the sheathing (especially if gypsum).

Hope this helps

 

[phuduhudu]

Thanks. At the moment the Z rail is also continuous but your suggestion is a good way to increase the lever arm on the eccentric load and so reduce the screw axial forces. However, I will still need to justify the axial and shear forces on the screws and on the thin gauge stud material and board which I am struggling to do.

 

[jjl317]

The 1.2 mm thickness appears to be about the same as 18 gage here in the US, and for exterior work, we generally see 16 gage (approximately 1.6mm) minimum.

Sometimes we will go back to the architect/design team and ask for the studs to be increased in thickness.

Unfortunately, we find this issue has been coming up more often, as many jobs are light gage stud framed, with relatively heavy cladding at a large cantilever to make room for insulation.

Good luck