Fastening metal panels through 2" insulation - Tek Screws go into bending?

[Jc67roch]

I am assisting a contractor with calculations for fastening corrugate metal panels to wall girts on a structure. There will be 2" of rigid foam insulation between the metal siding and the girts. Thus, the screws will penetrate throught the insulation and into the light gauge metal girt (hat channels) for fastening metal panel and insulation to the wall. The question has been raised, does this then put the Tek screws into bending as they carry the weight of the metal panels over a moment arm the lenght of the 2 inches of insulation? Or can I consider the Tek screws as holding the metal and insulation tight enough together that they act as a sandwich (due to friction/attachment) and thus the Tek screws only see shear?

 

[Jerehmy]

I'd try and use blocking.

 

[XR250]

Can they be supported at the base?

 

[a2mfk]

I don't think a light gage hat channel will offer much moment resistance. I think you would get local yielding as the screw bends the channel material. I would think due to the compressibility of the rigid insulation that you will get some good clamping (friction) force. The rigid foam insulation would also yield in any shear action, so I think the screw will mainly be in tension with some bending stress due to the channel.

The only load here is the weight of the rigid insulation and the metal siding?

 

[CANPRO]

"The only load here is the weight of the rigid insulation and the metal siding?" Plus the suction from the wind. If the channel is a light gauge...which it probably is, you're not going to get much pull out resistance from the screw.

I would think that the screws would be subjected to bending. As mentioned above, the rigid foam might reduce that somewhat...but how do you quantify it? I would assume it doesn't do anything.

Can you add another channel or clips where the edge of the panels are going to be to offer some solid support?

 

[a2mfk]

CANEIT- of course wind! I should not have left that out. And you are right, better check the pull out resistance of that hat channel closely. Probably not much seismic concern in such a light cladding.

It would seem there is some type of proprietary product for this exact application like a clip as CANEIT mentioned that would ensure you get the proper load transfer.

 

[SteelPE]

I have designed a few of these light guage girt systems in the past and I can tell you that there is not way a light guage girt will support the weight of a panel system. The systems are designed for maximum spans with maximum spacing of the girts. The systems can barely support the required wind loads. Sag rods are sometimes installed to prevent the sag of the girt system only.... not support the weight of the siding. The siding is typically supported at the base,floor or roof. So, the load on your fasteners would only be due to wind pressures only. Others may have varying opinions, but that's the way I was taught and the way I have seen others do it.

 

[hokie66]

Agree with Excel and SteelPE. Carry the gravity load of the panel at the bottom.

 

[dik]

Often use a liner panel which acts as a vapour barrier with joints sealed using double sticky butyl tape and 'Z' sections the thickness of the insulation and the outer cladding is screwed to the Z sections.

Dik

 

[Jc67roch]

Thank you all for the great input! The girts are attached to the building structure at 12" on center. A check of the girts as well as the pull out capacity of the Tek screws showed both to be adequate for the wind loads. Bending of the screws turned out to be the only questionable calc - and as CANEIT pointed out, a tough one to quantify. I did however find a great reference -

http://c.ymcdn.com/sites/

http://www.nibs.org/resource/resmgr/BEST/BEST1_015.pdf

- for continuous insulation systems which seems to show the best way to do this. Install a continuous sheathing on the structure first - as a thermal break - then the girts to that with the rigid insulation between the girts. The metal panels are then attached directly to the girts. Thus there is more of a continuous load path from the exterior without the insulation between, and depending on the fasteners.

 

[jayrod12]

If you're going to install the continuous sheathing behind the girts why bother installing the girts at all?