Z-Furring

[XR250]

OK, with all this new energy code regs, the Architechs are now going to "outsulation" systems. Typically, light steel studs, 5/8" denglass, 4" "Z" furring and then the cladding. The insulation is installed in the 4" cavity between the denglass and the furring. The furring typically has a 4" horizontal leg and 1 1/2" vertical legs. One vertical leg is attached thru the densglass into the stud and the other is fastened to the cladding. The furring is typically run horizontally continuous along the face of the wall spaced 24" apart in the vertical direction.

So here is my question. The dead load of the cladding causes the horizontal leg of the furring to bend downwards and, as such, the vertical leg to try to pull away from the connection to the stud. Since this connection only occurs every 16", the vertical leg wants to bend outwards between connections.
What would I use for the effective width of the furring for section modulus calculations?
I checked it using a 4" length of furring and the numbers work out, but I do not know if that is a reasonable assumption.
Since the vertical leg is not a stiffened element and st light gage (typically 18ga), it has very little resistance for outward bending between connections.
Only un-stiffened leg furring is available in these sizes (to my knowledge). They could get some custom made, but that would be more expensive.

Thoughts?

 

[msquared48]

Can you post a detail of the condition?

Mike McCann
MMC Engineering

 

[XR250]

Here you go..

 

[RFreund]

Interesting.
RE effective width - Possibly use the equations from SSMA for effective width of track subject to a point load (haven't thought that through much though). You would still be faced with the stiffness problem of the steel stud.

I believe we used a similar system on a job some time ago. If I remember correctly I think we ended up welding a small channel on to the columns (this was a SS20 type building, so columns were spaced 10' apart and were small HSS columns) which cantilever into the wall system. Then the bottom of the z-girt was fasten to the top of the angle. A little more work than fastening to the studs.

EIT

http://www.HowToEngineer.com

 

[hokie66]

What type cladding? Architects often worry about air when they should be worried about water.

 

[XR250]

@hokie

It is 3/4" plywood covered with a metal skin

 

[hokie66]

As the furring members are only carrying 16" x 24" of self weight and cladding between fasteners, I think intuitively that there is no problem. I think your 4" length is reasonable.

 

[XR250]

Sounds good

 

[Ron]

This is a very flexible system and depends to some degree on allowing the cladding to bear on something at the bottom. Given the inherent flexibility, much attention should be paid to waterproofing the system. All dissimilar materials should be isolated with a properly profiled sealant joint.

 

[XR250]

Ron,

I agree. They actually show it bearing on the bottom in most locations. There is one area where it is not, but hovering a 1/4" above the support. I think we are just going to have them make it tight. The unfortunate problem with light gage steel design is that the Arch's or, EOR's for that matter, do not pay much attention to it or draw something that is totally unreasonable. By the time I get it, the sub has already bid on it and I have to try to make it work within their material budget. There are plenty of Arch's and EOR's that I would care to strangle!