Back-to-Back C-channels

[JJMEI]

I am trying to come up with a clever way to connect (2) C15X33.9 so that there is at least a 3" gap for insulation. The reason for this is that one channel is exposed and only for aesthetics while the inside channel is structural. The 3" gap is supposed to create a thermal break as required by the energy code. What is the easiest most efficient way to connect these sections and still have a sufficient thermal break also, I would like to utilize the combined moment of inertia in the horizontal direction for lateral deflection criteria. Connected to the outside channel is 20 feet of storefront. What type of connection would allow me to do this? Has anyone ever dealt with this situation before? I appreciate the free consulting, by the way I'm an engineer in training.

 

[jayrod12]

how about back to back L2.5x2.5x0.188 oriented vertically at some spacing? That's how we do our brick angle ledgers to create a gap for insulation. It's obviously not a perfect thermal break but it's the best we are willing to do. It minimizes the thermal bridge.

 

[rb1957]

how about a 3" wide channel, oriented horizontally ? ... several short pieces, rather than a long continuous piece (less thermal losses).

btw, if you mean to use both channels structurally, then the connection between them needs to be structural, which seems at odds with your thermal issue.

another day in paradise, or is paradise one day closer ?

 

[jike]

For normal conditions, most structural engineers would use an intermittent structural connection between channels plus insulation between connections. For more critical conditions, there are thermal blocks and fasteners that are specially manufactured which can be used (Do a Google search).

On an extremely cold day the cold can be transmitted to the interior channel thru the structural connections. It can cause frosting on the interior channel or worse dripping.

 

[DaveAtkins]

For waterfront structures (bulkheads), when a double channel is used as a waler, sometimes the two channels are connected with short pieces of pipe.

DaveAtkins

 

[calvinandhobbes10]

Another suggestion: 3" wide x 1/4" thick 6" long steel plates (or whatever dims you need), intermittent say every two feet, laid flat on top and bottom of the channel, fillet-welded. Can be done shop or field, and is easy weld access. Ties it together like the built-up shape you're looking for. Can't completely avoid the thermal breaks.

Curious---what closes the insulation cavity at the head? Anything you can use (light-gage angle, etc)?

 

[JJMEI]

The T.O. steel is bearing for open web joists with a metal roof deck.

 

[KootK]

The T.O. steel is bearing for open web joists with a metal roof deck.

Are you grabbing both channels with the joist seat somehow then? Granted, that in itself would be a pretty terrible thermal break. If you're not grabbing both channels, you may have a rather undesirable torsion situation at play in the interior channel unless you're intending to make the two channels behave compositely for twist as well.

For more critical conditions, there are thermal blocks and fasteners that are specially manufactured which can be used (Do a Google search).

Along these lines, you could install thermal break pads like these at the connection locations. They come with isolation bushings etc for improved performance. They don't cost all that much; the only issue that I've had is lead time where contractors didn't stay on top of things. With regard to composite lateral behavior, the Fabreeka pads have great compression capacities so you could conceivably make something akin to a slip critical connection for horizontal shear using ordinary friction. You're never going to get real great quality control on the tension in the bolts but, for light duty serviceability criteria, perhaps you can make some compromises in that regard.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JJMEI]

Yes, the joist seat is actually extended out over both channels to support 2'-6" of overhang. So you're saying that if both channels are attached then I don't need to worry about torsion being a problem?

 

[jayrod12]

No that's not quite what he's saying.

He said that if you design the connection to ensure they act compositely in both vertical, horizontal and torsion load carrying then you may not have to deal with torsion.

But since your joist seat extends across both channels, it's less of a concern as the joist seat "should" evenly distribute the load between the two. How are you dealing with the thermal bridge from the joist seat?

 

[JJMEI]

The channels on the facade is the theme of this project, one because it's an addition so it has to match, and two because the architects want to add a ton more storefront and steel framing for a clerestory between the new and existing. So I guess the joist extensions are a small battle in a bigger war.