Curtain Wall Glazing Offset

[ron.sanpedro]

Can anyone explain the 1.25" offset between the center of the glazing and the center of the wall definition in a curtain wall? I am using the default Curtain Wall 1 as an example.
I would have expected an offset of 0 to result in the glazing centered in the wall, but the exterior face of the glazing is actually 3/4" in from the center. Indeed I cannot figure out how I would get this curtain wall to actually center in a wall if i wanted to. Is this a common condition in real world curtain systems? I am just trying to understand it, so I can explain it to others. If you understand why something is weird it suddenly becomes less odd.

Thanks,
Gordon

[D_Driver]

I will give it a go...What you typically see in curtain wall or storefront is just the outside of what can be a very complex system. While it may appear as a single extruded piece of alumimun, the frames can be up to 5 or 6 parts that are snapped or mechanically fastened together.

To simplify and over generalize; curtain wall frames typically have 2 general parts
the inner part looks like a rectangle with a tongue sticking out one end where the glazing joins and a second piece (an outer cap).
You can sometimes specify the length of both the inner and outter pieces
the outter cap comes in 1/2", 1 1/2", 2 1/2" sizes (give or take a quarter inch). Each manufacture has its own sizes, often series or models come in limited sizes. Likewise with the interior part that comes (roughly) in sizes that range from 3 to 8 inches depending on manufacturer and series or model.
Of course these systems come in inverse (Inside glazed) that are essentially backwords for a more expressed mullion on the exterior of the building.

Yes you can probably find a manufacture that will supply a 2 3/4" cap and a 2 3/4" backmember, but the backmember a different size than the cap is more common.

While I could not quote the exact manufacture that provies the default 3 1/4" back member, 1" glass, 3/4" front member (defaults in template), there probably is one.

Kawneer and Wausau are two companies I am familiar with.
http://www.alcoa.com/bcs/kawneer_namer/en/home.asp
Here is a generalized detail sheet for wausau
http://www.wausauwindow.com/resources/literature/SweetsCurtainwall.pdf