Filling the gaps between delegated design items

[structured]

In my office we often specify windows as a delegated design item and have them supported on CFMF walls which are also specified as a delegated design item.

My question for those of you who have similar experience is this; who is responsible for the design of the blocking between the window and the wall? Is it the SSE for the windows, the SSE for the CFMF walls, or the SEOR?

There are arguments for and against each of those options. My instinct is that the SSE for the windows should design it. The way that our specs are written they are already designing the anchorage for the window. We as the SEOR do not want to do it because we don't know for sure what the windows and CFMF are doing until after the submittal process begins. If we start designing blocking at that point it will be a change order for sure. The SSE for the CFMF will almost certainly refuse for similar reasons to us and since timber is not their specialty.

So, what do you think? Is there a standard practice here that I am missing? I've been scouring the codes, MasterSpecs, and industry articles for some guidance but so far have come up short.

 

[jayrod12]

What do you mean by blocking between the windows and metal framing. Typically around here the window openings are framed tight enough that there is only minor shims required between the window frame and the wall framing. The shimming is typically covered by the window sub.

 

[structured]

Jayrod12,

In the case of a building with a veneer and wall cavity the window will often be set out beyond the CFMF backup and over the wall cavity. To ensure that there is a solid substrate for the window to anchor to we place a piece of blocking (2x6 or 2x8 usually) on top of the CFMF and cantilevering out over the cavity underneath the window frame.

For reference, the way that we handle this situation when the backup material is CMU instead of CFMF is to provide a wide bond beam (10" or 12" wide if the rest of the CMU is 8"). This reduces the cavity width just below the window and provides the solid surface for window anchorage.

I personally don't like the blocking detail which I described in the first paragraph and would prefer to use a steel plate. Alas, I'm told "this is how we've always done it" more often than I would care to say...


I've attached an example of what I am talking about, taken from google images.

 

[jayrod12]

I get it.

Around here though, the building envelope is provided behind any veneer, therefore the window itself (being part of the building envelope) is provided at that plane, and the sills are manufactured wider to get out to the edge of veneer.

 

[structured]

See attached example

 

[jayrod12]

That's typical of old detailing. The new energy code for us does not allow that form of thermal bridging to occur. They would get crucified by the plans examiners for trying to put blocking through the insulation.

 

[structured]

I think what they are doing to address the thermal bridge issue is to hold the blocking back slightly and provide a small amount of insulation or FRP between the veneer and the blocking.

What do you see instead of this?

 

[jayrod12]

Glass line back at the face of the stud/sheathing, insulated sills, and the insulation on the face of the studs run tight to the sills.

 

[XR250]

As an almost former CFMF designer, it seems the GC always ended up with it.