ASHRAE Journal - November 2010 - 37

Figure 2a (left): Masonry Wall Sill Detail. Wood buck with plywood lip creates under window gutter. Note the foam “filet” “picture-framing” the opening. Figure 2b (right): Masonry Wall Head Detail. The foam “filet” is sprayed first to prevent the head flashing from being pulled upward leading to a negative slope. membrane to provide a drained opening (an under window “gutter” that directs water to the outside face of the SPF should the window assembly leak). This approach allows standard window installation approaches to be used. Also notice how the membrane extends around the opening onto the face of the masonry backup wall or on the face of the sheathing over the steel stud backup wall. This is called “raccooning” because of the eyes of a raccoon, not when your mascara runs or periorbital ecchymosis. With this approach, the windows can be installed either before or after the application of the SPF. Now for the secret double probation stuff that no one tells you about. Notice in the images the use of “gray” shading to “define” something called a spray foam “filet”? Upon application, SPF tends to pull away from some surfaces; shrinkage is a term sometimes used. This is a problem at openings. To prevent a gap from opening up between the edge of the extended buck and the SPF, the perimeter of the opening is “picture-framed” with SPF in a “filet” shape/geometry. Then the field of the wall is sprayed. It’s easy and it works, but you have to know that you have to do it. This is a big deal at the tops of windows because if you don’t do it the flashings can be pulled upward, resulting in a negative slope on the flashings. If you get a negative slope on a flashing, it will ruin your day. In fact, all flashing should have the “filet” treatment first before the field of the wall gets sprayed. Memo to architects: treat door openings the same way. Sort of. Don’t forget the head flashings, and don’t pretend that the masonry jamb trim can be used as part of the water control. Take the trim off the freaking doors first, OK? Install the “trim less” doorframes. Water manage the opening. Then install the trim. Concrete frame buildings shrink over time (Figure 4). Yup. They get shorter. Structural engineers don’t use the word “shrink” because that would make things understandable to non-structural engineers, and we can’t have that. Sometimes the word “creep” is used; sometimes the phrase “frame-shortening” is used. Sometimes it is just kept a secret among the structural engineers. This can be a lot of fun when you use brick because sometimes the brick expands when its moisture content equilibrates after it comes out of the kiln, and it does this equilibration while it is on the building. When the frame gets shorter, and the cladding gets taller, things get stressed, especially the structural engineer. Now, structural engineers are incredibly clever and resourceful folks. This problem was solved long ago with the use of “relieving angles”§ and “soft” joints. Check out the “soft” joints in Figures 5 and 6a and the concept of the “nested” track in Figure 6b. I told you they were clever. The relieving angle is shown in Figure 7. Also note how the relieving angle is held away from the wall

Figure 3a (top): Steel Frame Wall Sill Detail. Don’t forget the importance of the foam “filet.” Figure 3b (bottom): Steel Frame Wall Head Detail. A negative slope is not a positive development.

Frame shortening due to elastic shrinkage and creep

Figure 4: Frame Shortening. How much and how soon? More than you think and a decade or two. Around an inch or two over 20 stories, but this is very variable. on brackets (or “stand-offs”) to control thermal bridging.# See, they are being clever again.

§Lots of folks call them “shelf angles,” but I prefer the term “relieving angle” because it better describes their function. It’s that terminology thing with me again.

Can’t help it. #October 2007 ASHRAE Journal Building Sciences column “A Bridge Too Far” (“Clint Eastwood Thermodynamics” in Figure 2, Photos 6a and 6b).

November 2010

ASHRAE Journal

ASHRAE Journal - November 2010

Table of Contents for the Digital Edition of ASHRAE Journal - November 2010