• Field Notes 04.01.2012

    My December 8 blog post “Condo Heaven or Hell” discussed some common pitfalls with condo glazing work, but once the decision has been made to bite into that particular pie, there are some key issues to be aware of in detailing and executing the work. After all, it’s hard to turn down any work in this economy, so here are some thoughts to help minimize the risks.

    One of the features common to many condos is floor slabs exposed or expressed on the exterior. Sometimes, the windows or curtainwalls will be recessed to form balconies.

    Due to project economics and space constraints, such floors are often pre- or post-tensioned concrete slabs that are thinner (8-12” typically) than conventional floor slabs for office buildings and larger commercial structures, which can be up to two to three times thicker. That translates into more floors for the height of the building, less area to be covered with curtain wall and windows on the exterior slab.

    But it’s these pre- or post-tensioned slabs that create a lot of issues for condo curtainwall or window wall detailing. First off, at balconies, the detailing ought to include a curb so that the sill of the windows sits 6-8” or more above the exterior floor. A curb is best utilized at sliding or swing doors, where water penetration at the sill can be a problem. This arrangement allows water to drain away from the sill, so there’s no chance of standing water against the sill sealant or waterproofing detail.

    The curb, however, creates its own headaches, most notably achieving compliance with the Americans with Disabilities Act (ADA). A curb of any height can be a hurdle to literally get over if the condominium owner has any disabilities, and so the curbs take a second hit.

    Incorporating a curb into a poured-in-place tensioned slab is a nightmare for GCs and structural engineers. The sub who has to actually form and pour it is where the rubber hits the road, and the cost to make this all work can be prohibitive. The curb is frequently offered up to the “value engineering gods,” and ends up not included in the final details. Water management becomes more difficult without a curb.

    One alternative that’s often explored is putting a step in the slab, where the exterior portion of the slab is 4-6” (or more) lower than the interior part of the slab. I’m not going to profess to know enough about structural engineering, but suffice it to say, the structural engineers will require the slab thickness to be increased throughout the entire floor plate to accomplish this. That’s an expensive proposition. So the step down gets left “on the cutting room floor,” too, never making it to the construction documents.

    All that’s left then is to ensure a sill detail that has at least two lines of defense, as the exterior finished surface of the concrete will not be that much lower than the concrete directly under the window sill. The one exception to this: if there’s any sort of paving being applied to the balcony, then the concrete MUST be lowered at the window sill so as to not form a trough between it and the window sill. The sill should never sit in its own trough or channel, there would be no place for water to drain out of most systems, which is usually at the bottom of the sealant joint. If that’s lower than the exterior surface, the sill is going to sit in water, which is NEVER an acceptable detail, condo or not.

    This is just one of the design features of condos that the window or curtainwall detailer has to address. And all of these have to be blended together for the wall to work well from a performance point of view, as well as meet the aesthetics the architects want. That’s not an easily reached balance. In a later blog, I’ll address that balance, and who gets to win in the end. A couple of other problem areas to be dealt with in future blogs are:

    1. Thermal performance of exposed slabs
    2. The effects on curtainwalls and windows of poured-in-place concrete tolerance and building movement
    3. Sliding vs. swing doors on balconies
    4. ADA compliance vs. water penetration at doors

    Throughout this whole process, either in quoting or executing the work, a question that’s critical to ask is: “Is this type of detailing worth the risks?” If already under contract to perform the work, that question’s already been answered. Please, don’t roll over and play stupid – these details demand that you pay attention to this (and all the other condo related detailing yet to come) and do it right, regardless of what the architect, general contractor or owner may request for cost savings. If you choose not to, you may have to pay for the inattention at a later point in time.

    Here’s hoping your holidays were enjoyable. I got a great Christmas Eve present: The Eagles beat the tar out of the Cowboys! But alas, it didn’t carry much meaning once the Jets laid their egg against the Giants. But 2-0 this year against the Cowboys, I’ll take that any time.

    Posted by Blogger @ 8:35 am

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