All the Right Moves: Live Load Limits of Glazing Systems and Buildings

By Craig Carson

The luxury apartment market, which incorporates window wall products from regional and national glass and aluminum manufacturers, is a growing area. Generally, there are two types of these products: those developed by window manufacturers based on heavier versions of their window designs, and those from unitized curtainwall manufacturers that tend to use scaled-down versions of their unitized systems. It’s natural for a company to develop their products like this, given their background, comfort level and expertise.

Lately, though , I’ve had experience with some manufacturers that have a general lack of knowledge, or maybe consideration, of how their products need to interact with the design of the building structure. Many of the window wall systems I’ve seen from some well-known companies incorporate head and sill receptors that are needed for the installation, as they tend to be used in long expanses of the openings. I believe this is mostly a way to square the opening and expedite the installation. I have to warn you, though, when we’ve asked the manufacturers if they’ve allowed for the building movement, many say yes, that they’ve allowed for ¼-inch movement. That’s not enough to capture the live load deflection, however.


In almost all cases, these residential projects are built with a post-tensioned concrete slab structure that’s very flexible for weight distribution concerning live loads. On one of our projects we asked the architect to have the engineer of record (EOR) provide the anticipated live load deflections, and found some areas exceeded 1¼-inch. The typical live load defection that we would see on poured-in-place or steel and concrete composite decks would be +/- 3/8-inch or ¾-inch overall. The 1¼-inch that the EOR had designed would have required us to design the system to 2½-inch overall movement.

After we saw this, the EOR and architect, along with the general contractor, agreed it was unacceptable. They then introduced more tendons into the perimeter of the design and were able to bring the deflection back into an acceptable range of +/- 3/8-inch. However, this was still more than the standard window wall system we were using was designed to accommodate. Since this project was a design-assist and we had made a decision to team up with a known manufacturer, we looked to them to remedy the limit of their de-sign to incorporate the required movement, which they did.


When using a system that’s derived from a unitized curtainwall, the parameters are the same but, in my experience, are designed from the beginning to allow for this expected movement. I believe that’s because these companies, unlike window manufacturers, are used to incorporating the structural review of the building within their body of producing an estimate.

Be sure to request the anticipated live load report, review it to see if the design falls into the normal deflection range and provide it to your window wall supplier to ensure they provide you a system that will accommodate this movement.

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