Q: Can you help me understand the dynamics of steel stud curtainwall systems where the wall terminates into a concrete floor slab?
A: This is the one area of the exterior envelope that experiences what is termed “differential movement.” The definition of differential movement is when adjacent building elements, in this case the steel framing, the exterior cladding and the floor system, all move independent of each other. There are forces at play, typically environmental in nature, that cause these elements to move. Also, the directions of the forces are both horizontal as in wind loads and vertical as in the gravity loads on the floor system. This all leads up to providing allowance for differential movement at the head of the cold-formed steel curtain wall stud. To be effective, the allowance should carry through the exterior cladding. Also, there shouldn’t be any architectural feature that bridges the gap and is rigidly attached to both sides of that gap. A good design and installation not only takes into account the various forces at this location, but also the material properties and limitations of all the building materials to be installed. A systems’ approach where all the adjacent materials are considered must be taken.
Looking a little closer at these forces, it’s interesting to note that in high-rise structural steel buildings, solar gain on the exterior skin is the limiting criterion for design, even over floor deflection under live loads and concrete creep. This is based on studies that were done many years ago, and the building that required the analysis is now considered iconic and towers more than 100 stories in height. Structures that use steel framed curtain walls are not subjected to that much movement from solar gain. Commonly the vertical deflection of the concrete floor dictates the curtain wall/slab edge design. That information should not be left to the contractor to determine. It should be clearly provided within the contract documents.
In steel stud curtain wall design, there are three types of designs: infill, fly-by and spandrel. This discussion will focus on the infill design. The infill design is where the steel framing starts at the lower floor and terminates under the higher floor. A steel track is installed on the lower floor to receive the steel studs and a specially designed, long-leg track is installed on the bottom side of the higher floor. The legs of this track must be long enough to accommodate the anticipated movement, and the steel thick enough to adequately transfer the wind loads back to the structure. Special attention must be made to the interface of the stud to the top track. The track must not only be allowed to move vertically up and down, but also must adequately support the stud for load transfer. This is the critical part of the structural design of the steel framing system. The wind loads can be significant, with loads of 40 pounds per square foot not uncommon. The design of the stud is fairly straightforward; it’s a simple span condition with a uniform load. The allowable stud deflection will be set by the exterior cladding. The trick will be to transfer that uniform load to the structure while accommodating the vertical displacement of the floor system. Your steel framing manufacturer partner can be an invaluable source of information.
Both the gypsum panel on the inside of the curtain wall stud and the sheathing on the outside of the curtain wall stud must be installed in such a way as to allow for the floor movement. On the inside of the curtain wall, the gap between the floor and the top of the gypsum panel should be caulked with a sealant. The gap on the outside of the curtainwall system can be a little more challenging. All the materials must allow for this movement. This includes any air/water barriers, continuous insulation and the exterior cladding. The challenge is allowing for the movement, maintaining the continuity of the air/water barrier and meeting the aesthetic requirements of the project.
In many cases, the exterior side of the sheathing is in the same plane as the edge of the concrete floor system. That works well with maintaining the continuity of the barrier. Most air/water barrier manufacturers have an accessory that is called a “transition membrane.” This membrane can be visualized as a tape that bonds to the face of the sheathing, spans the movement gap and then bonds to the concrete above. Proper installation should have the membrane loop into the gap and create a “bellows.” After the membrane is installed, the air/water barrier is applied over its face. The question of how much of a gap this detail handle must be discussed with the barrier manufacturer.
The continuous insulation must maintain the required movement gap. That means there is a potential thermal shunt through the exterior envelope. Some cavity insulation has a “memory” and can compress and recover as the floor moves.
An important design decision is how to account for the movement—and do it with aesthetics in mind. The simplest solution is to include a horizontal joint right at the bottom side of the floor system. Even this may have issues to consider, such as how to support the cladding as it passes over the edge of the floor.
As you can see, this is a very complex design with many important factors that need to be considered. The successful application requires that the contractor research all the variables involved. A team approach to determine a successful installation is warranted. This entails conversations with the all involved material manufacturers and the architect of record, which might include an exterior envelope consultant.
Robert Grupe is AWCI’s director of technical services. Send your questions to [email protected], or call him at (703) 538.1611.
