Buildings Will Move
Robert Grupe / December 2015
Q: I’m working on a commercial mid-rise project. Is there anything that I should be aware of regarding interior partition details and the impact of building movement?
A: All buildings are not static; they will move. There are two sources of building movement. The environment induces forces. The most common are wind, thermal, moisture and seismic. The second source is structural in nature. The weight of the building and its occupants are examples. Structural loads lend to live load deflection on floor systems, or what is termed “creep” in a flat plate concrete floor. This movement, if unaccounted for in design or construction, can lead to the cracking of the gypsum panel.
Tall buildings are designed to limit the amount of drift or sway that a building can move when under a wind load. This is called “allowable drift,” and this limit is set by the ratio of the height of the building divided by 500. This is very similar to the deflection limits for floor and wall design. Unlike floor and wall design where the limit is based on minimizing the cracking potential for finish materials, the allowable drift is also concerned with the human response to movement. In design there is the allowable drift for the entire structure, and there is “interstory drift” as well. This is more critical to interior partition design and construction. It can be thought of as the bottom track of a steel stud partition being stationary while the top track moves in a parallel direction to the run of the wall. The wall is being racked from its natural position.
All building materials react to temperature change. They either expand or contract with temperature. Buildings are no different. Studies done in the past show that a structural steel building that is 10 stories tall will change half an inch over a 70 degree Fahrenheit change in temperature. The taller the building, the more it will move. A 50-story building will move 2 3/4 inches with the same change in temperature. Essentially, one side of a building will expand while the other side remains unchanged. This differential movement results in a racking force on the interior partitions.
An analysis on movement was made on one of the early modern high-rise buildings. What’s interesting to note is that the movement caused by this thermal change was three times greater than the movement caused through the more common deflection live load on the floors. Cracks that may occur in gypsum partitions then tend to run at a 45 degree angle. Conversely, excessive floor movement tends to cause a vertical crack.
Building movement from moisture changes are more critical in wood-framed buildings. The most recognized result was the occurrence of nail pops. Nail pops were the result of wood shrinkage over time.
Wood shrinkage will vary with the direction of the grain. The minimum wood shrinkage will be along the grain of the wood whereas the maximum will be across the grain. In platform framing, gypsum panels should be allowed to float over the floor lines, or some type of mechanism break the continuity of the gypsum.
Now that we understand how a building will move and the stresses that are induced in the partition, we can start to provide a means to counteract this phenomenon. The primary concept is to not tie the partition directly to the structure. The building will move, and the partition must be installed in such a way as to be structurally stable while accommodating the movement. The many existing “head-of-wall” designs that are available follow that concept. The intent is to rigidly attach the top track to structure while the studs and gypsum panels are allowed to “float.” This means that the track moves with the building and the wall below stays stationary. Of primary concern in those designs is to maintain the integrity of the fire resistance of the wall. They also provide relief from wind and thermal induced building movement. But that is not enough. It is important that some type of head-of-wall relief be used for all walls, not just those that require fire resistance.
The walls should be isolated from the structure at all locations. Partition assemblies and ceilings should not be directly tied to interior or exterior columns. There are available details to cover these conditions. Where the wall terminates at the exterior column, the first stud can be directly attached to that column. The next stud in that run of wall should be spaced no more than 6 inches from that first stud. The gypsum panels are then attached and only attached to the second stud. No fasteners are used in the first stud. The first stud then behaves as that top track, moving with the exterior column, while the rest of the wall remains stationary. There should be a gap between the gypsum panel and the exterior column. The gap is typically a nominal half-inch, and that gap should be filled with a sealant. The leading edge of gypsum panel should be covered with a trim piece, and the whole edge finished with joint compound.
There are two items to remember when working on mid- to high-rise construction. First, there are special precautions to be undertaken to minimize the potential for cracks in gypsum panels. Second, there are details and experience out there available for you to explore and provide that successful installation.
Robert Grupe is AWCI’s acting director of technical services. He can be reached at (703) 538.1611 or firstname.lastname@example.org.