Wall Prioritization
Q: What can you tell me about wall prioritization?
A: The term is typically used in regard to fire-resistive walls and buildings that fall under the International Building Codes. The intent is to assure the continuity of the required fire resistance throughout the building structure itself. In increasing numbers, authorities having jurisdiction are requiring either through plan review or actual inspection, that this critical continuity requirement is met. The impact on the contractor is potentially a significant change in framing practices and gypsum panel installation sequencing.
Definitions of the various types of fire-resistive walls were provided in a prior article. The International Building Code categorizes these walls as being a fire partition, a fire barrier or a fire wall. The key differential between these walls is what is termed “continuity.” The code has specific language as to the vertical continuity of the three walls. The fire wall is the most restrictive, the fire partition is the least restrictive. In the first case, the wall itself must be continuously vertical throughout the structure. This separates one building into two independent structures. The fire partition requires only the vertical continuity to be maintained in concealed spaces such as ceiling plenums with fire blocking.
The location and application of these walls are also dictated by the IBC. For example, fire partitions are used to separate individual dwelling units within the same building. They are also required for corridor walls. Fire barriers include shafts and exit passageways. Fire walls are used to separate a large building into two separate structures.
The continuity also has a horizontal component. The fire-resistive wall must be continuous, in a single-story context, across the required wall length. Where this gets challenging is where the fire-resistive wall intersects other walls along its run. The underlying theory is that where two walls meet, the wall with the highest fire-resistive rating has priority and therefore must be continuous through that wall intersection. An example of this would be where a two-hour fire-resistive wall intersects a one-hour, or even a non-rated wall.
The simplest condition would be where the lower rated wall terminates into a higher rated wall. Should a one-hour wall terminate into a two-hour wall, the two-hour wall must be continuous. Figure 1, which shows abutting detail, of ASTM C754, Standard Specification for Installation of Steel Framing Members to Receive Screw-Attached Gypsum Panel Products, would be a reference for this condition. To meet wall prioritization in the above example, the continuous wall has the two-hour rating. This means there is a double layer of 5/8-inch Type X gypsum panel on both sides of the cold-formed steel framing. The abutting partition, be it fire-resistive rated or not, would terminate into the second layer gypsum panel on the continuous wall.
There is a trade-off related to wall prioritization. If the wall that is abutting into the higher rated wall is to have a sound transmission classification, its net acoustical rating will be impaired. The continuity of the gypsum panel on the higher fire-resistive–rated wall will introduce a flanking path, thus reducing the sound performance. These conditions would be best analyzed by a recognized acoustical engineer.
The detail gets challenging if the abutting wall has the higher fire-resistive rating. In this condition the abutting wall must be continuous through the other wall. This includes both the gypsum panels and the steel framing. The gypsum panels of the one side of the lower rated wall must terminate into the higher rated wall. This will require that special attention is required by the contractor for framing and gypsum panel installation.
Another condition is shown as “Figure 2, Partition Corner” detail found in that same ASTM standard. There are two methods illustrated for making a partition corner. Sometimes this is called a 3-stud corner (figure 3) and a 2-stud corner. For wall prioritization reasons, many municipalities and authorities require the corner detail shown as figure 2. Here the one wall is continuous, both in gypsum and steel framing through the intersection. The steel track on the one wall is continuous through the intersection while the other is positioned in such a way as to allow for the continuity of the gypsum panel.
The third and final condition is the most challenging with the combination of figures 1 and 2. This would the case where the abutting wall in figure 1 has the higher rating, but instead of terminating into the other wall, it turns and continues along one of the legs of the other wall. The gap in the one steel track must be maintained as discussed earlier to allow for the continuity of the gypsum. This same track may have to be offset on one side by the thickness of a gypsum panel. This would be required to allow the gypsum panels on one side to be continuous and remain in plane. The offset should be considered during layout.
Wall prioritization can be a challenge and certainly a hazard to the contractor who is not aware of the required nuances in framing and sequencing. There are resources available to learn more about this and be prepared. The successful project requires that the critical information be understood prior to the setting of the first steel track.
Robert Grupe is AWCI’s director of technical services. Send your questions to [email protected], or call him directly at (703) 538.1611.
