Shaft Walls in High-Rises
Robert Grupe / April 2016
Q: I’ve heard that there is a change in the International Building Code on shaft wall construction for high-rise buildings. Can you clarify this new code provision?
A: Shaft wall enclosures on certain high-rise structures are now required to have impact resistant gypsum panels. The International Building Code references the shaft enclosures in question in “Section 403.2.3, Structural integrity of interior exit stairways and elevator hoistway enclosures.” The intent of the code is to create “impact-resistant enclosures around egress paths” as recommendation by Federal Emergency Management Agency in May 2002. It was one of the many that came as a result of a study conducted in the aftermath of Sept. 11, 2001.
New York City was the first to adopt language that would provide high-rise buildings with “hardened stairwells.” Soon after that, the code language was accepted on a nationwide basis in the IBC.
Buildings with an occupied floor 75 feet above the lowest level of fire department vehicle access are considered high-rise, but not all require impact-resistant panels. The impact-resistant enclosures are only for those high-rise buildings that are, because of occupancy, deemed Risk Category III or IV. Examples of these types of buildings include healthcare, education, power generation, emergency response and shelter. This is defined in IBC’s “Table 1604.5, Risk Category of Buildings and Other Structures.” Another qualifying category would be any building over 420 feet in height.
The basis for the requirement is physical testing under ASTM C 1629, Standard Classification for Abuse-Resistant Nondecorated Interior Gypsum Panel Products and Fiber-Reinforced Cement Panels. The standard addresses both surface and core damage from impact. There are two tests within the standard that determine impact resistance: soft and hard body. The hard body is simulated with a swinging ram apparatus with a concentrated force applied through a steel cylinder. The soft body is where impact energy is applied through a known leather bag filled with 60 pounds of steel pellets. The hard body can be seen as the impact of a door knob or hammer. The soft body is more like someone being pushed into the wall. Three “classification levels” of performance for both types of testing are based on the amount of energy required to either fracture the panel (soft body) or a breach (hard body) into the stud cavity.
A quick count of gypsum shaft walls listed in the Gypsum Association’s “GA 600-2015, Fire Resistance Design Manual” reveals that there are 47 different types of 2-hour fire resistant gypsum shaft wall assemblies. Most of those are based on two layers of 5/8” Type X gypsum board installed on one side of a specially designed steel shaft wall stud. On the other side of the stud and held in place by that stud without fasteners is a 1-inch gypsum liner panel. The intent of the code is to provide impact resistance on the side with the two layers of gypsum board, thus protecting the shaft enclosure from impact coming from outside the shaft.
In Section 403.2.3.1, the code requires that the “wall assembly” used for “interior exit stairways” and “elevator hoistway enclosures” must meet or exceed soft body impact classification level 2 (195 ft-lbf). This is somewhat interesting because ASTM C1629 testing is based on a per-panel basis.
The code then provides three options for selecting the wall assembly materials. The first option is to replace the two layers of Type X gypsum board with two layers of impact-resistant panels, both of which achieve a classification level 2 when tested for hard-body impact. Level 2 is stated as 100 ft-lbf. The impact panels need to have the same fire-resistant properties as the Type X panels they replaced. The second option was for a single layer of gypsum panel, but that single layer option must have a hard-body performance classification level of 3 or 150 ft-lbf. From a design standpoint, the wall must still meet the two-hour fire-resistance requirement, so there must be enough gypsum in the assembly to meet the fire rating. This could certainly be considered as an option where the wall must be finished on both sides of the shaft enclosure wall. That would allow for taking one layer of the typical two layers of 5/8” Type X and moving it to the other side of the wall, thus covering the 1 inch gypsum liner panel. The third option is more performance based. It stipulates that there are multiple panels installed on one side. These multiple panels, when tested “in tandem,” must meet the performance level category 3 or 150 ft-lbf. Moving a little further in the IBC are provisions for concrete and masonry shaftwall enclosures. Section 403.2.3.4, the last section, is based purely on performance. There is a required level of performance required no matter what type of wall is selected.
Impact-resistant panels are not required for all vertical shafts in all high-rise buildings. Not all impact-resistant panels have the same level of performance. Contractors should consult with their preferred gypsum partner to select the proper panel. Building codes are open to local interpretation, so a contractor should understand what is required in a given project’s location.
Robert Grupe is AWCI’s director of technical services. Send your questions to firstname.lastname@example.org, or call him directly at (703) 538.1611.