One of the many beneficial characteristics of gypsum board is the ease by which it can be cut to fit a specific dimensional application. Score one side of a sheet with a sharp blade, snap the core, and you can split a board in about ten seconds.
The downside to this attribute is the perception that gypsum board is “soft.” Because you can cut it with a knife, common logic dictates that it must not be very “tough” and durable-a concept reinforced by many Hollywood action movies that always seem compelled to incorporate a scene of the action hero d’jour smashing uninhibited through a drywall partition.
Can you smash a hole in a regular sheet of gypsum board with a hammer? Absolutely you can, and if you run around your house attacking the walls with a hammer, you will wind up with a lot of holes in your walls. If you are reasonable, however, you’ll find gypsum board to be quite strong and a multi-layer gypsum board system to be particularly durable.
Tests have proven that a single layer of 5/8-inch board installed on wood studs will resist the impact of a 50-pound leather bag of shot dropped on it from a height of approximately 30 inches and a double layer of material will resist the same impact from approximately 6 feet. A single-layer 5/8-inch wall will easily support a hanging item that imposes a load of 80 to 100 pounds when hung with a properly installed and sized toggle bolt.
And all of that is well and good for most construction applications. Unfortunately, in the modern world, some non-residential applications must provide a more robust impact resistance and, in some instances, traditional wallboard systems can’t comply with the specified criteria.
One specific instance is egress shafts in high-rise construction-specifically, stair towers and elevator hoist ways in buildings over 420 feet in height. Post- 9/11, much has been made of the need to ensure that vertical egress from high-rise buildings is not inhibited by the possible collapse of the walls that create the egress system enclosure; thus, construction standard and building code hearing committees have entertained numerous proposals to make egress systems more robust and impact-resistant over the past eight years.
Seeing the need for impact-resistant materials, the gypsum board manufacturing industry created impact-resistant products, most of which incorporate a membrane into the gypsum core that reinforces the board. These materials work quite well, often providing four to five times the impact-resistance of traditional board products, but their development did not answer one basic question: “How hard is hard enough?” For at the end of the day, the new materials were being compared only to traditional gypsum board, a material that is commonly perceived as being “soft”; however; many of the code proposals recommended a level of impact-resistance that was not exclusive to one particular material.
To satisfy the comparative analysis need, the gypsum board industry created a new standard: ASTM C 1629, Standard Classification for Abuse-Resistant Nondecorated Gypsum Panel Products and Fiber-reinforced Cement Panels. The new standard establishes classifications of abuse resistance for gypsum panel products and cement panels (think cement tile backer board on the latter item).
The new standard allowed the gypsum board manufacturing industry to specifically quantify the performance of gypsum panel and cement board materials and systems using four tests:
A surface abrasion test;
An indentation test;
A soft body impact test, and;
A hard body impact test.
The new standard also provided the gypsum board industry with the technical substantiation criteria to craft code proposals that would permit the use of gypsum core impact-resistant (i.e., abuse resistant) materials in high-rise egress shafts.
This latter point is important because, until the presence of the standard, many of the code proposals had attempted to impose levels of impact-resistance that were unquantifiable-they had no connection to potential, real-world impact-causing events. Rather, they were proposals that intentionally incorporated criteria considered to be unattainable using a gypsum board system; essentially they were written to promote the use of masonry shaft systems at the expense of gypsum board systems.
Working with representatives of New York, the gypsum industry first successfully proposed the inclusion of a reference to ASTM C 1629 into the New York City Building Code approximately two years ago. The city accepted the standard as a method of quantifying gypsum board systems, while also retaining language that would permit the use of masonry and other building materials. In effect, they set a benchmark for impact-resistance that could be quantified for any material.
With that success in hand, the gypsum industry proposed similar language for the International Building Code. The language was approved by the voting members of the International Code Council in 2008 and was published in the 2009 IBC.
The IBC language requires exit and elevator hoist way enclosures in buildings more than 420 feet in height to be constructed using wall assemblies that display a Level 2 Soft Body Impact Test Classification and incorporate materials complying with specific Hard Body Impact criteria, or masonry or concrete walls. Many gypsum and cement board building materials and systems comply with the soft and hard body criteria, and the specific materials can easily be integrated into traditional shaft wall enclosure systems. The new code language does not modify the fire-resistance requirements for the enclosure.
To accommodate different building systems, the code language also permits the use of other systems and building materials via language that ties the quantification of the materials back to the C 1629 method. An extremely hard and dense plaster system, for example, could easily be used in this application. Some glass block products also might meet the established criteria.
Interestingly, you probably could meet the criteria in the code using traditional gypsum board materials; however, a compliant system likely would require many layers of material and would be rather cumbersome to install.
Individual manufacturers produce the impact- and abuse-resistant products that comply with the code criteria. Because the materials are somewhat proprietary, the manufacturer should be consulted during the design and application stage of a project to ensure a proper application. W&C
All Things Gypsum: Hard Gypsum Board
October 1, 2009