Post-Flame Success: Post-Frame Firewall Surpasses 3-Hour Test
An hourly rated wall as much as a 3-hour wall may be required to separate building areas or occupancy types to meet the International Building Code (IBC) requirement for life safety. For architects, builders, and engineers, this usually means using a rated 8- or 10-inch concrete block wall on a concrete foundation. But what about using a wood-constructed firewall?
In this article, Leo F. Shirek, Co-Chair of the Technical and Research Committee of the National Frame Building Association (NFBA) and subcommittee member for the 3-hour firewall project, describes why and how NFBA has successfully developed a post-frame firewall. As Research and Development Manager at Mazomanie, WI-based Wick Buildings, Shirek and other NFBA leaders predict that a wood-constructed wall will prove an extremely competitive and cost-effective alternative to a traditional block wall system.Q: Please describe the recent 3-hour firewall test and explain its significance.
Basically, the post-frame building system is categorized as a Type-5 class structure under the IBC. Under certain conditions where occupancy separations are required like with a high-hazard manufacturing occupancy, a 3-hour firewall is required, often using a [concrete] block wall. Current 3-hour wall assemblies using a noncombustible framing system, as with light-gauge steel stud walls that have three or four layers of drywall on both sides, are also used. In all these cases, a foundation wall is required, which is more expensive than using the post-frame system as commonly used in the rest of the building.
We began testing work on this project in early 2011 knowing that, from an industry standpoint, we could create a wall that was much more cost effective than a typical block wall, which costs from $12 - $15 per square foot. We felt that a post-frame firewall with drywall could potentially halve that cost.
With a post-frame firewall, we eliminate a tremendous amount of inconvenience in terms of scheduling other work trades—including the block or foundation work. Generally the concrete block wall would be scheduled before you build the rest of the structure, whereas a post-frame wall can be built and incorporated into the natural flow of the building process possibly saving as much as a couple of weeks in completion time. Besides the scheduling convenience, it’s cost effective
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This wood-constructed firewall is a very unique product and quite a testimonial to our industry. Our subcommittee had numerous ideas that were incorporated into the final design and we also turned to a couple of industry experts to help us in our investigation to determine the final framing and sheathing design: Kuma Sumathipala of the American Wood Council and Robert H. White, PhD, of Forest Products Laboratory, Madison, WI, where we conducted six small-scale preliminary tests helped with decision making on the final design.Q: How did the small-scale test assemblies differ?
One area they differed in was the framing systems. We tested a known UL 3-hour light gauge steel assembly to represent our control assembly to gauge further testing results to. We tested a wood assembly with bookshelf girts. We also tested assemblies with 2 x 4 girts on each side 16 inches on center, typical to what the final design was. We tested the adequacy of the columns by experimenting with additional lumber plies as well as gypsum cover plates to provide additional fire protection. We ultimately used a 4-ply column, with the fourth ply as a sacrificial member to the fire. We also tested the effects of insulation in the cavity. As to gypsum sheathing, we varied the number of layers per side at 3 and 4 along with testing two grade types, C and X. Type C drywall, is a little bit more fire resistant but did not present itself as such in our test and that is one reason why we settled on 4 layers of type X. Our final assembly then was a post-frame structure with four layers of 2 x 6 laminations in the columns, spaced 8-feet apart with 2 x 4 girts 16 inches on center applied horizontally on each side. This frame then was sheathed with four staggered layers of 5/8 type X drywall on both sides. All joints were untapped.
Our assembly nearly made 4 hours (3 hours and 47 minutes) thus obtaining a 3.5-hour rated wall by Underwriters Laboratories (UL). We had our assembly nicknamed the “Fortress” by the UL testing group since they normally test stud walls and the like. This was a different challenge for them and you can see by the results we earned the name. It’s quite a testament to post-frame construction.
Q: Are you planning further tests?
Well, yes, in a way. With this test we do have the opportunity for obtaining other hourly ratings based off of this test through a UL engineering study for lower-rated walls. Possibilities are a reduced layered 2-hour post-frame system. Many possibilities exist and our committee will be exploring these. We are optimistic that this test will be a building block for other approvals and possibly other tests in the future that can benefit the industry. When you compare post-frame firewalls to other building systems, our structures provide a tremendous economic [advantage]. Post frame is much speedier to erect than a stud-wall system or light-gauge steel stud-wall system and especially block walls that generally requires an expensive foundationQ: Do UL test assemblies need to be followed to a T?
The UL-approved assembly came about by confirming the adequacy of a 10 x 10-ft tested sample of what will be built in the field. These tests follow the requirements of the UL 263 (ASTM E119) standard of the American Society for Testing Materials (ASTMN). There are stringent requirements that one must follow to pass this test. For this testing, we went to UL in Northbrook, IL, in December 2011. To pass the test, two duplicate test assemblies were run: the first one was a fire test to failure duration of 3 hrs and 47 minutes; and the second assembly fired for 1 hour, which was then subjected to a stringent water pressure test. There were a few notable points to the fire test.
- When the assembly frame was tested, a load was added to the structure to simulate maximum roof loads that the columns are capable of carrying on the building. Hydraulic pressure rams were used to put a load on the assembly of about 42,000 lbs—a lot of weight.
- UL is very thorough. They weigh every piece of lumber that is part of that assembly for calculation of the tare weight of the sample. The spacing of screws was quite methodical to eliminate the possibility of overpopulating fastener holes in the previous gypsum layers thus possibly affecting assembly performance. That’s good, prudent work.
- A blast of fire comes out of 20 holes during testing to bring the furnace to about 1800 degrees F. Thermocouples within the furnace and at various locations on the assembly sense the temperature, which is sent to and recorded on computers in a nearby control room. This log of data helps in any later engineering study for possible other approvals based off this test.
- An hour into the test, the exposed gypsum drywall loses its moisture and starts to shrink up. Joints start to open up. At 2 hours, two layers of drywall had fallen off, and a half hour later a flame began exposing the fourth layer. At nearly 3.5 hours, the last layer of drywall began breaking off, exposing the wood frame cavity.
Q: What is your overall message concerning this test?
Basically, the NFBA has tested an economical post-frame 3-hour wall system that can now be incorporated into a post-frame building without having to mix and match other types of framing systems. When a 3-hour wall is deemed necessary to meet the IBC code, a post-frame firewall provides additional savings to an already economical building system.
