Head of Wall Joints in Metal Buildings and Fire Resistance
By: W. Lee Shoemaker, Ph.D., P.E. and Daniel J. Walker, P.E.
Long one of the most popular choices for low rise buildings, metal buildings are more popular than ever. These structures are cost-effective, efficient and can be used for a range of end-uses, which makes them a good choice, particularly in today’s economy. As metal buildings have increased in size variations, end-use and design over the past few years, there has also been a corresponding increase in how to apply certain types of building codes to them.
The primary and secondary steel framing of metal buildings is non-combustible. There are many external and internal components that can be used with metal buildings such as steel plates, sheets and walls, masonry, concrete and they often use metal roofs as well. In large part due to their non-combustible nature, the International Building Code Type IIB Classification does not require many elements of single story metal building construction to contain passive fire protection.
There are many situations, however, where the local code will require fire-resistive walls in certain Type IIB buildings. This may be because of the building’s use, its proximity to other structures, its layout or a combination of these and other factors. While the walls need to be fire-resistant to meet these codes, the roof assemblies are often not required to be and are, in many cases, unrated. This situation has led to questions from a range of people involved in the building approval and inspection process about the fire resistance of head of wall, or HOW, joints between fire-resistant walls and unrated roof assemblies. The Metal Building Manufacturers Association recently performed research and testing to answer questions about this specific situation and provide guidance for future metal buildings as well as those who work on them and inspect them.
The IBC’s Type IIB construction designation requires non-combustible framing materials and compliance with allowable heights and areas for the design occupancy per the local building code. As a result of the Type IIB designation, the non-combustible steel roof is often permitted to be unprotected-unrated in terms of fire resistance. While the code may allow the roof to be unrated, in many cases the local code requirements for the specific building and its use will necessitate fire-resistive walls, even for Type IIB construction. These rated walls may be interior separation walls, corridor or exterior walls.
Sections 713.1 – 713.3 of the 2006 IBC contain the requirements for fire-resistant joints. This section of the code explicitly states that the code covers "joints installed in or between fire-resistance rated walls, floor or floor/ceiling assemblies and roofs or roof-ceiling assemblies.” While it is clear that the code refers to two fire-rated assemblies, it has not always been clear what the code allows when there is a rated wall and an unrated roof.
Building officials have interpreted this particular section of the IBC differently, but most often make the ruling that the code may include joints between rated walls and non-rated steel roofs in metal buildings. The MBMA conducted these tests to evaluate the head of wall joints with a fire-resistance rated wall and a non-rated roof and to determine the fire resistance of the HOW joints for this situation.
The Testing Process
The tests were undertaken and funded by MBMA and industry partner, the American Iron and Steel Institute, with the technical assistance of Hughes Associates Inc. The tests were successfully performed at Underwriters Laboratories in July 2007.
The tests undertaken at UL applied to three specific situations with fire-rated wall assemblies and non-rated roof assemblies: (1) the roof purlin is located inside the wall, (2) parallel to the wall and (3) perpendicular to the wall. The tests were completed successfully, and three new UL listings were issued: HW-D-0488, HW-D-0489 and HW-D-0490. The only difference between the HOW joints tested and those now specified in the UL Directory is the absence of a protective ceiling membrane. To address this, the UL issued a clarifying letter, which is quoted below.
After the wall and roof assemblies were constructed and instrumentation attached, two HOW joint tests were conducted. In one, the purlin was inside the wall assembly, and in the other test the purlin was perpendicular to the assembly. The tests were conducted to determine whether the HOW joint would meet the flame-through requirements and withstand a hose-stream test. Once these two tests were successfully completed, it was determined that the third case, with the purlin running parallel to the wall, would also meet the necessary requirements.
The test assemblies had a wall assembly consisting of gypsum board with minimum 5/8-inch thickness and constructed as specified by U400 or V400 Series Wall and Partition Design in the UL Fire Resistive Directory. The assembly for the HW-D-0488 rating, with the purlin inside the wall, included steel floor and ceiling runners, batts and blankets with a minimum of 6-inch thickness before compression, minimum 3.5-inch steel studs and a U-shape deflection channel that was secured to the bottom flange of the purlin. The bottom of the studs nested in the floor runner and the maximum separation between the top of wall assembly gypsum board and bottom of cripple wall gypsum board (at time of installation of joint system) is 2 inches (51 mm). The joint system is designed to accommodate a maximum 100 percent compression or extension from its installed width. A minimum 5/8-inch gypsum board rip strip was installed to cover the cripple wall above the ceiling deflection panel and to lap a minimum of 1 inch onto the wall assembly when the joint is open at full extension.
The same assembly was used for the HW-D-0489 test and rating, which has the purlin running perpendicular to the wall, with the exception of the ceiling deflection channel. In this case the deflection channel was installed perpendicular to the purlins and secured to the bottom flange of purlins. For HW-D-0490, with the purlins running parallel to the wall, the deflection channel is a U-shaped channel installed between and parallel to the purlins while secured to lateral bracing.
The joint system fire barrier was subjected to 500 complete cycles of movement. Based on its designed movement range of ±100 percent in compression and extension, the nominal 2-inch wide joint system was cycled through a range of 0 to 4 inches. The cycle rate for all 500 movement cycles was a minimum of 10 cycles per minute. Other than some abrasion of the gypsum board paper facing on each side of the wall assembly, the joint system withstood the movement cycling without demonstrating any significant distress.
The average furnace temperature recorded was in accordance with the standard time-temperature curve outlined in ANSI/UL 2079. The fire test was started with the joint system at the maximum width of 4 inches. No flaming was observed on the unexposed surface of the joint system during the fire exposure test. The fire exposure test was stopped at 60 minutes.
The temperatures of the joint system were measured by 10 thermocouples (a junction between two different metals that produces a voltage related to a temperature difference), which were located on the wall assembly. The limiting temperature for the assembly rating (maximum 325F rise above starting temperature at hottest point on joint system) was not reached during the fire exposure test.
These tests were successfully completed, and three new UL ratings were issued. The exact details and specifications of the HOW joint are provided in UL’s HW-D-0488, HW-D-0489 and HW-D-0490. There are several important conditions of the HOW assemblies that should be noted and will make the job of both erectors and code officials easier:
• The HOW joint used is representative of current metal building construction practices. The wall ratings are applicable with 1-hour rated gypsum board and steel stud wall assembly in any U400 or V400 series design.
• The roof insulation is continuously draped over the top of the wall. The testing demonstrated that the fire will not travel to the wall’s unexposed side via the vapor barrier. Some building officials had required this vapor barrier to be cut and re-attached to each side of the wall. This concern should now be alleviated.
• The HOW joint has now been qualified for vertical joint movement of up to 2 inches from the installed neutral position. The steel deflection channel within the wall and other wall construction features allow for this movement and are fully described in the UL listings.
• Firestop caulking is required and is very important to the intended fire performance of the joint.
• All the joint components used are generic with the exception of the fire caulking product, which was UL rated.
After these tests were completed and the new UL certifications issued, an explanatory letter to outline the application of these HOW listings to both unrated and rated roof construction was issued by UL. This letter is available, along with the complete test information, at www.mbma.com. It notes that "because Standard ANSI/UL 2079 specifically disallows testing non fire-resistive structures, no UL Certification can be granted from tests incorporating non-fire resistive structures.” However, the basic roof assembly that was tested in the HOW joint tests is essentially the same as described in P265, P268 and P516 in the UL Fire Resistance Directory, only without the fire-resistive membranes.
The UL letter continues: "Because Standard ANSI/UL 2079 is not applicable to joint systems installed between fire resistive and non-fire resistive structures, it was determined that, by testing the joining systems beneath the same roof structure described in Roof-Ceiling Design Nos. P265, P268 and P516 but without the ceiling membrane, the results of the tests could be applied to those P200 or P500 Series Roof-Ceiling Designs in the Fire Resistance Directory, which specify the use of the basic roof structure.”
The letter concludes: "The nominal 2 in. wide dynamic joint systems in the two head-of-wall joint system fire tests conducted for MBMA at UL on July 19, 2007, met the conditions of acceptance in Standard ANSI/UL 2079 for a 1 hr fire assembly rating following Class II movement cycling of 100 percent in compression and extension.”
For those contractors and builders who need to use a fire rated roof assembly, there are three UL ratings that have been developed through the MBMA: P516, P265 and P268. P516 employs Type X gypsum board for the protective ceiling, while P265 and P268 use proprietary acoustical panels.
These tests have shown that the HOW joints between an unrated roof assembly and a rated wall assembly meet all the pertinent code requirements of 1-hour fire assembly ratings. For contractors, builders and code officials, this should make it much clearer as to what is necessary when working with Type IIB buildings.
The MBMA’s research on this matter, and the information available, will enable designers and engineers to comply with fire-resistive code requirements when working on Type IIB metal buildings.
Lee Shoemaker, PhD, PE, is the director of research and engineering for the Metal Building Manufacturers Association.
Dan Walker, P.E., is staff engineer for MBMA and Thomas Associates.
For more information on the HOW assemblies, these tests and to see a full report on these ratings as well as the UL’s follow-up letter, visit www.mbma.com.