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September 11, 2018
Here are five facts about fire ratings and the provable performance of metal building systems.
1. Non-combustible. Unlike wood-framed buildings, which are made of combustible materials, metal buildings are constructed almost entirely of steel. The International Building Code recognizes that steel construction is non-combustible. This doesn’t mean that metal buildings are exempt from fire-protection requirements, but the code does recognize significant advantages to using non-combustible rather than wood-framed construction.
2. Allowable Building Height and Area. The primary advantage of metal over wood construction is in the allowable height and area of buildings. In many building categories, such as retail, office, and education, the area of unprotected (non-fire rated) metal buildings is allowed to be much larger than that of unprotected wood-framed buildings—in many cases more than 50% greater. This recognition of their non-combustible nature gives metal buildings a significant advantage, because of the higher cost of providing fire protection for wood construction. Larger metal buildings can be made than similarly protected wooden buildings even when fire-rated designs are used.
3. Fire protection. The requirements for fire protection that affect all buildings are driven by intended end use, occupancy, and site location. For location, the IBC fire-protection requirements are based on how close a building is to adjacent ones, diminishing as the distance increases. The fire-resistance rating requirements also go down with this lower risk, with the strictest requirements applying to combustible constructions such as wood. Additional stipulations for fire ratings are based on the building’s intended use, or occupancy type, and its construction type, such as wood, steel, or concrete.
4. UL ratings = proven performance. The Metal Building Manufacturers Association designed and tested many fire-rated assemblies for roofs, walls, columns, and joints at Underwriter's Laboratory. The newest of these listings are UL Design Nos. W404 and W413, which are 1- and 2-hour exterior-wall fire ratings, respectively. The new exterior-wall assemblies consist of noncombustible metal building wall framing, steel furring, gypsum board, and exterior metal wall panels, and have several insulation options. The assemblies were arranged to maximize the span of the girts (horizontal metal wall framings), which can be spaced as far apart as 90 inches on center (vertical spacing). This is important because most metal building projects incorporate the larger girt spacing for the first wall girt up from grade level to accommodate door framing. Another important reason for these tests was to ensure that buildings continue to meet changing energy code requirements. Codes have become more stringent, requiring thicker insulation for roofs and walls and sometimes requiring the specification of exterior rigid board insulation. These changes were introduced to save energy by reducing the thermal transfer between the temperature-controlled building interior and the outside environment. However, fire-resistance ratings are affected by the type of insulation used. In addition, foam plastic insulation can bring additional concerns about fire performance and safety, including a potential extra fuel source and smoke and fire propagation potential. The new MBMA/UL W404 and W413 assemblies address these issues by allowing for energy code compliance using both fiberglass and rigid board insulation while maintaining the achieved fire-resistive wall-assembly ratings. Underwriter's Laboratory conducted full-scale fire-endurance tests that adhered to the requirements of ASTM E119, “Standard Test Methods for Fire Tests of Building Construction and Materials.” Both assemblies passed the tests and are now known as UL Design Nos. W404 (1-hour rated fire wall) and W413 (2-hour rated fire wall). Both designs allow for use in new and retrofitted metal building, and they provide enough flexibility to meet future energy code requirements for higher insulation levels.
5. Fire-rating materials. The most common fire-rating material in metal buildings is gypsum wallboard. Other materials can be used, including concrete masonry for walls and spray-applied fire-resistive materials. All the MBMA’s fire-rated designs use the generic 5/8- inch “Type X” gypsum wallboard as a minimum, so any gypsum supplier’s Type X board will comply. Each design is specifies the thickness and the required number of gypsum layers, supports, screw spacing, and other details necessary to achieve the rating. For joints and penetrations, there are other UL ratings that call for special details, which might involve items like intumescent fire caulks that can expand in a fire to seal cracks that develop and prevent the passage of smoke and flames. For instance, the MBMA developed UL Design Nos. CJ-D-0005, CJ-D-0006, and CJ-D-0007, which are wall-continuity joint systems that maintain the joint between a fire-rated interior partition wall and a non-rated metal roof. Other details exist for rated interior walls that terminate against fire-rated metal building roofs. These are HW-D-0488, HW-D-0489, and HW-D0490. The Fire Resistance Design Guide for Metal Building Systems, published by the MBMA, is the complete source for fire-protection information. Metal building construction makes up at least 40 percent of nonresidential 1- and 2-story building construction in the United States. Chances are, you have worked with a metal building in the past, but if you haven’t, consider the strength and flexibility of a metal building system to meet your clients’ needs.
Source: Dan Walker, P.E. is the Assistant General Manager of the Metal Building Manufacturers Association in Cleveland, Ohio. To learn more, visit www.mbma.com