One way of providing passive fire protection for structural steel is through the application of spray-applied fire-resistant materials (SFRMs), also known as conventional coatings. During a fire, SFRMs expand and insulate structural steel to prevent failure that results from rapidly rising temperatures. Generally, they are available in two types: cementitious coatings and sprayed fiber coatings. Cementitious wet-mix products are mixed with water to form a slurry. The slurry is pumped under pressure to a nozzle, where the pressure atomizes it and carries it to the steel substrate. Cementitious products generally are available as gypsum plaster-based coatings or Portland cement-based coatings Sprayed-fiber products, used mostly outside the United States, are pneumatically pumped dry to a nozzle. In the nozzle, they are wetted with high-pressure water that carries the coating to the substrate. Typically, Cementitious and sprayed-fiber products are hidden from view, above a room’s ceiling or behind its drywall. Higher density rapid-rise coatings also help protect against petroleum fires, in which temperatures rise quickly. Intumescent Coatings Intumescent coatings provide passive fire protection as well. They consist of thin chemical films that swell and expand when exposed to high temperatures to form a durable, adherent, fire-resistant foam layer. This layer provides an insulating barrier during a fire. In contrast to conventional coatings, intumescents appear more paint-like when applied to structural steel, giving them greater potential for aesthetic and architectural applications when applied to exposed steel members. UL designates Intumescent coatings by usage, in three categories: 1) Interior conditioned space, with controlled temperature and humidity conditions; 2) interior unconditioned space, with variable temperature and humidity; and 3) exterior spaces that are permanently exposed to the elements. Interior intumescent coatings are typically known as thin-film intumescent coatings. Thin films are available as either water-based or solvent-based coatings. Thin films offer architects and engineers the ability to take full advantage of their aesthetic license. Exterior Intumescent Coatings Exterior intumescent coatings, known as mastic coatings, are for use in heavy industrial environments, or when steel is located on the outside of a building and still needs a fire rating. Examples are stadiums, theme parks, petrochemical plants, offshore drilling platforms, and certain high-rise buildings. Also, for locations with tight space restrictions like elevator shafts, intumescent coatings offer a thinner alternative to conventional cementitious fire protection. Many mastic coatings offer strong impact resistance and durability. Exterior products are permanently exposed to the elements, and must meet a weathering program for intumescent materials. In order to list with UL, they have to withstand rain, wind, humidity, UV-exposure, and other elements. The thickness of a coating for structural steel depends on the size of the steel member, the fire-protection rating required for the steel or assembly, and the protection the coating itself provides. Designers can measure the amount of fire protection required for steel member by using what’s called the “fire triangle.” One side of the triangle is the W/D ratio of the assembly, where W is the weight of the steel, and D is the heated perimeter of the structural steel exposed to a fire situation measured in inches; the second side is the required hourly rating for the assembly; and the third side then is the amount of fire protection material required. Cementitious coatings generally require no more than two to three coats, providing up to 1” of thickness in one pass. Thinner coats are recommended when using intumescent coatings for a better appearance. For intumescents, each coat is generally about 5-10 mil thick. Most intumescent products are designed for less than 200 mil of coating, but current thicknesses for full coats range from about 30 mil to 400 mil. Topcoats and Drying Time Drying time depends on the temperature and humidity of the environment—the ideal is 50° F with no more than 50% humidity. Topcoats are available in a range of colors. In general, topcoats are not required for interior use if the humidity does not exceed 70% and if the temperature is below 70° F.