Steel Solutions Center
11.1. Fire Protection Systems
Steel designated to receive a field-applied, contact-type fireproof coating should be shop cleaned of dirt, oil, grease, and loose mill scale by appropriate means. Cleaning off rust, dirt, and other materials that accumulate between the time of fabrication and the time of application of the fireproof coating is not the responsibility of the fabricator/erector; such responsibility should be assigned in the contract documents.
Fire resistance means the ability of building components and systems to perform their intended fire-separating and/or loadbearing functions under fire exposure. Fire-resistant building components and systems are those with specified fire resistance ratings based on fire resistance tests. These ratings, expressed in minutes and hours, describe the time duration for which a given building component or system maintains specific functions while exposed to a specific simulated fire event. Various test protocols describe the procedures to evaluate the performance of doors, windows, walls, floors, beams, columns, etc. The term ‘fireproof’ is a misnomer in that nothing is fireproof. All construction materials, components, and systems have limits and will, at some point, be irreparably damaged by fire.
Fire resistance ratings are assigned to construction components and systems, not materials. With relevance to fire, materials are classified for their combustion properties, and steel is non-combustible. Steel also has many other valuable structural and durability properties. Steel is used in many fire-resistant building components and systems where loadbearing structural steel members are usually coated, wrapped, or otherwise insulated/protected from the thermal effects caused by fire.
Spray-applied fire-protective materials generally fall into two broad categories: mineral fiber and cementitious. These materials are usually based on proprietary formulations, supplied in a dry form, and must be mixed and applied by closely following the manufacturer’s recommendations.
The mineral fiber mixture combines the fibers, mineral binders, air, and water. It is generally spray-applied with specially designed equipment that feeds the dry mixture of mineral fibers and various binding agents to a spraying nozzle where water is added to the mixture as it is sprayed onto the metal surface. In the final cured form, the mineral fiber coating is lightweight, non-combustible, chemically inert, and a poor conductor of heat (low thermal conductivity insulator).
Cementitious coatings incorporate lightweight aggregates (e.g., perlite or vermiculite) in a heat-absorbing matrix, usually, of gypsum and/or Portland cement. Some formulations also use magnesium oxychloride, magnesium oxysulfate, calcium aluminate, or ammonium sulfate. Various additives and foaming agents may be added to the mixture. Cementitious coatings are often classified by their density (as low, medium, and high).
11.1.5. Other than spray-applied fire-protective materials, what other materials or methods can be used to fire-protect steel?
In general, structural fire protection is achieved through one or more of the following mechanisms:
- Low thermal conductivity
- High effective heat capacity
- Heat absorbing physical (e.g., transpiration, evaporation, sublimation, ablation) or chemical (e.g., endothermic decomposition, pyrolysis) reactions
- Intumescence, i.e., formation of a thicker foam upon heating
- Radiation or reflection.
Concrete and masonry encasements are traditional fire-protective materials that can be used. Today, numerous gypsum board and mineral board products, ceramic wool wraps, and various types of intumescent coatings are more common alternatives, although the traditional materials can still be used.
It should be also noted that steel itself is an effective fire-protective material, when used in the form of sheets, to provide protective and reflective shield for other materials, or in the form of meshes or wraps to help other materials maintain their integrity under heat exposure.
Other fire-protection methods for structural steel involve rain screens (water sprinklers designed to protect steel members) or filling tubular structures with concrete or water. Fire-protection systems continue to evolve and improve as new products and systems are developed every year.
An intumescent coating chars, foams, and expands when heated. The reaction is reminiscent of the chemistry demonstration where sulfuric acid is poured onto sugar in a beaker – the sugar chars, foams, and expands out of the beaker.
The compounds of intumescent systems can generally be placed into four categories:
- inorganic acid or material yielding an acid at temperatures of 212-570°F
- polyhydric material rich in carbon
- organic amine or amide, as a flowing agent
- halogenated material
In addition, various binders and additives are mixed in to provide specific physical properties of the total system. In many instances, the system is made of several coats with different properties and functions, e.g. a topcoat will provide a durable finish surface while the base coat will provide a strong bond to the substrate. Extensive research and development have led to greatly improved formulations that do not use traditional compounds.
In most cases, steel that is to be fire-protected should not be painted or galvanized. However, when such steel must be painted, additional measures can be taken to ensure adhesion. It is always prudent to consult your fire-protection contractor/supplier and your authority having jurisdiction (AHJ) in the early stages of the project on this issue, if you expect any of your painted or galvanized steel to be fire-protected by spray-applied materials.
Spray-applied fire-resistive materials may be applied to primed or painted steel shapes provided they have passed the bond tests in accordance with ASTM E736 “Standard Test Method for Cohesion/Adhesion of Sprayed Fire Resistive Materials Applied to Structural Members.” These tests should indicate a minimum average bond strength of 80% and a minimum individual bond strength of 50%, compared to the bond strength of the same fire-resistive material when applied to clean unpainted/ungalvanized steel plate surface. Some AHJs will enforce similar requirements for galvanized surfaces. Producers of fire-resistive materials usually maintain a list of “pre-approved” paints that have already passed the ASTM E736 tests. They will also advise you on the applicability of their product to galvanized surfaces. Additional tests will be needed if the coated steel surface has not been pre-approved. Where the bond strength is found unacceptable, a mechanical bond may be obtained by wrapping the structural member with expanded metal lath (min. 1.7 lbs/sq. yd).
Sprinkler systems have proven to be exceptionally effective and reliable when properly designed, installed, and maintained. Strict regulations in the US allow for standard practices for sprinkler system inspection, test and maintenance programs. An annual NFPA report notes the effectiveness of sprinkler system performance as follows: “The NFPA has no record of a fire killing more than two people in a completely sprinklered building where the system was properly operating, except in an explosion or flash fire or where industrial fire brigade members or employees were killed during fire suppression operations.”