Galvanized steel is carbon steel coated with a layer of zinc to prevent rust and corrosion. The zinc layer acts as a sacrificial barrier, protecting the steel underneath from environmental exposure. When considering fire safety, the performance of this composite material is complex, as its two components—the zinc coating and the steel base—react very differently to high temperatures.
How High Temperatures Affect the Zinc Coating
The zinc coating on galvanized steel has a relatively low melting point compared to the steel itself. The zinc layer melts at approximately 787°F (419°C), a temperature easily reached in a typical building fire long before the steel is structurally compromised. If the temperature climbs higher, the zinc begins to vaporize or boil around 1665°F (907°C). This vaporization releases a white or bluish smoke, which is zinc oxide fume that forms when the zinc vapor meets air.
The vaporization process poses a health concern, particularly if the material is cut or welded during or after a heat exposure event. Inhaling these zinc oxide fumes can cause a temporary, flu-like illness known as metal fume fever. Symptoms include fever, chills, nausea, and a metallic taste in the mouth, usually appearing a few hours after exposure. The condition is not life-threatening and resolves within 24 to 48 hours, but it requires proper ventilation when working with heated galvanized steel.
Understanding Non-Combustible Material Ratings
To assess galvanized steel’s fire performance, it is helpful to understand the distinction between “non-combustible” and “fire-resistant” in building codes. Non-combustible materials, such as steel, do not ignite, burn, or release flammable vapors when exposed to fire or heat. Galvanized steel, like all steel, is inherently non-combustible.
Fire-resistant or fire-rated materials are designed to maintain structural integrity and prevent the spread of fire for a specified duration, typically measured in hours. While steel is non-combustible, an unprotected steel structure is not considered fire-resistant on its own, due to its tendency to lose strength rapidly under heat. For a galvanized steel component to receive a fire-resistance rating, it must be incorporated into a composite assembly or treated with specialized coatings.
The Effect of Fire Heat on Steel’s Strength
The primary danger fire poses to any steel structure, including galvanized steel, is the rapid loss of load-bearing capacity. While the steel base metal does not melt until around 2500°F (1370°C), its strength begins to degrade at significantly lower temperatures. This loss of strength is the primary cause of structural failure in steel-framed buildings during a fire event.
Steel’s yield strength, its capacity to bear a load without permanent deformation, is severely reduced as the temperature increases. Structural steel can lose approximately 50% of its strength when its temperature reaches 1000°F to 1100°F (538°C to 600°C). Since temperatures in a fully developed building fire can quickly exceed this threshold, structural elements can buckle and lead to collapse. The zinc coating does not alter this fundamental characteristic, meaning galvanized steel shares the same structural vulnerability to high heat as plain steel.