Steel, an alloy primarily composed of iron and carbon, is widely used in construction due to its strength and durability. Steel is inherently non-combustible, meaning it will not ignite, burn, or contribute fuel to a fire, even under intense heat. However, the material is not fireproof because prolonged exposure to high temperatures causes a severe loss of the structural capacity that makes it suitable for construction. This distinction means that while the steel itself is safe from burning, the structure it supports is vulnerable to collapse in a fire event.
Steel’s Reaction to High Temperatures
When steel is exposed to the intense heat of a building fire, its mechanical properties rapidly diminish. Structural steel possesses high thermal conductivity, allowing heat to be quickly transferred throughout a beam or column. As the internal temperature rises, the material softens and weakens significantly, long before it reaches its melting point of around 1,500°C (2,700°F).
One immediate consequence of rising temperatures is thermal expansion, where the steel members grow in size. If the steel components are restrained by surrounding structures, this expansion can induce immense internal stresses. These forces can push connecting elements, such as bolts and joints, past their failure points, leading to warping or permanent deformation of the structure.
The direct impact of heat on the steel’s strength and stiffness is more concerning than expansion. The material’s yield strength, the point at which it permanently deforms, decreases substantially with temperature. For example, at approximately 204°C (400°F), carbon steel retains about 90 percent of its room-temperature yield strength. The steel’s stiffness (elastic modulus) also diminishes, increasing the likelihood of buckling in slender columns and beams even under normal design loads.
Defining the Critical Failure Point
Structural engineers define steel failure not by melting, but by the temperature at which the steel can no longer support the applied load. This is commonly referred to as the “critical temperature.” For most conventionally loaded structural steel members, this temperature is cited around 538°C (1,000°F).
At this critical temperature, structural steel retains only 50 to 60 percent of its yield strength compared to ambient conditions. Since most structures are not designed with a 50 percent safety margin, this strength loss is insufficient to bear the full design load, leading to structural instability and collapse. The actual critical temperature can vary depending on the load applied; a highly stressed member may fail at a lower temperature, while an underutilized member may withstand temperatures up to 750°C.
The concept of a fire resistance rating, often determined by standards like ASTM E119, is a time-based measurement that relates directly to this temperature threshold. This rating specifies how long a structural element can withstand fire exposure while preventing the steel from reaching the critical temperature. The goal of protecting steel is therefore to delay the heat transfer long enough for the building to be evacuated or for the fire to be contained.
Engineering Solutions for Fire Resistance
Since unprotected steel quickly reaches its critical failure temperature, passive fire protection (PFP) methods are routinely implemented to insulate structural members. These protective layers work by slowing the rate of heat transfer from the fire to the underlying steel. The selection of a PFP method is based on the required fire resistance rating, which can range from one to four hours.
Spray-Applied Fire Resistive Materials (SFRMs)
One of the most common methods is the application of Spray-Applied Fire Resistive Materials (SFRMs), which are often cementitious or mineral fiber-based compounds. These materials are sprayed directly onto the steel surface, creating a thick, low-density insulating layer. SFRMs have a low thermal conductivity, effectively absorbing heat and delaying the temperature rise in the steel core.
Intumescent Coatings
Another effective solution involves Intumescent Coatings, specialized paints that are inert at normal temperatures. When exposed to fire, these coatings swell and char, expanding many times their original thickness to form a foam-like insulating layer. This char layer is an excellent insulator, providing an aesthetic finish while protecting the steel from rapid temperature increase.
Physical Encasement
Physical encasement provides a third category of PFP, utilizing materials like concrete, gypsum board, or specialized fire-resistant boards. Encasement physically shields the steel member, leveraging the endothermic properties of materials such as gypsum, which release chemically bound water when heated. This release of water absorbs thermal energy, delaying the steel’s temperature rise and ensuring the structure maintains its load-bearing integrity for the specified time.