Structural steel is the backbone of modern construction, known for its immense strength and durability at ambient temperatures. Although non-combustible, its structural capacity is highly sensitive to the extreme heat generated during a building fire. Understanding the specific temperatures at which steel’s strength is compromised is paramount for safety, engineering design, and effective fire protection strategies.
The Critical Temperature Range for Strength Reduction
Structural steel begins to experience a notable reduction in strength once its temperature rises significantly above room temperature. The material retains approximately 90% of its room-temperature yield strength up to about \(200^{\circ} \text{C}\) (\(400^{\circ} \text{F}\)). This initial range demonstrates a high degree of stability before thermal effects become a major concern for load-bearing capacity.
Strength loss accelerates rapidly between \(300^{\circ} \text{C}\) and \(400^{\circ} \text{C}\) (\(572^{\circ} \text{F}\) to \(752^{\circ} \text{F}\)). By \(427^{\circ} \text{C}\) (\(800^{\circ} \text{F}\)), the steel typically retains only about 60% of its original yield strength. This weakening means the steel is increasingly less able to support the design loads placed upon it.
The most important temperature for fire safety engineering is the critical failure point. This point is defined as the temperature at which the steel retains only 50% of its room-temperature yield strength, which is enough loss to cause structural collapse. This critical temperature is widely accepted to be around \(538^{\circ} \text{C}\) to \(566^{\circ} \text{C}\) (\(1,000^{\circ} \text{F}\) to \(1,050^{\circ} \text{F}\)).
The exact temperature for catastrophic failure, often called the limiting temperature, varies depending on the utilization ratio (how much load the member is supporting). A highly stressed column supporting a near-maximum load might fail at a lower temperature. Conversely, a lightly loaded member could withstand temperatures closer to \(750^{\circ} \text{C}\). In fire resistance testing, failure is often assumed at the \(538^{\circ} \text{C}\) point because it signifies a profound loss of load-bearing capacity.
How Heat Affects Steel’s Structural Integrity
The loss of strength in steel at high temperatures is governed by several physical and metallurgical changes. One immediate effect of heat is thermal expansion, causing steel members to elongate. Since the steel is restrained by the structure, this elongation induces immense compressive or tensile forces on connections and adjacent elements, which can lead to premature failure.
A more direct cause of structural failure is the reduction of the material’s yield strength. Yield strength dictates the maximum stress a material can endure before permanently deforming. As the temperature increases, the atomic bonds within the steel weaken, making it easier for the material to deform under the applied load. This reduction is the primary reason the steel can no longer support its designed weight at the critical temperature range.
Concurrently, the stiffness of the steel, measured by its modulus of elasticity, decreases significantly as temperature rises. This loss of stiffness means the steel member becomes more flexible and prone to buckling or excessive deflection under load. Since structural stability equations rely heavily on the modulus of elasticity, a minor temperature increase can have a disproportionate effect on stability.
At temperatures above approximately \(370^{\circ} \text{C}\) (\(700^{\circ} \text{F}\)), a time-dependent deformation called “creep” becomes a factor. Creep is the tendency of a solid material to slowly move or permanently deform under sustained mechanical stress. In a fire scenario, the constant load combined with high heat causes the steel to continuously deform, accelerating the path toward collapse.
Fireproofing Techniques for Structural Steel
Passive fire protection (PFP) is applied to insulate structural steel members and prevent them from reaching the critical failure temperature. The goal of fireproofing is to delay the steel from reaching \(538^{\circ} \text{C}\) to provide time for evacuation and firefighting efforts. This delay is quantified in fire-resistance ratings, typically expressed in hours (e.g., two-hour or three-hour ratings).
Concrete Encasement
One traditional and effective method is concrete encasement, where the steel member is fully surrounded by a thick layer of concrete. The concrete acts as an excellent thermal barrier, absorbing heat and significantly slowing its transfer to the steel core. While highly durable, this method adds considerable weight and bulk to the structure, which is often undesirable.
Spray-Applied Fire-Resistive Materials (SFRM)
Another common application is Spray-Applied Fire-Resistive Materials (SFRM). These are lightweight, cementitious or gypsum-based mixtures sprayed directly onto the steel. SFRM is a cost-effective solution that forms a thick, insulating layer, though it is vulnerable to damage from moisture or physical impact.
Intumescent Coatings
For aesthetically exposed steel, intumescent coatings are frequently used, resembling thick paints. When exposed to extreme heat, these coatings undergo a chemical reaction, causing them to swell or char, expanding up to 100 times their original thickness. This resulting foamy, insulating char layer creates a highly effective thermal barrier that keeps the underlying steel cool for an extended period.