# Post-Earthquake Fire Resistance in Structures: A Review of Current Research and Future Directions
## 6 Behavior of Materials at Elevated Temperatures
As a result, accurate fire simulations must incorporate temperature-dependent values for these properties to predict structural behavior effectively [68]. The thermal conductivity of steel, which dictates the rate at which heat is transferred through the material, decreases as temperatures rise. Specific heat--the amount of heat required to increase the temperature of a unit mass of steel by one degree Celsius--increases with temperature, particularly up to 700 \({}^{\circ}\)C. Moreover, steel's thermal expansion slightly increases with temperature, necessitating the inclusion of these values in thermal analyses to accurately model fire conditions [68]. The stress-strain relationship of steel also undergoes significant alterations at elevated temperatures, which is crucial for understanding the performance of steel structures during fires. For example, hot-rolled steel exhibits distinct changes in its stress-strain behavior as temperatures escalate [72]. The cooling process following exposure to high temperatures plays a critical role in determining the residual mechanical properties of steel, especially its residual strength and ductility. Studies have shown that different cooling methods, such as air cooling and water cooling, can lead to varying degrees of residual strength and ductility, influencing the post-fire performance of steel structures [67]. Interestingly, research suggests that multiple heating-cooling cycles have a negligible impact on the mechanical properties of certain hot-rolled steels, such as Q345, which is a high-strength (minimum yield strength of 345 MPa and minimum tensile strength of 470 MPa), low-alloy structural steel that is widely utilized in various industries due to its excellent mechanical properties and cost-effectiveness. Q345 steel primarily comprises iron (98-99%) with a small percentage (1-2%) of alloying elements such as carbon, manganese, silicon, phosphorus, and sulfur. This finding implies that the effects of cyclic heating and cooling on these steels are minimal and can be largely disregarded in some scenarios [67]. While current literature provides valuable insights into the macroscopic behavior of steel at elevated temperatures, it often lacks a detailed exploration of the microstructural changes that underpin these alterations in properties. Understanding these microstructural transformations could further enhance the predictive models used in fire simulations and the design of steel structures for fire resilience. Concrete undergoes complex transformations when exposed to high temperatures, significantly affecting its structural performance. These transformations encompass both physical changes, such as expansion, and a series of chemical reactions among its components--cement, aggregate, and moisture--each responding differently to heat [15]. Due to its low thermal conductivity, concrete transmits heat slowly within its cross-section, which can provide some protection to the embedded steel reinforcement against rapid temperature increases. However, damage caused by earthquakes, such as cracks or spalling, can accelerate heat penetration, thereby compromising the reinforcement's integrity [50]. As the temperature in concrete rises, its crushing and yield strengths decrease, making it less effective as a structural material, especially at temperatures exceeding 500 \({}^{\circ}\)C [50]. This deterioration is even more pronounced in concrete that has been previously damaged, such as from seismic events [50]. A critical issue is the potential for thermal spalling--a sudden and violent chipping or explosion of the concrete surface--which can significantly reduce the structure's load-bearing capacity. Several factors influence spalling, including the thickness of the concrete cover, with spalling being more common in beams and columns where the cover exceeds 4-5 cm. Additionally, the type of concrete plays a role; high-strength concrete (HSC), particularly that with moisture content exceeding 3-4%, is more prone to spalling.