# Post-Earthquake Fire Resistance in Structures: A Review of Current Research and Future Directions
## 6 Behavior of Materials at Elevated Temperatures  
Investigating the behavior of steel and concrete under high temperatures is crucial for understanding the performance of structures during post-earthquake fires (PEF). Elevated temperatures have a profound impact on the mechanical properties of these materials, including compressive strength, Young's modulus, yield strength, and ultimate strength [15; 63; 67; 68]. While concrete, being a heat-inert material, can provide some degree of fire resistance, high temperatures can lead to spalling, which reduces the cross-sectional area of concrete elements and exposes the reinforcing steel to further damage. The presence of cracks, whether induced by seismic activity or fire, can significantly affect heat propagation, thereby compromising the structural integrity [50; 69]. This issue becomes even more critical when the concrete cover that protects the reinforcing steel is damaged [70]. The localization of maximum temperatures within a fire underscores the importance of precise thermal analysis [15]. Furthermore, the residual strength and ductility of steel are significantly affected by the cooling methods applied after a fire, such as air and water cooling [67]. Ongoing research into the behavior of steel and concrete at elevated temperatures has led to the development of predictive formulae and constitutive models, which are essential for estimating material properties under these extreme conditions [15; 69]. Under an air cooling condition, the behaviour of both CFS-F (flat region of cold-formed steel specimens) and CFS-C (corner region of cold-formed steel specimens) can be understood as follows:  
\[20\ ^{\circ}\mathrm{C}\leq T\leq 1000\ ^{\circ}\mathrm{C},\quad\frac{f_{uPT}}{f_{u }}=0.992+3.97\times 10^{-4}T-1.76\times 10^{-6}T^{2}+1.37\times 10^{-9}T^{3} \tag{2}\]  
Under a water cooling condition, the behaviour of both CFS-F (flat region of cold-formed steel specimens) and CFS-C (corner region of cold-formed steel specimens) can be represented as:  
\[20\ ^{\circ}\mathrm{C}\leq T\leq 1000\ ^{\circ}\mathrm{C},\quad\frac{f_{uPT}}{f_{u }}=0.995+2.55\times 10^{-4}T-9.89\times 10^{-7}T^{2}+7.19\times 10^{-10}T^{3} \tag{3}\]  
In the given equation, \(f_{uPT}\) denotes the ultimate tensile strength of steel following exposure to a specified elevated temperature. This parameter reflects the potential degradation in mechanical properties due to thermal effects. Conversely, \(f_{u}\) represents the ultimate tensile strength of steel under ambient conditions, where the material has not been subjected to fire or elevated temperatures. The variable \(T\) corresponds to the maximum temperature to which the steel has been exposed, serving as a critical factor in assessing the thermal degradation of its structural integrity. Advanced methodologies, such as hybrid fire simulation (HFS), are being developed to better understand the complex interactions between thermal and mechanical behaviors in PEF scenarios [15]. In conclusion, a thorough understanding of the thermal and mechanical behavior of steel and concrete at elevated temperatures is essential for developing accurate PEF assessment techniques, designing resilient structures, and ensuring the safety of buildings and their occupants during post-earthquake fire incidents. Steel's mechanical properties are highly sensitive to changes under elevated temperatures, which significantly affect its structural performance during fire events. Due to its high thermal conductivity, steel rapidly loses strength and stiffness as temperatures increase. This reduction in mechanical properties is particularly critical in fire scenarios, where maintaining structural integrity is essential. Beyond 500 \({}^{\circ}\mathrm{C}\), the ultimate strength of steel decreases by approximately 50% [71]. Additionally, the modulus of elasticity and yieldstrength substantially decline with rising temperatures, primarily due to the weakening of atomic bonds within the material.