Final answer:
A solid steel I-beam begins to elongate and deform in a structural fire due to thermal expansion, generally starting to lose strength around 538°C. The change in length can be calculated using the coefficient of linear expansion for steel and the formula for thermal expansion.
Step-by-step explanation:
The question of when a solid steel I-beam begins to elongate during a structural fire involves the concept of thermal expansion in materials. This phenomenon occurs when a material's temperature increases, causing its atoms to vibrate more intensely and take up more space. Steel beams in construction are subject to elongation under high temperatures due to this thermal expansion.
To calculate the change in length (ΔL) caused by temperature change (ΔT), the formula is ΔL = αLΔT, where α represents the coefficient of linear expansion for steel. Although the precise temperature at which a steel I-beam will begin to visibly deform depends on various factors such as fire intensity and the properties of the specific steel alloy, steel generally starts to lose strength at around 538°C (1000°F) and significant deformation can occur beyond this temperature. For the purpose of fire safety and engineering, critical temperature consideration for steel is crucial for building design and assessing fire resistance.