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The stability of steel structures during a fire critically depends on X-bracing systems. Our study analyzed the response of a steel warehouse by varying the brace cross-section (from UPN160 to UPN400) across three distinct fire scenarios: standard ISO 834, wood, and plastic.

We found that increasing brace stiffness:

Reduces Critical Loads: Larger sections (e.g., UPN400) reduce bending moments at column bases and control lateral displacements, enhancing global stability.

Delays Instability: In slow-heating fires (ISO 834, wood), larger braces delay reaching the buckling load.

Limited Benefit in Rapid Fires: In the plastic fire scenario, characterized by a steep temperature increase, even the largest braces buckle almost simultaneously due to the rapid strength degradation of steel.

The results provide actionable guidelines for designers : for gradual fires, larger sections are effective; for rapid fires, additional protection is necessary. The optimal size choice (e.g., UPN300 for wood fire or UPN400 for plastic fire) must account for the specific fire load.

https://doi.org/10.1080/10168664.2025.2520616