How does Q355B i beam perform in fire conditions, and what is its critical temperature for design?

 Like all structural carbon steels, Q355B loses strength and stiffness as temperature increases. Its fire performance is not an inherent property of the grade but is a function of its temperature-dependent stress-strain behavior. For design, engineers refer to the reduction factors for stress-strain relationships of carbon steel at elevated temperatures, as given in codes like Eurocode 3 Part 1-2 or Chinese GB 51249. The key parameter is the critical temperature, which is not a fixed number for Q355B but is determined for each specific structural element in a fire design analysis. It is the temperature at which the element's strength (or load-bearing capacity) decays to the level of the applied load during the fire (factored down if considering fire limit state). However, a commonly referenced practical critical temperature used in simplified design methods is around 550°C. At this temperature, the yield strength of typical carbon steel is reduced to approximately 60% of its ambient yield strength (fy). The reduction factors are: ~1.0 at 20°C, ~0.78 at 300°C, ~0.63 at 400°C, ~0.47 at 500°C, ~0.33 at 600°C, and ~0.18 at 700°C. The design process involves ensuring that the steel temperature, calculated based on the fire exposure (standard fire curve like ISO 834 or a parametric fire) and the level of fire protection (if any), remains below this critical temperature for the required fire resistance period (e.g., R30, R60, R90). Q355B itself does not have superior fire resistance compared to other carbon steels; its performance is managed through fire protection measures such as intumescent coatings, spray-applied fire resistive materials (SFRM), or board/blanket systems that insulate the steel and delay its temperature rise. For unprotected steel, the temperature rises very quickly, leading to failure within minutes. Therefore, the fire design of a structure using Q355B focuses on selecting appropriate protection to keep the member below its critical temperature for the design duration.