Influence of Pre-flashover Fire on Structural Steel Temperature

Abstract

Current structural fire engineering practice generally assumes that the pre-flashover fire (growth) stage has a negligible influence on structural response and therefore considers only post-flashover conditions. The Eurocode parametric fire, widely used for structural fire design, follows this approach.
This thesis investigates the influence of incorporating pre-flashover temperature on the thermal response of structural steel members. A continuous gas temperature-time history is developed by combining a pre-flashover fire model based on the MQH correlation with the Eurocode parametric fire curve. Steel temperature evolution is then evaluated using the lumped mass approach in EN 1993-1-2:2024.
A sensitivity analysis is performed to identify key parameters affecting the difference in maximum steel temperature between the combined and conventional approaches. This is followed by a Monte Carlo simulation within a factorial framework, covering a range of compartment sizes and occupancies.
The results indicate that pre-flashover heating has negligible influence on unprotected steel members, due to the rapid heating associated with the post-flashover phase and the high thermal conductivity of steel. For the pre-flashover fire to have any meaningful impact on protected structural steel, the Monte Carlo simulation shows that three scenarios must meet: (i) slow-to-medium growing fires, (2) fuel-controlled post-flashover fire, and (3) light protection thickness. In such combinations, pre-flashover fire results a maximum steel temperature increase by more than 30 °C for the investigated compartment size.
Finally, the study provides a framework to quantify the influence of pre-flashover temperature history, supporting more informed and performance-based structural fire design within the context of the Eurocode parametric fire.

Popular Abstract

Hishe Kahsay Negash: Influence of Pre-flashover Fire on Structural Steel Temperature
IMFSE Master Thesis, Lund University, May 2026

A popular science summary
When engineers design a building for fire, they often focus on the hottest part of the event: the stage after flashover, when the room is fully involved in fire. This thesis asked a simple question: does the earlier growth phase, before flashover, also matter for the temperature of structural steel?
To answer this, the study combined two fire descriptions. One model represented the early growth stage, and the Eurocode parametric fire represented the later stage. By joining them into one continuous fire curve, the research followed the heat exposure from ignition through flashover and into the cooling phase.
The main finding was clear. For unprotected steel, the early fire growth had very little effect. Steel heats up quickly once the fire becomes fully developed, so the earlier warming is almost hidden by the much stronger later heating. But for protected steel, the picture changes. In slow-growing fires with good ventilation and thin insulation, the early phase can raise the maximum steel temperature by more than 30°C in the cases studied.
That increase matters because it reduces the safety margin in a design. In practice, a structure designed using only the later fire curve may underestimate the thermal demand in some scenarios. The effect was strongest when the fire grew slowly, the later fire was fuel-controlled, and the fire protection layer was light.
The broader message is not that current design methods should be abandoned. Rather, it shows that the first minutes of a fire can be important in some buildings, especially when the fire develops slowly. Including the growth phase gives engineers a more complete picture of what steel members really experience and supports more informed, performance-based fire design.

• author: Negash, Hishe Kahsay
• supervisor: Nils Johansson
• organization: Division of Fire Safety Engineering
• course: VBRM05 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Pre-flashover fire, MQH correlation, Post-flashover, Eurocode parametric fire, Critical steel temperature
• publication/series: LUTVDG/TVBB
• report number: 5762
• other publication id: LUTVDG/TVBB—5762--SE
• language: English
• id: 9229226
• date added to LUP: 2026-06-02 13:25:12
• date last changed: 2026-06-02 13:25:12