The impact of fire development on design resistance of structures

Sammanfattning: The current design methods used to determine fire progression and temperature-time development in fire compartments today are being questioned to not give accurate results in large and complex enclosures (larger than 500 m2). The established design methods proposed by Eurocode and used by fire safety engineers today are primarily the standard temperature-time curve and the parametric temperature-time curves. The parametric temperature-time curves are based on the heat and mass balance equations and both methods assume homogenous temperatures and uniform burning. These assumptions are being questioned for use in large enclosures such as open-plan compartments and compartments with multiple floors connected which are typically modern and common building types in today’s society. Today there are no established design methods developed to determine fire progression in large enclosures, but the Improved Travelling Fire Method (iTFM) and the New MT model II are new, alternative design methods which are prospects to become established engineering tools in the future. The iTFM is developed at the University of Edinburgh for travelling fires in large size compartments and the New MT model II is developed by RISE, Research Institutes of Sweden, for large tunnel fires. These two new design methods have been investigated and compared to established methods in a case study. Also localised fires from Eurocode with proposed interpretations by Ulf Wickström has been investigated and compared to the standard temperature-time curve and the parametric temperature-time curves. The new interpretation suggests that the given heat flux boundary conditions in Eurocode are interpreted as adiabatic surface temperatures based on given emissivities and convection heat transfer coefficients according to Eurocode. Through a case study the different methods were compared throughout reference buildings with constant material properties and fire loads, but with varying floor area and height. The result focused on if the new methods have more bearing on reality than the standard fire curve and the parametric temperature-time curves methods when determining fire progression and temperature-time development. Desired benefits with the new methods are to better predict and describe fire development in large enclosures. The referenceIIIbuildings were considered as occupancy class 2 (Vk2) and Br2 buildings with a load bearing fire resistance capacity demand of 30 minutes. This report is an early stage in the process of developing new fire models to improve the fire designing process when working with large compartments. The aim with the new, alternative methods and localised fires with proposed interpretation is to enable them to become engineering tools used by fire safety engineers in the future to create a more efficient and adapted design process. The results differ significantly depending on used method and reference building. The maximum temperatures conducted by the iTFM are in general higher than the standard fire curve and the parametric temperature-time curves. When applying the method to the reference building with high ceiling height and low spread rate the resulting temperatures were lower than the standard fire curve. The fire progression of the New MT model II is highly dependent on opening factor and time until temperature increase starts. In comparison to the parametric fire curves with the same opening factors the New MT model II resulted in considerably faster temperature development and higher temperatures. Localised fires with the new proposed interpretations resulted in adiabatic surface temperatures which were compared to the standard temperature-time curve after 30 minutes of fire and the maximum temperature of the parametric temperature-time curves. The comparison resulted in slightly lower temperatures for the localised fires with the new proposed interpretations compared to the standard temperature-time curve and similar temperatures compared to the parametric temperature-time curves in the case study. The results of the iTFM and the New MT model II differs significantly depending on physical parameters used in the calculation processes. The models are customizable and vary depending on fire scenarios and compartments and could possibly be future alternative methods when designing for fires in large compartments. Further studies and development together with real fire tests would provide the models with better accuracy and continuity. Localised fires with proposed new interpretations are a future prospect to become a future standard method for determination of maximum temperature of member surfaces in fire safety design.