Fire Imposed Heat Fluxes for Structural Analysis

The last two decades have seen new insights, data and analytical methods to establish the behaviour of structures in fire. These methods have slowly migrated into practice and now form the basis for modern quantitative structural fire engineering. This study presents a novel methodology for determining the imposed heat fluxes on structural members. To properly characterise the temperature rise of the structural elements, a post-processing model for computational fluid dynamics tools was developed to establish the heat fluxes imposed on all surfaces by a fire. This model acts as a tool for any computational fluid dynamics model and works on the basis of well resolved local gas conditions. Analysis of the smoke layer and products of combustion allow for heat fluxes to be defined based on smoke absorption coefficients and temperatures. These heat fluxes are defined at all points on the structure by considering full spatial and temporal distributions. Furthermore, heat fluxes are defined by considering directionality and both characteristic length and time scales in fires. Length scales are evaluated for different structural member geometries, while time scales are evaluated for different structural materials including applied fire protection. It is the output given by this model that provides the input for the thermal analysis of the structural members that is a necessary step prior to the structural analysis to be undertaken. The model is validated against the experimental results of the previously mentioned large scale fire tests, showing good agreement. In addition, comparisons are made to current methods to highlight their potential inadequacies

Heat Transfer to the Structure during the Fire

Chapter 7 in the book: The Dalmarnock Fire Tests: Experiments and Modelling, Edited by G. Rein, C. Abecassis Empis and R. Carvel, Published by the School of Engineering and Electronics, University of Edinburgh, 2007. ISBN 978-0-9557497-0-4The post-flashover Fire Test One of a furnished room in Dalmarnock provides a wealth of information including measurements in both the gas phase and on compartment boundaries (Chapter 3). Total heat fluxes at a number of locations in the room were obtained together with gas flow velocities at all major openings. Materials involved comprised thermally-thick and thermally-thin elements including ceilings, internal and external walls and small length-scale members. This chapter presents the spatial distribution of heat fluxes within the compartment and uses the data to validate an analytical model developed to act as an interface between the gas-phase fire environment and the solid-phase structural elements using a computational fluid dynamics (CFD) based approach. It is anticipated that the output of such a model can be used on a larger stage to conduct a structural analysis

Modelling of Structures in Fire: An Example of the Boundary Condition

The collapse of the World Trade Center Towers 1, 2 and 7 on September 11th 2001 has highlighted the need for proper understanding of the behaviour of structures in the event of a fire. A detailed analysis of the fires and the behaviour of the structures followed the events revealing numerous gaps of knowledge and uncertainties within the methodologies that are generally used by engineers and that are meant to deliver building designs that can be deemed as safe. Detailed modelling of the fire can only be achieved via Computational Fluid Dynamics (CFD) and of the structure using Finite Element Models. The integration of both models implies an adequate understanding of the boundary condition. This paper will analyse the boundary condition between the gas phase environment generated by the fire and the solid phase representing the structural members and protective elements. As an application example the performance of high-rise steel framed structures in the event of a large uncontrolled fire will be evaluated using the Fire Dynamics Simulator (FDS) CFD code and ABAQUS Finite Element Model

Furnace tests on unprotected and protected concrete filled hollow structural hollow sections

The accurate prediction of cross-sectional temperatures within concrete filled steel hollow (CFS) sections is critical for the accurate prediction of fire resistance. Whilst there have been many thermal and structural tests conducted on CFS columns, there are few that report the full cross-sectional thermal profile, and when they are reported, the sensor density is low, hindering the ability to validate models. This paper presents furnace tests and thermal modelling on 14 unprotected and 20 protected CFS sections, and examines the effect of several parameters on cross-sectional thermal profiles, as well as assessing the accuracy of both Eurocode thermal analysis guidance and intumescent fire protection design guidance. This paper shows that; (a) the assumptions within the Eurocode guidance can lead to large over-estimations in cross-sectional temperatures; (b) proposes new thermal modelling assumptions in three key areas; and (c) shows that the current intumescent fire protection design guidance is very conservative