Development of an Engineering Methodology for Thermal Analysis of Protected Structural Members in Fire

Abstract

A novel CFD-based methodology for thermal analysis of protected steelwork in fire has been developed to overcome the limitations of existing methodologies. This is a generalised quasi-3D approach with computation of a "steel temperature field" parameter in each computational cell. It accommodates both uncertainties in the input parameters and possible variants to the specification by means of many simultaneous thermal calculations. A framework for the inclusion of temperature/time-dependent thermal properties, including the effects of moisture and intumescence, has been established. The method has been implemented as the GeniSTELA submodel within SOFIE RANS CFD code. The model is validated with respect to the BRE large compartment fire tests. Sensitivity studies reveal the expected strong dependencies on certain properties of thermal protection materials. The computational requirements are addressed to confirm the practicability of the tool in simultaneously running a large number of parametric variants. Ultimately, the steel temperature field prediction provided by GeniSTELA provides far more flexibility in assessing the thermal response of structures to fire than has been available hitherto; hence it could be further used for the structural response analysis, demonstrating the potential practical use of the method to improve the efficiency and safety of the relevant structural fire safety design.Keywords: CFD; thermal analysis; quasi-3D; GeniSTELA; SOFIE INTRODUCTIONIncreasing interest in assessing the performance of structures in fire is driving the development of an array of modelling methodologies to be used in fire safety engineering design. Whilst traditionally most code-based design has been based on simple calculations, referencing measured fire performance in standard tests, the progressive shift towards performance-based design has opened the door to use of advanced methods based on numerical models. These approaches will not replace standard testing, but they can already be used in a complementary fashion, to extend the application of test data.Some simplified modelling methods have also been established, such as the protected member equation in Eurocode 3 (EC3) [1], but as with all semi-empirical methods the results will tend to be conservative and there are of necessity a number of simplifying assumptions. CFD-based methodologies can in principle provide a much more detailed description of the thermal environment and the effects of localised heating, which could be used in conjunction with thermal analysis models to examine structural performance.There are currently various approaches used to link the CFD-based models and the thermal analysis models. For example a range of advanced methodologies have been developed to couple the CFD model with the thermal and structural model, such as those examined in the FIRESTRUC [2] project with code pairs ANSYS-CFX, FDS-ANSYS, VESTA-STELA etc. All these implementations have certain aspects of pros and cons.