Reliability-based decision making for concrete elements exposed to fire

by Ruben Van Coile

Institution: Ghent University
Year: 2015
Keywords: Technology and Engineering
Record ID: 1074665
Full text PDF: http://hdl.handle.net/1854/LU-5972509


Fire is one of the most severe load conditions for structures and may cause premature failure of structural components, potentially resulting in human casualties and considerable economic losses. A number of recent fire-induced failures as for example the partial collapse of the Windsor Tower in Madrid (2005) and the collapse of the Architecture faculty of the TU Delft in the Netherlands (2008) have stressed the importance of structural fire design and structural fire safety, but up to now little research exists with respect to the safety level obtained during fire exposure. For concrete elements in the European Union structural fire design is governed by EN 1992-1-2. While this standard gives detailed material models and calculation rules, the safety level which is targeted by the standard is not stated explicitly. Furthermore, EN 1992-1-2 relates the fire performance of concrete elements to the idealized ISO 834 standard fire, further complicating the evaluation of the appropriateness of the standard with respect to real (natural or parametric) fire exposures. In this dissertation a numerical calculation tool is presented which allows to determine the safety level obtained by concrete elements during exposure to both standard fires and natural fires. The tool is based on a discretization of the cross-section and takes into account the non-linear and time-dependent temperature distribution in the cross-section, as well as the uncertainties which may exist with respect to basic parameters as for example the concrete compressive strength, the reinforcement yield stress and the concrete cover. While the initial evaluations are based on crude Monte Carlo simulations, improved computational efficiency is obtained by determining the probability density function describing the resistance effect during fire exposure, and the application of advanced structural reliability methods. Especially for concrete slabs a significant improvement is obtained by the introduction of the mixedlognormal distribution for describing the bending moment capacity during fire. It is observed that a more traditional choice for a standard lognormal distribution is not capable of accurately describing the distribution of the moment capacity and results in an unsafe approximation for the obtained safety level. Furthermore, parametric studies are executed and the obtained results stress the importance of the concrete cover for structural fire safety. The developed methodology is applied to determine the implicit target reliability index incorporated in the current Eurocode design format, and to generalize the Eurocode definition of the structural fire resistance time. This generalized reliability-based definition is in full accordance with EN 1992-1-2 but expands its applicability to existing structures and structures with reduced or improved quality control. Although the generalized methodology can be considered as a more refined tool not suitable for everyday applications, it gives very strong mathematical support – based on the current Eurocode…