Development of software for reliability based design of steel framed structures in fire

Abstract

Fire in building structures represents a risk both to life and property that cannot be fully eliminated. It is the aim of fire safety engineering to reduce this risk to an acceptable level through the application of scientific and engineering principles to evaluate the risk posed by fire and to determine the optimal set of protective measures. This is increasingly being achieved through performance-based design methods. Performance-based design sets out performance requirements, typically related to life safety and control of property losses, and the designer is free to choose the most suitable approach to meet these requirements. Accurate performance-based design requires the evaluation of the risks to a structure through the evaluation of the range of hazards that may occur and the resulting structural responses. The purpose of this research is to develop simplified methodologies for the reliability based design of steel framed structures in fire. These methodologies are incorporated into a software package, FireLab, which is intended to act as a tool for practicing engineers to aid in learning and applying performance-based design. FireLab is a Matlab based program that incorporates a number of different models for analysing the response of structural elements exposed to fire. It includes both deterministic and probabilistic analysis procedures. A range of simple fire models are presented for modelling compartment fires. A set of heat transfer processes are discussed for calculating the temperature distribution within common structural elements exposed to fire. A variety of structural models are discussed which may be used to model the effects of fire on a structure. An analytical model for the analysis of composite beams has been implemented in the software program. Interfaces between the software and 2 separate third party programs have also been created to allow for the analysis of composite beams using the finite element method. Analytical methods for the analysis of composite slabs under thermo-mechanical load have been implemented in the software. These methods account for the additional load carrying capacity that slabs have in fire due to the positive effects of tensile membrane action. A numerical analysis method for the vertical stability of structures subjected to multi-floor fires has been implemented using the direct stiffness method. This method uses an elastic 2nd order solution in order to check the stability of a column under the fire induced horizontal loads from sagging floors. These models of potential failure scenarios provide the basis for the probabilistic analysis methods. A variety of methods for reliability analysis are evaluated based on ease of use, accuracy and efficiency. A selection of these methods has been implemented in the software program. A selection of sample cases are examined in order to illustrate the procedures and to evaluate the important input variables. These methods provide the probability of failure of a structure under specific loads. The probability of failure is a useful parameter in comparing the level of safety between various design options. A more comprehensive framework is developed for the evaluation of the probable costs due to fire associated with a given design. This framework is based on an existing framework from earthquake engineering. It involves calculating the statistical spread of both the magnitude and likelihood of occurrence of fire and the resulting structural responses. The damage that occurs from the structural response may be then estimated. Finally, given the likely level of damage that will occur it is possible to estimate the cost of the damage either in terms of monetary cost of repair or downtime due to repair works. This method is applied to a variety of design options for a typical office building in order to illustrate the application of the framework.