The reliability based design of composite beams for the fire limit state

[摘要] In the past use was made of prescriptive design rules to provide for the fire limit state. ModernDesign Codes provide the scope and the means to design for fire in a performance based manner.The Eurocode provides guidance on the actions on structures exposed to fire as well as methodsto predict the structural behaviour of elements in fire.Structural designers can now incorporate the use of parametric fire curves to describe compartmentfires. These fire models are not an extension of the old nominal standard temperaturetime curves. Parametric curves are analytical models that are based on natural fire behaviour.The temperature in the fire compartment can be predicted in a scientific manner taking accountof fire loads, ventilation conditions and compartment characteristics.The combination of rational fire models and temperature dependant structural behaviour enablesdesigners to predict whether elements will fail during a fire. This is an improvement onthe empirical prescriptive fire resistance ratings, used to date.Multi-storey steel framed structures, with composite floors, were identified as structures withhigh inherent fire resistance and robust behaviour. The composite beams in the floor structurewere identified as critical elements when subjected to fire. The deterministic design and thereliability level of these elements were studied.Deterministic fire design procedures are presented that can be used to design unprotected compositebeams for the fire limit state. The reliability of the deterministic design procedures wasevaluated through a First Order Reliability Method.Parametric fire curves are suitable for reliability analysis due to the fact that they can bedescribed by stochastic variables. The fire load was determined to be the dominant variableinfluencing the reliability level of the composite beams. The ventilation conditions of the firecompartment also has important implications for the temperature development of the compositebeams.The reliability analyses results show that reasonably sized composite beams can be used as unprotectedelements in smaller fire compartments with moderate fire loads. It was found that astructural element's total probability of failure can be improved by the use of active fire fighting measures. The benefit of active fire fighting measures can be quantified by considering theirprobability of failure.By use of conservative assumptions and basic knowledge of fire engineering principles, rationaldesign methods can provide safe and economical solutions for fire design of composite beams.

[发布机构] Stellenbosch University