Fire Design of Concrete Structures based on a Levels-of-Approximation Approach
Concrete structures, contrary to steel or timber structures, have not been traditionally considered as significantly sensitive to fire conditions. As a consequence, their performance in fire conditions has generally been assessed on the basis of quite simplified rules provided in codes of practice, mostly dealing with minimum geometric requirements in terms of thickness and concrete cover. Accidents under fire conditions have nevertheless revealed a certain level of vulnerability of concrete structures in fire and have thus shown that code regulations are potentially insufficient to cover this aspect of design. As a reaction to this level of simplicity, some codes are currently available with specific and very detailed provisions with reference to fire design in concrete structures, such as EN 1992-1-2, covering a wide number of simplified and advanced design procedures. Unfortunately, the hierarchy of these procedures and which one is most suitable for a given situation is sometimes unclear for designers. In addition, it is probably not necessary to perform rather complex and detailed analyses for all types of structures, but the level of refinement can be adapted to the sensitivity of the structure to fire conditions and to its significance for the society. In this paper, a possible approach for consistent design of concrete structures under fire conditions is proposed based on the Levels-of-Approximation philosophy. The Levels-of- Approximation approach has been successfully introduced into the new Model Code 2010 for a number of problems (such as shear, punching and 2nd order effects) and allows refining the accuracy of the analysis when necessary. This allows keeping simple rules for most cases but provides a general frame for assessing complex or sensitive structures. In addition, it does not only incorporate calculation methods, but specifies which ductility requirements are to be fulfilled in order to ensure a correct applicability of each method.