Developing system-level fragility functions for performance-based fire engineering of buildings

This research contributes to the current efforts for developing probabilistic methods in structural fire engineering (SFE) to foster the shift towards performance-based design in the field. A methodology is presented to assess the risk related to structural failure due to fire in multi-story buildings. At the core of this methodology is the concept of fragility functions, which allows quantifying in a systematic way the effects of the uncertainties in both demand and capacity parameters on the reliability level.
The fragility functions, adopted here for the purpose of SFE, yield the conditional probability to reach predefined damage states as a function of an intensity measure of the fire. The damage states are grouped in categories such as ‘slight’, ‘moderate’, ‘extensive’, and ‘complete’, for estimating the magnitude of the damage. The fragility functions are defined for an entire building and depend on parameters such as the structural type and the fire resistance rating of the structural components.
As the functions give conditional probabilities, they are completed with probabilistic models for the occurrence of a fire in the building (i.e. likelihood of the hazard) and the expected value of the fire load density (i.e. intensity of the hazard). These models depend on the building occupancy type, surface area, and active fire protection measures. Finally, one can obtain the (total) probability to reach different levels of potential damage for buildings of different typologies, structural design, size and occupancy. In addition, similar to earthquake engineering, one can also estimate the probability of reaching a certain damage state for a given probability of fire hazard.
The methodology is illustrated for prototype multi-story steel buildings. An important contribution is the quantification of the safety level at the building scale, as opposed to an individual component. The results provide an insight for the discussion about the evolution of the design codes for a better standardization of this safety level.