Structural Fire Load and Computer Modeling

The Structural Fire Load (temperature-time relationship of hot gases) can be obtained analytically by using computer models of fire development.

The simplest model is a one-zone model for post-flashover fires, in which the conditions within the compartment are assumed to be uniformly distributed in space and represented by a single temperaturetime function. The parametric models discussed in previous chapter are the one-zone models. In general, zone models are simple computer models that divide the considered fire compartments into separate zones, where the conditions in each zone are assumed to be uniformly distributed in space.

Analytical zone models for predicting fire behavior have been evolving since the 1960s. Since then, the completeness of the models have gone through major development to multi-zones and multi-compartment for modeling localized and preflashover fires. The zone models also model the fire compartments in more detail, compared to that for parametric fires and time equivalence methods. The geometry of compartments, as well as the dimensions and locations of openings, can be modeled easily.

Zone modeling is the most common type of physically based fire model. It is deterministic model. It solves the conservation equations for distinct and relatively large regions. Its main characteristic is that it models the room into two distinct regions, one hot upper layer and a cooler layer below. The model estimates the conditions for each layer as a function of time only. There are many Zone modeling packages available now on a market. Below is a summary of current Zone models available from.

One more computer zone models should be added to Table 1: this is the “Ozone” model that had been developed at the University of Liege, Belgium. Ozone, Version 2.2.0, was originally developed as part of a European Coal and Steel Community project entitled the Natural Fire Safety Concept. It created considerable interest in Europe and it was suggested that it could replace the parametric temperature-time relationship in Eurocode 1. The parametric method in this case is done using the ‘Probabilistic Fire Simulator V2.1’, developed at VTT Technical Research Centre of Finland. Ozone belongs to the family of zone models. Ozone allows to model only one compartment.

The main hypothesis in this case is that the compartment is divided in two zones in which temperature distribution is uniform in time. Transition from two zones to one zone model is related to the notion of fire growth curve, shown below: This software was tested against experimental data from fire tests conducted by CORUS Research, Development and Technology, Sweden Technology Centre. In general the correlation between the fire test and predicted results was found to be poor.

There were concerns about the theoretical background to the first model. One major concern was the use of a “design” rate of Heat Rate Release (HRR) curve based on a t2 growth phase, a constant release phase and a linear descending branch after 70% of the fire load has been consumed. Further developments in the software have been made that allowed to solve the problems associated with using the software. However dissimilarities between measured and predicted temperatures may still exist. Out of many two-zone models mentioned above let’s review just the “CFAST” model.

Full version you can download here

Text by Dr. Leo Razdolsky, LR Structural Engineering Inc., Professor at Northwestern University, Evanston, IL., USA