Fire modeling: An introduction for attorneys

...by Dr. Vytenis Babrauskas, Fire Science and Technology Inc.

Introduction

Fire modeling is something which is often found to be mysterious by attorneys. Yet, understanding what it is, what it can do, and what it cannot do can be vital to successful development of some types of fire cases. The purpose of this note is to present the basic ideas so that they are understandable by the non-scientist. Thus, the information should be of value also to fire investigators, claims adjusters, and other individuals involved with fire losses. Most of them are not aware of either the strengths or the limitations of the fire modeling. Thus, in this note the objective is to explain the process in simple terms, so that a clear picture will emerge how fire modeling can and cannot be used.

What is a model?

Before we can discuss fire models, we must explain what a scientist means by 'model.' The meaning of this crucial term is essential to understand. A model of anything is, simply, a systematic representation of that thing. Thus, for example, we can have

thought models (or conceptual models)
scale models, and
mathematical models.

The above three examples are probably the main 'representations' which are used by scientists. A thought model is simply a proposed schema explaining how something works. Scale models are often used in structural engineering, fluid dynamics, and have occasionally been used in fire science. Model trains are familiar to all. A scale model in scientific work is simply a reduced-size object on which certain measurements will be made. The category which we want to discuss in this Note is the last type, the mathematical model. In general, a mathematical model will be a series of equations which describe a certain process. If the equations are simple enough, they can be solved on the hand calculator. More commonly, the equations are not so simple. Consequently, a computer is required for their solution. Thus, in the fire field, we would speak of "computer fire models." Nowadays, when one speaks of a "fire model," it is usually understood that one is referring to a "computer fire model." This is unnecessarily restrictive, however, and other types of models (such as scale models) remain legitimate scientific forms of model.

A "computer fire model" is normally realized as a computer program. This again, is most common, but not necessarily always true. A computer fire model, for example, could be realized as only a flowchart. From the above, one can understand why fire modeling is often taken to mean "use of computer programs for predicting fire," although this would be too restrictive a definition.

What do fire models do?

By now, fire modeling has been in use for more than two decades. This author's computer program COMPF was released in 1975 [1] and was the first computer program for predicting room fires to be developed in the U.S. Research in several other countries, however, goes back further. During the subsequent two decades, tremendous progress was made in the field. Today, many persons who have only a limited knowledge of fire science have already had a slight exposure to fire modeling. From this, they are apt to conclude that fire modeling is something which allows scientists/engineers to 'wave a magic wand' and to calculate the history of a fire just by working at their computer. On rare occasions, this can be true. But normally, the situation is not so straightforward.

The function of COMPF was to predict the fire history within a single room. The history was represented only after the time of 'flashover' within the room. Flashover is the point in a fire (it does not occur in all fires) when the room "fills with flames." The hazard greatly increases from that point on. Nowadays, various other types of computer fire models are also available to the scientist. What kind of fire characteristics, then, can a fire model predict? The list is limited only by the ingenuity of scientists, but we can cite characteristics which are already routinely being computed:

gas and surface temperatures
flow rates of gas through openings
heat fluxes impinging on surfaces
smoke obscuration
production of certain toxic gas species
strength reduction and structural failure of building elements
activation times for sprinklers and detectors

It can be noted that this list is weighted towards fluid mechanics and related themes. This is not surprising, since a majority of the researchers creating fire models have been fluid mechanics specialists. Models also exist for certain human behavior aspects (e.g., exiting through corridors and stairs) although these have so far been very little used for practical problem solving; thus their validity is generally unknown.

It should be noted that certain characteristics are usually not being computed. These include:

the ignitability of objects from small flames
the spread of fire over surfaces
the actual 'size' of the fire, that is, its heat release rate

A list of other fire characteristics that we cannot yet routinely predict has recently been publicized [2]. The three characteristics above are three exceedingly important aspects of fire, indeed heat release rate (HRR) has been referred to as the single most important variable in describing fire hazard [3]. Likewise, there will not be a fire without ignition and, in most cases, flame spread is also an essential trait of fire. The way that today's fire models normally solve a problem is by being given the HRR as input. The flame spread aspects are usually not made explicit. The most important role of flame spread is to progressively involve greater areas in burning, that is, to cause a growth of HRR. Thus, if we have a HRR versus time curve, the flame spread issue has already been solved. The initial ignition is, simply, assumed to have taken place, so no computation is made there either.

To make a computation using one of our state-of-the-art models, such as HAZARD [4], then requires that the modeler supply a HRR curve as input. In some cases, the HRR curve may already have been published in the literature for a 'similar' burning object. Compendia of data are available which present some useful, non-proprietary data [5]. However, the variety of items which can burn is essentially infinite, while the amount of publicly available data is quite tiny.

The situation is even more complicated when one realizes that more than one item can burn. Methods have been suggested for estimating second-item involvement [6]. However, under most conditions, such procedures entail a great deal of uncertainty. This can be due to: (a) irregular geometry of the item in question; (b) not well enough studied ignition response of the item; (c) inadequately detailed knowledge of local heat fluxes, etc. When one contemplates the uncertainties then associated with estimating the ignition for the third, fourth, etc. item, it becomes clear that the ignition sequence of a roomful of diverse items cannot be predicted with a reasonable degree of confidence.

Fire Testing

The solution to the above difficulty is actually straightforward: when data are not available, run a fire test. Model development is a difficult, specialist task. Thus, one cannot expect to say "improve the models," since progress could hardly be made on a time schedule to suit fire litigation needs, even if the resources were available. What is possible to do on relatively short notice is to organize fire tests.

Fire tests have their limitations, too. The largest fire that can be conducted indoors in a laboratory, under controlled and instrumented conditions is about 20 megawatts. Physically, this corresponds to one room or a couple of smallish rooms joined together. Fire models are much less restricted in that respect. They are available for computing multi-story, multi-room arrangements, and the rooms do not have to be small enough to fit under a laboratory's exhaust hood.

Thus, the practical solution is to combine fire modeling with fire testing.

Normally, the objects, walls, etc. associated with ignition and early fire growth are directly reconstructed in the laboratory by procuring exemplars and creating what is normally termed a sectional full-scale mockup. Full-scale denotes that real appliances are used, real wall thicknesses are employed, etc. Sectional denotes that only a slice out of the building is constructed in the laboratory and not the whole fire environment.

The presumed or alleged ignition sequence is then started in the laboratory test and measurements are taken of HRR, smoke production, temperatures, heat fluxes, and other fire variables. Fire modeling is then used to take the laboratory data of the initial fire stages as an input and to compute the subsequent stages of fire development. Thus, fire modeling can be viewed as a direct extension of fire testing, or vice versa.

The confidence in the results produced by the fire model is normally greater for the intermediate stages of the fire than for the late stages. During the late stages of fire, a number of additional events can happen. These include burn-through of partitions, collapse of beams, collapse of occupant goods (e.g., rack storage) and similar. Also, it may be expected that firefighting will make some difference on the outcome of the fire, and this may not be reasonable to try to predict mathematically. Models do exist which can allow the prediction of the collapse of structural members, but these require input data which may often be unavailable.

Tests vs. demonstrations

It is important to distinguish between a field demonstration and a large-scale laboratory test. Both involve setting up of an environment intended to recreate the scene of the fire origin. Both can be used to produce videos for jury viewing. However, a field demonstration does not collect HRR nor other fire data which could usefully serve as input to a fire model. Thus, demonstrations can only be used for video purposes.

The advantage of a demonstration is that it can be conducted in every town and city. A laboratory test, by contrast, requires use of a fire testing laboratory, and there are only a handful of such facilities in the country.

The costs, however, are not necessarily much lower for a demonstration. The bulk of the cost is normally associated with procuring exemplars, constructing the mockup, setting up video and other documentation, and witnessing of the test. Since a fire test laboratory already has the HRR and other instrumentation necessary, the marginal cost is small for setting up the instrumentation and collecting the necessary data. The actual laboratory test procedures [7] are, by now, quite well worked out, and time does not need to be allocated to research in this area.

Conclusions

Fire modeling can normally be considered as the prediction of fire characteristics by the use of a mathematical method which is expressed as a computer program.

The needs of fire litigation from fire modeling are specialized. Usually, there is a great deal of specificity about the sequence of fire ignition and the materials involved in the process. This commonly precludes the use of handbook data as input to fire models. Instead, it will usually be necessary to conduct a sectional full-scale mockup to obtain appropriate data describing the initial part of the fire. This information then serves as input to a fire model, using which the later fire development can be approximately predicted.

References

[1] Babrauskas, V., COMPF: A Program for Calculating Post-flashover Fire Temperatures (UCB FRG 75-2). Fire Research Group, University of California, Berkeley (1975).

[2] Babrauskas, V., Fire Modeling Tools for Fire Safety Engineering: Are They Good Enough? J. Fire Protection Engineering 8, 87-95 (1996).

[3] Babrauskas, V., and Peacock, R. D., Heat Release Rate: The Single Most Important Variable in Fire Hazard, Fire Safety J. 18, 255-272 (1992).

[4] Bukowski, R. W., Peacock, R. D., Jones, W. W., and Forney, C. L., HAZARD I Fire Hazard Assessment Method (NIST Handbook 146). [U.S.] Natl. Inst. Stand. Tech., Gaithersburg, MD.

[5] Babrauskas, V., Burning Rates (Section 3/Chapter 1), pp. 3-1 to 3-15 in The SFPE Handbook of Fire Protection Engineering, Second Edition, National Fire Protection Association, Quincy MA (1995).

[6] Babrauskas, V., Will the Second Item Ignite? Fire Safety J. 4,281-292 (1981/82).

[7] Babrauskas, V., and Grayson, S. J., eds., Heat Release in Fires, E. & F. N. Spon, London (1992).