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Role of risk analysis in fire engineering: a research perspective
Thomas, R.
NRCC-48144
A version of this document is published in / Une version de ce document se trouve dans : APEC Fire Safe Use of Timber in Construction Seminar,
Building Confidence in
Timber, Wellington, N.Z., May 24-26, 2005, pp. 1-27
Role of Risk Analysis in Fire Engineering:
A Research Perspective
Dr Russ Thomas
National Research Council of Canada
Introduction
The move of many countries to adopt building and fire regulation based upon outcome or performance based principles is posing some very interesting challenges to the fire research community. Many of the prescriptive solutions upon which the majority of traditional building and fire codes have been based were established over time on the basis of experience. Few of them had any formal measurement of performance and in many cases there was a complete lack of the knowledge required to establish an
appropriate measurement criteria. Often, where there were test methods, these did little but result in relative rankings of one solution/product over another and were often only distantly related to the hazard being managed. A good example of this approach is the use of standard fire ratings of assemblies where there is relatively universal agreement that the fire challenge used in the test is rarely realistic in terms of the challenge that the assembly would experience in a “real” fire today.
As a result, there is a growing need to establish a better understanding of the likely occurrence of various types of fires in our communities and also the need to have a good understanding of the life history of such fires. Although in it’s early years the research community focused considerable efforts upon recreating the fires that were occurring at that time (early 1900’s) there has been considerable changes in both the types and quantities of combustibles involved in current day fires. Unfortunately, in many cases our standard testing approaches has not kept abreast of these changes in terms of both types and quantities of the combustible materials present.
For many years, the research community has focused considerable efforts upon gaining a better understanding of the “fundamentals” of fire and fire behaviour. Although we now have a much better understanding of the science underpinning our discipline there are still many aspects of fire and it’s behaviour that we are still not able to measure. As we move towards the adoption of performance-based codes, the need to establish measurable performance criteria for all aspects of a structures fire performance poses a significant challenge to the fire research community.
The move to performance approaches has also highlighted the need for tools to aid both the designer and regulator in their design and review process. Although there have been a number of these tools around for a few years there has only been limited validation of
them, in part due to the time and costs associated with running extensive validation experiments.
With the focus upon “provable” performance the new codes are also beginning to challenge the research and testing laboratories to demonstrate that their test methods not only measure the performance against an appropriate criteria but that also, their
measurements are consistent both within and between laboratories. Many of the existing test standards either fail to completely specify the experimental procedure in sufficient and leave areas open to interpretation by those undertaking the testing.
In the remainder of this paper, I will look in more detail at a number of these issues and more clearly identify the challenges that we face in fire engineering in the modern performance based environment.
What performance do we expect?
We are all familiar with building codes expressing the fire resistance requirements for a structure in terms of 1 or 2 hours of fire resistance. In doing so, the criteria has been measured in terms of a standard fire resistance test in which a sample component or structure is subjected to a specified thermal exposure and the structure has to maintain its performance for the specified duration. The fire exposure used in most cases (see figure 1)
Figure 1. Standard fire time-temperature curve (red) and data from a residential fire (green).
is based upon a standard time temperature curve first established back in the early 1900’s. At the time, the dominant materials to be found in buildings were cellulosic in nature and the time-temperature curve reflected the fire growth that had been observed at that time. One other critical feature of this curve was the fact that it only represented a fire growth phase and there was no formal recognition of a diminishing fuel source and an eventual decay phase to the fire. Most natural fires go through three distinctive phases, a growth phase, a peak or plateau phase (depending upon the availability of the fuel source) and a decay phase resulting in final extinguishment when all available fuel is consumed (see figure 1 and the paper by Mehaffey in this seminar).
In the move to Performance based regulation two issues become evident. The first is that generally the performance requirements within the code are written at a fairly high level and often in the case of “fire” the requirements are specified that the structure should resist the impact of fire for a sufficient duration that the occupants have sufficient time to escape or that the building should remain structurally sufficient (not collapse onto
adjacent structures) in the event of a fire. The other issue, often overlooked, is that many structures have been constructed using traditional materials and techniques which have resulted in an inherent level of fire resistance. These implicit levels of fire resistance have often not been formally specified and so when new materials and techniques come into play, these structures no longer provide the expected levels of fire performance. A number of Codes developing bodies are now trying to establish the levels of “inherent” fire resistance provided by the traditional materials to avoid new materials and practices compromising the current levels of provided fire safety.
Both of these issues require some substantial body of research to be undertaken to
establish the “real” performance of our buildings and structures. In the first case we need to establish the parameters of the fire (most probable fire?) that a particular structure is likely to be exposed to (the design fire for that structure). Then we need to establish the real duration that the structure resists the fire (for example, a typical one hour fire separation between occupancies in a residential building would not survive a “natural” design fire for that type of occupancy for a full hour before failing). In some situations, where there is only a life safety requirement in the code and no requirement for
protecting the building structure, the critical factor may not be the structural sufficiency of the building in the event of fire but rather the ability to maintain a viable egress path before the occupancies are overcome by the products of combustion from the design fire.
What are appropriate fire challenges to use in fire engineering?
As outlined above, there is a need to gain a better understanding of the real fires, or natural fires, that our buildings and structures can reasonably be expected to be exposed to in the event of a fire. In any risk-based approach to fire engineering, we also need to have a good understanding of the relative frequency of a fire event occurring and the type and impact of such fires.For many years most countries have attempted to collect fire statistics in varying levels of detail and completeness. In most countries, the data on fire deaths are considered to be
fairly accurate although even here there are often some problem cases such as fire deaths in the context of a vehicle accident or double counting due to jurisdictional overlap. In general though these are often the most accurate data available. At the other end of the spectrum are data on the occurrence of fires in the community, where many cases never result in a call to the fire service and therefore never make it into the official statistics. Studies, such as the that undertaken in the UK’s “British Crime Survey” (Ford, 2004) indicates that about one in five domestic fires are actually reported to the fire service and end up as part of the official fire statistics.
In most countries, on a long-term basis (Geneva Association, 2004), the majority of fire fatalities occur in residential settings. Within the residential setting, the kitchen is the site of the majority of fires although most fatalities are associated with either the bedroom or the living room (UK Fire Statistics, Canadian Fire Statistics, etc.). Although the general trends in these data are clear, the reliability of much of the data is questionable. The reason that I touch upon the issue of fire statistics is that in developing a risk based approach to fire engineering we need some “reliable” estimates of the probability of events occurring to produce our risk assessments. From a research perspective we often have to make some significant, and often gross assumptions, about the frequency of the occurrence of these phenomena. Also, there is the question of the applicability of the source of our estimates – would you be happy to use data on the frequency of living room fires from say California (where the external temperature is mild for a large portion of the year) to apply it in the Canadian North West Territories where the external temperature can be below zero for over half the year? The same type of issue also relates to the issue of human performance in evacuating from a building in different environments.
Evacuating from a house after an alarm sounds in summer in the southern USA is likely to be somewhat faster than the time it would take an occupant to evacuate from a house in northern Canada in winter with an external temperature below -35C, if only because one would require some additional preparation to emerge into the potentially lethal external environment!
In terms of the fire loads found inside our modern structures, these are also often
underestimated and do not always take into account variations that often take place in the normal range of operation of facilities. For example, in North America it is not unusual to find shopping malls, during the Christmas and other festive seasons, with a
considerable variety of temporary display sales points taking up a third of the width of the egress paths in the Mall. Often, the fire load contained in and on these temporary displays represents a very significant additional burden often not included in the original design parameters. In addition, the egress paths are also significantly compromised, resulting in an additional elevation of risk.
Work is underway in Canada to establish current fire loads, both in terms of types of material and quantities, for a range of commercial and residential settings. So far these studies have identified some significant differences from previously published work. Currently the research is now focusing upon characterizing the major parameters e.g. heat release, products of combustion, etc., of the fires that result from igniting these fuels. The results of this work will provide material for establishing a range of design fires for use in modeling the fire performance of new and existing structures
Knowing and measuring the sources of risk
Although fuels and fuel loads clearly represent components that we have to include in our considerations when carrying out a performance based design there also other significant sources of uncertainty that we need to consider. These uncertainties range from
variability in the performance of component materials to variability associated with the actual test methods themselves. For example, if one were to consider the density of gypsum board as a surrogate to the available water within the board for retarding it’s failure when exposed to fire, then one study which looked at a range of products that were all rated as Type X board indicated considerable variability in that product (see Figure 2). This variability in density is quite likely to be reflected in somewhat equivalent variability in their fire endurance performance.
Figure 2. Variability in density of two thickness of gypsum wallboard from four different manufacturers.
Although structural fire testing has been undertaken for nearly a hundred years there has been relatively little formal comparisons ether between labs or for that matter within labs using the same assemblies. Where systematic studies have been undertaken within labs the variability between tests tend to be in the range of two to three minutes. From other studies, such as those looking at the differences in the thermal impact of different furnace lining materials, it would appear that there is likely to be differences between labs, depending in part on the furnace lining (not specified in many of the standards) and other non specified parameters, of up to 15%. Such data would indicate that test outcomes on
specific assemblies that may just pass, or just fail, on one occasion may well have an opposite result on another occasion in the same or different lab!
In the end though, what part do these tests play in predicting the performance of the outcomes in real world fires (those that we are in fact expected to protect against in our performance designs). There has been relatively little experimental work undertaken to date in deriving such data and what work there has been done is not of sufficient quantity that we have a good understanding of the uncertainty associated with the data. We also have to question how relevant some of this is to the requirements that we are being asked to address by the codes – in most cases if the criterion is to ensure that the occupants of the building are to egress the building safely then the question is “how long has the structure to stay in place to provide for safe egress in the event of a fire?” Studies to date seem to indicate that the viability of an egress path is likely to be compromised, through the impacts of products of combustion, well before most structural components that impact the egress path are likely to fail.
For example, in the event of a fast growing residential fire a number of studies have indicated that the egress paths are only likely to remain viable for only two to three minutes after a smoke alarm triggers. Even our existing Standard test data would indicate that in that time span, most of our typical constructed structural elements are likely to stay in place. The real concern of most regulators is to ensure that new materials and processes do not compromise the existing levels of safety that have been achieved over the years.
Tools and their limits
The focus of this seminar is on the fire safe use of timber in construction and a
considerable part of that safety is the risk associated with using incomplete or inaccurate information (data) or applying tools outside of the range of their appropriate use. In recent years we have seen the development of a number of very useful tools that aid the designer in completing a fire safe design. Most of these tools though have been based on a limited number of cases, due in no small part to the high costs of conducting such studies, and as a result there has been only limited validation of the of the tool. In the majority of cases we are working with stochastic processes where we are trying to capture the probable range of events/values that are likely to occur. As a result, one needs to apply a reasonable level of caution in the use of such tools and be prepared to question the range of situations in which the tool has been validated (in the case of software based tools one may also wish to enquire as the processes used to verify the tool as well).
An example of an appropriate use of such a tool would be where the tool is used to select a design of an assembly prior to testing. Such tools can reduce the costs associated with the design and development phase and can result in considerable cost savings. There are a growing number of design aid tools becoming available from the research labs around the world and many of these provide useful functionality provided they are used by competent individuals who know and understand the limitations on their use.
Another type of tool that is becoming available to the practitioner community are the new fire engineering guideline documents being produced by some of the professional
societies (e.g. SFPE) and by the international code developing community (International Fire Engineering Guidelines). These guideline documents provide guidance not only on the process of undertaking performance based fire engineering but they also provide clear pointers on the use of available paper based and computational tools. These documents, along with resources such as the SFPE Handbook of Fire Protection Engineering provide the best currently available knowledge and reference data for undertaking performance-based fire engineering.
Conclusions – A way forward
In this paper, I have tended to focus upon the challenges and issues that we as researchers have to resolve and refine over the next few decades to ensure that we continue to
improve the discipline of fire safety engineering. There is a growing body of knowledge and tools to aid the fire engineer in carrying out their work and these will be built upon and refined as our knowledge and understanding increases. In discussing these tools, I have emphasised, perhaps too much, the need to understand the limitations inherent in the tools. But a good engineer needs to always bare in mind the limits of their tools so that they are not extrapolating beyond an acceptable level of risk.
Finally, we need to keep abreast of the developments in materials and new construction processes so that we are able to ensure that our current levels of fire safety are not compromised and in doing so we may be able to achieve even higher levels of fire safety in the future.
References
Canadian Fire Statistics (2001) At: http://www.ccfmfc.ca/stats/statsReports_e.html Ford, G. (2004), Fires in the Home: findings from the 2002/3 British Crime Survey. At: http://www.odpm.gov.uk/stellent/groups/odpm_control/documents/contentservertemplate /odpm_index.hcst?n=4844&l=3
Geneva Association (2004), World Fire Statistics – Newsletter No. 20, At: http://www.genevaassociation.org/wfsc.htm
UK Fire Statistics (2004) At:
http://www.odpm.gov.uk/stellent/groups/odpm_control/documents/contentservertemplate /odpm_index.hcst?n=4844&l=3
Role of Risk Analysis in Fire
Engineering: A Research Perspective
Dr Russ Thomas
National Research Council of
Canada
2
Outline
Context for research: an
outcome-based environment
Knowing and measuring the sources of
risk
Managing risks through research
Tools and their limits
Our way forward
3
Context for research: an
outcome-based environment
International trend to move towards
Performance-Based Codes and
Regulations
Impact of moving beyond the
prescriptive “Cook-Book” solutions
Code requirements now at high-level
4
Impact of “high-level” code Objectives
Expectations are that “performance
solutions” will meet or exceed the
performance of traditional approaches
An expectation of “provable” performance
Problem! Do we know the real performance
offered by traditional solutions? (a 1 hour
fire rating – what does it provide in a “real”
or “natural” fire?)
5
Traditional Approach to Fire Performance
Provided a method to “rank order”
products – does little to predict
performance in modern fires
Developed when cellulosic materials were
the dominant source
Creating the need for “design fires” that
6
7
Knowing and measuring the
sources of risk
Fire Statistics: a less than reliable tool
Perhaps one in five fires make it into
the statistics that are collected (except
for deaths and major losses)
But even so we are able to gather
some working estimates of frequency
and impact to establish risk
8
Other Sources of Uncertainty
Measurement Uncertainty – how
consistent are our tests?
Multiple variables in our experiments
and difficult to control the impact of all
of them
Materials variability (quality and
9
10
Managing Risks Through Research
Last 50 years of fire research has
focused upon developing fundamentals
Recognizing the need for a Science
Based engineering approach
Still need better measurement tools
Need to look where the problem is and
11
House Fires: An example
Most people in Canada who die in
fires, die in house fires
New codes require to protect
occupants against the impact of fire
Although most house fires occur in the
kitchen, most deaths occur in the living
or bed rooms
12
House Fires: An example
Cont.
What is the “real” risk in a house fire
Failure to warn occupants in time?
Failure of occupants decision making?
Failure of safe egress route?
Structural failure?
13
Possible Sequence of Events
Initiation of fire
Smoke alarm sounds
Occupants evacuate
Environment untenable
Structure no longer
usable for egress
Time
14
House Fires: An example
Cont.
Requires a realistic fire challenge
(Design Fire)
Based upon expected file load and
materials mix
An appropriate fire scenario
15
Tools and their limits
How can our research support
practice?
Standards and Guideline Documents
such as the “International Fire
Engineering Guidelines”
16
Need to recognise the limits
Tools are useful within limits of use
Many based upon sparse date and
limited verification and validation (for all
the reasons pointed out in the first part of
the talk)
Useful in assessing if one is “in the
ballpark”
17
Our Way Forward
Developing a better understanding of the
“real” fire challenge (or “natural fires”) –
Design Fires
We need to focus on what we need to
manage (the risk) rather than focus on what
we can currently measure (e.g. we have
been looking for the car keys at night under
the lamp-post rather than where we dropped
them in the dark – lets get a flash-light!)
18
Our Way Forward
Cont.
Better understand the level of fire
safety our existing solutions offer to
help ensure that they are not
compromised by new materials and
procedures
Still need to develop better and more
comprehensive data on material
properties
19
Our Way Forward
Cont.
We can optimize fire performance with
other performance objectives and in so
doing identify better solutions (see
presentation on fire and sound
performance)
Understand the interaction between fire
20
