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A system for describing the expected hazards of building fires
Fire Safety Journal, 2 ( 1 9 7 9 / 8 0 ) 181 - 189 © Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
181
A System for Describing the Expected Hazards of Building Fires* G. N. B E R L I N
National Fire Protection Association, 470 Atlantic Avenue, Boston, MA 02210 (U.S.A.)
SUMMARY
The Building Firesafety Model serves as an efficient tool for systematically evaluating complete building designs or specific elements of a fire code, such as those standards regarding the flame spread rating of wall coverings and the location of exits. This model enables the evaluation of fire safety implied by compliance with codes such as the HUD/MPS, NFPA 101, and the model building codes used throughout the United States. Furthermore, the model provides the flexibility to evaluate the use of new materials, while ensuring an acceptable level of fire safety without causing undue or unnecessary costs. Both the designer and code official may use this tool, once it has been perfected, as a guide to predict the building's fire safety performance before it is constructed. By using the model, these professionals may jointly assess the trade-off in a variety of architectural and engineering designs incorporating the use of innovative materials. Projects involving, for example, low-cost housing, new community development, structural rehabilitation, and energy conservation systems could also be analyzed. Thus far, the Building Firesafety Model has been used to evaluate: (1) full-scale fire tests; (2) alternative single-family dwelling designs; (3) fire door in a multifamily dwelling; (4) smoke control procedures in a care-type facility; (5) stove hoods in mobile homes; (6) escape potential in the Beverly Hills Supper Club; (7) the effect of room size on fire safety. These applications have illustrated the importance of considering the complex interactions * P a p e r p r e s e n t e d a t t h e I n t e r n a t i o n a l Symposium on Fire Risk E v a l u a t i o n in I n d u s t r y , S t o c k h o l m , Sweden, May 7 - 9, 1 9 7 9 .
among factors related to the occupant capabilities; building and room dimensions; barriers; variations in fuel loading, ignitability, and flame spread rating; and environment characteristics such as wind conditions and humidity. The Building Firesafety Model also provides a framework for evaluating the trade-off between individual and community fire detection and suppression capabilities. Such innovation could, in the short run, increase construction costs; but, in the long run, decrease the cost of building ownership by reducing the losses due to fire. This will eventually lead to a reduction in insurance costs and, perhaps, taxes due to decreased reliance on community fire suppression services.
INTRODUCTION
Building codes are intended to provide a legal basis for ensuring a satisfactory level of safety for the general public. Fire safety requirements constitute a major part of most building codes in effect today throughout the United States. These requirements are based, in large part, on the best judgment of many technical experts regarding factors that are intended to reduce injury and to protect property. Despite the fact that these codes are frequently updated, new materials, construction methods, and interior finishes often provide a situation that is not covered explicitly. In addition, traditional fire safety standards, while they require each element of a building to meet some minimum specification, do not provide a methodology for evaluating the elements in conjunction with each other. In some cases, changes in construction or materials provide a safer environment or at least seem to provide the same level of safety at less cost. It is desirable to identify those
182
changes that improve the level of fire safety or that reduce construction costs. To enable the introduction of desirable changes within the framework of the existing codes, a practice of establishing code "equivalency" has been used. When faced with situations not explicitly covered in the code, local officials determine whether the proposed situation meets or exceeds the level of safety implied by the code. These officials use their professional judgment both to interpret the implications of the various sections of the code and to establish the relative performance of the new materials or design based upon the best available data. The practice of code equivalency can lead to numerous problems relating to the efficiency and equity of such procedures. The process of granting variances is often not followed uniformly even within the same local area, partially because the sparsity of the data precludes rigorous technical justification. This situation presents a difficult position for the code official who is legally responsible for enforcing the local codes. This situation is further complicated by the realization that the level of fire safety varies according to the weather, number of occupants, fire origin, fuel type and configuration, the quality of the construction materials and methods, etc. For example, a three-hour door may occasionally be penetrated in less than three hours; but sometimes it will not be breached for a much longer time. Such variation may be due, in part, to fires of different intensity or to the heterogeneity of certain materials used in the construction of the particular door. For whatever reason, the threat to life and damage to property due to fire varies from one fire to another, from one building to another, from one fuel configuration to another, and so on. A means of summarizing these risks is by expressing the descriptors of fire in probabilistic terms. For example, there is a probability of " x " that a fire will extend beyond the room of fire origin for a smoldering couch fire in a typical single-family dwelling. Therefore, the relative likelihood of a specified level of fire safety must be used to describe the risks from fire. This paper presents a systematic procedure for describing the hazards of building fires. This procedure, called the "Building Firesafety
Model" describes the rate of fire growth, the likelihood of entrapment, and the extent of structural damage. The modeling approach provides a systematic, flexible mechanism for quantifying fire safety objectives and for comparing the levels of fire safety in alternative building designs. Furthermore, the model incorporates actual fire test data instead of professional judgment as the primary source of input information. In addition, fire incident data can be used for partial validation of the results.
BUILDING F I R E S A F E T Y MODEL
The Building Firesafety Model is a computer simulation that is based on a state transition formulation of fire behavior [2]. The model describes the temporal and spatial characteristics of fire development and the associated combustion products, and serves as a framework for presenting the natural and yet unexplained variation of fire behavior. As will be illustrated, it can also be used for investigating the effect of actions that alter the fire environment. The states of the Building Firesafety Model correspond to the critical stages in fire development and are termed fire "realms". Starting with a preburning condition, the realms describe the progressive stages of fire growth within the room of fire origin and the extension throughout the dwelling. Realms are defined by several measurable criteria such as flame height, heat release rate and upper room air temperature. Fire development does not necessarily involve passing through all consecutive realms. Since transitions can describe fire recession as well as growth, some fires may never reach the higher realms. Also, it may be inappropriate to identify the intermediate stages in the description of some rapidly growing fires when there is no chance of the fire developing in some other way. Table 1 identifies the critical events currently used to determine the beginning of realms for fires in a single-family dwelling. Information a b o u t the physical mechanisms of fire development, the rate that combustion products are generated, and the opportunities for suppression was used to define the realm boundaries. Nevertheless, additional research
183 TABLE 1 Critical events defining realms for single-family dwellings* Realm
Critical events
1
Preburning. Situations prior to ignition and following termination of a fire are represented by this realm. Sustained burning. There is sustained burning of the fuel, which includes smoldering. Vigorous burning. The heat release rate exceeds 2 kW in the fire room. It is expected that the flame height may be approximately 25 cm and that the upper room air temperature has increased by 150 K above ambient temperature. Interactive burning. The upper r o o m air temperature exceeds 425 K in the fire room. It is expected that the flame height may reach 120 cm and that the heat release rate may exceed 50 kW. Remote burning. The upper room air temperature exceeds 725 K in the fire room. It is expected that the external heat flux returning to the fuel surface exceeds 5 kW/ m 2. This realm also includes secondary ignitions beyond the r o o m o f fire origin, with the change in upper r o o m air temperature in this area less than 15 K. Room involvement. There is full room involvementS.
*These definitions are the result of discussions involving Geoffrey Berlin, Dave Russell and Robert Thompson of NFPA, John deRis of Factory Mutual, and Rexford Wilson of Firepro.
is required before the precise realm definitions can be established. Two types of statistical distributions are required to characterize the transition of fire from one realm to another, as illustrated in Fig. 1. A discrete distribution indicates the conditional probability of transition from a particular realm to each of the other realms. A continuous distribution indicates the variation in realm time. Since the nature of the transition and time in a realm are usually interdependent, the characteristics of the temporal distributions are provided for each possible transition. The characteristics of these statistical distributions can be based entirely on fire test
Fig. l. Realm transition descriptors.
data. Data from tests involving single items and full-scale tests were analyzed to determine the transition descriptors illustrated in Table 2 for a smoldering couch fire in a standard size living room. For example, it was found that from Realm 3 there is a 75% chance of growth to Realm 4 and a 25% chance of recession to Realm 2. Also based on these data, normal and uniform distributions were selected to describe the temporal distributions for the transitions between Realms 3 and 4, and between Realms 3 and 2, respectively. To illustrate how experimental data could be used for determining the transition descriptors, it was necessary to base this information on limited data from the variety of sources listed in Appendix A. While the process for obtaining the descriptors is well established, the values may change as more data become available. The combined risks due to the accumulation of one or more of the combustion products are described as causing either slight, moderate, or extreme stress. Step functions describing the time for the progression from one stress level to the next depend on the type of material burning during each realm. Separate functions are used to describe the stress variation in different locations and the effect of features that influence the spread of the combustion products. Figure 2 illustrates a few step functions for two rooms in a typical single-family dwelling. This figure indicates that the combustion products will
184 TABLE 2 Transitions descriptors for a single-family dwelling F i r e t y p e : s m o l d e r i n g c o u c h fire w i t h c o t t o n c u s h i o n s . R o o m o f o r i g i n : living r o o m . Realm transitions From
To
2 2 2 3 3 3 4 4 4 5 5
1 3 6 2 4 6 3 5 6 4 6
Transition probabilities
Temporal distributions
0.33 0.67 0.25 0.75 0.25 0.75 0.08 0.92
Type
Parameter values (min.)
Uniform Log-normal
a = 2 p = 8.45
/3 = 5 o = 0.78
Uniform Normal
a = 1 /1 = 5 . 5 5
~ = 2 o = 3.22
Uniform Uniform
a = 1.5 a = 0.5
/3' = 9 (7 = 3 . 5
Uniform Log-normal
a = 0.6 p = 5.18
j3 = 6 . 0 o = 4.18
Key p = mean o = standard = lower bound /~ = u p p e r b o u n d *See Appendix A for references.
• Single-family Owellmg • Fire Originating in Living Room • Smoldering Couch Fire
Extreme
J•
Realm 5 ] . . . . . . . . .
I.
M°derate t
I--?
,
I
Slight
Step Functions For A Bedroom
I I Realm 2
---- I
'1
10
20
3
40
50
70
80
Time (in minutes)
Realm 5
Extreme "~ I. . . . . .
" - -" -- " - -- -- "" -- ";
Moderate /~1
s,,g.t
I
Step Functions For Living Room
N IR-,m2 T,r_ /I
I
i 10
l 20
I 30
l 40
1 50
I A
;0
80
Time(in minutes)
F i g . 2 . S t r e s s level s t e p f u n c t i o n s . S i n g l e - f a m i l y d w e l l i n g , fire o r i g i n a t i n g in l i v i n g r o o m , s m o l d e r i n g couch fire.
cause an ext rem e stress in the bedroom after 60 minutes even though the fire has n o t grown b e y o n d Realm 2. A measure of escape potential is used to describe one effect of the combustion products. For this project, escape potential is defined in terms of the routes available from any room to a location of safety [1]. A tunnel, anot her fire c o m p a r t m e n t , or the outdoors, can serve as a location of safety depending on the dwelling type. Once the stress level reaches or exceeds some predetermined level, all escape routes through, or from, this location are eliminated. This level m ay be based on information describing when the level of com bust i on products presents an untenable condition, when the occupants m ay perceive t hat a r o o m is impassable, or when there is structural failure. Table 3 illustrates the relative effects of blocking either the kitchen or hallway in the particular singlefamily dwelling design. As might be expected, the consequences of a fire in a central location such as the hallway will adversely affect the n u m b e r of escape routes m ore drastically than a fire in a r e m o t e location.
185 TABLE 3
Evaluation of fire safety
Effect of blocked rooms on the number of escape routes in a typical single-family dwelling
The model provides a tool for assessing the expected risks from fire by focusing attention on the basic facets of fire safety -- the probability of injury and damage. Both facets of fire safety can be described by static and dynamic measures. Static measures are those facets that can be determined directly from a description of the building and fuel configuration. Dynamic measures describe factors that are affected by the variation in fire development and the accumulation of the combustion products. These measures must be described in statistical terms. Table 4 indicates a number of static and dynamic measures that could potentially serve as descriptors of fire safety. The susceptibility of humans to the combustion products, and the behavior of occupants during a fire, are extremely variable. Therefore, a number of surrogate measures are used to describe the potential for injury, such as the number of escape routes, the time until entrapment occurs, and the stress levels along the egress paths. The larger the number of independent escape routes, the longer the time until entrapment, and the lower the stress levels the better the chances of an occupant escaping unharmed. However, it is difficult to predict accurately w h o will be injured and the severity of any injury.
bedroo=
1~
oo 1
kitchen dining area
< bedroom 2
Number of directed escape routes Location All rooms Kitchen passable blocked
Hallway blocked
Kitchen Hallway Living r o o m Bathroom Bedroom 1 Bedroom 2 T OTAL
2 x 2 0 1 1 6
3 5 3 1 2 2 16
x 4 2 1 2 2 11
NOTE: An " x " denotes a r o o m that is considered to be impassable. TABLE 4 Measures o f building fire safety Fire safety objectives
Factors
Measures
Escape route characteristics Time for occupant reaction Levels of the combustion products Delays in detection
Static 0 Maximum length of escape routes 0 Number of remote escape routes 0 Exitway dimensions 0 Toxicity o f combustion products
Reduce injury
Dynamic 0 Escape route utilization 0 Time until entrapment 0 Passability of escape routes Minimize damage Fuel loading and configuration Compartmentation Suppression system effectiveness Construction characteristics
Static 0 Rates of energy release 0 Fire resistance of barriers and structural components Dynamic 0 E x t e n t of flame spread 0 Extent of smoke spread 0 Effectiveness of suppression activities
186 TABLE 5 Candidate fire situations for evaluating building fire safety Fire situation
Location
Item ignited
Ignition strength
Environment
1
Living room
Smoldering
Night, Winter
2
Living room
Smoldering
Night, Summer
3 4 5 6 7 8
Bedroom Bedroom Bedroom Bedroom Kitchen Basement/utility room
Large, highly combustible furniture Large, highly combustible furniture Bedding Bedding Bedding Bedding Grease/off Oil or gas-fired heating unit
Smoldering Smoldering Flaming Flaming Flaming Flaming
Night, Winter Night, Summer Night, Winter Night, Summer Day, Fall/Spring Day, Winter
Environment Key Fall/Spring: Moderate ventilation: No heat or air conditioning, window half open. Winter : Windows closed, heating system operational. Summer : Air conditioning operating, windows closed. Night : Bedroom: Doors closed. Day : Bedroom: Doors open.
Similarly, it is difficult to predict accurately the extent that operations will be disrupted by fire of a certain character. The ability of a firm to continue operations depends on many factors such as the availability of other space, equipment, and labor to replace what was lost. Nevertheless, the area involved by flame and by the combustion products will likely provide basic data from which to derive estimates of damage and the extent that operations are curtailed. For example, Fig. 3 illustrates a possible approach to describing the expected structural damage resulting from a fire that has reached a particular realm. This same approach could be taken to describe the expected damage resulting from the combustion products reaching a particular stress level. To assess the overall safety of a particular building design, it is necessary to simulate the most likely fire scenarios for the range of occupancy uses, interior finishes, and furnishings, as illustrated in Table 5. The collection of quantitative measures are then used to reach an assessment of the safety of the particular building design as illustrated in Table 6. While this list is not exhaustive, it does indicate a format for presenting fire safety criteria. The measures that are included on future lists will identify those aspects of escape and property damage t h a t are considered to be important in assessing the absolute level of fire safety. While these mea-
7
Fire Realm
D 10
30
I
65
0
85 100
Percentage of Structure
Damaged
Fig. 3. Projected structural damage.
sures are the performance metrics of fire safety, the particular values for M. P, T, etc., in such measures indicate the socially accepted level of risk. The values for these measures serve as a hazard profile that can be compared with corresponding values for other designs that have been approved by the appropriate governmental agencies as a basis for judging the acceptability of the building design, i.e., code equivalency. Once specific levels of fire safety have been adopted, the Building Firesafety Model can be used to determine the correct response to each question. Perhaps, any negative response would indicate a failure of the building to be considered as "code equivalent".
187 TABLE 6 Evaluation of fire safety objectives Building Environment Analysis 1
Fire situations Yes
2 No
Yes
3 No
Yes
No
Selected Fire Safety Measures 1. Is the maximum distance to the nearest location of safety less than M meters? 2. Does it take longer than T minutes before entrapment occurs? 3. Is the probability of growth from Realm 2 less than P? 4. During the first S minutes, are fires within an occupant's suppression capability? 5. Is the expected structural damage less than X percent of its value?
TABLE 7 Partial fire safety evaluation for a single-family design Fire type: smoldering couch fire. Room of origin: living room Measure
Fire safety measures
A
Percentage of total fire duration that at least two directed escape routes are available. Time when entrapment occurs with a probability of 0.33. Average time before fires grow beyond an occupant's suppression capability. (Time to reach Realm 4.) Percentage of fires that have not grown beyond an occupant's suppression capability in ten minutes. (Growth beyond Realm 3.) Average extent of structural damage.
B
C D E Design description
A (%)
B (rain)
C (min)
D (%)
E ($)
Floor plan 1 ($27 360) Fire department arrival (e.g.; 9.5 rain) Room of origin suppression* Sprinkler system* *
44.8 81.6 48.2 76.3
21 ~o 20 oo
17.1 13.1 16.3 14.3
96 97.2 95.4 96.6
16 2 14 3
944 435 832 411
*Ten minutes in Realm 4. **Activates in living room and bedroom 1 after 2.5 min at a "Slight Stress Level".
Table 7 indicates the values for several dynamic measures based on one type of fire in a single-family dwelling. These measures are derived from the results of 500 simulated fires. The first column summarizes the outcome of these " u n d i s t u r b e d " fires. Figure 4 provides information about the nature of these fires -- at approximately 45 minutes, half the fire will have terminated. Figure 5 focuses on the expected status of the fire after 5 or 15 minutes. Approximately 80% of
the fires are still in the incipient stage after 5 minutes and the remaining 20% will have terminated. However, after 15 minutes, the situation is much more complex with significant percentages of fires being in Realms 3, 4 and 5, which represent very serious situations. Table 7 also illustrates the capability of the Building Firesafety Model to describe changes in the fire environment. The simulation of a fire can be interrupted to alter the
188
100
FireRiskat 5 Mpnutes
0.S0. ' ~
4 5
6
~
0.60, Realm
2
Frequency
0.20. 0
1 S
10
16 20 25 310 3; Time (in minutes)
4;
45
Fig. 4. Simulated fire development.
pattern of fire growth and the spread of the combustion products. The following conditions can be used to identify when a change is to take place: (1) a specified time after ignition; (2) a specified time in a certain realm; and (3) a specified time at a certain stress level in any location. The change is then described in terms of new values for the realm transition descriptors and stress level step functions. For instance, changes could represent the collapse of a wall, the breaking of a window, the closing of a fire door, and the effect of different suppression systems. Available data were used to illustrate each of these simulation interrupts. The arrival of the fire department, for example, is represented as a specific time after ignition. This time includes the delays in detection, notification, dispatch, response, and setup. Note that the value of Measure C decreases because the fire department, once on the scene, often prevents the extension of the fir.e that develops slowly. As a result, the rapidly growing fires constitute a larger portion of the fires that reach Realm 4. The actuation of a sprinkler system is represented by a specific time at a certain stress level. The values presented in Table 7 depend upon the accuracy of the times when the changes take place and the description of the change. Since there is very little information in the proper form available at this time, the results presented in this figure are only for illustrating the capability of the model and not for describing the effec-
FireRiskat 15Minutes
Fig. 5. Realm status.
tiveness of the actual alternative suppression systems.
ACKNOWLEDGEMENTS
I should like to express may appreciation to Joseph Swartz, David Russell, Rita Fahy and Robert Thompson of NFPA and to Edward Connelly of Performance Measurements, Inc. who were major contributors to this research effort. The financial support for this project was provided by the Office of Policy Development and Research, U.S. Department of Housing and Urban Developm e n t (Contract H-2316).
REFERENCES 1 G.N. Berlin, The use of directed routes for assessin escape potential, Fire Technol., 14 (1978) 126 - 135. 2 G. N. Berlin, E. M. Connelly, F. Fahy, D. Russell and J. A. Swartz, Firesafety Systems Analysis for Residential Occupancies, National Fire Protection Association, Boston, M A , July, 1978.
189
APPENDIX A
TRANSITIONS DESCRIPTORS
T h e t r a n s i t i o n d e s c r i p t o r s are b a s e d o n a subjective e v a l u a t i o n o f fire t e s t d a t a f r o m n u m e r o u s s o u r c e s and o f a c t u a l r e s p o n s e observation.
R. L. Alpert, A. T. Modak and J. S. Newman, The Third Full-Scale Bedroom Test of the Home Fire Project, Vol. 1, Test Description and Experimental Data, Factory Mutual Research Corporation, Norwood, MA, 1975. H. D. Bruce, Experimental dwelling room fires, Rep. No. 9141, U.S. Forest Products Laboratory, U.S. Department of the Interior, Madison, WI, 1959.
E. Budnick, Fire spread along a mobile home corridor, NBSIR-76-102, Nat. Bur. Stand., U.S. Department of Commerce, Washington, DC., 1976. P. A. Croce, A. E. Emmons, A study of room fire development: the large scale bedroom fire test, FMRC Serial No. 210112, Factory Mutual Research Corp., Norwood, MA, 1974. J. B. Fang, Fire buildup in a room and the role of the interior finish materials, NBS Tech. Note 879, Nat. Bur. Stand., U.S. Department of Commerce, Washington, DC., 1975. J. M. Hogg, A model of fire spread, Fire Research Report, Home Office Scientific Advisory Branch Rep., No. 2/71, London, England, 1971. B. T. Lee and W. J. Parker, Naval shipboard fire risk criteria -- berthing compartment fire study and fire performance guidelines, NBSIR-77-1072, Nat. Bur. Stand., U.S. Department of Commerce, Washington, D.C., 1976. F. J. Vodvarka and T. E. Waterman, Fire behavior, ignition to flashover, IIT Research Institute, Chicago, IL, 1975.
181
A System for Describing the Expected Hazards of Building Fires* G. N. B E R L I N
National Fire Protection Association, 470 Atlantic Avenue, Boston, MA 02210 (U.S.A.)
SUMMARY
The Building Firesafety Model serves as an efficient tool for systematically evaluating complete building designs or specific elements of a fire code, such as those standards regarding the flame spread rating of wall coverings and the location of exits. This model enables the evaluation of fire safety implied by compliance with codes such as the HUD/MPS, NFPA 101, and the model building codes used throughout the United States. Furthermore, the model provides the flexibility to evaluate the use of new materials, while ensuring an acceptable level of fire safety without causing undue or unnecessary costs. Both the designer and code official may use this tool, once it has been perfected, as a guide to predict the building's fire safety performance before it is constructed. By using the model, these professionals may jointly assess the trade-off in a variety of architectural and engineering designs incorporating the use of innovative materials. Projects involving, for example, low-cost housing, new community development, structural rehabilitation, and energy conservation systems could also be analyzed. Thus far, the Building Firesafety Model has been used to evaluate: (1) full-scale fire tests; (2) alternative single-family dwelling designs; (3) fire door in a multifamily dwelling; (4) smoke control procedures in a care-type facility; (5) stove hoods in mobile homes; (6) escape potential in the Beverly Hills Supper Club; (7) the effect of room size on fire safety. These applications have illustrated the importance of considering the complex interactions * P a p e r p r e s e n t e d a t t h e I n t e r n a t i o n a l Symposium on Fire Risk E v a l u a t i o n in I n d u s t r y , S t o c k h o l m , Sweden, May 7 - 9, 1 9 7 9 .
among factors related to the occupant capabilities; building and room dimensions; barriers; variations in fuel loading, ignitability, and flame spread rating; and environment characteristics such as wind conditions and humidity. The Building Firesafety Model also provides a framework for evaluating the trade-off between individual and community fire detection and suppression capabilities. Such innovation could, in the short run, increase construction costs; but, in the long run, decrease the cost of building ownership by reducing the losses due to fire. This will eventually lead to a reduction in insurance costs and, perhaps, taxes due to decreased reliance on community fire suppression services.
INTRODUCTION
Building codes are intended to provide a legal basis for ensuring a satisfactory level of safety for the general public. Fire safety requirements constitute a major part of most building codes in effect today throughout the United States. These requirements are based, in large part, on the best judgment of many technical experts regarding factors that are intended to reduce injury and to protect property. Despite the fact that these codes are frequently updated, new materials, construction methods, and interior finishes often provide a situation that is not covered explicitly. In addition, traditional fire safety standards, while they require each element of a building to meet some minimum specification, do not provide a methodology for evaluating the elements in conjunction with each other. In some cases, changes in construction or materials provide a safer environment or at least seem to provide the same level of safety at less cost. It is desirable to identify those
182
changes that improve the level of fire safety or that reduce construction costs. To enable the introduction of desirable changes within the framework of the existing codes, a practice of establishing code "equivalency" has been used. When faced with situations not explicitly covered in the code, local officials determine whether the proposed situation meets or exceeds the level of safety implied by the code. These officials use their professional judgment both to interpret the implications of the various sections of the code and to establish the relative performance of the new materials or design based upon the best available data. The practice of code equivalency can lead to numerous problems relating to the efficiency and equity of such procedures. The process of granting variances is often not followed uniformly even within the same local area, partially because the sparsity of the data precludes rigorous technical justification. This situation presents a difficult position for the code official who is legally responsible for enforcing the local codes. This situation is further complicated by the realization that the level of fire safety varies according to the weather, number of occupants, fire origin, fuel type and configuration, the quality of the construction materials and methods, etc. For example, a three-hour door may occasionally be penetrated in less than three hours; but sometimes it will not be breached for a much longer time. Such variation may be due, in part, to fires of different intensity or to the heterogeneity of certain materials used in the construction of the particular door. For whatever reason, the threat to life and damage to property due to fire varies from one fire to another, from one building to another, from one fuel configuration to another, and so on. A means of summarizing these risks is by expressing the descriptors of fire in probabilistic terms. For example, there is a probability of " x " that a fire will extend beyond the room of fire origin for a smoldering couch fire in a typical single-family dwelling. Therefore, the relative likelihood of a specified level of fire safety must be used to describe the risks from fire. This paper presents a systematic procedure for describing the hazards of building fires. This procedure, called the "Building Firesafety
Model" describes the rate of fire growth, the likelihood of entrapment, and the extent of structural damage. The modeling approach provides a systematic, flexible mechanism for quantifying fire safety objectives and for comparing the levels of fire safety in alternative building designs. Furthermore, the model incorporates actual fire test data instead of professional judgment as the primary source of input information. In addition, fire incident data can be used for partial validation of the results.
BUILDING F I R E S A F E T Y MODEL
The Building Firesafety Model is a computer simulation that is based on a state transition formulation of fire behavior [2]. The model describes the temporal and spatial characteristics of fire development and the associated combustion products, and serves as a framework for presenting the natural and yet unexplained variation of fire behavior. As will be illustrated, it can also be used for investigating the effect of actions that alter the fire environment. The states of the Building Firesafety Model correspond to the critical stages in fire development and are termed fire "realms". Starting with a preburning condition, the realms describe the progressive stages of fire growth within the room of fire origin and the extension throughout the dwelling. Realms are defined by several measurable criteria such as flame height, heat release rate and upper room air temperature. Fire development does not necessarily involve passing through all consecutive realms. Since transitions can describe fire recession as well as growth, some fires may never reach the higher realms. Also, it may be inappropriate to identify the intermediate stages in the description of some rapidly growing fires when there is no chance of the fire developing in some other way. Table 1 identifies the critical events currently used to determine the beginning of realms for fires in a single-family dwelling. Information a b o u t the physical mechanisms of fire development, the rate that combustion products are generated, and the opportunities for suppression was used to define the realm boundaries. Nevertheless, additional research
183 TABLE 1 Critical events defining realms for single-family dwellings* Realm
Critical events
1
Preburning. Situations prior to ignition and following termination of a fire are represented by this realm. Sustained burning. There is sustained burning of the fuel, which includes smoldering. Vigorous burning. The heat release rate exceeds 2 kW in the fire room. It is expected that the flame height may be approximately 25 cm and that the upper room air temperature has increased by 150 K above ambient temperature. Interactive burning. The upper r o o m air temperature exceeds 425 K in the fire room. It is expected that the flame height may reach 120 cm and that the heat release rate may exceed 50 kW. Remote burning. The upper room air temperature exceeds 725 K in the fire room. It is expected that the external heat flux returning to the fuel surface exceeds 5 kW/ m 2. This realm also includes secondary ignitions beyond the r o o m o f fire origin, with the change in upper r o o m air temperature in this area less than 15 K. Room involvement. There is full room involvementS.
*These definitions are the result of discussions involving Geoffrey Berlin, Dave Russell and Robert Thompson of NFPA, John deRis of Factory Mutual, and Rexford Wilson of Firepro.
is required before the precise realm definitions can be established. Two types of statistical distributions are required to characterize the transition of fire from one realm to another, as illustrated in Fig. 1. A discrete distribution indicates the conditional probability of transition from a particular realm to each of the other realms. A continuous distribution indicates the variation in realm time. Since the nature of the transition and time in a realm are usually interdependent, the characteristics of the temporal distributions are provided for each possible transition. The characteristics of these statistical distributions can be based entirely on fire test
Fig. l. Realm transition descriptors.
data. Data from tests involving single items and full-scale tests were analyzed to determine the transition descriptors illustrated in Table 2 for a smoldering couch fire in a standard size living room. For example, it was found that from Realm 3 there is a 75% chance of growth to Realm 4 and a 25% chance of recession to Realm 2. Also based on these data, normal and uniform distributions were selected to describe the temporal distributions for the transitions between Realms 3 and 4, and between Realms 3 and 2, respectively. To illustrate how experimental data could be used for determining the transition descriptors, it was necessary to base this information on limited data from the variety of sources listed in Appendix A. While the process for obtaining the descriptors is well established, the values may change as more data become available. The combined risks due to the accumulation of one or more of the combustion products are described as causing either slight, moderate, or extreme stress. Step functions describing the time for the progression from one stress level to the next depend on the type of material burning during each realm. Separate functions are used to describe the stress variation in different locations and the effect of features that influence the spread of the combustion products. Figure 2 illustrates a few step functions for two rooms in a typical single-family dwelling. This figure indicates that the combustion products will
184 TABLE 2 Transitions descriptors for a single-family dwelling F i r e t y p e : s m o l d e r i n g c o u c h fire w i t h c o t t o n c u s h i o n s . R o o m o f o r i g i n : living r o o m . Realm transitions From
To
2 2 2 3 3 3 4 4 4 5 5
1 3 6 2 4 6 3 5 6 4 6
Transition probabilities
Temporal distributions
0.33 0.67 0.25 0.75 0.25 0.75 0.08 0.92
Type
Parameter values (min.)
Uniform Log-normal
a = 2 p = 8.45
/3 = 5 o = 0.78
Uniform Normal
a = 1 /1 = 5 . 5 5
~ = 2 o = 3.22
Uniform Uniform
a = 1.5 a = 0.5
/3' = 9 (7 = 3 . 5
Uniform Log-normal
a = 0.6 p = 5.18
j3 = 6 . 0 o = 4.18
Key p = mean o = standard = lower bound /~ = u p p e r b o u n d *See Appendix A for references.
• Single-family Owellmg • Fire Originating in Living Room • Smoldering Couch Fire
Extreme
J•
Realm 5 ] . . . . . . . . .
I.
M°derate t
I--?
,
I
Slight
Step Functions For A Bedroom
I I Realm 2
---- I
'1
10
20
3
40
50
70
80
Time (in minutes)
Realm 5
Extreme "~ I. . . . . .
" - -" -- " - -- -- "" -- ";
Moderate /~1
s,,g.t
I
Step Functions For Living Room
N IR-,m2 T,r_ /I
I
i 10
l 20
I 30
l 40
1 50
I A
;0
80
Time(in minutes)
F i g . 2 . S t r e s s level s t e p f u n c t i o n s . S i n g l e - f a m i l y d w e l l i n g , fire o r i g i n a t i n g in l i v i n g r o o m , s m o l d e r i n g couch fire.
cause an ext rem e stress in the bedroom after 60 minutes even though the fire has n o t grown b e y o n d Realm 2. A measure of escape potential is used to describe one effect of the combustion products. For this project, escape potential is defined in terms of the routes available from any room to a location of safety [1]. A tunnel, anot her fire c o m p a r t m e n t , or the outdoors, can serve as a location of safety depending on the dwelling type. Once the stress level reaches or exceeds some predetermined level, all escape routes through, or from, this location are eliminated. This level m ay be based on information describing when the level of com bust i on products presents an untenable condition, when the occupants m ay perceive t hat a r o o m is impassable, or when there is structural failure. Table 3 illustrates the relative effects of blocking either the kitchen or hallway in the particular singlefamily dwelling design. As might be expected, the consequences of a fire in a central location such as the hallway will adversely affect the n u m b e r of escape routes m ore drastically than a fire in a r e m o t e location.
185 TABLE 3
Evaluation of fire safety
Effect of blocked rooms on the number of escape routes in a typical single-family dwelling
The model provides a tool for assessing the expected risks from fire by focusing attention on the basic facets of fire safety -- the probability of injury and damage. Both facets of fire safety can be described by static and dynamic measures. Static measures are those facets that can be determined directly from a description of the building and fuel configuration. Dynamic measures describe factors that are affected by the variation in fire development and the accumulation of the combustion products. These measures must be described in statistical terms. Table 4 indicates a number of static and dynamic measures that could potentially serve as descriptors of fire safety. The susceptibility of humans to the combustion products, and the behavior of occupants during a fire, are extremely variable. Therefore, a number of surrogate measures are used to describe the potential for injury, such as the number of escape routes, the time until entrapment occurs, and the stress levels along the egress paths. The larger the number of independent escape routes, the longer the time until entrapment, and the lower the stress levels the better the chances of an occupant escaping unharmed. However, it is difficult to predict accurately w h o will be injured and the severity of any injury.
bedroo=
1~
oo 1
kitchen dining area
< bedroom 2
Number of directed escape routes Location All rooms Kitchen passable blocked
Hallway blocked
Kitchen Hallway Living r o o m Bathroom Bedroom 1 Bedroom 2 T OTAL
2 x 2 0 1 1 6
3 5 3 1 2 2 16
x 4 2 1 2 2 11
NOTE: An " x " denotes a r o o m that is considered to be impassable. TABLE 4 Measures o f building fire safety Fire safety objectives
Factors
Measures
Escape route characteristics Time for occupant reaction Levels of the combustion products Delays in detection
Static 0 Maximum length of escape routes 0 Number of remote escape routes 0 Exitway dimensions 0 Toxicity o f combustion products
Reduce injury
Dynamic 0 Escape route utilization 0 Time until entrapment 0 Passability of escape routes Minimize damage Fuel loading and configuration Compartmentation Suppression system effectiveness Construction characteristics
Static 0 Rates of energy release 0 Fire resistance of barriers and structural components Dynamic 0 E x t e n t of flame spread 0 Extent of smoke spread 0 Effectiveness of suppression activities
186 TABLE 5 Candidate fire situations for evaluating building fire safety Fire situation
Location
Item ignited
Ignition strength
Environment
1
Living room
Smoldering
Night, Winter
2
Living room
Smoldering
Night, Summer
3 4 5 6 7 8
Bedroom Bedroom Bedroom Bedroom Kitchen Basement/utility room
Large, highly combustible furniture Large, highly combustible furniture Bedding Bedding Bedding Bedding Grease/off Oil or gas-fired heating unit
Smoldering Smoldering Flaming Flaming Flaming Flaming
Night, Winter Night, Summer Night, Winter Night, Summer Day, Fall/Spring Day, Winter
Environment Key Fall/Spring: Moderate ventilation: No heat or air conditioning, window half open. Winter : Windows closed, heating system operational. Summer : Air conditioning operating, windows closed. Night : Bedroom: Doors closed. Day : Bedroom: Doors open.
Similarly, it is difficult to predict accurately the extent that operations will be disrupted by fire of a certain character. The ability of a firm to continue operations depends on many factors such as the availability of other space, equipment, and labor to replace what was lost. Nevertheless, the area involved by flame and by the combustion products will likely provide basic data from which to derive estimates of damage and the extent that operations are curtailed. For example, Fig. 3 illustrates a possible approach to describing the expected structural damage resulting from a fire that has reached a particular realm. This same approach could be taken to describe the expected damage resulting from the combustion products reaching a particular stress level. To assess the overall safety of a particular building design, it is necessary to simulate the most likely fire scenarios for the range of occupancy uses, interior finishes, and furnishings, as illustrated in Table 5. The collection of quantitative measures are then used to reach an assessment of the safety of the particular building design as illustrated in Table 6. While this list is not exhaustive, it does indicate a format for presenting fire safety criteria. The measures that are included on future lists will identify those aspects of escape and property damage t h a t are considered to be important in assessing the absolute level of fire safety. While these mea-
7
Fire Realm
D 10
30
I
65
0
85 100
Percentage of Structure
Damaged
Fig. 3. Projected structural damage.
sures are the performance metrics of fire safety, the particular values for M. P, T, etc., in such measures indicate the socially accepted level of risk. The values for these measures serve as a hazard profile that can be compared with corresponding values for other designs that have been approved by the appropriate governmental agencies as a basis for judging the acceptability of the building design, i.e., code equivalency. Once specific levels of fire safety have been adopted, the Building Firesafety Model can be used to determine the correct response to each question. Perhaps, any negative response would indicate a failure of the building to be considered as "code equivalent".
187 TABLE 6 Evaluation of fire safety objectives Building Environment Analysis 1
Fire situations Yes
2 No
Yes
3 No
Yes
No
Selected Fire Safety Measures 1. Is the maximum distance to the nearest location of safety less than M meters? 2. Does it take longer than T minutes before entrapment occurs? 3. Is the probability of growth from Realm 2 less than P? 4. During the first S minutes, are fires within an occupant's suppression capability? 5. Is the expected structural damage less than X percent of its value?
TABLE 7 Partial fire safety evaluation for a single-family design Fire type: smoldering couch fire. Room of origin: living room Measure
Fire safety measures
A
Percentage of total fire duration that at least two directed escape routes are available. Time when entrapment occurs with a probability of 0.33. Average time before fires grow beyond an occupant's suppression capability. (Time to reach Realm 4.) Percentage of fires that have not grown beyond an occupant's suppression capability in ten minutes. (Growth beyond Realm 3.) Average extent of structural damage.
B
C D E Design description
A (%)
B (rain)
C (min)
D (%)
E ($)
Floor plan 1 ($27 360) Fire department arrival (e.g.; 9.5 rain) Room of origin suppression* Sprinkler system* *
44.8 81.6 48.2 76.3
21 ~o 20 oo
17.1 13.1 16.3 14.3
96 97.2 95.4 96.6
16 2 14 3
944 435 832 411
*Ten minutes in Realm 4. **Activates in living room and bedroom 1 after 2.5 min at a "Slight Stress Level".
Table 7 indicates the values for several dynamic measures based on one type of fire in a single-family dwelling. These measures are derived from the results of 500 simulated fires. The first column summarizes the outcome of these " u n d i s t u r b e d " fires. Figure 4 provides information about the nature of these fires -- at approximately 45 minutes, half the fire will have terminated. Figure 5 focuses on the expected status of the fire after 5 or 15 minutes. Approximately 80% of
the fires are still in the incipient stage after 5 minutes and the remaining 20% will have terminated. However, after 15 minutes, the situation is much more complex with significant percentages of fires being in Realms 3, 4 and 5, which represent very serious situations. Table 7 also illustrates the capability of the Building Firesafety Model to describe changes in the fire environment. The simulation of a fire can be interrupted to alter the
188
100
FireRiskat 5 Mpnutes
0.S0. ' ~
4 5
6
~
0.60, Realm
2
Frequency
0.20. 0
1 S
10
16 20 25 310 3; Time (in minutes)
4;
45
Fig. 4. Simulated fire development.
pattern of fire growth and the spread of the combustion products. The following conditions can be used to identify when a change is to take place: (1) a specified time after ignition; (2) a specified time in a certain realm; and (3) a specified time at a certain stress level in any location. The change is then described in terms of new values for the realm transition descriptors and stress level step functions. For instance, changes could represent the collapse of a wall, the breaking of a window, the closing of a fire door, and the effect of different suppression systems. Available data were used to illustrate each of these simulation interrupts. The arrival of the fire department, for example, is represented as a specific time after ignition. This time includes the delays in detection, notification, dispatch, response, and setup. Note that the value of Measure C decreases because the fire department, once on the scene, often prevents the extension of the fir.e that develops slowly. As a result, the rapidly growing fires constitute a larger portion of the fires that reach Realm 4. The actuation of a sprinkler system is represented by a specific time at a certain stress level. The values presented in Table 7 depend upon the accuracy of the times when the changes take place and the description of the change. Since there is very little information in the proper form available at this time, the results presented in this figure are only for illustrating the capability of the model and not for describing the effec-
FireRiskat 15Minutes
Fig. 5. Realm status.
tiveness of the actual alternative suppression systems.
ACKNOWLEDGEMENTS
I should like to express may appreciation to Joseph Swartz, David Russell, Rita Fahy and Robert Thompson of NFPA and to Edward Connelly of Performance Measurements, Inc. who were major contributors to this research effort. The financial support for this project was provided by the Office of Policy Development and Research, U.S. Department of Housing and Urban Developm e n t (Contract H-2316).
REFERENCES 1 G.N. Berlin, The use of directed routes for assessin escape potential, Fire Technol., 14 (1978) 126 - 135. 2 G. N. Berlin, E. M. Connelly, F. Fahy, D. Russell and J. A. Swartz, Firesafety Systems Analysis for Residential Occupancies, National Fire Protection Association, Boston, M A , July, 1978.
189
APPENDIX A
TRANSITIONS DESCRIPTORS
T h e t r a n s i t i o n d e s c r i p t o r s are b a s e d o n a subjective e v a l u a t i o n o f fire t e s t d a t a f r o m n u m e r o u s s o u r c e s and o f a c t u a l r e s p o n s e observation.
R. L. Alpert, A. T. Modak and J. S. Newman, The Third Full-Scale Bedroom Test of the Home Fire Project, Vol. 1, Test Description and Experimental Data, Factory Mutual Research Corporation, Norwood, MA, 1975. H. D. Bruce, Experimental dwelling room fires, Rep. No. 9141, U.S. Forest Products Laboratory, U.S. Department of the Interior, Madison, WI, 1959.
E. Budnick, Fire spread along a mobile home corridor, NBSIR-76-102, Nat. Bur. Stand., U.S. Department of Commerce, Washington, DC., 1976. P. A. Croce, A. E. Emmons, A study of room fire development: the large scale bedroom fire test, FMRC Serial No. 210112, Factory Mutual Research Corp., Norwood, MA, 1974. J. B. Fang, Fire buildup in a room and the role of the interior finish materials, NBS Tech. Note 879, Nat. Bur. Stand., U.S. Department of Commerce, Washington, DC., 1975. J. M. Hogg, A model of fire spread, Fire Research Report, Home Office Scientific Advisory Branch Rep., No. 2/71, London, England, 1971. B. T. Lee and W. J. Parker, Naval shipboard fire risk criteria -- berthing compartment fire study and fire performance guidelines, NBSIR-77-1072, Nat. Bur. Stand., U.S. Department of Commerce, Washington, D.C., 1976. F. J. Vodvarka and T. E. Waterman, Fire behavior, ignition to flashover, IIT Research Institute, Chicago, IL, 1975.