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Fundamentals of designing buildings for fire safety
:.
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National Research
Conseil national
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Council Canada
de recherche~
Canada
FUNDAMENTALS OF DESIGNING BUILDINGS FOR FIRE SAFETY
by T.Z. Harmathy
Appeared in
A P ~
A I . Y Z ~ D
Proceedings of Conference on Building Fire Safety
Chung-Li, Taiwan, Republic of China
September 22, 1984
p.
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19
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. . -DBR Paper No. 1243
Division of Building Research
ABSTRACT
Factors that control the various phases of building fires
(ignition and initial fire spread, pre-f
lashover fire growth,
and fully developed fire) are examined together with the smoke
problem that may be associated with any of them.
Techniques
for designing building elements that maintain prescribed levels
of fire safety are discussed, and the role of psychological and
sociological factors outlined.
Les facteurs qui influent sur les diverses phases des incendies
dans les bstiments (inflammation, propagation initiale du feu,
dgveloppement du feu avant l'embrasement g6nsral et embrasement
gsngral) sont examinb, ainsi que le problgme de la fum6e qui
peut Gtre associs
5
n'importe laquelle de ces phases.
Les
techniques de conception d1616ments de bstiments qui assurent
les niveaux prescrits de s6curitE incendie sont traitses, et le
r61e des facteurs psychologiques et sociologiques est dgcrit.
PROCEEDINGS OF
CONFERENCE ON B U I L D I N G F I R E SAFETY
SEPTEMBER 2 2 , 1984
CHUNG-LI, TAIWAN, REPUBLIC OF CHINA
FUNDAMENTALS OF DESIGNING BUILDINGS
FOR FIRE SAFETY
T.Z. Harmathy
Head
Fire Research Section
Division of Building Research
National Research Council of Canada
Ottawa. Canada
A fire-safe building can be defined as one for which there is a high probability that all occupants will survive a fire without injury, and in which property damage will be confined to the immediate vicinity of the fire.
There are numerous, mostly complementary, ways of achieving fire safety, not all of which are related to building design. Those that are, concern (i) layout and dimensioning of the building and its constituent parts, (ii) provision of safety devices and facilities, and (iii) selection of construction materials.
In North America the minimum requirements for safety are dealt with in building codes. The designer is allowed, however, t o use equivalent or better solutions and t o choose safer materials. All in all. the level of fire safety depends t o a large extent on the conscience of the designer, and the provision of safety at a minimum cost depends on his expertise. There are three ways a designer can further the cause of fire safety in buildings: (i) by taking measures t o reduce the probability of compartment fires reaching the flashover stage, (ii) by ensuring that, if flashover does occur, the probability of fire spreading t o other compartments will remain below a specified limit, and (iii) by employing techniques that prevent the smoke from spreading through the building.
Building before the Outbreak of Fire
The nature of a building fire and the course the fire is likely t o take depend on certain conditions prevailing before the outbreak of fire.
The distribution of drafts prior t o ignition is a profoundly important factor. Their intensity increases with building height. T o emphasize the role they may play in the fire process, the building in this study will be pictured as one of several storeys. Naturally, all conclusions will be applicable t o low buildings as well, in a toned-down sense.
Drafts are brought about by two factors: the temperature difference between the building interior and the outside atmosphere, and the air leakage characteristics of the building components employed. Owing t o the former, drafts'are especially strong during the winter heating season. For this reason the winter situation will be discussed.
Leakage of building elements results from the presence of small channels, usually invisible, through which air can pass. As the flow of air through them is analogous t o flow through orifices, the aggregate area of these small channels per unit area of the building element is often referred to as "equivalent orifice area". The intensity and direction of air currents is illustrated in Fig. la, which shows the situation in a nine-storey building on a calm day after the shut-down of the air-handling system. (The shut-down is effected by devices installed in compliance with mandatory code regulations.) If the leakage characteristics of the building envelope are uniform with height, air will infiltrate below the mid-height. After passing through perhaps one or two partitions, it will enter the "shafts" (stairwells, elevator shafts), rise to the upper storeys, and exfiltrate t o the outside atmosphere. (Because of the important role the stack-like shafts play, the phenomenon is often referred t o as stack effect.) Naturally, strong winds may bring about substantial changes in the intensity and distribution of air currents.
Figure 1 Illustration of smoke problem in a 9-storey office building (a) air currents
((b) smoke distribution (fire on first storey)
Since the equivalent orifice area of the outside walls is usually smaller than that of internal partitions, it can be assumed that the principal resistance to themovement of air is offered by the building envelope. With this assumption, the total rate of air infiltratioq can be expressed as follows' :
where V is the mass flow rate of air, w is a constant (orifice factor), a is the equivalent orifice area for the outside walls,
P
is the perimeter of the building, C is a constant (related t o the gas constant), g is gravitational acceleration, T, is the (absolute) temperature of the outside atmosphere, Ti is the (absolute) temperature of the building interior, and h , is the height of the building.m i t i o n and Initial Fire Spread
Since a t least four of every five fires start from relatively small ignition sources
]
, the risk of outbreak of fire is directly related t o the use in the building of products not resistant to ignition by small energy sources. Ignition is a very complex problem; the scope of this paper allows n o more than a cursory discussion. Those who wish t o acquire a deeperunderstanding are advised to read such review articles as those by Fang3 and Thomas ?
The factors that control the ignition of solids are partly intrinsic to the materials and partly extraneous. Their roles depend to a great extent on whether ignition is piloted or spontaneous, i.e., whether it occurs with or without the aid of a flame, spark, or glowing wire. Munch speculation is related to defining the conditions immediately preceding ignition in terms of such intrinsic factors as the geometry of the solid and the physical and thermodynamic properties of the material and its pyrolysis products, and such extraneous factors as the ambient atmospheric conditions, and the nature and total energy of the ignition source.
One can achieve a fair insight into the material-intrinsic factors of ignition by examining the energy balance imme- diately following ignition. (The understanding so acquired is not complete; some fire-retardant-treated plastics show substantial resistince t o ignition but bum just as rapidly as their untreated counterparts, once ignited .) Clearly, sus- tained combustion is possible only if the flame that remains attached to the surface of the solid after removal of the ignition source is capable of evolving energy a t a rate sufficient, discounting the energy dispersion t o the surroundings, (i) to m a i n t a i ~ the surface a t the temperature level of pyrolysis, and (ii) If pyrolysis is endothermic, to proyide the heat for the pyrolysis, the process that feeds the flame with gaseous fuel. The energy requirement for maintaining the surface at the level of pyrolysis temperature is related t o the heat capacity of the solid, the product,pc (where p is density and c is
specific heat) if the solid is thin, and t o its thermal inertia, the groupJkpc (where k is thermal conductivity), if it is thick.
This simple visulization of the post-ignition energy balance suggests that the most important factors abetting ignition are: (i) high radiant heat output by the flame, which in turn is determined by the size of the flame, its luminosity, and the heat of combustion of the gaseous combustion products; (ii) low pyrolysis temperature; (iii) low (endoihermic) heat of pyrolysis; and (iv) low heat capacity (for t h h materials) or low thermal inertia (for thick materials).
Studies conducted by deRis 6 , Lastrina et a1
,
and~emandez- el lo'
indicate that these are also the principal factorscontrolling the velocity of flame spread across the surface of an ignited object in the early stage of fire, when spread is as yet unaided by heat emitted from neighbouring burning objects or hot compartment boundaries. Oneis led t o believe, therefore, that products that tend t o ignite easily also tend t o bum rapidly at the onset of fire
This rule becomes somewhat clouded, however, when applied to lightweight foam plastics of very low thermal inertia, to materials that melt on heating before reaching pyrolysis temperature, and t o char-forming materials. With foam plastics, the energy of the ignition source and the surface area exposed to the source dictate whether or not ignition will occur. Although the surface temperature of such materials rises quickly to the level of pyrolysis if exposed t o even a small energy source, the heat penetration, because of the extremely low density and thermal inertia of the material, will remain shallow and the production of pyrolysis gases following removal of the ignition source may not be sufficient to supply combustion energy at a rate necessary t o keep the process going. If, however, the ignition energy is large enough to produce a sizable initial flame and the energy supply t o the surface is perhaps fortified by radiative feedback from nearby objects or hot compartment boundaries, the burning will quickly spread over the entire surface of the material.
Keeping the temperature of the surface of melting materials (of which polyethylene, polypropylene, and polystyrene are prime examples) at the level of pyrolysis may be difficult if their orientation is such that the melt flows away from the sight of the flame. With char-forming materials, of which cellulosics are of principal importance, pyrolysis produces a porous carbonaceous coating on the surface. Depending on the shape and orientation of the surface, the char layer gradually build up, and by blocking radiation from the flame, it may eventually quell pyrolysis and cut off the fuel supply to the flame.
Even if flaming combustion is stopped, a charring material may continue to undergo combustion of a different kind: smoldering. Whereas flaming combustion of charring materials usually consists of three kinds of simultaneous reactions: gas phase combustion, pyrolysis, and char oxidation, smoldering consists of two kinds only: the consumption of the surface char by oxidation and the renewal of the char zone by pyrolysis driven by the heat produced in the oxidation. (Some authors extend the meaning of smoldering t o the pyrolysis of non-charring materials without flaming, under strong radiative fluxes.) Cellulosic materials pf complex surface structure and low thermal inertia, such as loose-fill cellulosic insulation, are especially prone to smoldering.
The so-called "oxygen index" method ' I 0 provides a convenient way of arranging materials according t o their pro- pensity for sustaining flaming combustion, following ignition by a small-energy pilot flame. Table 1 gives the oxygen indices for the most common materials used in furnishings and building products. Unfortunately, the oxygen index does not reflect the increased or decreased propensity associied with the energy of the ignition source, the nature of the ignition source, and the shape, mass, and surface texture of the material.
Coping with Preflashover Fires
For some time following ignition, the source item bums in approximately the same way in a compartment as it would in the open. Then, as flames spread over the surface of the source item, and perhaps t o other contiguous items, the process of burning becomes influenced more and more by factors characteristic of the compartment as a whole. Heat is fed back from the surrounding objects, especially from the compartment boundaries, and augments the rate of burning. A layer of hot smoky gases builds u p below the ceiling. As Fig. 2 shows, intense radiant energy fluxes originating mainly from the hot ceiling and the adiacpnt smoke layer gradually heat u p the contents of the compartment. If the rate OF
burning is sufficient to raise the radiation level t o 1.7 to 2.1 w / c m 2 13
,
all combustible items in the compartment willignite in quick succession: flashover occurs. (Experimental studies 14"' indicate that the attainment of a temperature of 500 t o 6 0 @ ~ by the hot gas layer can also be regarded as flashover criterion.)
TABL8 I
-
Oxygen i n d e x -for a Few c o m o n m a t e r i a l s l 1 l 2 M a t e r i a l Oxygen 1ndexa Carbon, porous Epoxy, c o n v e n t i o n a l Foam r u b b e r Neoprene Polyamide ( n y l o n ) P o l y c a r b o n a t e P o l y e s t e r (FRP) P o l y e t h y l e n e P o l y i s o c y a n u r a t e foam, r i g i d Polymethyl m e t h a c r y l a t e P o l y p r o p y l e n e P o l y s t y r e n e P o l y s t y r e n e foam P o l y s t y r e n e foam, flame r e t a r d a n t P o l y t e t r a f l u o r o e t h y l e n e ( t e f l o n ) P o l y v i n y l c h l o r i d e P o l y u r e t h a n e foam, f l e x i b l e P o l y u r e t h a n e foam, r i g i d Urea-f ormaldehyde Wood, w h i t e p i n e Wood, s u g a r maple Wood, plywood 19.7'oxygen i n d e x = minimum oxygen c o n c e n t r a t i o n . e x p r e s s e d a s volume p e r c e n t . r e q u l r e d t o s u p p o r t f l a m i n g combustion
F l g u r e 2 P r e - f l a s h o v e r f i r e
have t o escpae or be rescued. For this reason a thorough understanding of the chain of events that connects the ignition of the source item with the flashover has become, in recent years, one of the major objects of theoretical and experi- mental fire research 16'17'18
Whether a small fire dies out or grows into a large fire depends on tour factors: ( I ) rate of heat release by the object f m t ignited, (ii) total "fire load", i.e., the total amount of combustible material in the compartment, (iii) nature of the compartment lining materials from the point of view of supporting combustion and spreading fire, and (iv) thermal inertia of these lining materials. If these factors create a condition favorable to unlimited fire growth, flashover will ensue and the entire compartment will eventually become involed in fire.
The first two factors (rate of heat release by the item first ignited, and total fire load) depend largely on the nature of the compartment furnishings; they are subject t o statistical probabilities and are beyond the control of the building designer. The designer does, on the other hand, have at least partial control over the other two factors: the combustion and flame spread characteristics and the thermal inertia of the compartment boundarties.
As t o the combustion and flame spreading characteristics of the compartment boundaries, unfortunately there is no reliable test method that could predict the burning behaviour of various materials once the fire has grown beyond its incipient stage. Benjamin lgaffirmed(with data borrowed from Castino et al 20 ) that for lining materials the sequence of merit with respect t o spread of flames, as derived from the most commonly used standard performance test (the tunnel test ), is n8t.necessarily valid under advanced pre-fl&over conditions. This is not surprising, as the rate of flame spread depends rather strongly on external radiation to the burning objectzz'*": Changes in the merit rating due to changed radiation levels are consistent with Tewarson and Pion's 25 findings that different materials respond differently
to changes in external radiation.
The most important requirements t o prevent the fast development of fires are covered in building codes, which regulated what can be built into a building, and fire codes, which control what can be brought into it. Typical items that fue codes are concerned with include movabel partitions, floor covering and decorating materials, drapes and curtains, for use in buildings of dense occupany. These items must be subjected to various performance tests 2 6 that
will, it is hoped, yield some idea of their propensities for becoming ignition sources and propagating fire.
The building code regulations that bear on the time to flashover are those that restrict the use of combustible lining materials. Conventionally, interior finishes having flame spread ratings higher than 150 (in Canada) or 200 (in the U.S.A.) are not allowed in buildings of dense occupancy. Further restrictions are imposed on the flame spread ratings of linings used in exits. A recent addition to building codes requires that foam plastics, which have been known t o spread fire much faster under realistic fire conditions than in performance tests, be covered with insulating linings.
The safety of a building can further be improved by circumspect design. The building designer knows the intended use of the building and, therefore, has at least a rough idea oC the types of articles that may be brought into the various compartments upon completion of the building. He can add valuable minutes t o the time t o flashover b$ avoiding extensive use of conibustible linings in those compartments that are most likely t o be furnished with fabric-covered (upholstered) items, or in which clothing articles are kept or stored. He can further heighten the level of fire safety by providing closets and built-in cabinets for the storage of clothing and paper products. In the design of theatres, lecture rooms, atriums, lounges, etc. he can specify slightly elevated or recessed walk-ways or built-in planters along walls that are t o be lined with combustible materials, and thus prevent the occupants or the interior decorator from placing upholstered furniture close t o those surfaces.
In closing this subject, it may be appropriate to mention briefly the sprinkler system, because its chief function is t o prevent incipient fires from reaching the flashover stage. Except for buildings with very large uncompartmented spaces, the use of a sprinkler system is optional, but its use is often rewarded by the reduction of other building code
27
requirements and lower ihsurance premiums. The principles of designing (1 sprinkler system are well known and will
not be discussed here.
Coping with Fully-Developed Fires
Once fire has grown beyond the flashover stage, human survival in the fire compartment becomes impossible; the strategy of defense from this point on is t o prevent the fire from spreading t o other compartments. It has long been believed that fue resistant compartmentation provides the complete answer to the problem of spread of
fire
through buildings. This concept pictures a building as composed of number of compartments perfectly isolated from each other,6
and fires as spreading by the successive destruction of compartment boundaries.
The idea of perfectly isolated compartments is, of course, a crude abstraction. Fire must have access t o air; it cnanot develop in a fully isolated space. The fire compartment must communicate with at least one other inside o r out- side space, e.g. through an open door, broken window or any kind o r ceiling or wall opening. There must be at least one route along which it can spread by convection: by the advance of flames and hot gases. Thus, the defense against the spread of fully developed fue has two components: countering the potential of fire for destructive spread and coun- tering its potential for convective spread. Observations over the past several decades have clearly indicated that the potential of fire for convective spread far outweighs its potential for destructive spread.
Fire resistant compartmentation is the traditional technique of preventing the spread of fire through a building by destruction. Fire resistance is a measure o f the ability of building elements to yithstanq the spread of fires by deS- truction or (possibly) thermal conduction. It is determined by subjecting a specimen of the element to a standard fire test za
.
In that test the specimen is exposed t o h q t gases on oneside (except for building columns, which are exposed on all sides) in a furnace whose tepperature is controlled t o follow a prescribed temperature-time curve. The fire resis- tance is interpreted as the period of satisfactory performance. According t o the test specification, collapse, development of through-holes, or rise of temperature at specified locations on or in.the specimen mark the failure of the specimen.Recent studies indicate that the potential of fires (real-world fires as well as test fires) for destructive spread can be
29
quantified by the popalized heat load
.
The normalized heat load, H, is defined as follows:where q is the heat flux penetrating the boundaries of an enclosure (building compartment or test furnace) exposed t o fire, t iatime,
=*
is duration of fire exposure, an- is the thermal inertia of the enclosure boundaries.As Eq. (2) shows, normalized heat load is the heat absorbed during fire exposure by the boundaries of the encloseur (per unit surface area) divided by the thermal inertia of the boundaries.
According t o fie theorem of uniformity of normalized heat load, H is approximately the same for the f i e enclosure as a whole as for the individual boundary elements. This implies that the destructive spread potential of fire is the same for all boundary elements of the compartment. (The concept of normalized heat'load is not applicable, however, t o boundary elements made entirely from materials of very high thermal inertia, for example, ffom unprotected steel or aluminum.)
Table 2 lists the thermal inetia for a number of compartment lining materials. (The values represent averages ap- plicable t o the usual temperature iqtervals encountered in fues. When information is lacking on compartment size and lining materialsJkpcV00 J m-'
s2
may
be used for common plasterboard-lined compartments.)TABLE 2
-
Thermal i n e r t i a s of compartment boundary m a t e r i a l s&
T
&PC
M a t e r i a l J m -2 s -{ K - 1 M a t e r i a l m - 2 s - b K - l H a r b l e 2273 P l a s t e r b o a r d 742 Normal-*eight c o n c r e t e 2192 V e r m i c u l i t e p l a s t e r 667 B r i c k 1520 Wood 436 L i g h t w e i g h t c o n c r e t e 931 M i n e r a l wool 86Drawing on the concept of normalized heat load, determining fire resistance requirements is as simple as requirink that building elements be able t o endure, in a test fire, a normalized heat load (denoted by H") equal t o or greater than
the normalized heat load expected on the same building elements in real-world fires (denoted by H'). In reality, the problem is somewhat more complex. To understand why, it is necessary to examine the variables on which
H'
depends and the factors that cloud the meaning of H".The most important variables on which the normalized heat load in real-world compartment fues depends are 30 :
AF
floor area of compartment,At
total area of compartment boundaries,h
c height of compartment,&
surface-averaged thermal inertia of compartment boundaries,@
ventilation factor characterizing the rate of air flow into the compartment.L
specific fire load (mass of combustibles per unit floor area).Values for the fust three variables are available from building plans. The fourth may be calculated from the planned use of building materials. The last two, ventilation factor and specific fire load, are random variables. Ventilation may vary with climatic conditions, building height, and other factors related to both the building design and the state of the. compartment at the outbreak of fire. There is little information on its frequency distribution; values close to its min- imum (to be discussed later) probably occur with high frequency.
The specific fue load is largely a function of building occupancy, but even for the same occupancy i t may vary in an unpredictable fashion. A typical cumulative plot is shown in Fig. 3.
S P E C I F I C
F I R E
L O A D L. k g l r n 2F i g u r e 3 C u s t o m a r y p r e s e n t a t i o n o f i n f o r m a t i o n o n s p e c i f i c f i r e l o a d
In room-bum experiments the values of the ventilation factor and specific fire load can be carefully set. Under
30
such conditions the following approximate equation is applicable t o the normalized heat load
whche
8
is, in a loose sense, the fraction of the fuel energy released in the flaming combustion insdie the fire compart- ment. (This interpretation of 13 is strictly appropriate only for poorly-ventilated fires. At very high air flow rates,6
0.79
Jfi
d =
{
whichever is less1
The mathematical model on which these two equations are based has been discussed and compared with 13 other models in Ref. 31.
Although Eqs. (3) and (4) were derived for compartments with cellulosic fire loads, their application t o compart-
3 2
ments with noncellulosic fue loads will result in an error on the safe side
.
A recent experimental study has revealed that for c e ~ ~ u l b s i c fire loads the average error of H', as calculated from Eqs. (3) and (4). is 7percent.S i n c e a n d L are random variables, by virtue of Eqs. (3) and (4) the normalized heat load in real-world compart- ment fires, H' , is also a random variable.
The temperature history of standard test fires is uniquely defined, and therefore the normalized heat load on peci- mens of building elements in fue tests,
HI',
is a function solely of the duration of fire test. (This statement is only a p proximately correct. There are secondary factors that may influence the nature of the relation between H" and the33
duration of a fue test
.
) That function is shown in Fig. 4 for a highly efficient test furnace heated with "black" gases (curve 1) and for the floor test furnace of the Division of Building Research, National Research Council of Canada l a b oratory (curve 2). Herer
denotes length of exposure t o the test fire in hours. If the test specimen performs satisfact- orily during that time,rquantifies the fue resistance of the building element the specimen represents.9
1
I
I
I
1
0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5
L E N G T H OF E X P O S U R E T O S T A N D A R D T E S T F I R E . T, h
Figure 4 Correlations betbeen 8" and T for standard fire tests
Curve 1
-
high-efficiency furnacesCurve 2
-
floor test furnace at DBRINRCC (estimated)The relation between normalized heat load in standard fire tests and length of testing is described by the following equation:
P E - R C E N T I L E S P E C I F I C F I R E L O A D
9 9 . 9 9 9 . 5 9 9 9 5 9 0 5 0 4 . 0
Figure 5 C o r r e l a c i o n b e w e e n
B
and the p e r c e n t i l e s p e c i f i c f i r e l o a d in the cumulative p l o t (upper s c a l e ) , or f a i l u r e p r o b a b i l i t y (lower s c a l e ) TABLE 3-
Information on f i r e l o a d 3 " S p e c i f i c Fire Load kg a - 2-
L 8 L Standard Occupancy Median D e v i a t i o n O f f i c e 24.8 8 . 6 School 1 7 . 5 5 . 1 Hospital 25.1 7 . 8 Hotel 14.6 4 . 2whereris now in hours. Eq. (5) is an approximate representation of curve 2 in Fig. 4.
When fire resistance is determined by standard fue tests, the test value, r,is a random variable. (For some elements the fire resistance can be determined by calculation.) Its randomness is due to two factors: (i) the test specimen may not fully represent of its real-world counterpart, mainly because of difference in workmanship or material properties; and (ii) the measured fue resistance, on account of small differences in fue test facilities or loading practices, may show some variation from one laboratory to another. According to an ASTM study, the coefficient of variation forr is some- where between 0.01 and 0.1 5, depending on the type of building elemtnt. Clearly, because of the randdomness ofs,H" is also a random variable.
Since fire safety design must address reasonably adverse conditions that may arise in real-world compartment fires, the design values of the two random variables,@and
L.
should be selected to represent such adverse conditions. A glance at Eq. (3) will reveal that the normalized hear load, H' (i.e., the destructive potential of real-world fire) decreases m o n e tonically with increase in ventilation factor,@. Thus, as far as ventilation is concerned, the most deterimental conditions occur at the lowest possible value of @. i.e., when the air flow rate into the compartment is not augmented by drafts or winds. Under such conditions the ventilation factor can be determined by the dimensions of the ventilation opening, as follows:where p.is the density of atmospheric air,Avis the area of ventilation opening, g is gravitational constant, and hv is the height of the ventilation opeing.
Focusing attention now on the other random variable, specific fire load, L, Eq. (3) shows that the normalized heat load increases monotonically with an increase in L. Unfortunately, there is no straightforward rationale to pick anadvene value for L acceptable for design.
How, then, should its design value be selected? If the median value is selected (see the cumulative plot in Fig. 3), the design may be inadequate in roughly 50 percent of the cases. If a value much higher thanLis selected, the cost of fire protection may greatly increase. Specific fire loads applicable to eigher 80 percent or 95 percent of cases used to be considered as potential design values. (The meaning of this 80th percentile, L B O , is illustrated in Fig. 3.)
Assuming normal distribution, any design value that may be selected can be described by the following equation:
-
where L is the median of specific fire load (see Fig. 3). and oL is standard deviation. (Information o n y a n d
a
based on Swedish data 34 , is presented in Table 3.) is a dimensionless factor whose value depends on the percentile specific fire load (in the cumulative plot) selected for the design. The relation betweena and the percentile fire load in the cumu- lative.plot is hown in Fig. 5. (For the 80th and 95th percentiles,Lm and L 9,, ,3 = 0.84 and 1.64, repectively.)Because three of the input variables needed in the design and selection process, namely, L,@ and
r
(if determined by test), are random variables, the design fire resistance requirement, rd, .is subject to some uncertainty. The question,
.
that arises is this: how to calculate,rd so as to ensure that the probability of failure in fire of the building elements to be selected remains below a target level. Two methods have been described in a recent paper3'
The first method of designing for target failure probability is based on the assumption that the specific fire load, L, is the only variable whose randomness need be considered in the d e s k and selection process. This assumption is realistic: as discussed earlier, with the selection of @* = Qmi.
,
a safe condition is ensured as to ventilation, and the coefficient of yariation for the fm test results, r, is usually not more than 0.1 5.If the specific fire load, L, is the only random variable, the failure probability,
P,
, can be controlled at any desired level by an opportue selection of the percentile specific fm load in the cumulative plot and, in turn, by the factor t8Clearly, if the 80th percentile, L
,!
, is selected as design value, the probability tkat the compartemtn boundaries will survive the fire is 80 percent and the failure probability is 20 percent, i.e.,P,
= 0.2. Thus the/ versus specific fire loadcan be converted into a
B
versus P , plot simply by changing the abscissa scale. T o facilitate the design, theB
versusPf correlation is also shown in Fig. 5.
A value for L, as calculated from Eq. (7) w i t h b selected from Fig. 5, when used in Eq. (3) will yield a design value
for H'; this, in turn, if substituted in Eq. (5) (remembering that the condition of satisfactory performance of a building element in fire is H"
>
H') will yield a design value for fire resistance, r d ,that ensures that the faliure probability for thebuilding element wlll not exceed P,
.
The second method is-more elegant; It is based on the 'second moment analysis'. It can be expanded t o cover the other principal random variable, the fire resistance test results, 7 . According t o this technique, the probability of failure for the compartment boundaries will not exceed P f if the fire resistance requirement for the boundary elements, 7 is determined from the following modified form of Eq. (5):
where
and in the latter equation according t o Zahn's procedure
and
-
in Eq. (9) is t o be chosen from Fig. 5 t o reflect the selected allowable level of failure probability, P.oL/L in Eq.(lO) is t o be selected from Table 3, and values for o L / r in Eq. (1 1) are available from the already mentioned ASTM study (usually between 0.01 and 0.15). The ventilation factor is again taken into account with its most adverse value, j.e,
@d
=
@",in.
In general, the fire resistance of the building elements actually used greatly exceeds the value contemplated by
design. The fire resistance of various building elements is always listed by the testing laboratories in rounded-down values: 0.5, 0.75, 1.0, 1.5, 2.0,
...,
h. Assume that the design value for fire resistance is r d = 1.12 h. The applicable building elements have t o be selected from among those whose fire resistance is listed as 1.5 h. Yet, because of the round- ing , a building elenlent listed as having a fire resistance of 1.5 h usually has a fire resistance somewhwere between 1.5 and 2.0 h, or, in the present example, 34 to 79 percent higher than the design value.Thus, the very system of listing the fire resistance of building elements (as determined by fue tests) and the practice of selecting components for a building from among those on the list may ensure a quite substantial, though usually unknown, margin of safety as t o the ability of the selected elements to withstand the spread of fire by destruction.
So far the design t o counter the potential of fully developed fires to spread by destmctuion has been covered.
As
mentioned earlier, the design must also address the problem of spread by convection.
12
defined as follows:
r a t e o f h e a t e v o l u t i o n o u t s i d e compartment p
= -
t o t a l r a t e o f h e a t e v o l u t i o n from f i r e l o a d For fires involving cellulosic fuel, its approximate expression is
Although the threat of convective fire spread can be characterized quantitatively by thep-factor, there is as yet n o established method of using its value in fire safety design. A possible approach would be to require extra safety measures for countering the convective spread of fires whenever the design value of the p-factor exceeds a specified limit, say 0.4.
In spite of the apparent lack of a philosophy on how t o deal with convective fire spread, in most situations com- monsense considerations suffice. The danger of fire spread is more severe if uncombusted volatiles have a means of entering the inside of the building, e.g., through a corridor, than if they leave through windows t o the outside atmosphere As Fig. ](a) shows, spontaneous air currents in the lower s h r e y s of high-rise buildings during the winter heating season will drive flames towards the corridors. Consequently, equipping the lower storeys of such buildings with self-closing doors may prove to be the best investment in fire safety.
In the upper storeys of high-rise buildings, the air currents will aid the movement of uncombusted pyrolysis products towards the building envelope. T o prevent storey-to-storey spread of fire along the facade of the building, flame deflec-
38
tors have been suggested (Figure 6). When activated by flames, they block the vertical passage of the flames.
-
F i g u r e 6 F l a m e d e f l e c t o r s i n o p e r a t i o n
So far methods of coping with the spread potential of fires of cellulosics only has been discussed. Althouth statis- tical data seem to indicate that cellulos~cs still constitute the bulk of fire load, the use of plastics is growing year by
year. The question naturally arises, how t o counter the spread potential of fires of plastics.
Unfortunately, little information is available on tllc su.bject. From a theoretical study3' the following conclusions have been drawn.
-
From the p o ~ n t of view of potential for destructive spread, fires of cellulosics are usually more dangerous than fires of non-charring plastics. From the point of view of potential of convective spread, the opposite is true.-
For both types of fire load, the destructive spread potential decreases with increasing ventilation (as characterized by the ventilation parameter).-
The convective spread potential of fires decreases with increasing ventilation if the fire load consists of non-charring plastics, and increases with increasing ventilation if the fue load consists of cellulosics.Coping with the smoke Problem
2 - 3 9
Fire statistics reveal that more people die in burning buildings from inhalation of toxic fire gases than f r o d heat-inflicted injuries. Even in deaths caused by bums, smoke is often a contributing factor, since dense smoke obscures the vision of the occupants and prevents them from reaching safety.
The seriousness of the smoke problem depends on three factors. These are, in order of improtance, (i) the extent t o which materials of high smoke-producing propensity are used, (ii) the intensity of the drafts in the building at the time of fire, and (iii) the toxicity of the pyrolysis and combustion products of the combustible contents of the building.
A number of experimental techniques for measuring the smoke producing propensity of materials have been review-
ed by Hilado and Murphy
''
The experimental results depend not only on the chemical composition of material, but also on such factors as the nature and amount of additives, the density and thickness of the sample material, the nature of thermal exposure, and the mode of ventilation. Representative values developed by a gravimetric technique" are listed in Table 4 for a few common plastics.TABLE 4
-
R e p r e s e n t a t i v e v a l u e s of t h e smoke p r o d u c i n g c h a r a c t e r i s t i c s of s e l e c t e d m t e r i a l s 4 ' P e r c e n t Smoke M a t e r i a l Based on I n i t i a l Weight A c r y l i c , u n i d e n t i f i e d 0.33 Linoleum 0.52 P o l y c a r b o n a t e 0.89 - 1.34 P o l y c h l o r o p r e n e r u b b e r , f i l l e d , f i r e r e t a r d a n t 0.80 P o l y e s t e r , b r o m i n a t e d , r e i n f o r c e d 1.70 Polymethyl m e t h a c r y l a t e 0.08 P o l y p r o p y l e n e , f i r e r e t a r d a n t 1.64 P o l y s t y r e n e 4.86 P o l y v i n y l c h l o r i d e , f l o o r i n g 0.21 P o l y v i n y l c h l o r i d e , f l e x i b l e , f i r e r e t a r d a n t 2.36 P o l y v i n y l c h l o r i d e , r i g i d 1.33 Wood, hard 0.05-
0.13 Wood, s o f t 0.08-
0.23 Wood, board 0 , 0 6-
0,57There has not yet been any attempt to restrict the use of materials on the basis of their propensity to generate toxic gases. The most likely reason is that carbon monoxide, which may be produced by any material as a result of
incomplete combustion, is still believed t o be the only toxic gas worth considering. Accumulated data indicate
"
,.however, that other toxic gases such as hydrogen cyanide, hydrogen chloride, nitrogen dioxide, and sulfur dioxide may be the cause of fire deaths or injuries more often than is commonly believed.
The effect of the intensity of drafts on the spread of smoke is similar t o their effect on the spread of fire. Yet, smoke is not a combustion-carrying medium but merely an aggregate of fire gases and airborne particles; it is much more mobile than the fire that breeds it and can disperse throughout a building in a much shorter time. The air currents that arise in a nine-storey building during the winter heating season were described earlier and illustrated in Fig. la. Fig. 1 b shows how the same air currents would distribute smoke on the various levels of the building within a mere 1 0 t o 15 minutes from the onset of fue on the first floor. (Smoke contamination of the second floor would be the result of ver- tical leakage currents not mentioned in this paper.)
The most obvious step the building designer can take t o alleviate the smoke problem is to avoid specifying lining materials known t o be heavy smoke producers or to generate highly toxic decomposition and combustion products. Yet, this "passive': method of defence is rarely sufficient. From among the "active" methods, three will be discussed here briefly: smoke dilution, provision of refuge areas, and building pressurization.
In milder climates, where the role of stack effect in smoke dispersioh may not be significant, the technique of dilut- ing the smoke is often used in keeping certain vital parts of the building, such as lobbies and stairwells, relatively free
43
of smoke. It is believed that dilution with fresh air in a 100 t o 1 proportion will ensure safety conditions with respect
to both visibility and toxicity. The information needed for the design of smoke dilution systems includs the equivalent orifice area for the boundaries of the space t o be kept smoke-free and the Fate of smoke generation by the fire. (The latter can be estimated as described in Refs. 32 and 37).
449 15
Detailed studies have revealed that the time for evacuating a building in case of fire is approximatedly propor-
tiopal to the building height and, depending on 'the occupant concentration, may take much longer than the expected
duration of an average fire. Consequently, complete evacuation of a building above a certain height, say 10 t o 15 storeys,
does not seem practicable. The danger of exposing the occupants t o smoke can be greatly reduced by providing pressuriz- ed refuge areas, preferably in the vicinity of a stairwell, where the occupants can stay in relative safety for the duration of the fire. The required rate of air supply t o those areas is not likely t o be determined by the leakage chvacteristics of its boundaries, but rather by the need for maintaining tolerable conditions for the assembled occupants. The required
4 6
minimum flow rate of fresh air is 0.42 m3 lmin per person
.
The most effective way of preventing the spreed of smoke is t o pressurize the entire building or major parts of it. Smoke travel through the shafts t o the upper storeys of the building is prevented if the pressure everywhere in the build- ing, or at least in the vertical shafts, is raised above that of the outside atmosphere. The required rate of air supply is
i.e., roughly three times the rate of infiltration of air into the building under normal conditions. The pressure difference against which the supply fan has t o work can be'calculated as outlined inRef. 38.
The most convenient way of achieving pressurization is by injecting outside air into all shafts at the top of the building. Additional advantages (and savings in energy consumption) can be gained by preheating the air t o only slightly above the O°C level (provided that the outside temperature is below the freezing pointb
The discussion of smoke control techniques has been restricted here t o the simplest high-rise buildings, those with uniform compartmentation and with shafts that run the full height of the building. In more complex stituations, the
47.48r49
design is rarely possible without a computer-aided analysis Moreover, even far simpler buildings, invoking the
computer may be necessary if building pressurization is combined with other techniques5h5.'
52
A supplement t o the National Building Code of Canada contains an exhaustive survey of measures for providing
fire safety in high buildings. Some of them are just commonsense solutions and impose very little restriction on the design.
Many clauses in building codes relate t o facilitating escape from fire-stricken buildings. Regulations cover the width of exits as function of occupant concentration, distance between exits, access routes t o exits, location and illumination of
exit signs, and the maximum length of dead-end corridgs. The installation of fue detectors and fue alarms has been made mandatory in certain buildings, mainly those. of high occupant concentration. In addition, in buildings with air circulating systems, the installation of smoke detectors in the main ducts is also required. Detection of smoke in the main return duct is followed by the shutdown of the return fan and by the actuation of the fire dampers of the duct system.
If the fire.load is higher than usual and consists mainly of plastics, the use of fire drainage in the corridors may prove beneficial. Fig. 7 shows a section of a corridor equipped with fue drainage facilities. The corridor is divided into a number of imaginary cells, 1, by retractable fold-up drop curtains, 2, made from thin metal sheets and non-combustible cloth. If flames penetrate one of the cells, they activate drop curtains on either side of the penetration. The curtains slide down in grooves, 3, but stop a certain distance from the k o r to allow for controlled ventiiation of the fue. Shortly afterwards, the flames will trigger the opening of an access gate, 4, which will connect the cell containing the fire with a drainage duct. 5, running along the entire height of the building and opening into the atmosphere above the roof level.
F i g u r e 7 T y p i c a l c o r r i d o r i n a b u i l d i n g e q u i p p e d w i t h f i r e d r a i n a g e s y s t e m : 1
-
c o r r i d o r c e l l ; 2-
f o l d - u p d r o p c u r t a i n ; 3-
c u r t a i n g r o o v e s ; 4-
a c c e s s g a t e s ; 5-
d r a i n a g e d u c tIf properly designed, the fire drainage system will perform a threefold function: (i) it will draw air into the corridor cell at a rate that ensures relatively low heat load on the cell, (ii) it will keep the pressure in the cell below the level of the n e i g h b o u ~ g spaces and thus hinder the spread of flames and smoke, and (iii) it will remove the flames and smoke from the cell in a safe and organized manner. (Further details of the design of fire drainage facilities are available in Ref. 38.)
Since operation of the fue drainage system does not rely on the availability of water and electricity at the time of fire (the charring remnants of the fuel can be extinguished with some chemical suppressant), its application may offer special advantages in remote, poorly serviced communities, or with buildings the contents of which are sensitive to water damage. The disadvantage of the system is that with its use the normal loss expectancy is an entire cell and, there- fore, it is not suitable forth protection of buildings with valuable contents.
Psychological and Sociological Factors in Fire Safety Design
The question naturally arises as to how much fm safety is really necessary. The answer t o this is not only a matter
of science and economics, it also concerns psychology and sociology. Human adaptation is an important factor. Count- less examples show that people can live safely in extremely flammable shacks, using candles or oil-lamps for illumination;
they can also fall victim to fire in modem buildings equipped with all amenities and fm safety facilities. An average
.person is subject to a behaviour pattern reminiscent of the principle of action-reaction in physics. Perhaps it could be described as the following generalized form of Parkinson's law: human laxity expwds to fill the space allowed by the restraining factors.
There are, however, great deviations from this behaviour pattern, governed primarily by the individual's understand- ing of his responsibilities toward society which, in turn, depends on his educational level. This point has not yet been brought out clearly, probably due t o the fact that statistical data on fire losses are often collected on the wrong bases, e.g., area, racial origin of the occupants, or type of housing. The profound underlying factor behind these statistical
data is revealed in a British report 53 ; the propensity for high fm losses in an area with a low level of household ame-
nities did not decrease after a massive redevelopment of the area, including provision d amenities.
The high incidence of fires in modem times is due largely t o negligence, which is a symptom of a deficient sense of responsibility. It manifests itself in various ways and is the chief source of the troubles that plague society. Education is the cure. Problems that have a strong social component cann.ot be solved by technology alone.
Concluding Remarks
Although the most important aspects of the design of buildings for fire safety are governed by building codes, a competent design team can increase the level of safety beyond that provided by the stereotyped application of regulatians, usually without any additional expenditures, or even at substantial savings to the builder. It is extrenely important to realize, however, that fire safety is not something that can be added on after completion of the building plans. To be really effective, the design must take into account the problem of fire safety from the first step.
Discussion in this paper was confined t o the more or less conventional types of residential, business, and institu- tional buildins. With buildings erected for housing large crowds or people of reduced mobility, a variety of other fue safety problems will inevitably arise, many of them not covered adequately by building codes. The inclusion of a fue
safety expert in the design of such buildings is not just a wise move, it is essential.
Nomenclature A area, m c specific heat, J kg-' K-' C constant, 353.3 kg K m-3 g gravitational acceleration, 9.8 m
c2
h height, m I -H normalized heat load, s K
1
-
H' normalized heat load under real-world conditions, s K
-
H' median of H', sYK
1
H"
normalized heatjoad pertaining to experimentally derived fue resistance, S ~ K-
H" median of H", K k thermal conductivity, W m-' K-'Jkp
thermal inertia, J m-2<:
K r 1L
specific fm load, kg m-2-
L
median of L, kg m-P
perimeter of building, mPr failure probability, dimensionless
q heat flux penetrating the enclosure boundaries, W m - 2
T absolute temperature, K
V
.
rate of air infiltration, kg s-'
W rate of air supply for pressurization, kg s C 1 Greek letters
a equivalent orifice area (of the order of 0.0005), dimensionless
B
factor (Fig. 5), dimensionless6 factor (Eq. (4)), dimensionless
p factor, characterizing the potential of fire to spread by convection, dimensionless
p density, kg m-3
o standard deviation
r time of exposure t o standard test fire; fire resistance, h
-
r median of r , h
r* duration of real-world fire exposure, s
cD
ventilation factor, kg s -w orifice factor,Zo.6, dimensionless
Subscripts a of atmospheric air
B
of building C of compartment d design value F of floor H' pertaining t o H' H" pertaining t o H" i of building interior L pertaining to L min minimumt total for the compartment boundaries
V of ventilation opening
T pertaining t o T
8 0 pertaining t o the 80th percentile 95 pertaining to the 95th percentile References
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