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Fire and the spatial separation of buildingsMcGuire, J. H.

113 -7 7

NATIONAL RESEARCH COUNCIL

CANADA

CONSEIL NATIONAL DE RECHERCHES

e9s?s

Fire and the Spatial

Separation of Buildingsby

J. H. McGuire

ANALYZED

Reprinted from

Fire Technology, Vol. 1, No. 4

November 1965, pp. 278-287

Technical Paper No. 212

of the

Division of Building Research

OTTAWA

February 1966

NRC 8901

/ 8 7cc?1 t7*il

Price 25 cents

LE FEU ET LA DISTANCE SEPARANTLES BATIMENTS

SOMMAIRE

On sous-entend que les valeurs des distances de s6para-tion des bdtiments calcul6es sur la base d'un rayonnementmaximal pr6viendront la propagation d'un incendie par

rayonnement pendant une dur6b ind6finie. Les valeurscalcul6es des distances d6passent toutefois ce qui est r6alisablepratiquement. L'auteur pr6sente des tables de distances des6paration suffisantes pour pr6venir la propagation des in-cendies par rayonnement pendant un laps de temps per-mettant aux sapeurs-pompiers de commencer leurs op6ra-tions. L'auteur explique comment ces tables ont 6te calcul6eset 6tur''emplr

{e leur

-:d\

:C\

:+--

F1-iC

6-o=-oo-9Q-A

-a:@-T

-CD

REPRIIVTED FROM

FIRE TECHNOLOGY

Vo l . I No .4 NOV. 1965

FT.12

Norn: This paper is a contribution of the Division of Building Research, NationalResearch Counciil,-Canada, and is published with the approval oT the Director of theDivision. Acknowledgement is due-to Mr. G. Williams-Leir for programming the com-pr.rter to solve the configuration factor equations and to Mr. P. Huot for carr5ring outthe computations.

Fire and the Spatial

Separation of Buildings

Copyrisht 1965 NATIONAt tlRE FROTTCTION ASSOCTATION

60 EATTERYMARCH ST., BOSTON, MASS, O21IO

J. H. McGUIRE, SFPE

Diuision of Building Research

National Research Council (Canada)

It has been implied that spatial separations based on peak radia-tion levels will prevent ignition by radiation, indefinitely. Thespecified distances, however, exceed practical limits. Separationscalculated to prevent ignition by radiation long enough for fire ex-tinguishing operations to be initiated have been tabulated. Theauthor explains how the tables were derived and discusses problemsthat may be encountered in their use.

,TtHE spread of fire from one building to another separated from the firstI by a vacant space may result from one or more of the following mecha-

nisrrs:

o Flying brands.

o Convective heat transfer.

o Radiative heat transfer.

Flying brands may initiate secondary fires at substantial distancesfrom the primary fire, e.g., at least a quarter of a mile. It is not, therefore,practical to consider the spatial separation of buildings as a means of com-bating this hazard. Regulation of the choice of exterior cladding materials,particularly on roofs, minimizes such ignitions, and their extinguishmentis usually easy, provided they are detected at an early stage.

Convective heat transfer will cause ignition only if ttre temperature ofthe gas stream is several hundred degrees Celsius. Such high gas temper-atures are only to be found in or very near the flames emanating from thewindows of burning buildings.

Ignition by radiation from a burning building can occur at distancessubstantially greater than those to which flames generally extend. It isthis mechanism, therefore, that will be the factor governing the specifica-tion of the spatial separation of buildings from the fire point of view.

Prinrod in U.S.A.

Separation of Buildings 279

The remainder of this paper is devoted to the formulation of a tech-nique for prescribing separation distances between buildings with a view to

reducing the likelihood of spread of fire by radiative heat transfer.

T O L E R A B L E R A D I A T I O N L E V E L

When discussing the possibility that combustible materials will be ig-

nited by radiant heat, the lowest level of intensity that proves to be sig-nificant is 0.3 cal/sq crnfsec; below this, most materials cannot be ignited

in the presence of a pilot flame. Unfinished, untreated fiberboard does notobey this generalization and will ignite in the presence of a pilot flame ateven lower intensities. In the present context, however, this feature willnot be considered on the grounds that untreated, unfinished fiberboard is

very unlikely to be a material exposed to radiant heat from fire in anadjacent building.

The mechanism discussed above involves the presence of a pilot flame,which constitutes a local high-temperature source. When a building is onfire and is exposing another to radiation, sparks and flying brands con-stitute the local high-temperature sources. In many cases, where a radia-

tion level greater than 0.3 cal/sq cm/sec is incident on a building, a sparkor flying brand will pass through the evolved streams of combustiblevolatiles.

It will therefore be assumed, that the spatial separation of buildingsshould be such that a fire in one building should not subject the facade ofanother to levels of radiation higher than 0.3 cal/sq cm/sec.

R A D I A T I O N L E V E L S F R O MB U R N I N G B U I L D I N G S

ExppnrupNrar- FrNnrNcs

The radiation levels to be expected from burning buildings were investi-gated in the course of a program of full-scale burns known as the St.Lawrence Burns, carried out by the Division of Building Research, Na-

tional Research Council, during the winter of 1958.1 The following were theprincipal findings:

. The nature of exterior cladding - brick or clapboard - did notnoticeably influence radiation levels.

. Peak radiation levels at some distance from the buildings coincidedwith those that would result if window openings, at an appropriate hypo-

thetical temperature, were taken to be the only sources of radiation.

. Peak radiation levels from buildings with highly flammable linings

were twice those from buildings with noncombustible linings.

o Radiation levels were affected by wind direction, those on the lee-

ward side of a building being, in general, much greater than those on the

windward side.

Using the second result described above, it was found that peak hy-pothetical radiation levels at window openings on the leeward sides of

280 Fire Technology

the buildings came to nearly 40 and 20 cal/sq cmfsec, respectively, forbuildings with highly flammable and noncombustible linings. These valuesare, in fact, much greater than the maximum level to be expected at win-dow openings - about 4 cal/sq cm/sec - because it is assumed thatradiation from the flames above windows is actually emanating from thewindow openings themselves. Spatial separation calculations using theseresults gave a range of values that were inordinately large and virtuallyimpractical. An attempt, therefore, was made to justify basing spatialseparation calculations on lower levels of radiation from burning buildings.

In re-examining the results of the St. Lawrence Burns, it was noticedthat, although the fires had been arranged to develop very rapidly, radia-tion levels did not exceed about one-flfth of the peak values listed, i.e.,40 and 20 cal/sq cmf sec until at least 16 min had elapsed. As fire fightingis in progress at this stage for the great majority of fires, it is possiblethat

spatial separation would perform adequately if it merely gave protection

against the spread of fire during this period. In many cases, spatial separa-tion calculated on this basis would, in fact, protect a building indefinitely,for the radiation levels previously discussed are maxima and would notalways prevail.

Frnr.o Sr:unrns

To throw more light on the possible hazard. of adopting the less stringentapproach just referred to, it is worth examining the results of the veryIimited number of field investigations carried out to date.

It is preferable to discuss this question in terms of a quantity called theconfiguration factor rather than in terms of radiation levels, which obvi-ously are not recorded during a fire. A configuration factor is defined asthe ratio of the radiant intensity at the receiving swface to that at the(one or more) radiating surfaces. Assuming that these are at, a uniformblack body temperature, a configuration factor is calculated solely fromthe relative geometry of the radiating and receiving surfaces. If it isassumed that radiation may be represented as emanating solely from win-dows and other openings, then this latter calculation is usually feasiblefollowing a fire.

The configuration factors that would be specified on this basis, to offerprotection against the peak levels of radiation measured at the St. LawrenceBurns would be 0.3/40:0.0075 (hazardous cases) and 0.3/20:0.015(normal cases). To guard against radiation levels of about one-fifth thepeak value would call for configuration factors of 0.035 (hazardous cases)and 0.07 (normal cases).

The first record of configuration factor calculations made on this basisduring field investigations may be found in a British technical paper pub-

lished in 1950.' The results relate to two fires. For the first, the resultsrefer to the condition of a number of window frames in the exposed buitd-ing and are given in Table 1. The exposing building was a multistoryclothing store.

Separation of Buildings

Tanr-n L. Damage Related to Configuration Factor

Configwation foctor Condition of window frame

0.093

o.L12

Paint blistered

Paint blistered, little charring

Surface charring

Burned

Burned

The second fire gave only one result - a timber billboard with a con-

figuration factor of 0.092 ignit€d.

A fire that occurred in Winnipeg in 1956 also offers interesting informa-

tion on this subject. At one stage, an exposd building had a configuration

factor of 0.05 but did not becorne involved in the fire. Shortly afterwards,

,another building ignited, raising the configuration factor of the exposd

building to 0.1. Many of the window frames of the exposed building then

ignit€d.

The choice of a corifiguration factor of 0.07, based on the St. Lawrence

Burns rreeults, appears to be eompatible with the above field obeervations

and appropriate for normal use. The high intensities recorded during some

of the St. Lawrence Burns are so disturbing, however, that it is suggested

that a configuration factor of 0.035 should form the basis of separation

calculations involving buildings that can be expected to burn extra vigor-

ously.

Since the above suggestion was adopted in the 1960 edition of the Na-

tional Building Code of Canada, a field fire investigation involving two

dwellings has further justified it. The two dwellings were separated by a

dietanc€ of 17 ft, which is 2 ft gxeater than the 15 ft given by a configuration

factor of 0.035 together with a constant addition of 7 ft, i.e., as for Table

2. Despite this substantial separation, ignition still occurred, suggesting

that the distances prescribed are not excessive. The fire was started withgasoline, which pertly explairrs the very rapid development and the attain-

ment of poak radiation levels before the arrival of the fire department.

T H E T A B L E S

DnnlettoN

Tables 2 and 3 are samples of calculations based on configuration factors

of 0.035 and 0.07, respectively, for particularly hazardous and normal con-

ditions. In other words, the specified separations theoretically reduce the

radiant intensity at an exposed building to 0.035 or 0.07 times the equiva-lent intensity at the window openings of the exposing building.

Further distances of 7 ft (particdlarly hazardous) and 5 ft (normal)

have been added, following the basic calculations, to account for the fact

that flamee have a horizontal projection and that the equivalent radiating

0.067

0.067

0.081

?42 Fire Technology

Tnnr,n 2. Building &parations (hazardous conditions)

Width of Per centcompartment of window

(ft) opening

Height of comportment (ft)

12.5 25 37.5 1007550

20

63DD

45

32

4035.53022.5

53.54739.529

64564633

8674604l

100t o

5025

100t o

5025

100755025

100' t c

5025

100l o

5025

30

60

100

200

100I D

5025

53.5473928

73645338

8876.56345

t20.5104.58559.5

L52131105.57I

204.5173.513685

87.5

76.5

62.5

44.5

106

92.5' /D .b

O.t.A

147

L27.5

LO4

72.5

186.5

161

130

89

255

218

173.5

1t2.5

7 l

61.5

50

34.5

100

86.5

7L

49.5

t2t.5

105.5

86

60.5

169

146.5

119.5

83

2L5.5

186

151

103.5

297

254.5

203.5

135

84 93.5

72 79.5

57.5 62.5

37.5 39.5

119.5 135.5

103 116.5

83.5 93.5

57 62

146 166.5

126.5 L43.5

L02.5 116

71 78.5

205.5 235.5

L77.5 203.5

L44.5 165

100.5 114

263 303

227.5 26t.5

184.5 212.5

L27.5 146.5

366 423.5

315 365

253.5 294.5

17L 200.5

106

90.5

7L.5

16

136.5

r13.585.550

surface is thus in front of an actual building facade. The two dimensions,

7 ft and 5 ft, were results given by the St. Lawrence Burns during peak

levels of radiation. As the separations ane not intended to offer protection

in these circumstances, it might well be that these dimensions are somewhat

excessive and should be reduced by some 2 or 3 ft.

To cater for the almost infinite variety of window shapes and distribu-

tions that exist in building facades, a variable "percentage window open-

ing" has been introduced. Where windows are uniformly distributed and

are close together in comparison with the spatial separation distance, thisaction will not introduce noticeable error-

Calculation of Tables 2 and,3 was made on a computer suitably pro-

grammed by a colleague in the Fire Section of NRC.*

-._*Amplified ve{Fions of thee tableq, together with others involving, for example,different story heighte, and different increments in percentage window opening, areavailable on r€queot from the Division of Building Research, National ResearchCouncil.

Separation of Buildings 283

Pnnctrcer- Appr,tcattot r

The first feature to be considered in applying tables of this nature is

whether or not the adoption of an average value of percentage window open-

ing will give a valid distance of separation. If, say, windows occupy agreater proportion of exterior wall area at one end of a building facade than

at the other, then a greater separation is called for in that region. To be

on the safe side, it would be desirable to require a separation in that par-

ticular region based on the adoption of a higher value of percentage window

opening while retaining the true values of the height and width of the

building.If an individual window or other opehing proved to be very large, i.e.,

to have dimensions comparable with the separation dimensions, further

modification would be necessary. It would be essential to provide forgreater separation that that given on the assumption that a particular

window was the only radiator. Without delving more deeply into the

evaluation of configuration factors,3 it is not practical to offer recommenda-

Tasr,p 3. Building Separations (normal conditions)

Width of Per centcompartment of window

(ft) opening

Height of compartment (ft)

12.5 roa755037.525

43D J

30

20

60.5

52.5

42.5

29

100' l D

5025

100755025

100I O

5025

100755025

100755025

100755025

10

20

30

60

100

28

24.5

20.5

1 5

o ,

32.5

27

19

44

38

31

2l

58

49.5

39

24.5

69.5

58

44

26

83.5

67

48

26.5

J '

32

26

18

51

44

36

25

61

53

43

29.5

83.5

71.5

57

37.5

103.5

88

69

42.5

134

111

83

46.5

73.5

63.5

51.5

35

LO2

87.5

70.5

47

t28

109.5

8 7 '

55.5

r7 t .5

t44

110.5

64.5

48

4T

32.5

20.5

69

59

47.5

31.5

84

72.5

Dt' .D

39

tt7.51018154

t49t27.5101.566

201.5170.5r32.580.5

55.5 60.5

46.5 50

36 37.5

2L.5 22

81.5 91 .5

69.5 77.5

55 60

34.5 36.5

100.5 1L4

86.5 97

69 76.5

44.5 48.5

t42.5 163

122.5 140

98.5 118

65 73

782.5 2tO.5

156.5 180.5

125.5 L44.5

82.5 95

251.5 292.5

214.5 250

169 198.5

106.5 127200

284 Fire Technology

tions that are both simple and valid. Having derived separation distancesbased on a mean percentage window opening and on the window openingitse[ it must be left to the designer to assess an appropriate separation.

Radiation levels at a fixed distance from a building facade will decreaseas the distance from the center of symmetry increases; therefore, it wouldbe reasonable to relax spatial separation requirements near the corners ofbuildings. A number of sample calculations indicate that appropriateseparations near the corners of buildings range between 65 and 95 per centof those listed in Tables 2 and 3. It might be reasonable to suggest a re-laxation to 80 per cent of the value tabulated. The resulting separationrequirements are illustrated in Figure 1.

3 5 0 r g s 9

8 0

ira o % D l

g-

D = D r o R D 2 w H I c H E V E R

I S T H E L A R G E R

Figure 7. Boundary conditions at the corners of buildings.

Figure 1 also gives the conditions required beyond the extreme cornersof the building. In the case illustrated on the left of Figure 1, it might beconsidered some hardship that the boundary of the restricted area extendsbeyond the projection of the imperforate fire resistant side wall. This re-striction can be eliminated by ensuring that there are no window open-ings in the section CE of the adjoining wall.

The above measure has made use of two virtually self-apparent defini-tions. First, the equivalent building facade whose width and height will belooked up in either Table 2 or Table 3 will probably not coincide with theactual building facade. It is only necessary to include those openings thatwill be radiating freely during a fire. Thus, each story of the average build-ing will be separated from iLs neighbor by appropriately fire resistant con-struction and can be treated separately in the present context. Second,openings may be described as portions of the facade that might collapse andfall out during the course of a fire. Thus, any portion that does not meet

eoz.3rl ..

F I R E R E S I S T A N T W A L LN O O P E N I N G S

6 0 % D l

/-r sov.

Separation of Buildings 285

the integrity requirements associated with fire resistance considerations

will fall in this category. There is no call for it to meet any temperature

requirements.

A complicating feature to be catered for is that the exterior wall of a

building is often irregular in shape (Figure 2). In such cases, preliminary

considerations should refer to a line joining the extremities of the exterior

wall. Where the building is entirely contained behind this line no further

steps are required, for so far as radiation levels are concerned the irregular

external wall is closely represented by an imaginary one having the same

percentage window openings and located on the line referred to. Where a

portion of the building projects beyond the line, separation requirements

will be largely fulfilled by a composite boundary line as illustrated in Fig-

we 2. It is made up of a boundary line as calculated above, together with

one referring solely to the projecting portions of the building.

B U I L D I N G

I

B O U N D A R YL I M I T S

Figure 2. Boundary mnditions for irregularly shaped buildings.

Building codes usually discuss the location of a building with relation

to the lot line rather than to another building. It is difficult to see how this

type of specification can be soundly framed. The only practical suggestion

that has so far been conceived is that buildings should be separated from

their lot lines by half the distances derived according to the principles here

discussed. Where this rule is adopted for two adjacent buildings that are

mirror images of each other, the separation between the two will, in fact,

be appropriate. For dissimilar buildings, however, this will not be the case,

and the separation may be more than adequate if the one building catches

fire and less than adequate if the other ignites. It is doubtful whether this

B U I L D I N G

286 Fire Technology

incompatibility will ever be resolved. The only mitigating feature is that,in most cases, the building exposed to unnecessary hazard, will be thesmaller of the two - the greater the difference in size, the greater thehazard.

C O M P A R I S O N W I T H O T H E R W O R K

The Joint Fire Research Organization in the United Kingdom has de-

veloped recommendations concerning the spatial separation of buildingsalong the same lines as those described in this paper.a The choice of thelevel of radiation to be considered tolerable at an exposed building is thesanne, both being based on JFRO results.

In specifying the radiation levels to be expected from burning buildings,it is stated that the radiant contribution from flames issuing from windowsmay virtually be neglected. It is assumed that windows and other open-ings radiating at a temperature not exceeding 1,100'C will be the onlysources of radiation. In terms of configuration factors, the recommenda-tion is that calculations should normally be based on a value 0.075.

This value corresponds closely with the one used in this paper, althoughit is not claimed that separations based on the latter (together with theadditional 5 ft always included) will prevent the spread of fire unless firefighting is undertaken before the fire attains its peak. The British reportimplies that the separations wilI be adequate in their own right.

The British report also suggests that where fire loads are low, 5 lb/sq ftor less, much less stringent separations based on a configuration factor of0.15 are acceptable. It is probable that this relaxation is appropriate for

certain types of buildings now being constructed. Relaxation of the separa-tions suggested in the present paper could, in fact, conveniently be achievedwithout computing additional tables. By multiplying the percentage

window openings by a factor of 2, Lhe "normal" tables, instead of beingbased on a configuration factor of 0.07, would be based on one of 0.14 (with

the constant addition of 5 ft). The values thus obtained would correspondclosely to those given in the British table, except for the 5-ft addition re-ferred to.

The lowest value of percentage window opening available in the tablewould then be 2 X20% :4070, and it might be considered some hard-ship not to have lower values available. However, the use of lower valuesmight be somewhat dangerous. Separation values would be small andmight well become comparable to the dimensions between windows. Suchconditions would invalidate the use of the variable "percentage windowopening," which assumes a continuous distribution of very small windows.

A relaxation, such as the above, might be recommended where walllinings and the contents of a building have very low flammability ratingsand constitute a low fire load, say less than 5 lb/sq ft.

In the British report, the absence of especially stringent requirementswith regard to buildings that might burn extra vigorously would seem

Separation of Buildings

undersirable as far as Canadian conditions are concerned. The results

of the St. Lawrence Burns and the field investigation of the dwelling house

fire, here reported, emphasize the need for a stringent requirement here in

Canada.The British report states that "A wall clad with timber would be con-

sidered as an opening, since the burning timber would act as a source of

radiation . . ." The St. Lawrence Burns results suggest that the radiation

level from clapboard cladding can be neglected, provided the wall remains

intact and is fairly thick. A plane vertical sheet of thick timber will burn

vigorously only if it receives supporting radiation or convection on' its

front side, or alternatively, supporting conducted heat from the reverse

side.

R E F E R E N C E S

r "The St. Lawrence Burns," G. W. Shorter, J. H. McGuire, N. B. Hutcheon, andR. F. Legget, NFPA Quarterly, Vol. 53, No.4 (April 1960), pp. 300-316.

, "Radiation from Building Fires," R. C. Bevan and C. T. Webster, Investigationson Building Fires - Part III, National Building Studies Technical Paper No. 5, 1950.

3 "Heat Transfer by Radiation," J. H. McGuire, Fire Research Special ReportNo. 2, 1953.

a "Heat Radiation from Fires and Building Separation," M. Law, Joint Fire Re-search Organization Technical Paper No. 5, 1963.

287