NBSIR 73-266

Fire Spread on Exterior Wails Due to

Flames Emerging from a Window in

Close Proximity to a Reentrant

Wall Corner

B. C. Son and J. B. Fang

Center for Building Teclinology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

April 1973

Final Report

Prepared for Office of Policy Development and Research

Department of Housing and Urban Development

Washington, D C. 20410

NBSIR 73-266

FIRE SPREAD ON EXTERIOR WALLS DUE TO

FLAMES EMERGING FROM A WINDOW IN CLOSE PROXIMITY TO A REENTRANT

WALL CORNER

B. C. Son and J. B. Fang

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards Washington, D. C. 20234

April 1973

Final Report

Prepared for Office of Policy Development and Research

Department of Housing and Urban Development

Washington, D. C. 20410

U. S. DEPARTMEiMT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director
' moo :
Table of Contents

Abstract 1

1.0 Introduction 1

2.0 Test Arrangement 2

3.0 Instrumentation 3

4.0 Test Results 4

Test Number 1 4

Test Number 2 5

5.0 Discussion and Conclusions 7

Appendix A - Log of Test Observations 9

Test Number 1 9

Test Number 2 10

Appendix B - References 13

Appendix C - SI Conversion Units 14

Figures

FIRE SPREAD ON EXTERIOR WALLS DUE TO FLAMES EMERGING FROM A WINDOW IN

CLOSE PROXIMITY TO A REENTRANT WALL CORNER

BY

B. C. Son and J. B. Fang

Abstract

As a part of the research program concerning the reconjmended criteria for

fire safety in Operation BREAKTHROUGH, two full scale fire tests were

performed on a mockup of a reentrant corner, i.e., the interior corner

formed at the intersection of the exterior walls of adjacent buildings,

such as townhouses and garden apartments.

In each test, two wall specimens representing exterior walls were erected

perpendicular to a wall containing a window opening into a fire room.

One wall was located 1 foot east and the other one 5 feet west of the

edges of the window. The objective of the reentrant corner fire test

was to study the potential ignition and spread of fire from the room to

an adjacent exterior combustible wall..

In the first test, charring on the east wall, but no surface ignition was

observed during the test. The peak temperature measured did not exceed

350°C (660°F) . In the second test, surface ignition occurred on the east

wall 9 minutes after the wood crib, representing the combustible contents

of the room, was ignited. No significant changes were observed on the

west wall during either test.

The instantaneous heat flux incident on the east wall just prior to

i
ignition and the total heat energy absorbed were estimated to be on the

2 2 order of 1.0 W/cm and 175 Joules/cm respectively.

Key Words: Exterior wall, fire spread, fire test, ignition, Operation

BREAKTHROUGH, reentrant corner.

ii
1.0 INTRODUCTION

In the Operation BREAKTHROUGH Program, for separate tenancy housing, a

flame spread index of not more than 75 was imposed on materials for

exterior, mutually perpendicular walls, one of which contains a window

opening, the edge of which is less than 5 feet from the adjacent wall.

Due to the lack of specific fire performance data for the reentrant wall

corner situation the flame spread index was conservatively set at a

value of 75 based on the Indices of representative exterior construction

materials. As a part of the research program to validate this recommended criteria for fire safety, two full scale fire tests were initially

performed at the Fire Test Laboratories of the National Bureau of

Standards, Washington, D.C., utilizing a representative exterior plywood facing material having a flame spread index of 103.

The objective of this reentrant wall corner fire testing was to study

the potential ignition and spread of fire on exterior combustible walls

due to flames emerging from a window opening in close proximity to an

adjacent wall. In such an arrangement, the burning rate of the combustible contents, ventilation conditions, the size and configuration of the

window, the distance from the window to the wall, the f lairanability of

the exterior siding, and environmental conditions will effect the ease

and extent of potential fire spread. The room fire, provided by the

burning of wood cribs, was intended to provide realistic levels of

radiation and convection on the exterior wall due to flames emerging

from the window.

1
2.0 TEST ARRANGEMENT

For these tests, a framework was erected for mounting wall specimens at

the required distance and orientation from a fire room containing a

single window. This test arrangement was located within a large building which represented essentially draft free conditions. The fire room

was 6 ft. wide by 10 ft. 5 in. long and 10 ft. high and had a window

opening located 3 ft. above the floor on the south wall (see figures

1 and 2). The window opening was 38 in. high and 3 in. deep; its

width was 24 in. for Test Number 1 and 32 in. for Test Number 2.

The fire room had three 6 in. by 12 in. low level openings for the

inlet of air supplied through a duct and a blower. The walls and

ceiling of the fire room, or burn room, were lined with one layer of

5/8-in. thick Type X gypsum board sprayed with vermiculite plaster.

The gypsum board on the ceiling was secured with 3/8-16 NC bolts,

1-1/2 in. long on 24 in. centers held with 3/8 in. expansion shells.

Gypsum boards on the side walls were secured with the same bolts on

16 in. centers. The floor in Test Number 1 was concrete. Since the

floor was damaged during Test Number 1, in Test Number 2 the floor

was covered with fire sand and then fire bricks were placed evenly

with a gap between each brick of 1/2-in. Exterior wall specimens were

erected on each side of the window opening (24 in. x 38 in. in Test

Number 1, 32 in. x 38 in. in Test Number 2) perpendicular to the

south wall of the fire room. One wall was located 1 ft. east, and

the other 5 ft. (in Test Number 1) and 4 ft. 4 in. (in Test Number 2)

west, of the edges of the window. Each wall was 12 ft. long and

2
16 ft. high and composed of a single layer of 1/2 in. thick A-C grade

exterior plywood in 4 x 8 ft. sheets over one layer of gypsum board

backing. The plywood, selected as a representative exterior wall facing

material, had a flame spread index of 103 as measured by ASTM E162.''''"'^

The burn room was loaded with wood cribs representing the fire load of

combustible contents. Fifteen wood cribs, equally spaced, were placed

on the floor in three rows along the north-south axis. Each wood crib

was made of multiple layers of nominal 2-in. by 2-in. by 14-in. long

kiln-dried wood sticks nailed into a lattice arrangement as shown in

figure 1. For Test Number 1 the cribs were five layers high and weighed

approximately 18 pounds each (4.4 pounds per square foot of fire load);

while in Test Number 2 the cribs were seven layers high and weighed

approximately 24 pounds each (6.3 pounds per square foot of fire load).

The 4.4 and 6.3 pounds per square foot of fire load are considered repre-

[21 sentative of the combustibles in a typical residential room condition

Liquid heptane in a sheet metal trough 90 in. long, 6 in. wide placed

under each row of wood cribs was used to ignite the wood.

3.0 INSTRUMENTATION

Instrumentation consisted of 48 thermocouples, two radiometers and a

radiation pyrometer. A total of 48 Chromel Alumel (Type K, No. 24 gage

0.020 in. diameter wire) thermocouples were installed as shown in figure 2.

3
Four ASTM* thermocouples mounted in protective 1/2 in. diameter wrought

iron pipes were located in the burn room and 44 thermocouples were on the

plywood surfaces, (32 on the east wall and 12 on the west wall). The

radiometer (Location A in figure 2) was mounted flush with the west wall

panel facing the east wall panel in order to measure the radiation from

the east wall. It was 5 ft. away from the plane of the window and even

with the top of the window. The radiation pyrometer (Location B in

figure 2) was centered facing the window at its mid-height and at a

distance of 10 ft. from the window.

The field of view for the particular lens of the pyrometer used at

10 ft. was 24 in. in diameter. In Test 2, an additional radiometer

(Location C in figure 2) was mounted in the west wall 5 in. from the

south wall of the fire room and at the same height as the top of the

window. It was used to measure the radiation from the flame exiting

from the window.

4.0 TEST RESULTS

Test Number 1

The complete log of test observations is given in Appendix I. In Test

Number 1 approximately 1540 cfm (cubic feet per minute) of air was supplied to the burn room starting at 1 min:30 sec after ignition. At

*The thermocouple was of the type used to measure the furnace temperature

during a fire test in accordance with Standard Methods of Fire Tests of

Building Construction and Materials (ASTM E119-70)[3J.

4
approximately 5 min. of test time some flame emerged from the window and

caused steaming and charring of the east wall plywood adjacent to the

window. As the test progressed, the flame projection from the window

became intermittent. No surface ignition was obtained on the plywood

for either the east or west wall.

Figure 3 is a plot of temperature isotherms on the east wall after

5 minutes. Similar plots at 10, 16 and 20 minutes are shown in figures

4, 5, and 6. The peak temperatures measured did not exceed 350°C

(660°F). Figures 7 and 8 are plots of temperature readings versus time

of the nine thermocouples on the east wall and of the six thermocouples

on the west wall, respectively.

The rate of burning of the wood cribs was estimated to be 14 pounds per

minute. For a heat of combustion of 7500 Btu/pound, the rate of heat

release was on the order of 1.0 x 10^ Btu/minute. The burn room timetemperature curve compared to the standard curve specified in ASTM

E119-70^ ^ is shown in Figure 9.

Test Number 2

Since flame projection beyond the burn room through the window in Test

Number 1 was less than anticipated, several changes were made in Test

Number 2. The fire load was increased from 4.4 pounds per square foot to

6.3 pounds per square foot. The air supply to the burn room was 1300 cfm

at the beginning of the test and increased to 1750 cfm at test time of

4 minutes. The burn room time- temperature curve compared to the standard

curve is shown in figure 10.
In this test surface ignition occurred at 9 minutes on the east wall.

The magnitude and progression of flaming is shown in the photograph

(figure 11) . Figures 12 and 13 are plots of temperature isotherms on the

east wall at 5 and 10 minutes after initiation of the test. No significant changes were observed on the west wall during the test. Figures 14

and 15 are plots of temperature readings versus time of the nine thermocouples on the east wall and of the six thermocouples on the west wall,

respectively.

The instantaneous heat flux absorbed by the east wall just prior to

ignition and the total heat energy absorbed, were estimated to be on the

2 2 order of 1.0 W/cm and 175 J/cm respectively. These values were

[41 calculated by a heat balance equation using an integral method based

on the assumption that the plywood wall could be represented as a semiinfinite slab with constant thermal properties, and by use of the experimental surface temperature data at the location where ignition occurred.

Figure 16 shows the temporal distributions of radiant flux and blackbody temperature levels measured at several locations and the average

fire room temperature plotted for comparison. It can be seen that

the blackbody temperature at the window opening and the projected flame

were considerably higher than the average room temperature during the

early stage of the fire test. This is due to the large thermal lag and

to conduction loss to the cold walls through the iron pipes mounting the

thermocouples in the fire room. As shown in Figure 16, the radiant flux,

6
monitored continuously by the two radiometers mounted on the west wall,

was mainly due to the flame emerging from the window opening and due to

flaming of the east wall. The intensity measured at Location C was found

2 to be 2.8 W/cm at the peak fire condition.

5.0 DISCUSSION AND CONCLUSIONS

Comparing figure 9 and figure 10, in the early stage of the tests the

fire room temperature in Test Number 2 is slightly lower than that in Test

Number 1. This difference in fire room temperatures, during the early

test stage, could have been created by variations in the wood crib stack

height, changed geometric configuration, radiation absorption and initial

heat release, etc.

Ignition of the exterior plywood panels located 1 ft. from the edge of

the window in a reentrant corner did not occur due to the burning of a

room fire load of 4.4 pounds per square foot of floor area. However,

surface charring occurred and peak surface temperatures of 350°C (660°F)

were measured. When the fire load was increased to 6.3 pounds per square

foot, and air was continuously introduced into the burn room, surface

ignition took place at 9 minutes on the plywood in the vicinity of the

window opening.

The incident irradiance and the total amount of heat energy absorbed at

the exposed surface of the east pljrwood wall prior to ignition were

2 2 estimated to be approximately 1.0 W/cm and 175 J/cm respectively.

7
Since there was no ignition of the exterior plywood (flame spread index103) on the west wall (4-1/2 to 5 feet from the opening) for either fire

load and since there was ignition of the plywood on the east wall (1 foot

from opening) for only the higher fire load it would appear that the

choice of a limiting flame spread index of 75 for the exterior combustible wall surface material in a reentrant wall corner configuration was a

sound technical judgment. However, to refine this index, additional

testing and analysis involving variations in the window to wall separation distance and wall materials would be required.

The ultimate goal of continued testing would be to develop a model of the

radiation and convective heat transfer from flames emerging from a window

to a wall of specified surface flammability properties and to calculate

the separation distances required to prevent ignition for any window and

wall arrangement.

8
APPENDIX A

Log of Test Observations

Test Number 1

Time Observations

min: sec

0:00 Start test. Ignition of heptane fuel,

1:00 Black smoke from the heptane fuel and

wood cribs. The wood cribs ignited.

1:30 Flame is rising up inside the fire room.

Air (1540 cfm) supplied to the fire room.

2:00 Flames starting to project from the fire

room window.

5:00 Flame is shooting out at an angle of

about 45° and the maximum length from the

window is approximately 4 ft.

5:30 Charring and steaming observed on the east

wall right above 0 line around No. 53

thermocouple

7:00 Flame projecting out from the window is

reduced.

9
10:00 A sudden large flame is shooting out of

window.

12:00 The pljrwood surface is buckling near

No. 53 thermocouple.

16:00 Most of the wood cribs have been consumed,

No visual change has been observed on the

west wall. Charring and sparking but no

flaming ignition has been observed on the

east wall surface.

END OF TEST

Time

min:sec

0:00

3:00 " \ < Wood cribs are burning severely.

A: 00 The air supply is increased to 1750

3, . feet /minute

10

Test Number 2

Observations

Ignition of heptane fuel. The blower is

turned on to supply 1300 cfm of air.
Flames start to project from the fire

room window.

Charring is observed on the east wall

around 2 foot line near thermocouple

No. 47.

Smoke and sparking is observed at the same

site. Flame from the window impinging on

the east wall.

Surface ignition is occurring over a wide

area of the east wall.

Fog spray is applied to extinguish the

fire on the east wall to prevent further

spread

The east wall catches fire again.

Extinguishment and reignition have

occurred several times.

Flaming inside the fire room diminishes.

11
The air blower is turned off. Most of the

wood cribs has been consumed. No visual

changes have been observed on the west wall.

END OF TEST

12
APPENDIX B

REFERENCES

"Standard Method of Test for Surface Flanmability of Materials

Using a Radiant Heat Energy Source," American Society for Testing

and Materials Designation E162-70 , available at 1916 Race Street,

Philadelphia, Pennsylvania 19103.

Fire-Resistance Classifications of Building Constructions, Building

Materials and Structures Report BMS 92 and BMS 149 , National Bureau

of Standards, Washington, D.C.

"Standard Methods of Fire Tests of Building Construction and

Materials," American Society of Testing and Materials Designation

E119-70 .

T. R. Goodman, "The Heat Balance Integral—Further Considerations

and Refinements," ASME Journal of Heat Transfer , Vol. 83, pp 83-86,

(1961).

13
APPENDIX C

SI Conversion Units

In view of present accepted practice in this country in this technological

area, common US units of measurement have been used throughout this paper.

In recognition of the position of the United States as a signatory to the

General Conference on Weights and Measurements which gave official status

to the metric SI system of units in 1960, we assist readers interested in

making use of the coherent system of SI units by giving conversion factors

applicable to US units used in this paper.

Length '. ' -

1 in = 0.0254 meter

1 f t = 0. 3048 meter

Mass

1 lb = 0.45 kilogram " '

Stress

2

1 psf = 47.88 newton/meter

2 1 psi = 0.332 newton/meter

1 plf = 13.49 newton/meter

Temperature

Temperature in °F = 9/5 (temperature in "C) + 32°F

Energy •

1 BTU = 252 cal = 1055 Joule

14
15

TEST NO. 1 5 MIN

Figure 3. Temperature Isotherms (Deg C) on the East Wall at 5 ninutes

test n am' l- r 1 .

17
TEST NO. 1

10 MIN

18
TEST NO. I

16 MIN
TEST NO. 1

20MIN

127 132 125" 109 100° 75° 70 57

175° 150° 125° 100° 75°

i'igura 6. Temperature Isotherms Ce-Q C) on the i.ast VJall at

20 minutes, tesL iium! er 1.

20

66 ,T) 67 .4, 6F .

Figure 8. 6 THERMOCOUPLE TEMPERATURES ON WEST WALL ADJACENT TO WINDOW

(test number 1). 22
1 100

1000_

900_

eoo_

700_

600_

500_

400_

r /

/

r /

/

/

/ /

/ /

300j / /

/

/ 200.

100.

' /

X

PIVERRGE FURMflCE TEMFERflTURE

- STHNDflRD El 19

^"-

"Yr "T7

TIDE (MINUTEj

16 18 1,

FIGURE 9. AVERAGE FIRE ROOM TEMPERATURE FOR TEST NO. 1(4.4 psf FIRE LOAD)

WITH STAj-nIDARD E 119.

23
1 100.-,

0 5 E I Vz "Ti Ti ^4

TIME (MINUTE)

FIGURE 10. AVERAGE FIRE ROOM TRMPERATURE FOR TEST NO. 2 (6. 3 psf FIRE LOAD) COMPARED

WITH STAiMDARD E 119. oa
The plywood exterior wall

ignited by flames emerging

from a window opening 9 min.

after start of test.number 2
TEST NO. 2

5MIN

26
TEST NO. 2

10 MIN

Figure 13. Te:nperature Isotherms (Deg C) on the east wall at 10 minutes,

test number 2.

27

FIGURE 15. 6 THERMOCOUPLE TEMPERATURES ON WEST WALL ADJACENT TO WINDOW

(test number ) . oq
TIME (MINUTE)

I'lCURK lb. TEMPORAL DISTRIBUTION OF RADIATIVE FLUXES MEASURED AT SEVERAL LOCATIONS

AND BLACKBODY TEMPERATURE OF THE WINDOW AND THE PROJECTED FLAMES FOR TEST

30
lil-^LlCC': ' Pi'lC DATA

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.!• AX I' M..in I I.H Fire Spread on Exterior Walls Due to j'lai.'x^s Eracrgi.ng f.roi: a Window in Close Proxiiuity to a J'.eentranl VJall CoiMier

7. ALM IlOlUS) B.C. Sen and J..13. Fanf;

9. P1;rF01U11\(, ORCANI/ATION NAMF AND AOIiI'^LSS

NA'I IONAL bUREAU Ol' STANDARDS

DHP AR TMl'N'I OF COMMHRCF

WASillNGTON, D.C. 20234

12. Sponsoring Oij-,.\n iz.it i(jn NarnL' and Address Oft ice of Policy Development and Research

Department of Housing and Urban Development

Washington, D.C. 20410

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13. Type of Report S; Period

Covcrc-d Final i

14. Spt>ii!:'-)rinf; A^;t.ncy Cov'e

15. SUPPLFMFN FARV NOTES

16. ABSTR.'\C'r (.A 200-word or less fLsotudl summary of rriost sij;n]ficant inforniat io n, If document, includes a signif ica-.!t I hiblioi?.r,ip!-,y or lircraturc survey, mention it here.) j As a part of the research program concerning the recommended criteria for fire safety | in Operation BREAKTHROUGH, two full scale fire tests were performed on a mockup of a reentrant corner, i.e., the interior corner formed at the intersection of the exterior

vjalls of adjacent buildings, such as tovjnhouses and garden apartments.

In each test, tv.-o wail specim.ens representing exterior walls v.^ere erected perpendicular

to a wall containing a window opening into a fire room.. One v/all was located 1 foot

east and the other one 5 feet west of the edges of the window. The objective of the

reentrant corner fire test was to study the potential ignition and spread of fire

from the room to an adjacent exterior combustible wall.

In the first test, charring on the east wall, but no surface ignition v/as observed

during the test. The peak temperature measured did not exceed 350'G (660°F) . In

the second test, surface ignition occurred on tlie east wall 9 minutes after the wood

crib, representing the comibustible contents of the room, was ignited. No significant

changes__ were observed on the west wall during either test.

The instantaneous heat flux incident on the east wall just prior to ignition and the

total heat energy absorbed were estim.ated to be on the order of 1.0 W/cra? and

175 Joules/cm^ respectively.

17. KF:^' \i,ORF)S (Alphabeticai order, sep-.nrted by - e:nicolon <) exterior wall; fire spread; fire test; ignition; Operation E.RE AKTHROUGE. ; reentrant

corner

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33

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