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Fire resistance of barriers in modelling fire spread
Fire Safety Journal 22 (1994) 399-407 1994 Elsevier Science Limited Printed in Northern Ireland 0379-7112/94/$07.00 ELSEVIER
Short Communication Fire Resistance of Barriers in Modelling Fire Spread D a v i d G. Platt Department of Civil Engineering, Queens Building, University of Bristol, Bristol, UK, BS8 1TR (Received 16 May 1991; 2nd revised version received and accepted 11 October 1993)
ABSTRACT This communication examines the findings from a survey of fire tests carried out in New Zealand. The survey compares the quoted Fire Resistance Ratings with the times at which the structures actually failed for different failure criteria. In particular, the differences between insulation and integrity failure for light timber-framed structures are discussed. The apparent imbalance between possible failure modes has implications for those involved in the analysis of structural fire safety.
INTRODUCTION
In the process of modelling fire spread one is often faced with the problem of having to compare the calculated severity of a compartment fire with the expected resistance of a structure. For example, let us assume that the severity of a compartment fire, expressed as the expected duration, is calculated to be 40 min using one of the standard formulae, 1"2 and that the fire resistance of the compartment walls is 60 min. These values represent the expected, or probable, values and will each have an associated probability density function. The problem of whether fire will spread through the barrier can be expressed by comparing the two density functions, as shown in Fig. 1. The probability of fire spreading through a barrier at a given time, say t, is the probability that the barrier has a fire resistance equal to t, and that the duration of the fire exceeds this value. Summing the probabilities for 399
400
David G. Platt
FireDuration
.o Time Fig. I.
Comparison of probability density function for fire duration and fire resistance.
all possible values of time gives the 'overall' probability that fire will spread through the barrier. To carry out such an analysis we need to know the expected or mean value and some measure of the spread of probable values. A measure of the spread is usually given in the form of a standard deviation or coefficient of variation. Although a knowledge of the form of the distribution is desirable, it is often unobtainable and standard distribution types are assumed, such as a log-normal or Weibull. The function of this discussion is to examine in more detail the values used for the expected fire resistance of a structure and the associated coefficient of variation. Of particular interest is the difficulty that arises in determining a clear value for the resistance of a structure because of the current convention used in testing. No attempt has been made in this discussion to look at the values used for the fire duration.
FIRE RESISTANCE TESTING Fire resistance tests for barriers are performed using one of the standard testing procedures such as BS 476: part 21 or ASTM E, 119. They are usually carried out on a 'one-off' basis, partly because of the high cost, but mainly because sponsors only require one successful test result to obtain a Fire Resistance Rating (FRR). As a result, there is little information on the expected variable for a particular type of barrier. The following review of values given by Elms and Buchanan 3 indicates the wide range of values that have been taken as representative for the coefficient of variation:
401
Fire resistance o f barriers
Elms and Buchanan 4 used a figure of 15% for the coefficient of variation, following a discussion with Denis Bastings and Hugh Baber at the Building Research Association of New Zealand. Ling and Williamson 5 report replicated fire tests on eleven identical clay tile walls for which the coefficient of variation was 38%. Lie 6 uses an average value of 40% in his study, varying it from 20% to 70% in a sensitivity study. Bender et al. 7 found a value of 8 to 12% in a computer model of glued laminated beams. White et al. s carried out replications of fire tests on unprotected timber joist floors, obtaining a value for coefficient of variation of 4% for low applied loads and 12% for high applied loads. Schneider et al. 9 quote values for concrete and steel structures in the range 12% to 20%. Mehaffey and Harmathy 1° suggest an upper limit of 15%. Elms and Buchanan also reviewed a series of tests that were performed for the New Zealand Fibrous Plaster Association. Their results followed a very strong trend of increasing F R R for increasing thickness, indicating a low coefficient of variation about the m e a n trend. The average values of test failure time were all approximately 1.25 times the F R R approved from the test results as shown in Fig. 2. A further survey of fire tests carried out in New Zealand indicated that the coefficient of variation is between 5% and 13%. The larger the approved F R R the smaller the coefficient of variation (see Table 1). The barriers tested were predominantly light timber-framed walls sheathed with gypsum plaster and other lightweight structures. The sample size used in this study was relatively s m a l l - - a r o u n d 40---and future work will be required to expand the scope.
~.0-
• - FRR Approvals • - Test Results
/
/ 4.0-
1/" ../"
3.0-
//"
2.0/. ! 1.0I 6
1 13
I 19
I 26
3I 2
Lining Thickness [mm]
Fig. 2. Fire resistance ratings for fibrous plaster walls, taken from Elms and Buchanan)
David G. Platt
402
TABLE 1 Statistical Fire Resistance Data
Approved FRR (min)
30 60 90 120
Mean test fire resistance (rain) 37.4 70.1 99.9 130-6
SD of test results
Coefficient of variation of test results
Ratio of test fire approved FRR
4.7 6.6 6-8 7.1
12-6 9-4 6-8 5.4
1-25 1.17 1-11 1.09
Most standards are designed so that the approved F R R is less than the actual time at which the structure fails. For example, with BS 476: part 21 the actual failure time is rounded down to the nearest approved F R R value, which could be ~, 1, 11, 2, 3, 4 or 6 h. The difference between the actual failure time and the approved F R R is shown in Fig. 3. The graph suggests that the mean values of test failure times lie between 1.09 and 1.25 times the approved FRR. The larger the approved F R R the smaller the fractional discrepancy between the test results. The effect of this is that the mean or expected fire resistance of the barrier in Fig. 1 is likely to be greater than the nominal resistance that is specified in the design brief of a building.
M O D E S OF F A I L U R E The New Zealand test failure results have been further subdivided into the times at which a particular failure occurred. A test specimen in a fire resistance test is said to have failed at the time when it fails to meet any one of the following criteria: • insulation: the unexposed face of the specimen must not increase in average temperature by more than 140 °C, • integrity: the specimen must remain in place and prevent the passage of flames or hot gases; • stability: a load-bearing specimen must continue to support the imposed load. At the Building Research Association New Zealand ( B R A N Z ) testing
403
Fire resistance of barriers
~. ~' ~ £ 30~ # /
luretime
equalto approvedFRR
n 30
u i n P 60 90 120 Approved FRR[minutes]
Fig.
3.
Fire test failure times compared with approved FRRs.
facility, fire tests are generally not terminated at the first mode of failure. Where possible, the tests are continued until an integrity or stability failure is recorded. Therefore, for the same sample of results it is possible to obtain a ratio of the approved F R R against mean test failure times, for each mode of failure. Such a comparison has been carried out for insulation and integrity failure modes. Stability failures have not been included in this comparison because the sample size of tests that failed by the stability criterion was very small. From the test results analysed, most of the non-load-bearing structures failed by the insulation criterion. In general, the mean insulation failure times are the same as those given in Table 1. The mean integrity failure times were from 1-4 times the approved FRR for low values of FRR to 1.1 times the approved FRR for high values of FRR. In general, this corresponds to an increase of 12 min from the approved FRR. These results are shown in Fig. 4.
~tegr
•ii
o
M
/" o
.
/
~ 60 / / eoftestfailuretime ~ ~ 1 /~//" equalto approvedFRR I~//
I
I
I
I
30
60
90
120
II
Approved FRR[minutes1
Fig.
4.
Actual fire test failure times for different modes of failure compared with approved FRRs.
404
David G. Platt
FIRE SPREAD RESULTING FROM FAILURE As pointed out above, most of the non-load-bearing structures failed by the insulation criterion. Such a mode of failure would only result in fire spread if the unexposed surface of the structure was in contact with a material which would ignite and support combustion at about 160°C (assuming an ambient temperature of 20°C and temperature rise of 140°C). As most c o m m o n materials will not spontaneously ignite at 160°C, ~ it seems likely that the most probable cause of fire spread will be an integrity or stability failure. Wallpaper, for example, requires a temperature around 470 °C to support spontaneous ignition. Traditionally, the apparently low value of 140 °C temperature rise for insulation failure has been justified on the assumption that if, for instance, bales of wool, cotton or sawdust were stored up against the barrier, then a temperature rise of 140°C could result in spontaneous combustion. However, Schwartz and Lie, ~ after conducting tests to investigate the unexposed surface temperature, concluded that the value of 140 °C was too low and suggested an average temperature rise of 222°C (400°F). In tests they found that cotton waste required a temperature of 371 °C to support glowing combustion and sawdust 357 °C. However, they did not examine raw wool. It seems reasonable to conclude that in most buildings it is unlikely that a barrier temperature of 160°C would result in spontaneous combustion of say wall-charts or book-shelves. Thus, in a real fire, the expected fire resistance may be greater than the approved Fire Resistance Rating. The actual fire resistance of the barrier will depend upon the material it is made of and its construction. With the predominantly light timber-framed walls analysed in New Zealand, an integrity failure usually occurred soon after the insulation failure. In such cases, the external temperature is unlikely to have reached a critical value sufficient to support spontaneous combustion. In these circumstances, it may be more appropriate to take the fire resistance of the barrier as that given by the integrity failure time. The situation is different when dealing with concrete or masonry walls. Almost entirely, these walls fail on the insulation criteria but unlike the gypsum plaster walls, which suffer severe internal damage, these walls may still offer significant reserves against an integrity or structural failure. In these cases, it is possible that the external temperature will rise to a value where it will support spontaneous combustion of nearby materials before an integrity or stability failure occurs. It is therefore not valid to extrapolate the previous results for such structures, as much higher insulation temperatures may become
Fire resistance of barriers
405
violated before an integrity or stability failure occurs. Fire test results on concrete and masonry walls still need to be investigated to determine typical variations between insulation, integrity and stability failures.
IMPLICATIONS F O R M O D E L L E R S Engineering models of fire spread are often used to predict the extent of fire spread within a building. As pointed out in the Introduction, these models are often based on a comparison of the expected fire severity with the fire resistance of a barrier or some other structure. This initial analysis of the F R R of the barriers would indicate that the expected values taken from fire tests are likely to underestimate the resistance of the barrier to the spread of flames. To compensate for this, the modeller could modify the quoted F R R of the barriers. In the case of lightweight timber-framed structures, the F R R may be increased by 12 min, as suggested by Fig. 4. In this situation this would avoid the problems associated with using a value of insulation failure that appears unbalanced when compared with the other failure criteria and the likely spread of fire. It may be argued that such a change is unwarranted, especially for larger values of FRR, given the level of uncertainty that still exists in other values used to calculate the overall fire safety. It is the author's opinion that changes of between 10 and 25% are significant enough to be included, given that their inclusion does not involve a significant increase in the complexity of the analysis. However, in the case of concrete masonry and other 'heavy' forms of construction, the situation is further complicated by the seemingly low value adopted for the insulation criteria, and requires further investigation.
A NEED FOR CHANGE This is certaintly not the first study to suggest that the balance between insulation, integrity and stability failure criteria does not appear to be well distributed, the insulation criterion being the more dominant mode of failure. A change in the testing standards and also a possible separation in the functions of a structure are recommended, such that a structure is given a measure of its structural performance and a measure of its performance in resisting fire spread. The intention of such change is a more rational approach to the design of fire-safe buildings. It may
406
David G. Platt
be argued that, from the engineering point of view, it is better to err on the side of safety and accept the built-in safety margins inherent within the current testing procedure and the wealth of knowledge that has accumulated around it. However, as the foundations of fire science expand and develop, then should not also the scientific and engineering methods that are based upon these foundations?
CONCLUSION The results of analysis on the failure times of lightweight timber barriers subjected to standard fire resistance tests indicate that the expected mode of failure is likely to be insulation. Such a mode of failure is unlikely to result in fire spread. A more realistic value for the fire resistance of a structure would be the expected integrity failure time. The analysis shows that integrity failure varies from 42 min for an insulation failure time of 30 min to 132 min for an insulation failure time of 120 min. It is r e c o m m e n d e d that further analysis be carried out that compares the failure times for a variety of structural forms such as concrete and masonry walls. It is also r e c o m m e n d e d that a review of the current testing requirements for insulation failure of barriers be carried out.
ACKNOWLEDGEMENT The Building Research Association of New Zealand is thanked for its financial support of this work and the provision of statistical data.
REFERENCES 1. CIB, Design guide for structural safety, Fire Safety Journal, 10(2) (1986). 2. Harmathy, T. Z., On the equivalent fire exposure. Fire and Materials, 11 (1987) 95-104. 3. Elms, D. G. & Buchanan, A. H., The effect of fire resistance ratings on likely fire damage in buildings. Department of Civil Engineering, Research Report 88/4, University of Canterbury, NZ, 1988. 4. Elms, D. G. & Buchanan, A. H., Fire resistance ratings spread analysis of buildings. BRANZ Research Report R35, New Zealand, 1988. 5. Ling, W. C. T. & Williamson, R. B., Using fire tests for qualitative risk analysis. A S T M S T P 762. American Society for Testing and Materials, Philadelphia, PA, 1982.
Fire resistance of barriers
407
6. Lie, T. T., Safety factors for fire loads. Canadian Journal of Civil Engineering, 6 (1979) 617-29. 7. Bender, D. A., Woeste, F. E., Schaffer, E. L. & Marx, C. M., Reliability formulation for the strength and fire endurance of glued laminated beams. US Forest Products Laboratory, Research Paper FP1 460, 1985. 8. White, R. H., Schaffer, E. L. & Woeste, F. E., Replicate fire endurance tests of an unprotected wood joist floor assembly. Wood and Fibre Science, 16(3) (1984) 374-90. 9. Schneider, U., Bub, H. & Kersken-Bradley, M., Structural fire protection levels for industrial buildings. ACI SP 80-6, Fire Safety of Concrete Structures, 1983. 10. Mehaffey, J. R. & Harmathy, T. Z., Failure probabilities of construction designed for fire resistance. Fire and Materials, 8 (1984) 96-104. 11. Schwartz, K. J. & Lie, T. T., Investigating the unexposed surface temperature criteria of Standard ASTM Ell9. Fire Technology, 21(3) (1985) 169-80.
Short Communication Fire Resistance of Barriers in Modelling Fire Spread D a v i d G. Platt Department of Civil Engineering, Queens Building, University of Bristol, Bristol, UK, BS8 1TR (Received 16 May 1991; 2nd revised version received and accepted 11 October 1993)
ABSTRACT This communication examines the findings from a survey of fire tests carried out in New Zealand. The survey compares the quoted Fire Resistance Ratings with the times at which the structures actually failed for different failure criteria. In particular, the differences between insulation and integrity failure for light timber-framed structures are discussed. The apparent imbalance between possible failure modes has implications for those involved in the analysis of structural fire safety.
INTRODUCTION
In the process of modelling fire spread one is often faced with the problem of having to compare the calculated severity of a compartment fire with the expected resistance of a structure. For example, let us assume that the severity of a compartment fire, expressed as the expected duration, is calculated to be 40 min using one of the standard formulae, 1"2 and that the fire resistance of the compartment walls is 60 min. These values represent the expected, or probable, values and will each have an associated probability density function. The problem of whether fire will spread through the barrier can be expressed by comparing the two density functions, as shown in Fig. 1. The probability of fire spreading through a barrier at a given time, say t, is the probability that the barrier has a fire resistance equal to t, and that the duration of the fire exceeds this value. Summing the probabilities for 399
400
David G. Platt
FireDuration
.o Time Fig. I.
Comparison of probability density function for fire duration and fire resistance.
all possible values of time gives the 'overall' probability that fire will spread through the barrier. To carry out such an analysis we need to know the expected or mean value and some measure of the spread of probable values. A measure of the spread is usually given in the form of a standard deviation or coefficient of variation. Although a knowledge of the form of the distribution is desirable, it is often unobtainable and standard distribution types are assumed, such as a log-normal or Weibull. The function of this discussion is to examine in more detail the values used for the expected fire resistance of a structure and the associated coefficient of variation. Of particular interest is the difficulty that arises in determining a clear value for the resistance of a structure because of the current convention used in testing. No attempt has been made in this discussion to look at the values used for the fire duration.
FIRE RESISTANCE TESTING Fire resistance tests for barriers are performed using one of the standard testing procedures such as BS 476: part 21 or ASTM E, 119. They are usually carried out on a 'one-off' basis, partly because of the high cost, but mainly because sponsors only require one successful test result to obtain a Fire Resistance Rating (FRR). As a result, there is little information on the expected variable for a particular type of barrier. The following review of values given by Elms and Buchanan 3 indicates the wide range of values that have been taken as representative for the coefficient of variation:
401
Fire resistance o f barriers
Elms and Buchanan 4 used a figure of 15% for the coefficient of variation, following a discussion with Denis Bastings and Hugh Baber at the Building Research Association of New Zealand. Ling and Williamson 5 report replicated fire tests on eleven identical clay tile walls for which the coefficient of variation was 38%. Lie 6 uses an average value of 40% in his study, varying it from 20% to 70% in a sensitivity study. Bender et al. 7 found a value of 8 to 12% in a computer model of glued laminated beams. White et al. s carried out replications of fire tests on unprotected timber joist floors, obtaining a value for coefficient of variation of 4% for low applied loads and 12% for high applied loads. Schneider et al. 9 quote values for concrete and steel structures in the range 12% to 20%. Mehaffey and Harmathy 1° suggest an upper limit of 15%. Elms and Buchanan also reviewed a series of tests that were performed for the New Zealand Fibrous Plaster Association. Their results followed a very strong trend of increasing F R R for increasing thickness, indicating a low coefficient of variation about the m e a n trend. The average values of test failure time were all approximately 1.25 times the F R R approved from the test results as shown in Fig. 2. A further survey of fire tests carried out in New Zealand indicated that the coefficient of variation is between 5% and 13%. The larger the approved F R R the smaller the coefficient of variation (see Table 1). The barriers tested were predominantly light timber-framed walls sheathed with gypsum plaster and other lightweight structures. The sample size used in this study was relatively s m a l l - - a r o u n d 40---and future work will be required to expand the scope.
~.0-
• - FRR Approvals • - Test Results
/
/ 4.0-
1/" ../"
3.0-
//"
2.0/. ! 1.0I 6
1 13
I 19
I 26
3I 2
Lining Thickness [mm]
Fig. 2. Fire resistance ratings for fibrous plaster walls, taken from Elms and Buchanan)
David G. Platt
402
TABLE 1 Statistical Fire Resistance Data
Approved FRR (min)
30 60 90 120
Mean test fire resistance (rain) 37.4 70.1 99.9 130-6
SD of test results
Coefficient of variation of test results
Ratio of test fire approved FRR
4.7 6.6 6-8 7.1
12-6 9-4 6-8 5.4
1-25 1.17 1-11 1.09
Most standards are designed so that the approved F R R is less than the actual time at which the structure fails. For example, with BS 476: part 21 the actual failure time is rounded down to the nearest approved F R R value, which could be ~, 1, 11, 2, 3, 4 or 6 h. The difference between the actual failure time and the approved F R R is shown in Fig. 3. The graph suggests that the mean values of test failure times lie between 1.09 and 1.25 times the approved FRR. The larger the approved F R R the smaller the fractional discrepancy between the test results. The effect of this is that the mean or expected fire resistance of the barrier in Fig. 1 is likely to be greater than the nominal resistance that is specified in the design brief of a building.
M O D E S OF F A I L U R E The New Zealand test failure results have been further subdivided into the times at which a particular failure occurred. A test specimen in a fire resistance test is said to have failed at the time when it fails to meet any one of the following criteria: • insulation: the unexposed face of the specimen must not increase in average temperature by more than 140 °C, • integrity: the specimen must remain in place and prevent the passage of flames or hot gases; • stability: a load-bearing specimen must continue to support the imposed load. At the Building Research Association New Zealand ( B R A N Z ) testing
403
Fire resistance of barriers
~. ~' ~ £ 30~ # /
luretime
equalto approvedFRR
n 30
u i n P 60 90 120 Approved FRR[minutes]
Fig.
3.
Fire test failure times compared with approved FRRs.
facility, fire tests are generally not terminated at the first mode of failure. Where possible, the tests are continued until an integrity or stability failure is recorded. Therefore, for the same sample of results it is possible to obtain a ratio of the approved F R R against mean test failure times, for each mode of failure. Such a comparison has been carried out for insulation and integrity failure modes. Stability failures have not been included in this comparison because the sample size of tests that failed by the stability criterion was very small. From the test results analysed, most of the non-load-bearing structures failed by the insulation criterion. In general, the mean insulation failure times are the same as those given in Table 1. The mean integrity failure times were from 1-4 times the approved FRR for low values of FRR to 1.1 times the approved FRR for high values of FRR. In general, this corresponds to an increase of 12 min from the approved FRR. These results are shown in Fig. 4.
~tegr
•ii
o
M
/" o
.
/
~ 60 / / eoftestfailuretime ~ ~ 1 /~//" equalto approvedFRR I~//
I
I
I
I
30
60
90
120
II
Approved FRR[minutes1
Fig.
4.
Actual fire test failure times for different modes of failure compared with approved FRRs.
404
David G. Platt
FIRE SPREAD RESULTING FROM FAILURE As pointed out above, most of the non-load-bearing structures failed by the insulation criterion. Such a mode of failure would only result in fire spread if the unexposed surface of the structure was in contact with a material which would ignite and support combustion at about 160°C (assuming an ambient temperature of 20°C and temperature rise of 140°C). As most c o m m o n materials will not spontaneously ignite at 160°C, ~ it seems likely that the most probable cause of fire spread will be an integrity or stability failure. Wallpaper, for example, requires a temperature around 470 °C to support spontaneous ignition. Traditionally, the apparently low value of 140 °C temperature rise for insulation failure has been justified on the assumption that if, for instance, bales of wool, cotton or sawdust were stored up against the barrier, then a temperature rise of 140°C could result in spontaneous combustion. However, Schwartz and Lie, ~ after conducting tests to investigate the unexposed surface temperature, concluded that the value of 140 °C was too low and suggested an average temperature rise of 222°C (400°F). In tests they found that cotton waste required a temperature of 371 °C to support glowing combustion and sawdust 357 °C. However, they did not examine raw wool. It seems reasonable to conclude that in most buildings it is unlikely that a barrier temperature of 160°C would result in spontaneous combustion of say wall-charts or book-shelves. Thus, in a real fire, the expected fire resistance may be greater than the approved Fire Resistance Rating. The actual fire resistance of the barrier will depend upon the material it is made of and its construction. With the predominantly light timber-framed walls analysed in New Zealand, an integrity failure usually occurred soon after the insulation failure. In such cases, the external temperature is unlikely to have reached a critical value sufficient to support spontaneous combustion. In these circumstances, it may be more appropriate to take the fire resistance of the barrier as that given by the integrity failure time. The situation is different when dealing with concrete or masonry walls. Almost entirely, these walls fail on the insulation criteria but unlike the gypsum plaster walls, which suffer severe internal damage, these walls may still offer significant reserves against an integrity or structural failure. In these cases, it is possible that the external temperature will rise to a value where it will support spontaneous combustion of nearby materials before an integrity or stability failure occurs. It is therefore not valid to extrapolate the previous results for such structures, as much higher insulation temperatures may become
Fire resistance of barriers
405
violated before an integrity or stability failure occurs. Fire test results on concrete and masonry walls still need to be investigated to determine typical variations between insulation, integrity and stability failures.
IMPLICATIONS F O R M O D E L L E R S Engineering models of fire spread are often used to predict the extent of fire spread within a building. As pointed out in the Introduction, these models are often based on a comparison of the expected fire severity with the fire resistance of a barrier or some other structure. This initial analysis of the F R R of the barriers would indicate that the expected values taken from fire tests are likely to underestimate the resistance of the barrier to the spread of flames. To compensate for this, the modeller could modify the quoted F R R of the barriers. In the case of lightweight timber-framed structures, the F R R may be increased by 12 min, as suggested by Fig. 4. In this situation this would avoid the problems associated with using a value of insulation failure that appears unbalanced when compared with the other failure criteria and the likely spread of fire. It may be argued that such a change is unwarranted, especially for larger values of FRR, given the level of uncertainty that still exists in other values used to calculate the overall fire safety. It is the author's opinion that changes of between 10 and 25% are significant enough to be included, given that their inclusion does not involve a significant increase in the complexity of the analysis. However, in the case of concrete masonry and other 'heavy' forms of construction, the situation is further complicated by the seemingly low value adopted for the insulation criteria, and requires further investigation.
A NEED FOR CHANGE This is certaintly not the first study to suggest that the balance between insulation, integrity and stability failure criteria does not appear to be well distributed, the insulation criterion being the more dominant mode of failure. A change in the testing standards and also a possible separation in the functions of a structure are recommended, such that a structure is given a measure of its structural performance and a measure of its performance in resisting fire spread. The intention of such change is a more rational approach to the design of fire-safe buildings. It may
406
David G. Platt
be argued that, from the engineering point of view, it is better to err on the side of safety and accept the built-in safety margins inherent within the current testing procedure and the wealth of knowledge that has accumulated around it. However, as the foundations of fire science expand and develop, then should not also the scientific and engineering methods that are based upon these foundations?
CONCLUSION The results of analysis on the failure times of lightweight timber barriers subjected to standard fire resistance tests indicate that the expected mode of failure is likely to be insulation. Such a mode of failure is unlikely to result in fire spread. A more realistic value for the fire resistance of a structure would be the expected integrity failure time. The analysis shows that integrity failure varies from 42 min for an insulation failure time of 30 min to 132 min for an insulation failure time of 120 min. It is r e c o m m e n d e d that further analysis be carried out that compares the failure times for a variety of structural forms such as concrete and masonry walls. It is also r e c o m m e n d e d that a review of the current testing requirements for insulation failure of barriers be carried out.
ACKNOWLEDGEMENT The Building Research Association of New Zealand is thanked for its financial support of this work and the provision of statistical data.
REFERENCES 1. CIB, Design guide for structural safety, Fire Safety Journal, 10(2) (1986). 2. Harmathy, T. Z., On the equivalent fire exposure. Fire and Materials, 11 (1987) 95-104. 3. Elms, D. G. & Buchanan, A. H., The effect of fire resistance ratings on likely fire damage in buildings. Department of Civil Engineering, Research Report 88/4, University of Canterbury, NZ, 1988. 4. Elms, D. G. & Buchanan, A. H., Fire resistance ratings spread analysis of buildings. BRANZ Research Report R35, New Zealand, 1988. 5. Ling, W. C. T. & Williamson, R. B., Using fire tests for qualitative risk analysis. A S T M S T P 762. American Society for Testing and Materials, Philadelphia, PA, 1982.
Fire resistance of barriers
407
6. Lie, T. T., Safety factors for fire loads. Canadian Journal of Civil Engineering, 6 (1979) 617-29. 7. Bender, D. A., Woeste, F. E., Schaffer, E. L. & Marx, C. M., Reliability formulation for the strength and fire endurance of glued laminated beams. US Forest Products Laboratory, Research Paper FP1 460, 1985. 8. White, R. H., Schaffer, E. L. & Woeste, F. E., Replicate fire endurance tests of an unprotected wood joist floor assembly. Wood and Fibre Science, 16(3) (1984) 374-90. 9. Schneider, U., Bub, H. & Kersken-Bradley, M., Structural fire protection levels for industrial buildings. ACI SP 80-6, Fire Safety of Concrete Structures, 1983. 10. Mehaffey, J. R. & Harmathy, T. Z., Failure probabilities of construction designed for fire resistance. Fire and Materials, 8 (1984) 96-104. 11. Schwartz, K. J. & Lie, T. T., Investigating the unexposed surface temperature criteria of Standard ASTM Ell9. Fire Technology, 21(3) (1985) 169-80.