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Effect of thermal insulation and heat sink on the structural fire
endurance of steel roof assemblies
National Research Conseil national
I
+
Council Canada de recherches CanadaEFFECT OF THERMAL INSULATION AND HEAT SINK
ON THE STRUCTURAL FIRE ENDURANCE OF
STEEL ROOF ASSEMBLIES
A ? J r v . m
by W. W. Stanzak and L. Konicek
Reprinted from
Canadian Journal of Civil Engineering Vol. 6, No. 1, March 1979
p. 32-35
DBR Paper No. 843
Division of Building Research
Effect of thermal insulation and heat sink on the structural fire endurance
of steel roof assemblies
W. W. STANZAK A N D L. KONICEK]
Fire Research Section. Division ofBuilding Research, National Research Council of Canada, Ottawa, Onr., Canada K I A OR6
Received May 17, 1978 Accepted November 1,1978
Designing for energy conservation in buildings calls for considerably better insulated roof structures than have been used and fire tested in the past. This requires that information be developed on the effect of such increased insulation on the temperature of structural steel and steel roof decks duringa fire exposure.
Fourteen small-scale fire tests are summarized that show that for combustible and noncombus- tible insulating materials a critical thickness exists beyond which additional insulation does not increase the temperatures developed on structural steel.
La conception des bitiments en fonction des imperatifs de conservation de I'energie impose le recours a des toitures mieux isolees que celles qui ont ete construites et mises a I'essaijusqu'ici. 11 faut dont realiser les recherches qui permettraient de determiner I'effet d e cette accroissement d'isolation thermique sur la temperature atteinte au cours d'un incendie par une charpente d'acier et un toit a pontage metallique.
En depouillant les resultats de 14 essais au feu realises a petite echelle, les auteurs ont trouve qu'il y a une epaisseur critique d'isolant combustible et non combustible au-dela d e laquelle un accroissement d e I'epaisseur d'isolant ne fait plus croitre la temperature
a
laquelle est portee la charpente.Can. J . Civ. Eng., 6.32-35 (1979) [Traduit par la revue]
Introduction cause heat transfer by conduction depends directly AS part of the energy conservation effort, more On temperature difference7 it can readily be aPPre-
roof insulation is being on contemporary ciated that events at the unexposed surface (such as
buildings. hi^ raises the question of whether and adding insulation) will not significantly hfluence in what manner the fire endurance of tested roof temperatures closer t o the exposed surface during the assemblies is affected by the added insulation. short duration of a typical fire test (1 or 2 h).
Figure 1 shows the typical temperature distribu- Thus, although basic considerations of heat trans-
tion through a steel-supported floor-ceiling assembly fer indicate that with increased insulation all tem- with 2.5 in. (63.5 mm) normal weight concrete top- peratures inside a structure will be higher, the highly ping (density
-
145 lb/ft3 (2324 kgIm3)) on 1.5 in. transient nature of a fire exposure suggests that (38 mm) cellular steel deck, during the course of a temperatures at or near the fire-exposed surfacestandard fire test. The ceiling membrane in this test be affected only minimally. *he present was
was 1 in. (25.4 mm) gypsum sanded plaster. An in- carried out to develop experimental data in support
sulated roof assembly would show a similar tempera- this thesis.
ture distribution pattern, but because the heat- Some of the available literature anticipates the re-
storing concrete topping is absent the time-scale on sults. In a series of tests on prestressed concrete roof the abscissa would be approximately double, i.e., assemblies with and without insulation, it is reported
the fire endurance time about one half. (Abrams 1976) that "a comparison of the data ob-
Examination of this typical temperature distribu- tained shows that use of the insulation did not result
tion shows that the largest temperature difference in higher strand temperatures. In fact, measurements
occurs between the furnace and the supporting steel that at and greater, average
(beam and deck). That is, it occurs through the pro- strand temperatures were consistently a few degrees
tective membrane ceiling. ~h~ temperature differ- less in the insulated specimen." Noting that the ence through the most massive portion of the as- prestressing strands are near the fire-exposed sur- sembly, the concrete topping, is relatively small. Be- face of the specimen, this experimental finding is consistent with the laws of heat transfer. (Abrams 'Present address: Department of Indian and Northern gives no explanation for this behaviour.)
Affairs, Ottawa, Ont., K I A 0H4. Also, earlier work (Stanzak and Harmathy 1968)
03 15- 1468179/010032-04$01.00/0
STANZAK AND KONICEK
0 10 20 30 40 50 60 70 80 90 100 110 I20 I30 140 150 TIME. MINUTES
FIG. 1 . Typical temperature distribution.
at the National Research Council of Canada clearly I
demonstrated that the heat storage capacity of an I
assembly has a very pronounced effect on the struc- tural fire endurance time. In a series of three fire tests, the moisture content of a sand-filled steel pan deck supported by a wide flange steel beam was 0, 5, and 1 0 x by weight. The fire endurance times (col- lapse) of the three otherwise identically protected
assemblies were 109, 135, and 180 min, respectively. 1 '
Fourteen fire tests were conducted to add to the information available. The factors under investiga- tion were the effects on the temperature of the steel deck of: (1) concrete heat sink; (2) addition of com- bustible insulation; and (3) addition of noncom-
bustible insulation. I
The fire tests were conducted in an electric furnace I
with the specimens oriented in a horizontal position. r-
Power input was controlled so that the furnace tem- 33"
perature closely followed that prescribed by two standards, ASTM E l 19-76 (American Society for
A
-
Testing and Materials 1976) and ULC-S101-1977(Underwriters' Laboratories of Canada 1977). Sketches of a typical test assembly and temperature measurement points are shown in Fig. 2. Details of the test specimens and results are presented in Table 1. In all cases the fire endurance time was
taken be that required an average FIG. 2. Plan and cross section of roof assembly with marked
C A N . J . CIV. ENG. VOL. 6.1979
TABLE 1. Details of test specimens and results obtained
Thermal resistance
Thickness of concrete of insulation (R)
fill (normal weight) Insulation (ftZ h "F/Btu) Fire enduranm
Series Membrane (in.) (in.) (mm) (mZ.K/W) (min)
1 Light* 0 0.5 (13) 2.28 (0.40) 65
(Flutes filled) Noncombustible
0.5 0.5 (13) 2.28 (0.40) 88 Noncombustible 1 .O 0.5 (13) 2.28 (0.40) 105 (R = 0.11) Noncombustible 1.5 0 . 5 (13) 2.28 (0.40) 109 Noncombustible 2.0 0.5 (13) 2.28 (0.40) 112 Noncombustible
2 3 in. (16 mm) 0 1 .O (25) 2.94 (0.52) Critical temperature
ceiling tile Combustible content not reached
0 1.5 (38) 4.40 (0.77) 124 combustible content 0 3.0 (76) 8.82 (1.55) 138 Combustible content 0 4.0 (100) 11.76 (2.07) 130 Combustible content
3 3 in. (16 mm) 0 0.5 (13) 2.28 (0.40) Critical temperature
ceiling tile Noncombustible not reached
0 1 .O (25) 4.56 (0.82) Critical temperature
Noncombustible not reached
0 2.0 (51) 9.12 (1.61) 140 Noncombustible 0 3 .O (76) 13.68 (2.41) 163 Noncombustible 0 4.0 (102) 18.24 (3.21) 217 Noncombustible Light* 0 3.0 (76)
-
60 Noncombustible Light 0 5.0 (127)-
60 Noncombustible*A sheet metal membrane with asbestos paper facing.
prescribed for unrestrained s p e c i ~ e n s in the two test standards previously cited. In evaluating the results it should be recognized t h a t structural steel supporting a steel deck will have its temperature during fire exposure influenced by a lesser degree than that of the steel deck. Therefore, the discussion and conclusions to follow apply to both structural steel and steel deck.
Discussion of Results
1. Figure 3 shows the effect of the heat sink pro- vided by normal weight concrete on the structural fire resistance of a steel deck. As can be seen, the increase in fire endurance time with thickness beyond
1 in. (25 mm) is minimal.
2. In the test with 1 in. (25 mm) combustible insula- tion, which burned through in less than 60 min, atmospheric cooling prevented failure of the steel; i.e., a quasi-steady-state condition with the steel at approximately 1000°F (538°C) existed for some time
and the test was terminated. With 1.5 in. (38 mm) insulation, it did not burn through quickly enough to prevent failure of the steel, which occurred at slightly over 2 h. As can be seen in Table 1, increasing the insulation beyond 1.5 in. (38 mm) had little further effect on the structural fire endurance of the steel.
3. With thin noncombustible insulation, sufficient cooling to the atmosphere occurred to prevent failure of the steel deck. Failure did occur at 140 min with 2 in. (51 mm) of insulation, and it can be assumed that a critical situation exists between a thickness of 1 and 2 in. (25 and 51 mm), where the fire endurance time was decreased because the cooling to atmo- sphere no longer had a significant effect on tempera- tures near the exposed surface of the assembly. Be- yond a certain (undetermined) critical point, signifi- cant increases in structural fire endurance were ob- tained with the thicker insulation. This situation is explained by the fact-that while the insulation has a low density, it also has some heat capacity.
STANZAK A N D K O N I C E K 35
C O N C R E T E T H I C K N E S S , i n c h e s
FIG. 3. Dependence of fire resistance on thickness of con-
crete topping.
Conclusions
1. Where concrete cover on a steel roof deck or other structural steel is 2 in. (51 mm) or greater, addition of materials to the unexposed surface has no significant effect on the structural fire endurance of the assembly.
2. For combustible insulations directly on a steel deck, a thickness can be found beyond which an in- crease of insulation has no significant effect on the structural fire endurance. This critical thickness of insulation depends on its charring rate under test conditions, as well as its physical properties at ele- vated temperatures. For the insulation tested, in-
creasing the thickness beyond 1.5 in. (38 mm) had no significant effect on the structural fire endurance.
3. For noncombustible insulations placed directly
on a steel deck, a thickness can be found beyond which further increase in thickness of insulation re- sults in a marked increase in structural fire endurance. The transition thickness depends on the thermal properties of the insulation and was about 1.5 in. (38 mm) for the material studied.
4. As a rule of thumb, it may be assumed that where the total thermal resistance of the material above the steel deck is 6 ft2 h "F/Btu (1.1 mZ.K/W) or greater, addition of other material, combustible or noncombustible, will not decrease the structural fire endurance. (This conclusion is based on a study of the temperature rise curvks of the steel deck, and is subject to refinement on the basis of additional fire test data.)
5. In general, the thermal storage capacity of an assembly has a greater influence on the structural fire endurance than the thermal resistance.
Acknowledgement
This paper is a contribution from the Division of Building Research, National Research Council of Canada and is published with the approval of the Director of the Division.
ABRAMS, M. S. 1976. Fire tests of hollow-core specimens with
and without roof insulation. Prestressed Concrete Institute Journal, 21, pp. 40-49.
AMERICAN SOCIETY FOR TESTING A N D MATERIALS. 1976. Fire
tests of building construction and materials. Philadelphia, PA, ASTM Designation E l 19-76.
STANZAK, W. W., and T. 2. HARMATHY. 1968. Effect ofdeckon
failure temperature of steel beams. Fire Technology, 4(4), pp. 265-270.
UNDERWRITERS' LABORATORIES OF CANADA. 1977. Standard
methods of fire endurance tests of building construction and materials. Scarborough, Ont., ULC-S101-1977.
