Page 179 - ICC IEBC 2018

RESOURCE A

perature increases of 250°F (121°C) above ambient or 325°F units are essentially the same as item W-8-M-82. Fire endur-
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(163°C) above ambient at any one place with the membrane ance: 3 / hours.
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being exposed to the fire. The temperature is measured at the These walls show 1 hour greater fire endurance by the
interface of the assembly and the protective membrane.
addition of the 2-inch (51 mm) air space.
Rule 2: The fire endurance of a construction does not Rule 4: The farther an air gap or cavity is located from the
decrease with the addition of further layers.
exposed surface, the more beneficial is its effect on the fire
Harmathy notes that this rule is a consequence of the pre- endurance.
vious rule. Its validity follows from the fact that the addi- Radiation dominates the heat transfer across an air gap or
tional layers increase both the resistance to heat flow and the cavity, and it is markedly higher where the temperature is
heat capacity of the construction. This, in turn, reduces the higher.
rate of temperature rise at the unexposed surface.
The air gap or cavity is thus a poor insulator if it is located
This rule is not just restricted to “thermal” performance in a region which attains high temperatures during fire expo-
but affects the other fire test criteria: direct flame passage, sure.
cotton waste ignition, and load bearing performance. This
means that certain restrictions must be imposed on the mate- Some of the clay tile designs take advantage of these fac-
rials to be added and on the loading conditions. One restric- tors. The double cell design, for instance, ensures that there is
tion is that a new layer, if applied to the exposed surface, a cavity near the unexposed face. Some floor/ceiling assem-
must not produce additional thermal stresses in the construc- blies have air gaps or cavities near the top surface and these
tion, i.e., its thermal expansion characteristics must be similar enhance their thermal performance.
to those of the adjacent layer. Each new layer must also be Rule 5: The fire endurance of a construction cannot be
capable of contributing enough additional strength to the increased by increasing the thickness of a completely
assembly to sustain the added dead load. If this requirement is enclosed air layer.
not fulfilled, the allowable live load must be reduced by an Harmathy notes that there is evidence that if the thickness
amount equal to the weight of the new layer. Because of these of the air layer is larger than about / inch (12.7 mm), the
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limitations, this rule should not be applied without careful heat transfer through the air layer depends only on the tem-
consideration. perature of the bounding surfaces, and is practically indepen-
Particular care must be taken if the material added is a dent of the distance between them. This rule is not applicable
good thermal insulator. Properly located, the added insulation if the air layer is not completely enclosed, i.e., if there is a
could improve the “thermal” performance of the assembly. possibility of fresh air entering the gap at an appreciable rate.
Improperly located, the insulation could block necessary ther- Rule 6: Layers of materials of low thermal conductivity
mal transmission through the assembly, thereby subjecting are better utilized on that side of the construction on which
the structural elements to greater temperatures for longer fire is more likely to happen.
periods of time, and could cause premature structural failure
of the supporting members. As in Rule 4, the reason lies in the heat transfer process,
though the conductivity of the solid is much less dependent
Rule 3: The fire endurance of constructions containing on the ambient temperature of the materials. The low thermal
continuous air gaps or cavities is greater than the fire endur- conductor creates a substantial temperature differential to be
ance of similar constructions of the same weight, but contain- established across its thickness under transient heat flow con-
ing no air gaps or cavities. ditions. This rule may not be applicable to materials undergo-
By providing for voids in a construction, additional resis- ing physico-chemical changes accompanied by significant
tances are produced in the path of heat flow. Numerical heat heat absorption or heat evolution.
flow analyses indicate that a 10 to 15 percent increase in fire Rule 7: The fire endurance of asymmetrical constructions
endurance can be achieved by creating an air gap at the mid- depends on the direction of heat flow.
plane of a brick wall. Since the gross volume is also increased
by the presence of voids, the air gaps and cavities have a ben- This rule is a consequence of Rules 4 and 6, as well as
eficial effect on stability as well. However, constructions other factors. This rule is useful in determining the relative
containing combustible materials within an air gap may be protection of corridors and walls enclosing an exit stairway
regarded as exceptions to this rule because of the possible from the surrounding spaces. In addition, there are often situ-
development of burning in the gap. ations where a fire is more likely, or potentially more severe,
from one side or the other.
There are numerous examples of this rule in the tables. For Rule 8: The presence of moisture, if it does not result in
instance:
explosive spalling, increases the fire endurance.
Table 1.1.4; Item W-8-M-82: Cored concrete masonry, The flow of heat into an assembly is greatly hindered by
nominal 8 inch thick wall with one unit in wall thickness and the release and evaporation of the moisture found within
with 62 percent minimum of solid material in each unit, load cementitious materials such as gypsum, portland cement, or
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bearing (80 PSI). Fire endurance: 2 / hours.
2 magnesium oxychloride. Harmathy has shown that the gain in
Table 1.1.5; Item W-10-M-11: Cored concrete mansonry, fire endurance may be as high as 8 percent for each percent
nominal 10 inch thick wall with two units in wall thickness (by volume) of moisture in the construction. It is the moisture
and a 2-inch (51 mm) air space, load bearing (80 PSI). The chemically bound within the construction material at the time

160 2018 INTERNATIONAL EXISTING BUILDING CODE ®
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