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Technical Notes 16B - Calculated
Fire Resistance
[June 1991] (Reissued Aug. 1991)
Abstract: Fire-resistance periods for building components are
normally determined by physical tests conducted according to ASTM
E 119 Standard Methods of Fire Tests of Building Construction and
Materials. This Technical Notes addresses analytical
methods to determine fire ratings for building construction not
specifically tested under ASTM E 119. Described are procedures for
calculating the fire resistance of clay masonry walls including:
1) effects of plaster; 2) effects of air spaces; 3) multi-wythe
construction; and 4) equivalent thickness for hollow masonry
units.
Key Words: brick, building codes, fire ratings, fire resistance period.
INTRODUCTION
Fire resistance is determined by standard test methods and used by model building codes to provide both life safety and property protection. Standards to establish fire resistance are developed and adopted by a consensus group. Model codes contain mandatory requirements which local jurisdictions adopt as law to govern building design and construction.
Fire resistance is a property of all types of building construction and is related to whether the materials are combustible or non-combustible. Clay masonry is a non-combustible material and has excellent fire resistance in building construction. Accepted practice has been to determine the fire resistance of clay masonry walls by ASTM E 119 Method for Fire Tests of Building Construction and Materials.
There are some instances when a particular type of building construction has not been physically tested by the ASTM E 119 test method. This Technical Notes provides an analytical method to determine the fire rating for building construction not specifically tested by ASTM E 119. This method is commonly known as the calculated fire resistance. This Technical Notes, the third in this series, concentrates on calculated fire resistance as it applies to model building code requirements. Other Technical Notes in this series address the ASTM E 119 test method and fire resistance ratings for bearing wall applications.
DEFINITIONS
To fully understand the calculated fire resistance procedure, two terms must be defined. These two definitions will be referenced throughout this Technical Notes.
Fire Resistance Period: the property of a material or assembly to withstand fire or give protection from it for a specific period of time. As applied to elements of buildings, fire resistance is characterized by the ability to confine a fire, to continue to perform a given structural function, or both.
Fire Rating: a time required, usually expressed in the lowest full hour, for an element in a building to maintain its particular fire resistance properties. Model building codes establish the required fire ratings for various building elements.
THEORY
The resistance of clay masonry walls to fire is a well established fact and has been found to be a function of wall mass or thickness. Fire resistance tests have been conducted on walls of solid and hollow clay units. During the ASTM E 119 fire test, the fire resistance of clay masonry walls is usually established by the temperature rise on the unexposed side of the wall specimen. Few masonry walls have failed due to loading or thermal shock of the hose stream.
The method of calculating fire resistance periods is described in NBS BMS 92, "Fire Resistive Classifications of Building Construction", National Bureau of Standards, 1942. The construction must be similar to others for which the fire resistance periods are known, or of composite constructions for which the fire resistance periods of various components are known. Regarding the derivation of the general calculated fire resistance formulae, the authors state:
"In most cases the fire resistance period will be determined by the temperature rise on the unexposed side of the wall, and it is on this criterion that the following method of interpolation and extension is based.
According to the general theory of heat transmission, if walls of the same material are exposed to a heat source that maintains a constant temperature of the surface of the exposed side, and the unexposed side is protected against heat loss, the time at which a given temperature will be attained on the unexposed side will vary as the square of the wall thickness.
In the standard fire test, which involves specified conditions of temperature measurement and a fire that increases the temperature at the exposed surface of the wall as the test proceeds, the time required to attain a given temperature rise on the unexposed side will be different from where the temperature on the exposed side remains constant at the initial exposure temperature for any period. It has been found that comparisons fairly consistent with test results can be obtained by assuming the variation to be according to some lower power of n than the second. The fire resistance period of the wall can be then expressed by the formula:
R = (cV)n Eq. 1
where: R = fire resistance period, hr
c = coefficient depending on the material, design of the wall, and the units of measurement of R and V V = volume of solid material per unit area of wall surface, and
n = exponent depending on the rate of increase of temperature at the exposed face of the wall
For walls of a given material and design, it was found that an increase of 50 percent in volume of solid material per unit area of wall surface resulted in a 100 percent increase in the fire resistance period. This relationship gives a value of 1.7 for n. The lower value for n as compared with 2 for the theoretical condition of constant temperature of the exposed surface is to be expected as the rising temperature at the exposed surface would tend to shorten the fire resistance period of walls qualifying for relatively higher ratings."
The fire resistance period of a wall may be expressed in terms of the fire resistance of the conjoined wythes of the wall as follows:
"If R1, R2, R3, etc. = fire resistance periods of walls (or component laminae of walls) having volumes of solid material per unit area of wall surface of V1, V2,V3,etc., respectively, also letting c and n be defined as above, then for walls in general, R1 = (c1V1)n, R2 = (c2V2)n, and R3= (c3V3)n."
The fire resistance period of the composite wall using the form of Eq. 1 will be:
R = (ctVt)n, Eq. 2
where: Vt = V1 + V2 + V3
and Ct = (c1V1 + c2V2 + C3V3) / V
therefore, R = (c1V1 + c2V2 + c3V3)n
= (R11/n + R21/n + R31/n)1.71 Eq. 3
Substituting 1.7 for n and 0.59 for 1/n, the general calculated fire resistance formula becomes:
R = (R10.59 + R20.59 + R30.59 ... + Rn0.59)1.7 Eq. 4
where: R1, R2, and R3 ... R1 = known fire resistance periods of the component laminae, in hr
The calculated fire resistance has been expressed in terms of the fire resistance periods of the component laminae of the wall, which need not be of the same material and design.
For walls of similar materials but of different thicknesses, the formula can be adjusted to the following form:
R2 = R1 (V2 / V1)1.7 Eq.5
where V1 and V2 are the volumes of solid materials per unit area of wall surface, and R2 and R1 are the corresponding fire resistance periods.
Using this theory, the fire resistance period of a wall assembly can be determined from the fire resistance periods of the component laminae. Either the fire resistance periods determined by fire tests or the fire rating established from the fire tests can be used. Use of actual fire resistance periods determined by tests will provide more accurate results than the use of fire ratings. This is because fire ratings are based on the lowest full hour achieved. As an explanation, a wall with a fire resistance period end point of 2 hours and 40 minutes will only attain a fire rating of 2 hours. Further, many fire tests of clay masonry walls are terminated once the wall attains the desired fire rating rather than continuing the fire test to its end point. The calculated fire resistance, using either the fire resistance period or fire rating, can thus be used to verify that the wall assembly equals or exceeds the fire rating required by the building code enforced in the area in question.
CALCULATED FIRE RESISTANCE
Multi-Wythe Construction
The theory behind calculated fire resistance indicates that a multi-wythe wall, i.e., a wall consisting of two or more dissimilar materials, has a greater fire resistance than a simple summation of the fire resistance periods of the various layers. Equation 4 developed from NBS BMS 92 yields a calculated fire resistance if the fire resistance periods for each dissimilar material are known. The fire resistance periods can be used to determine the total fire resistance of a multi-wythe wall composed of a combination of concrete, concrete masonry or clay masonry. The fire rating of the multi-wythe wall will be the maximum number of full hours thus determined.
Tables 1 through 5 are extracted from known fire resistance tests performed under ASTM E 119. Tables 1, 2, 4, 5 and 7 can be used to calculate fire resistance period for walls constructed of a combination of clay masonry, concrete masonry or concrete walls.
1Units shall comply with the requirements of ASTM C 62, C 126, C 216 or C 652.
21 in. = 25.4 mm.
3A 9 in. wall has a 120 min rating if the hollow spaces near combustible members are filled with fire resistance materials for the full thickness of the wall and for at least 4 in. above and below and between the combustible members.
4Units shall comply with the requirements of ASTM C 34.
1units shall comply with the requirements of ASTM C 34, C 56, C 212 or C 530.
21 in. = 25.4 mm.
3Ratings are for dense hard-burned clay or shale.
4Cells filled with blue, stone, slag, cinders or sand mixed with mortar.
5Ratings are for medium-burned clay tile.
1Equivalent thickness is the average thickness of the solid material in the wall. It is found by taking the total volume of a wall unit, subtracting the volume of core spaces, and dividing this by the area of the exposed face of the unit.
2Values between those shown in the table can be determined by direct interpolation.
3Where combustible members are framed into the wall, the thickness of solid material between the end of each member and the opposite face of the wall, or between members set in from opposite sides, shall not be less than 93% of the thickness shown in the table.
4Units shall comply with the requirements of ASTM C 55, C 73, C 90 or C 145.
5Extracted from the 1991 Edition of the Standard Building Code, Southern Building Code Congress International, Table 3103.1.
Effects of Plaster
Sanded gypsum plaster applied to either face of a clay masonry wall will increase the fire resistance period of the wall. Tests of solid clay masonry unit walls show the effect of one coat of plaster on the fire exposed side of a specimen to be approximately the same as for one coat of plaster on the unexposed side. No test have been performed with plaster on only the unexposed side of hollow clay tile walls, but there is no reason to suspect performance different from that of solid clay masonry walls.
The calculated fire resistance formula to include the effect of sanded gypsum plaster on either one or two sides becomes:
R = (Rn0.59 + pl)1.7 Eq. 6
where: R = calculated fire resistance of the assembly, hr
Rn = fire rating of individual wythe, hr pl = thickness coefficient of sanded gypsum plaster
The thickness coefficients of plaster for use in the formula for determining the calculated fire resistance of plastered clay masonry walls are taken from NBS BMS 92. The constants were derived from available test results. The average thickness of plaster applied in the series of tests ranged from 1/2 inch (12.7mm) to 3/4 inch (19.1mm). These thicknesses are most likely to be used in new building construction or are found to be applied on existing walls in a building where the fire rating is not known.
Values of pl for use in Equation 6 are given in Table 6. Thickness coefficients of sanded gypsum plaster should be selected based on the actual thickness of plaster applied to the wall and whether one or two sides of the wall is plastered. When using Equation 6, values for R1 must be hourly ratings. Thickness coefficients for other types of plaster materials have not been determined to date.
Fire Resistance Periods for Concrete Walls (minutes)
1Extracted from the 1991 Edition of the Standard Building Code, Southern Building Code Congress International, Table 3102.1B.
2The fire resistance period for this thickness exceeds 240 minutes.
3Dry unit weight of 35 pcf or less and consisting of cellular, perlite or vermiculite concrete.
Effects of Air Spaces
A continuous air space separating wythes of masonry will increase the calculated fire resistance of masonry walls. A continuous air space can occur between wythes of masonry or between masonry and concrete. The calculated fire resistance formula then becomes:
R = (R10.59 + R20.59 + ...Rn0.59 + as)1.7 Eq. 7
where: R = calculated fire resistance of the assembly, hr
R1, R2 and R1 = fire rating of the individual wythes, hr as = coefficient for continuous air space
NBS BMS 92 indicates that a continuous air space from 1/2 inch (12.7 mm) to 3 1/2 inch (89 mm) may be estimated by the use of a value 0.3 for each continuous air space. Thus a value of 0.6 would be used in multi-wythe wall with two continuous air spaces. Values for R1 in Equation 7 must be in hours.
Hollow Clay Masonry Walls
Although many fire tests on hollow clay masonry walls have been conducted, it would be virtually impossible to test all combinations of unit size and shape used in construction. Fire resistance periods can be readily obtained or estimated from the unit's equivalent thickness. It is accepted practice to determine the fire resistance of concrete masonry units based on the type of aggregate used to manufacture the units and the equivalent thickness of solid material in the wall. Recent developments in the clay masonry industry have led to the use of the equivalent thickness method for determining the fire resistance of hollow clay masonry units which conform to ASTM C 652 Standard Specification for Hollow Brick Made From Clay or Shale. The equivalent thickness method permits the determination of fire resistance period of hollow clay units that may not have been physically tested under ASTM E 119. The fire resistance periods for hollow clay masonry units included in Table 7 are based on the equivalent thickness principle.
1Values listed are for 1:3 sanded gypsum plaster.
1Equivalent thickness is the average thickness of solid material in the wall. It is found by taking the total volume of a wall unit, subtracting the volume of core or cell spaces and dividing by the area of the exposed face of the unit. 2Values between those shown in the table can be determined by direct interpolation.
3Where combustible members are framed in the wall, the thickness of solid material between the end of each member and the opposite face of the wall. or between members set in from opposite sides, shall not be less than 93% of the thickness shown in the table.
4Units shall comply with the requirements of ASTM C 652.
5Extracted from the 1991 Edition of the Standard Building Code, Southern Building Code Congress International, Table 3104.4.
Equivalent thickness is the average thickness of solid material in the wall. It is found by taking the total volume of a wall unit, subtracting the volume of core or cell spaces and dividing by the area of the exposed face of the unit. The equivalent thickness of hollow clay masonry units can be calculated from the actual thickness and the percentage of solid material in the unit. Both of these measurements can be obtained when the units are sampled and tested according to ASTM C 67 Methods of Sampling and Testing Brick and Structural Clay Tile.
The equivalent thickness is based on the equation:
where: ET = equivalent thickness, in.
V = net volume (gross volume less void area), in.3 l = length of hollow brick, in.
h = height of hollow brick, in.
When the walls are plastered with sanded gypsum plaster, the effect of the plaster may be included by using Equation 6 once Rn is calculated using the equivalent thickness method for the hollow clay masonry unit. However, Equation 6 can only be used when plaster consists of sanded gypsum materials. To determine the effects of other plaster materials for hollow clay masonry units, the only option is to include the thickness of the plaster when determining the net volume, V, in Equation 8.
EXAMPLES OF CALCULATED FIRE RESISTANCE
Multi-Wythe Construction
A composite wall consists of 4 inch (100 mm) nominal face brick with a mortared collar joint and 4 inch (100 mm) siliceous aggregate concrete wall. The fire resistance rating is determined as follows:
From Table 1
4 inch nominal face brick, R1= 60 min (1 hr)
From Table 5
4 inch siliceous concrete wall, R2 = 77.3 min (1.29 hr)
Fire Resistance is calculated by Equation 4
R = (R10.59 + R20.59)1.7 = [ (1.0)0.59 + (1.29)0.59 ]1.7
= 3.70 hr
Fire Rating is expressed in multiples of hours with 4 hr as a maximum required in all model building codes. A wall with a calculated fire resistance of 3.70 hr cannot attain a 4 hr fire rating. Thus, the fire rating is 3 hr.
Effects of Plaster
A 6 inch (150 mm) solid brick masonry wall has a 120 min (2 hr) fire rating from Table 1. Determine the fire rating with 3/4 inch (19.1 mm) thick sanded gypsum plaster on one and two sides.
Using Equation 6 and Table 6 with 3/4 inch thick plaster on one side
R = (Rn0 59 + pl)1.7 = [(2)0.59 + 0.45]1.7
= 3.13 hr. Fire Rating = 3 hr
Using Equation 6 and Table 6 with 3/4 inch thick plaster on two sides
R = [(2)0.59 + 0.90]1.7 = 4.45 hr. Fire Rating = 4 hr
Effects of Air Spaces
A cavity wall consists of 4 in. (100 mm) nominal solid clay masonry units, a 2 in. (50 mm) air space and 8 in. (200 mm) limestone, cinder or unexpanded slag concrete masonry unit. The concrete masonry unit has actual dimensions of 8 x 8 x 16 inches and is 50% solid. The fire rating is determined by Eq. 7 as follows:
From Table 1
4 in. nominal solid clay unit, R1= 60 min or 1 hr A 2 in. air space, as = 0.30
An 8 x 8 x 16 CMU composed of limestone, cinder or unexpanded slag has an equivalent thickness of
= 4 in., From Table 4, fire rating = 120 min (2 hr) Fire Resistance is calculated by Eq. 7: R = (R1 0.59 + R2 0.59 + as)1.7
= [(2)0.59 + (2)0.59 + 0 3]1.7
= 5.77 hr. Fire Rating = 4 hr
Hollow Clay Masonry Walls
Determine the equivalent thickness and fire rating for a nominal 8 x 4 x 12 in. (200 x 100 x 400 mm) hollow clay masonry unit with the coring pattern shown in Figure 1.
FIG. 1
First, determine the net volume of the unit.
Gross volume = t x h x I
= (7.625) (3.625) (11.625) = 321.3 in3
Next, determine core area volume
= 2 [(4.625) (3.625) (2.875)] + (4.625) (3.625) (0.625) = 106.9 in3
Net volume = 321.3 - 106.9
= 214.4 in3
% Solid =214.4/321.3 = 0.667, unit is 66.7% solid
Determine equivalent thickness, Et by Eq. 8
From Table 7 a hollow clay masonry unit, unfilled, with an Et = 5.09 attains a 240 min or 4 hr fire rating.
SUMMARY
This Technical Notes has addressed analytical methods to determine fire ratings for building construction not tested under ASTM E 119, commonly known as calculated fire resistance. Procedures for calculating the fire rating of multi-wythe construction, plastered clay masonry walls, the effects of air spaces within wall construction and the equivalent thickness of hollow clay masonry units are presented.
Fire ratings play an important part of building wall selection. The information presented in this Technical Notes will be useful for calculating the fire rating of untested wall assemblies composed of clay masonry and combinations of clay masonry with other construction materials such as concrete and concrete masonry.
The information and suggestions contained in this Technical Notes are based on the available data and the experience of the engineering staff of the Brick Institute of America. The information contained herein must be used in conjunction with good technical judgment and a basic understanding of the fire resistance properties of brick masonry. Final decisions on the use of the information presented are not within the purview of the Brick Institute of America and must rest with the project architect, designer, owner or all.
REFERENCES
2. "Fire Resistance of Various Masonry Walls", G.E. Troxell, University of California at Berkeley, 1967.
3. Fire Test Report #83-13, "Two Hour Fire Resistance Tests of Higgins Brick Company Solid Grouted 5-Inch Hollow Brick Units" by Fisher and Williamson.
4. International Conference of Building Officials Research Reports #1957, #2730 and #4062.
5. NBS Research Paper No. 37, Fire Resistance of Hollow Load Bearing Wall Tile, National Bureau of Standards, 1928.
6. "Standard Building Code, Chapter 31" Southern Building Code Congress, International, 1991 Edition.
7. Structural Engineers Association of California, "Annual Report for the Fire Ratings Committee", 1962.
8. Technical Notes on Brick Construction 16 Revised, "Fire Resistance", Reissued May 1987.
9. Williamson, R.B., "Standard Fire Tests of Unloaded Hollow 6-inch Brick/Block Panels (filled with lightweight Aggregate; Vermiculite)".