/ Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

NATIONAL BUREAU OF STANDARDS REPORT

7058

ESTIMATION OF THE FIRE ENDURANCE OF STEEL

PROTECTED BY SPRAYED INSULATION THROUGH USE OF

AN ANALOG COMPUTER

by

D. Gross

and

J. Ducraafiis*

U. $. DEPARTS OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1002-12-10120 January 23, 1961 7058

Estimation of the Fire Endurnnce of Steel

Protected by Sprayed Insulation Through Use of

An Analog Computer

by

D. Gross

and

J. Ducmahis*

^Participating in Junior Scientist

and Engineer Program 1959

(MPORTANT. NOTICE

NATIONAL BUREAU OF STANDARDS REPORTS are usually preliminary or progress accounting documents

Intended for use within the Government. Before material in the reports is formally published it Is subjected

to additional evaluation and review. For this reason, the publication, reprinting, reproduction, or open-literature

listing of this Report, either In whole or in part, is not authorized unless permission is obtained in writing from

the Office of the Director, National Bureau of Standards, Washington 25, 0. C. Such permission is not needed,

however, by the Government agency for which the Report has been specifically prepared If that agency wishes

to reproduce additional copies for Its own use.

U. S. DEPARTMENT OF. COMMERCE

NATIONAL BUREAU OF STANDARDS

Estimation of the Fire Endurance of Steel Protected by Sprayed

Insulation through Use of an Analog Computer

by

D. Gross and J. Ducmanis

ABSTRACT

Electrical analog computer solutions are

presented for estimating the fire endurance of

steel protected by sprayed insulation- upon exposure to a "standard" fire. Resistors and

capacitors comprising the electrical model were

selected to represent the measured or estimated

thermal properties. Comparisons with experimental

results are given and possible explanations for

the causes of differences are suggested.

1. INTRODUCTION

In connection with investigations of the effectiveness of fireretardapt insulating materials s;^rayed direct].y: on structural steel

members, it was considered desirable to compare the predictions of

an electrical analog computer with experimental results on one type

of sprayed insulation. Based upon estimates of its thermal properties, comparisons were made for the following constructions:

(a) sprayed insulation in 1/2-in., 1-in., and 2-in. thicknesses on

two-foot square. No. 16 U.S.G. steel plates, (b) sprayed insulation

of nominal 1/2-in. thickness on a 6r by 6-in. by ft high, wide

flange stanchion steel column and (c) a 6-in dia by 24-in. long

cylinder of this insulation material. This report presents^'and

discusses the results of the analog computations, and compares the

results with experimental data. Details concerning the thermal

and fire-retardant properties of eight sprayed insulations, including

the one selected for this study, are given in NBS Report Nos. 5821,

5943 , and 6558.

2. MATERIAL

The spray insulation for this study was selected on the basis

of its performance in the preliminary fire endurance tests. It was

a proprietary fibrous material (about 75^ glass and 25^ asbestos)

plus binders, fillers, etc.*

* Material used was the fifth listed on page 2 of NBS Report No. 5821
2

3. EXPERIMENTAL DETAILS

(a) Two-foot Slabs . The sprayed insulation was applied to

a two-foot square section of No. 16 U.S.G. steel plate. Specimens

of three thicknesses, 1/2 in., 1 in., and 2 in. were prepared by

spraying into enclosures whose side walls were of welded angle of

the the desired height. Each specimen, after drying, was placed

insulation side down to form the top surface of a gas-fired furnace.

Above the steel plate of the specimen were, in order, a air space, a 1-in. thick refractory board, an air space and a large steel pan containing circulating water at nearly constant

temperature. The results are shown in Figures 1, 2, and 3*

(b) Stanchion Column . The column was a 6-in. by 6-in. by

11 ft wide flange stanchion steel column onto which was sprayed

a nominal 1/2-in. coating of the insulation. The spraying was

done by a commercial contractor using commercial equipment. Prior

to test, a survey of the actual thickness of insulation was made

by measuring thq penetrations of a fine needle at four elevations

on the column corresponding to the elevations at which thermocouples were installed,. As shown in Figure 4, the actual thickness varied from about 1/8 in. to 1-1/8 in. with an estimated

mean thickness possibly slightly greater than 1/2 in. The column

was tested under load in a standard fire test [l]. The results

are shown in Figure

(c) Solid Cylinder . A sprayed solid cylinder of the same

insulajtion was prepared at the same time as the column. The

insulation was sprayed into a cardboard concrete form 6 in. I.D.

by 24 in. long, which, when removed, provided a simple solid

cylinder of the same insulation as was applied to-;the column. The

cylinder was dried and weighed prior to test and found to have a

density of 0.33 gm/cc for the two-foot slabs. The latter density

was also assumed for the column, since no "tamping" or compression

was done on the column or slabs insulation as was the case for the

cylinder. The cylinder was tested at the same time and in the same

furnace as the column. The results are shown in Figure 6.

In all cases, slabs, column and cylinder, thermocouples

were placed on the test specimens for indicating temperatures

within the specimen. The unusual temperature record of the first

cylinder test prompted a repeat test on the same cylinder with the

same thermocouples. The results are also shown in Figure 6 and

are discussed later.
- 3

-

ANALOG COMPUTATIONS

The similarity between heat flow through thermal circuits

and current flow in resistance-capacitance electrical circuits

provides the basis for the electrical analog method [2]. The

properties assumed for the sprayed insulation are listed in

Table 1. The estimation of thermal conductivity with temperature is based upon available handbook values for similar

materials in combination with an accurate determination for the

actual sprayed insulation at room temperature. In each case,

the resistors and capacitors comprising the electrical model

were selected to represent the thermal resistance and heat

capacity for the mean temperature expected at each section.

The magnitude of these components for each test depends upon

scale factors chosen to maintain the proper electrical and

thermal time relationship.

(a) Two-foot Slabs . In Figures 1, 2 and 3? "the analog

solutions for the three different slab thicknesses have been

drawn for direct comparison with the experimental curves. Also

shown are the electrical networks used. The following assumptions

or approximations were made:

(1) Because of its high thermal conductivity, the

steel plate was represented by a single capacitor only. The

temperature variation of specific heat for iron was taken from

the Handbook of Chemical Engineering and the capacitor was selected to represent the heat capacity for the mean temperature at

the plate.

(2) Because of the low heat capacity of air, the 3/^

air gap was represented by a single resistor only. The use of

a simple resistor is an approximation inasmuch as heat transfer

across an air gap is a combination of radiation, convection and

conduction. To properly represent this type of heat transfer

would require the appropriate non-linear resistor sensitive to

the absolute voltage applied. A slight change in the "effective

conductance" of the air gap for the 2-in. test was made as an

approximate allowance for the change in heat transfer with change

in mean temperature level.

( 3 ) The assumed properties of the refractory board

were as follows: K - 0.0005,* P = 0.812 and s - 0.19 cgs units

and were assumed temperature-independent. This is not too severe

an?, assumption since the teitperature gradient across the board is

* subsequent ly measured as .00067 at ll8°F on pentane hot plate
- 4 -

not too large and the effect of refractory board property changes

upon the steel plate temperature were not of the same order of

magnitude as those of the sprayed insulation.

( 4 ) The upper air space was simulated by a non-linear,

voltage-sensitive resistor arranged to represent heat transfer

by radiation and free convection from a horizontal plate facing

upward. Due to the proximity of the steel pan, the assumption

of a free convection coefficient is somewhat questionable.

(b) Stanchion Column . Figure 5 is a comparison between

the analog and experimental curves on the steel column protected

with 1/2-in. of sprayed plaster insulation. The electrical network employed is also shown.

v,. .(c) Solid Cylinder. Figure 6 is a comparison between the

analog and experimental cuf^ves for a 6-in. diameter^^ solid cylinder of sprayed insulation. Because the initial test yielded

unusual temperature curves, the results of the repeat test are

considered useful for comparison. Radial heat flow is reduced to

the one-dimensional case by selecting values of 1/R and C proportional to the (mean) radius of the section. The density used for

this case was 0.5g/cc, as measured.

5. Discussion

In comparing the analog results from the three slab tests

with the corresponding experimental data, it is seen that (a)

good agreement was obtained for the 1-in. thick insulation, (b)

the analog solution was slightly higher for the 1/2-in. thick

insulation and (c) the analog solution was considerably lower for

the 2-in. thick insulation. Since the good agreement for the 1-in.

thick insulation involved the arbitrary selection of an effective

conductance for the air gap, several determinations were subsequently

made to evaluate the validity of this assumption. It was found

that when one or two sheets of asbestos millboard (p = .86,k - .00029

c = 0.20) were used in place of the steel plate and insulation,

the temperature curves in Figure 7 were obtained. The good agreement between the analog and experimental results for these tests

gives added confidence to the selection and representation of the

air gap by a single linear resistor for approximate analyses.
- 5 -

The differences in the 1/2-in thick insulation results may

he due to a number of factors, of which the following are immediately suggested:

1. Over-rating of the temperature effect upon the thermal

conductivity of the sprayed plaster.

2. An exothermic reaction which would tend to increase

the experimental temperature curve particularly for

the thicker specimens.

3 . General lack of knowledge of the physical, chemical

and thermal transformations during rapid heating of

sprayed insulation.

The analog temperature curves for the insulated steel

column in Figure 5 and the solid cylinder in Figure 6 compare

favorably with the experimental curves. Since there were no

complex heat flow paths or external heat losses in these two

cases, the closer agreement is reasonable.

The criterion generally employed for determining the fire

endurance of an insulated steel member is the time period

during which the steel temperature remains below 1000°F (538°C).

This corresponds to the point at which the steel starts to lose

strength rapidly and consequently can no longer act as an effective structural member or fire barrier. The fire endurance

times as determined by means of the analog computer are compared

in Table 2 with the experimentally measured values.

The anomolous temperature record, see Figure 6 , of the

first cylinder test led to some qualitative investigations of

the behavior of thermocouples imbedded in this type of sprayed

insulation. A discussion of the results obtained may be useful

in the interpretation of this as well as future experimental

data.

Following the second cylinder test, one of the thermocouples

located along the center line of the cylinder was removed and

checked for calibration. At a temperature of 91°C within a heated

oven, the test thermocouple reading was within one degree of a mercury-in-glass thermometer reading and identical to that from

another thermocouple which had not been fire-exposed. At this

temperature, at least, the calibration of this thermocouple was

not seriously in error.
- 6 -

The effect of the sprayed insulation on the temperature

readings during a fire test was examined in a series of tests

employing an electrically heated furnace set at about 700°C,

a porcelain crucible or stainless steel wire basket, some

sprayed insulation and several thermocouples. The following

conclusions were indicated from the results obtained:

1. Sprayed insulation was not incombustile , but

generated heat when exposed to high temperatures;

there was a significant temperature rise (greater

than 50 degrees C) above the furnace temperature

when a small sample was placed in a furnace at

about 700°C. This occurred when bare or insulated

thermocouples were used for the measurements.

2. The reading of a thermocouple which passes through

the sprayed insulation without an electrically

impermeable covering will be affected by the surrounding insulation at high temperatures. This AiS'as determined using No. 24 B&S (.020 in. dia) insulated

chromel-alumel thermocouple wire, the type generally

used in National Bureau of Standards fire tests.

The same effect does not occur during the burning of

the thermocouple insulation in air at these temperatures.

3. This effect results from electrical conductance (a

partial short circuit) through the sprayed insulation.

It was determined that the electrical resistance

between two separate and bare chromel and alumel wires

placed in close proximity within the sprayed insulation

may decrease to lOK ohms or less during exposure to

high temperatures. The same result was obtained with

insulated chromel and alumel wires inasmuch as the

thermocouple insulation also breaks down at these

temjDeratures . After a period of time at these temperatures, the resistance was found to increase and

approach the initial open circuit resistances.

A possible explanation for the anomolous temperature behavior of the first cylinder test may therefore be formulated as

follows
- 7 -

The thermocouple located along the vertical center line

of the cylinder passed up through the sprayed insulation and

passed through a fairly sharp temperature gradient. At a time

when the center was still cool, the surface layers experienced

a rapid temperature rise corresponding to the application of

the standard fire exposure. At the high temperatures, there

was a breakdown of the thermocouple insulation and also a decrease in the electrical resistance of the surrounding spray

insulation. The emf output of the thermocouple circuit represented a combination of (a) the low temperature at the thermocouple junction along the cylinder centerline and (b) the high

temperature at the location of the (partial) short near the

cylinder surface. Consequently an erroneously high reading

was obtained. This may have been due, in part, to heat generated by combustible material in the sprayed insulation. After

a period of time, the electrical resistance of the sprayed

insulation increased and the partially shorted condition disappeared. The readings then decreased and approached the true

temperatures at the thermocouple junctions. The fact that the

temperature record for the second cylinder test did not show

this unusual effect means that the sprayed insulation had undergone an irreversible physical, chemical or electrical transformation during the high temperature exposure of the first

tes t

.

This effect was not observed in the fire tests of the

sprayed column and two-foot slabs inasmuch as the thermocouple

leads were not brought out through sharp temperature gradients

within the sprayed insulation.
- 8 -

6 . References

[1] American Society for Testing Materials. "Standard Method's

of Fire Tests of Building Construction and Materials,"

ASTM Designation E 119-59 T.

[ 2 ] Robertson, A. F. and Gross, D.,"An Electrical-Analog

Method for Transient Heat-Flow Analysis," J. Res NBS,

61 , 105-115, Aug 1958.

USCOMM-NBS-DC
Table 1. Assumed Thermal Properties of Sprayed Plaster

Temperature Density

Specific

Heat

Thermal

Con(jluctivity

°C gm/cm3 cal/gm ®C" cal/gm °C

23 0.33^ 0.000117

100 0.22 .00015

200 0.25 .00018

^00 0.28 .00026

600 0.29 .00036

800 0.29 .00048

Remarks

Measured
Table 2. Fire Endurance of Steel and Sprayed Insulation Assemblies

Comparison of Experimental and Analog Results

Fire Endurance Time

( 1000°F) , min

Experiment Analog

1/2-in. Sprayed Insulation on Steel Plate 68 37

1-in. Sprayed Insulation on Steel Plate IhO 125

2-in. Sprayed Insulation on Steel Plate >300 >300

1/2-in. Sprayed Insulation on Steel Column 36 44
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U.S. DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colo., is suggested in the following listing of the divisions and sections engaged in technical work. In

general, each section carries out specialized research, development, and engineering in the field indicated by its

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