APR 1 31962

NBS MONOGRAPH 45

Fire Tests of Precast Cellular

Concrete Floors and Roofs

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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Government Printing Office, Washington 25, D.C.
UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Fire Tests of Precast Cellular

Concrete Floors and Roofs

J. V. Ryan and E. W. Bender

National Bureau of Standards Monograph 45

Issued April 12, 1962

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. - Price 15 cents

Contents

Page

1. Introduction 1

2. Test specimens 1

2.1. General description 1 2.2. Reinforcement and cover 2

2.3. Construction of specimens 4

3. Test method and equipment 4

3.1. Moisture-content determinations 5

4. End-point criteria 6

5. Results and discussion 6

5.1. Plank thickness and cover 7

5.2. Reinforcement 9

5.3. Concrete density 9

5.4. Estimates of fire endurance 9

6. Conclusions 10

7. References 11

8. Appendix 11

III

Fire Tests of Precast Cellular Concrete Floors and Roofs

J. V. Ryan and E. W. Bender

The results of an investigation of lightweight, precast cellular concrete planks are

given. Fire tests were made of two floor and five roof specimens made up of these planks.

Variables included density of the cellular concrete, thickness and span of the planks, reinforcement, and cover for the latter. A steel beam encased in blocks of cellular concrete was

included in one floor specimen. The flexural strengths of 14 individual planks were determined. The investigation showed fire endurances up to 2 hr for 6-in. thick slabs tested

under load and up to 4 hr for other slabs not loaded. Estimates were made of the probable

results to be expected for slabs of thicknesses other than those actually tested.

1. Introduction

The framework and foundations of a building

represent a large part of the total cost in multij story construction. They must carry both the

I live loads imposed by occupancy and the dead

i load of the finished building. Significant econoI mies can be achieved in the cost of buildings by

; reducing the loads to be carried, and thereby permitting less expensive framework and foundation.

Since reduction in allowable live load limits the

usefulness of the building, research has been aimed

at reducing the dead weight of building elements

and materials.

I Research aimed at the reduction of the dead

weight of concrete structiu'es led to development

of lightweight aggregates as substitutes for dense

natm-al aggregates, and to development of cell- forming processes for producing cellular concretes.

The cell-forming processes included the use of

aluminum powder and other gas-forming agents,

the addition of preformed foam into the mixer,

the addition of chemicals that form and retain

air bubbles produced during mixing, and the use

of excess water. Although known as "gas concrete," "foam concrete," or "aerated concrete,"

according to the particular process employed, all are included in the more general term "cellular

concretes" [1].*

2. Test

2.1. General Description

The floor and roof panel specunens were made

up of planks of cellular concrete specially cast in

the necessary lengths for this study by a manufacturer. They were representative of commerciall}^ produced planks used in floor and roof

construction. The mix constituents and ratios

used in their production were not revealed.

I However, the particular cellular concrete is

! understood to be of the gas-formed type.

'Figures in brackets indicate the literature references on page 11.

Cellular concretes have been developed that

have densities as low as 20 Ib/ff*, are sawable,

and workable with general carpentry tools. Such

cellular concretes are being manufactured commercially and used to produce prefabricated blocks

and panels for floor, roof, and wall assemblies.

The economies achieved through the use of

cellular concretes are not limited to those associated with dead-weight reduction. Because of

their cellular structure, they have significantly

lower thermal conductivities than conventional

dense concretes. Therefore, the use of cellular

concretes often permits the elimination of additional insulating materials that would otherwise

be necessary.

A research program was carried out on floor

and roof slabs assembled from precast planks of a gas-formed cellular concrete to determine the fire

endurances of representative specimens and the

effects thereon of variables such as amount of cover for the reinforcing bars, overall thickness,

amount and distribution of reinforcement, and

density of the concrete. Five standard fireendurance tests of roof assemblies and two of

floor assemblies were conducted in a large floor

fm-nace. Each specimen consisted of several

planks grouted together. A steel I-beam, protected by cellular concrete blocks, was tested in

conjunction with one of the floor specimens.

Specimens

The planks were 18 in. wide and had a length

of either 13 ft 5 in. (to span the short dimension

of the furnace opening), or 17 ft 11 in. (to span

the long dimension of the furnace opening).

They were supplied in thicknesses of 5, 6, and

8 in., representative of the range of normal production. The nominal density of tlie planks for

floor use was 44 Ib/ft^, and that of the rool' planks

was 31 Ib/ft^ The measured densities ranged

from 42.6 to 46.5 Ib/ft^ for tlie floor planks, and

from 35.8 to 42.2 Ib/ft^ for the roof planks.

For four of the seven standard tests, the

specimen panels were made up of twelve G-in.

1
SOUTH

EAST

PLAN

ALL SPECIMENS EXCEPT NO. 380

A

NORTH SOUTH

18'- 0"

WEST

2

3

4 13'-

5

6

7

8

9

EAST

PLAN

SPECIMEN NO. 380 111111 1 T

SECTION A-A 1 T

SPECIMENS NO. 357, 371.372 AND 379

GROUT

^—

r

SECTION B-B

SPECIMEN NO 380

4-

1 1 SECTION A-A

SPECIMENS NO. 358 AND 373 Figure 1. Arrangemenilof planks within the furnace opening.

The individual plank numbers are as used in section 8, appendix.

planks laid across the furnace. Three specimens

of this group were roof panels and differed either

in the reinforcement design of the planks or in the

load applied during the test; the fourth was a

floor panel. One other roof panel was made up

of nine 8-in. planks laid along the length of the

furnace opening. For the remaining two fullscale tests (one floor and one roof), four planks of

each of the three thicknesses supplied, 5, 6, and

8 in., were I'aid across the furnace openmg, with

the 6-in. section in the center. Figure 1 shows

the different assemblies.

After an exploratory test in a small furnace to

determine the feasibilitj^ of the procedure, a

structural steel I-beam protected by cellular concrete blocks was tested along with the floor panel

of graduated thickness. The beam was a 10-in.

I-beam weighing 25.4 lb/ft, of 17 ft 7% in. length

to span the long dimension of the furnace. The

floor panel (with planks crosswise of the furnace)

rested on the top of the beam. The sides and

bottom, or soffit, of the beam were encased in

precast blocks to provide a cellular concrete cover of 2% in. on the bottom and l^Ke in. on the

sides.

Flexural strength tests were made on several

individual planks, which had not been exposed to

fire. 2.2. Reinforcement and Cover

The term cover refers to the thickness of concrete between a steel reinforcing bar and the

nearest surface of the reinforced concrete assembly,

or to the thickness of protective material encasing a structural steel member. The cover of greatest

interest in connection with reinforced concrete

flexural members is that between the bottom

tensile reinforcement and the surface exposed to

fire.The reinforcement in the specimens consisted

of nominally round, hot-rolled, plain steel bars,

in various patterns or combinations of number

and size. The basic pattern of reinforcement

consisted of longitudinal bars at two levels (one

in the tensile zone and one in the compressive

zone), plus crossbars, anchor bars, and spacers.

The crossbars maintained the spacing between

longitudinal bars at a single level and positioned

them in the form. The anchor bars provided

anchorage for the ends of the lower level, or tension bars. The spacers, which were formed

of sheet metal, maintained the spacing between

levels as determined by the overall thickness

and cover. The longitudinal reinforcing bars

were located and spaced so that those of the

upper level had the same cover from the top

surface as did those of the lower level from the

bottom surface. The amount of cover was

varied among the specimens. The variations

iwithin the basic pattern of reinforcement are

illustrated in figure 2 and the cover for each

specimen is given in table 1

.

2
Table 1. Summary of specimen details and results

No.

357

371

379

380 358

372

373

373

Type

RooL...

Roof,-.

Roof—

.

Roof-...

Roof....

Floor...

Floor...

I-Beam.

Thickness

68

6

56

8

Clear span

ft in. 13 5

13

13

17

13

13

13

13

13

13

13

17

Reinforce- ment

A,

BiA,

«A, E

B2

B,

B,A2D

BsC

Concrete

Cover

/4

Wi

m

IHm

^2%

Density

Nominal Measured

lb/ft 3 31

31

3131

31

3131

4444

44

44

lb/ft 3 35.8

35.8

42.2 3636

36

36

44.5

46.5

45.4 42.6

36.7

Computed

Applied tensile Time to Limiting

load steel initial condition

stress end point

lb/ft 2 lb/in.2 hr: min

74 17, 800 0:57 Load failure. 74 37, 300

56 14, 300 0:53 Load failure. 56 14, 900 1:29 Max surf. temp.

70 16, 100 1:13 Ignite waste. None 3:13 Ignite waste. None 4:07 Max surf. temp. None

70 12, 600 1:40 Max surf. temp. None 4:20 Max surf. temp. None

None

n7,700 20, 000 2:22 Load failure.

f See figure 2 for details of reinforcement.

b FoUowiug the test, the cover In three planks was found to have been IH in., rather than the intended % in.

« Contained shear reinforcement; see text for description.

d Thickness of side and soffit blocks used to protect steel beam.

« Load equally divided among four application points.

^ 13-2

- 3'- 1 ''z •+• 3'- 1 '/a" H- 3'- I '/i H4V

11

UPPER LEVEL

^CROSS BAR LOWER LEVEL ANCHOR BAR

"

PATTERNS A|- A;

UPPER LEVEL

LOWER LEVEL

PATTERNS B|- Bg-Bs

4^

4"h 3'- I '/a 3'- I 3'- I - •

I 3-1 V Hit

•I ' 3'-6'-

UPPER LEVEL

I h-8'.'t-8'H-8'H K8"-f8'-4-8"Hl- I i^"

LOWER LEVEL PATTERN E

UPPER LEVEL

LOWER LEVEL PATTERN C

UPPER LEVEL

LOWER LEVEL

PATTERN D

18

' 13"

TYPICAL (A|) CROSS SECTION

PATTERN A| Aj Bj c D E

UPPER % \ \ % \

LOWER ''a % %

CROSS ANCHOR \ \

NOMINAL DIAMETERS OF BARS IN INCHES

Figure 2. Reinforcement details.

In the planks used for one of the 6-in. roof panels

(test No. 379), shear reinforcement was provided

in addition to the reinforcement already described.

The shear reinforcement consisted of }^-in.-diam

rod bent in a multiple-V shape, each V section

thus formed bemg about 4}^ in. deep and 5% in.

long. A continuous length of this shear reinforcement was placed along each side of the assembly of

reinforcement bars and wire tied to the upper and

lower bars nearest each side of the plank. The

3
Figure 3. Steel beam partly encased.

ties were single strands of 0.027-in. wu-e and varied

as to spacing, some being 5/4 in. and others lOji in.

apart. In these planks the sheet metal spacing

clips were supplemented by M-in. diam steel rods

welded to the upper and lower bars nearest the

sides of the plank, 3 in. from their ends.

2.3. Construction of Specimens

In making up the test panels in the furnace, the

planks were placed in contact with one another,

and a grout consisting of 3 parts portland cement

to 1 part sand, by weight, was poured between the

panel and the furnace restraining frame and into

the groove formed along each joint between planks

by a kerf % in. wide by 1/^ in. deep along one edge

of each plank. The kerfs and adjoinmg edges of

the planks were wet down immediately before

pouring the grout, and periodically for a week or ten days thereafter, to prevent unduly rapid

drying of the grout, and thereby to improve the

bond. An electric heater was placed in the furnace, and for the week immediately preceding each

test a temperature of 125 to 150 °F was mamtained in an attempt to remove excess moisture

from the assembly.

The steel I-beam, tested m conjunction with the

floor panel of graduated thickness, was placed

along the longitudinal center line of the furnace

and attached at both ends to steel plates by means

of standard AISC B2 connectors [2]. These

plates, in turn, were bolted to the furnace frame

in such a way that the beam was supported,

restrained against rotation of the ends, but could

expand longitudmally. The beam was encased in

precast blocks of cellular concrete, after the floor

planks were in place. The soffit blocks (9 in.

wide, 17^jU in. long, and 2% in. thick) were

attached to the beam by means of formed wire

clips. Transverse wires of the clips were driven

into the ends of the blocks, holding them in good

to fair contact with the beam. The soffit blocks

extended about 2}^ in. beyond the beam flange on

each side. The joints between blocks were filled

with mortar consisting of 1 part Imae, 1 part

Portland cement, and 5 parts crushed cellular

concrete by volume. A bed of this mortar was

placed on the upper surface of the soffit blocks, and

the side blocks (9 in. wide, 18 in. long, and VYu

in. thick) were placed thereon. The vertical

joints between blocks and the spaces between

blocks and floor planks were flUecl with mortar.

The joints were wetted before the application of

the mortar and periodically thereafter for the

next 10 days to prevent excessively rapid drying.

Throughout the foUowmg week, an electric heater

was kept in the furnace chamber and temperatures

in the range of 125 to 150 °F were maintained in an attempt to remove some of the excess moisture.

Figure 3 shows the beam partly encased.

3. Test Method and Equipment

The tests were conducted in accordance with when the initial end-point criterion was reached,

the accepted standard test methods (ASTM desig- but some were continued in order to obtain more

nation E119) [3]. Not all the tests were stopped information about the behavior of the specimen.

4
This practice deviated from the standard test method, under which tests are stopped when the

j initial end point is reached and the specimens are subjected to the hose-stream and the doublereloading tests. 1 Inasmuch as the results would

not have been indicative of the true performance

of the specimens, because of their extended fire

exposure, neither the hose-stream nor the doublereloading test was performed on any of the speci- mens. The fire-endurance limits were determined

by the initial end point and are therefore in accord- ance with the standard test method, even though

the fire exposure was continued.

The specimens were tested in the furnaces at

the National Bureau of Standards designed for

the purpose of testing floor and roof specimens.

The small furnace, used for the exploratory test,

took specimens 2 ft by 2 ft in size, while the large

fm-nace, used for the standard tests, was constructed in such a manner as to accommodate a specimen 18 ft long by I3}i ft wide. Both fur- naces were box-shaped, open at the top, so that

the bottom face of the specimen was exposed to

the flames.

To measure the temperature in the chamber of

the large floor furnace, twelve thermocouples, en- cased in porcelain insulators and enclosed in iron

pipes sealed at one end, were placed in the furnace

so that the junctions of the thermocouples were

approximately 1 ft below the exposed surface of

the specimen. In the small furnace, four thermocouples were used. They were of the same type

and were placed in the same manner as those in

the large furnace. The fm-nace fires were controlled to approximate as nearly as feasible the temperatui'es set forth in the standard test method,

which include 1,000 °F at 5 min, 1,300 °F at 10

min, 1,550 °F at 30 min, 1,700 °F at 1 hr, 1,850 °F

at 2 hr, 2,000 °F at 4 hr, and 2,300 °F at 8 hr and

longer.

The fire-exposure severity, which is defined as the ratio of the area under the curve of average

furnace temperature to the area under the standard time temperature curve, is required to be

between 90 and 110 percent for tests of 1 hr or

less, between 92.5 and 107.5 percent for tests over

1 hr but not over 2 hr, and between 95 and 105

percent for tests longer than 2 hr.

In order to obtain temperature data on the

unexposed surfaces of the specimens, a total of

12 chromel-alumel thermocouples whose junctions

were in contact with the unexposed surface of the

specimen were used in each test. They were

distributed over the surface in a symmetrical

pattern, with junctions and short lengths of the

thermocouple wire coiled under standard felted

asbestos pads 6 in. square by 0.4 in. thick.

Temperatures of the reinforcing bars in the

specimens were measured by means of chromelalumel thermocouples tied to the bars of some

planks, at the time of fabrication. There were 5

1 Both these supplementary tests have been deleted from the Standard E119 since the study reported herein was started.

thermocouples on the lower bars, and 3 on the

upper bars, of each of the selected planks. Planks

with thermocouples were located in positions 3,

6, and 9 (see fig. 1) of specimens 357, 371, and 372;

positions 3, 5, and 7 of specimen 380; and positions

3, 4, 5, 7, 9, and 10 of specimens 358 and 373.

No planks with thermocouples were provided for

specimen 379.

The temperatures on the I-beams in the ex- ploratory and full-scale tests were measured by

chromel-alumel thermocouples which had their

junctions peened in small holes drilled into the

beams. Twenty-four thermocouples were used on

the beam in the full-scale test (373). The thermocouples were located in four groups of six, the

groups being spaced 42 to 43 in. on centers. Two

of the thermocouples of each group on the beam

were placed on the bottom of the top flange, one

on each side of the web, and two on the top of the

lower flange.

The recommendations of the manufacturer who

produced the cellular concrete planks were followed in loading the specunens. The superimposed loads were applied by means of hydraulic

jacks and were distributed evenly over the speci- mens. The manufacturer indicated that the

"normal" loads were based on a design tensile

stress of 18,000 lb/in. ^ in the lower reinforcing

steel. However, some specimens were tested at

loads other than normal, to provide an indication

of the effect of extent of loading on fire endurance.

Values of the stresses in the lower reinforcing bars,

as computed on the basis of simple beam theory,

are given in table 1. The fom"-point load applied to the steel I-beam

was transmitted through four planks of the

graduated floor specimen, thereby partially loading

the latter. No other external load was applied to

either of the graduated specimens.

The beam was loaded with a live load intended

to produce a steel stress of 20,000 lb/in. ^ The

deflections were measmed by a system of wires

attached to the specimen and passed over pulleys

to a scale, where riders on the wires indicated the

amount of change from the initial level.

3.1. Moisture-Content Determinations

Samples were taken from the specimens in the

furnace a day before each test in the study,

except the first, and from extra planks. The

samples were cylindrical cores, cut from the full

thickness of the individual slabs. They were

taken from near the corners of the specimens in

the furnace. The holes were filled with the

mixture used to grout the planks together.

Moisture determinations were made by drving

specimens at 105 °C (221 °F). The speciiiiens

were the cjdindrical cores in some cases, and tliin

wafers cut from the cores in others. Some were

from planks as received, others as tested (after

heating of the furnace chamber) and others from

5
planks stored at 50 percent relative humidity.

Some of the specimens were conditioned in various

ways, after removal from the planks, others were

not. One group of wafer specimens were placed in

50 percent relative humidity after having been

dried at 105 °C in order to permit measurement

of the moisture regain. The results of these

measurements are summarized in table 2. It is doubtful that the moistiu'e contents for

the fire-test specimens can be related to the

observed fire endm'ances, or to behavior in the

fire tests. The moisture-content specimens were

taken from near the edges of the fire-test speci- mens. Their moistm-e contents may therefore

show somewhat higher values than those of plank.=! nearer the center of the exposed area.

Table 2. Moisture content data determined hy oven-drying,

at 105 °C for 24 to 72 hr, cores removed from planks

heated after assembly into fire test specimens

i-VJL trail lllUlblpUrtJ Test Age lion liCCll/l-llgtr inill cnrtfiTYi on c furnace weight

mos Days %

358 2H 7 1 5.4

371 7H 10 4 8.1 372 8 8 2 1.9

373 9 6 2 2.2

379 7 2 14.6 380 10 6 2 2.5

Additional tests with cores, and thin wafers from cores, indicated equilibrium moisture content of about 2 percent wlien stored at 50 percent relative humidity, both for specimens as received and oven-dried. It required

much more than 30 days for equilibrium to be reached throughout a 6-in. plank.

4. End-Point Criteria

The previously mentioned standard test methods state that the endurance of a specimen is the

earliest time when any one of the following endpoint criteria is reached or observed:

(a) The specimen shall no longer sustain its

design load;

(b) The average temperature on the unexposed

siu-face rises 250 °F above the initial temperature;

(c) The maximum one-point temperature on the

unexposed surface rises 325 °F above the initial

temperature;

(d) Cracks or openings shall develop in such a manner as to allow the passage of flames or gases

hot enough to ignite cotton waste.

Although not included in the test methods as

criteria for failm-e, the following data are considered to be of general interest and are tabulated,

in part, in this report.

(e) The times at which steel structural members

or reinforcement attained an average temperature

of 1,000 °F at any appropriate level or section.

(f) The times at which steel structural members

or reinforcement attained a temperature of

1,200 °F at any one point.

It should be noted here that while criteria

(b), (c), and fd) are quite specific in defining

failure, (a) is not, and it is sometimes difficult to

judge when load failure occurs.

5. Results and Discussion

However, an objective method of determining

this end point has been developed which defines a

critical deflection and a critical rate of deflection.

The method has been shown to give results

reasonably consistent with those determined by

an experienced operator in charge of test [4]. The critical deflection is defined as D=U/S00d,

and the critical rate of deflection as R=U/150d

per hour, where L is the span, or length of the

specimen between supports, and d is the depth ^

of the specimen. The time when both of these

critical values has been reached is reported in

this study as the time of load failure.

The principal results of the tests made in the

large furnace are given in table 1, and representative time-temperature and time-deflection curves

are shown in figures 4 and 5. The observations

of the behaviors of the individual specimens,

made durmg the tests, are summarized in the test

logs in the appendix. The results of flexural

strength tests on individual planks are summarized

in table 3.

- "Depth" is determined by the type of construction: for reinforced concrete slabs it is measured from the top of the slab to the bottom of the main

reinforcement. This differs from "effective depth," which is measured to the centroid of tension reinforcement and which was used to compute the

steel stresses given in table 1.

Table 3. Results of flexural tests

Individual planks placed on supports at 12 ft spacing and subjected to load

at center only. The failures of the 13 ft 5 in. planks were judged due to

yielding of the steel; those of the 17 ft 11 in. planks to .shear in the concrete

.

Thickness Length Reinf. pattern » Nominal

density of concrete

Test No. b Max.

loads

Deflection

at ma.x. load

in. ft:in. Iblft 3 lb in. 6 13:5 B 1 31 358 2440 1.83

6 13:5 A2 44 372 4275 1.90

8 13:5 B , 31 358 1900 1.72

8 13:5 C 44 373 4590 1.40

8 17:11 E 31 380 5790 dL 30

» See figure 2. b Number of test in which other similar planks were exposed to fire.

' Mean of 2 to 5 specimens.

d Sudden failm-e, deflection reading not possible for 4 out of 5. Of the five panels tested under load, the perlormances of two were limited hy load failure,

two by unexposed surface temperature rise, and

one by ignition of cotton waste over a crack. The

only criterion applicable to a beam tested under

load is load failure; this was reached on the one

I-beam tested. The two panels assembled of 5-,

6-, and 8-in. planks were not subject to the limiting condition of load failure; other luuiting conditions were reached on the 5-in. section of each

specunen and on the 6-in. section of one. For

these two specimens, the attamment of the defined

6
TFCT Mn "^71

[ CD 1 IMW. O / 1

REFERENCE

- CURVE " FURNACE AVG DEFLECTION

/

y LOAD OFF

1 HR SMIN /V( 1

1

/'

max" L

AVG

OWER BARS /

- /

'' /

SURFACE MAX r

/ r-7

/

/ 1

/ SURFACE AVG -r

=1

30 60 TIME, MINUTES

60 90 TIME, MINUTES

150

FiGUEE 4. Time-temperature and time-deflection curves for 6-in. roof {371) and floor {372) specimens.

criteria was secondary in importance to the development of information on the effect of thickness.

Although the ignition of cotton waste occurred in one of these tests, it is not considered useful data

for comparisons.

During the test (373) of the floor without load,

flames issued from the exposed surface of one of the

5-in. planks, possibly from burning of plastic tape

used with the thermocouples. This was an exception to the general behavior observed in this study.

The temperatures of the remforcing bars in one of

the 5-in. planks in this specimen rose much more

rapidly than those for the other, presumably due

to the flaming. Since the flaming was not representative, and in fact cannot be definitely explained, the data from this plank have been

disregarded.

5.1. Plank Thickness and Cover

Each of the two graduated specimens, tests 358

and 373, provides a basis for comparisons of performance of planks of different thickness. Except

for the 8-in. roof planks in test 358, all planks had

the same cover. Within each test, all the individual planks were of the same nominal density

There were differences in the number and size of

reinforcing bars, but only those resulting from

design on the basis of span, thickness, and other

variables.

Among the roof planks (358), the limiting

temperature rises on the unexposed surface were

reached on the 5-in. planks at 3 hr 39 min for the

average and 3 hr 37 min for the maximum, and

at 4 hr 10 min and 4 hr 7 min respectively on the

6-in. planks, but were not reached on the 8-in.

planks. Temperatures of 1,000 °F average and

1,200 °F maximum respectively were reached on

the lower level reinforcing bars at 2 hr 9 min and

2 hr 14 min respectively for the 5-in. planks, and

2 hr 6 min and 2 hr 17 min for the 6-in. planks.

The data for temperatures of the reinforcing bars

in the 8-in. planks were not applicable in evaluation of the effect of thickness because the

amount of cover used differed from that for the

5- and 6-in. planks.

In order to avoid complete collapse of the I- beam, test 373 was stopped immediately after the

3 hr 50 min data readings, although no Hmiting

condition apphcable to floor slabs had been attained. It was estimated by extraploation of the

data obtained that the limiting maximum temperatm-e rise would have been reached on tlie 5-in.

planks at 4 hr 20 min. Temperatures of 1,000 °F

average and 1,200 °F maximum were reached at

2 hr 50 min and 3 hr 6 min respectively on the

7
LJ

cr

UJ

II 2TIME. HOURS

3.4

Figure 5. Time-temperature and time-deflection curves for steel beam, and floor slab made up of 5-, 6-, and 8-inch thick planks.

The load was concentrated on planks directly over the beam and was removed when the latter failed. lower-level reinforcing bars of the 5-in. planks,

and at 3 hr 12 min and 3 hr 40 niin for the 6-in.

planks. It was estimated that the 1,000 °F

average would have been reached at 3 hr 52 min

for the 8-in. planks; the 1,200 °F maximmn was

not reached.

The steel teinperatm-es chosen are generally asso- ciated with loss of steel strenghth such that load

failure is imminent for reinforced concrete structm"al elements designed with a safety factor

of two for the reinforcing bars. Since neither of

the two graduated specimens were subjected to

loads appropriate to the plank designs, the times

at which the reinforcing bar temperatm^es reached

1,000 °F average and 1,200 °F maximum may

differ appreciably from those that would have

been observed in tests of similar planks loaded in

accordance with accepted design practices. Under

such load the development of cracks and the

sloughing or spalling of concrete cover may differ

from and lead to earlier and more rapid steeltemperatme rise than would be observed under

comparatively light load. However, the times

observed in these tests should be valid indications of the relative performances to be expected

for the various thicknesses.

The effect of cover was not easy to evaluate

because all specimens with %-in. cover were tested

under load and all those with 1%-m. cover were

tested without load. However, it was discovered

after test 357 that three planks with 1%-in. cover had been supplied inadvertently along with the

nine planks with the intended %-in. cover. Moreover, the former had only half as much reinforcement as the latter. Therefore, another test, 371,

was conducted essentially duplicating the earlier

specimen except that all planks had /4-in. cover and the applied load was reduced by 25 percent.

Despite the reduced load in the latter test, the

specimen failed under load at 53 min, compared

with 57 min in the earlier test. Therefore the

greater cover gave appreciably longer fire endur- ance to the assembly.

The method of varying cover by changing the

effective depth of the reinforcing bars would

change the allowable load, the steel stress, or require a change in the amount of reinforcing

steel. Ordinarily all of tliese would be held

constant and change of cover would be accomplished by change of overall thickness. The

unusual procedm-e in this study resulted in the

8
effect of cover variation's not being masked by

the effect of thickness.

5.2. Reinforcement

Longitudinal reinforcing bars having nominal

diameters of ji, Yu, and % in. were used in the

specimens. Shear reinforcement was also used

in one specimen. For the most part, the number

and size of reinforcing bars incorporated in the

test specimens were such as to satisfy design

criteria related to span, load, and stress in the

steel. In general, the number and size of bars

were the same among planks having the same

thickness, span, concrete density, and cover for

the bars. However, there was an exception to

this general rule, as can be seen from comparison

of reinforcement in specimens 372 and 379.

The spans, thickness, concrete density, number of

reinforcing bars, and cover were equal (within

practical limits) for these specimens, but the

former had %-in. lower level and Ke-in. upperlevel bars, whereas the latter had ^e-in. lower

and K-in. upper bars plus full-length shear reinforcement. The specimen in test 372 was subjected to an applied load equal to Iji that applied

in test 379. The initial limiting criterion in each

test was unexposed surface temperature rise, that

in test 372 having been reached at 1 hr 40 min

and that in test 379 at 1 hr 30 min.

The deflection data would be more indicative of

the efi'ect of variation of the reinforcement than

would be the surface-temperatm-e data. Unfortunately, these tests were not continued to load

failure. The available deflection data do indicate

that load failure, for the specimen in test 372,

would have been about 40 min later than that for

the specimen in test 379. From this, it appears

that the }U-in. increase in bar diameter was much

more effective than the addition of shear reinforcement.

5.3. Concrete Density

The nominal density of the ceUular concrete

specified for floors was 44 Ib/ft^, whereas that for

roofs was 31 Ib/ft^ The actual densities of the

concretes as received were obtained from fullsized planks, with correction for the weight and

volume of the reinforcement, or from cores re- moved from planks; these were within ±2.5 Ib/ft^

of the nominal densities for the floor planks and

were about 5 Ib/ft^ high for the roof planks, with

one exception (test 379). In this specimen the

average density of the cellular concrete, based on

the total weight of two extra planks, was found to

be 42.2 Ib/ft^ although they were marked as roof

planks. Even after correction for moisture content, the densities of the roof planks were in the

range of 33 to 37 Ib/ftl

The specimens of tests 371 and 379 were very

similar, differing only as to concrete density, and

the presence of shear reinforcement in the latter.

Each was subjected to the same applied load.

Each was limited by temperature rise on the

unexposed surface, reached at 1 hr in test 371

(density 35.8 Ib/ft^) and at 1 hr 30 min in test 379

(density 42.2 Ih/it^). The times at which 1,000

°F average and 1,200 °F maximum temperatures

were reached on the lower level reinforcing bars

were 59 min and 1 hr 4 min, respectively, in the

former test, and over 1 hr 30 min in the latter.

Similar comparisons of the effect of concrete

density may be made from the unexposed surface

temperatme data of the 5-in. planks in tests 358

and 373 as well as from the temperature data for

the reinforcing bars of the 5-in. and 6-in. planks

of the same tests. These data indicate that, within the ranges of

density and other variables covered in this study,

the cellular concrete will provide increased resist- ance to heat penetration at higher densities.

Such behavior agrees with that to be expected

if the thermal diffusivity of the concrete decreased

as the density increased. Thermal diffusivity a is defined as c(=k/pc, where k is thermal conductivity, p is density, and c is specific heat. Since the

cellulur concretes of various densities may be presumed not to vary greatly as to chemical composition, the values of specific heat should be reason- ably constant. Although the values of thermal

conductivity tend to increase with density, apparently the net effect for the cellular concrete in the

particular range of densities investigated is one of

lower diffusivity for that of high density than for

that of low density. This is in agreement with

previously published data [5]. However, the same

data indicate that this trend is reversed at densities of about 60lb/ft^ and that diffusivity increases

with increased density above that level.

5.4. Estimates of Fire Endurance

Data from standard fu"e tests are often used as the basis for estimates of the probable fire endurances of similar constructions not actually tested.

The usefulness of such estimates is dependent on

the amount of test data available on similar constructions, the simplicity of the basic construction,

and the degree of similarity between the constructions actually tested and those for which estimates

are made. Another important factor is the basis

for the method employed in making the estimates.

A method has been developed on the basis of

many years of experience in tests of solid, essentially homogeneous walls [6]. It relates fu-e en- durance to the 5/3 power of the thicloiess. Since

the cellular concrete fioors and roofs approximated

homogeneity very closely, it was believed that a

similar relationship might hold for them, although

probably with the thickness raised to a different

power than that for walls. A plot of the data

confirmed this assumption. Data for load failure,

unexposed surface temperature limiting rise, and

1,000 °F average and 1,200 °F maximum temperatures on the lower-level reinforcement were

plotted and lines faired through the points. The

exponents for the various lines were in the range

0.64 to 0.80.
Specimens were tested at 5-, 6-, and 8-in. thicknesses. The hnes developed from the data plots

were extended from 3 to 10 in., representing extrapolation over 2 in. of thickness at each end. The

times at which the aforementioned load and temperature conditions might be expected were picked

off the lines and are given in table 4. This table

gives only the values from the lines; hence the

values at 5, 6, and 8 in. do not agree in all cases with those given in table 1, the actual values for

the particular specimens tested. The data from

table 4 were analyzed to determine which failure

criterion would be reached first; the resulting estimated fire endmances are given in table 5. Although fire endurance ordinarily is stated to the

nearest minute, the estimates obtained from table

4 were rounded off to the nearest 5 min up to 2

hr and to the nearest 10 min for longer times.

This was done because the uncertainties in the

estimates in table 5, due to the combined effects

of uncertainty in each data point, in fairing the

lines through the data, and in rounding off the

figures, are dependent on the extent of extrapolation and the time. An estimate of about 6 hx for a 10-in. nonloaded roof slab is probably within 30

min of the true value whereas for a 10-in. loaded

roof slab the estimate of 1 In- 30 min is probably

within 10 min. For 3-in. slabs, the uncertainty

would be about 10 inin in 2 hr and 5 min in 30

min.

The times at which the high temperatures of

the reinforcing bars were reached were not considered in developing table 5, since the reinforcement temperature is not a defined hmiting condition in the standard test method [3]. Since no

specimens with 1%-in. cover were tested under

load, load-failure estimates were not made for

them in table 4. However, the tabulated estimates for 1,000 °F average and 1,200 °F maximum

may be taken as rough indications of when such

failure might occur, provided that the specimens

were loaded to develop the design stress in the

steel. The ignition of cotton waste on the unexposed

surface is an applicable end point, and occurred

twice in the study reported herein. It always

occurs at a local failm'e, such as a crack or spall.

Such local failures often indicate a point not

representative of the overall specimen; therefore

they are difficult to predict. Consequently, no

attempt was made to estimate times for this

failure in drawing up tables 4 and 5.

TabLE 4. Estimated times to various conditions

The estimates were obtained from lines faired through data in the range of 5

to 8 in. and extrapolated at both ends.

Overall thickness

Unexposed surf, temperature rise Load

failure

Lower reinf.

Ijar temperature

250 avg 325 max 1,000 avg 1,200 max

Roofs, loaded, ?4 iu. cover

in. hr : min hr : min hr : min hr : min hr : min

3 0:50 0:42 0:36 0:36 0:37

4 1:01 0:52 0:44 0:44 0:45

.5 1:11 1:00 0:51 0:51 0:52

6 1:20 1:08 0:58 0:58 0:59

7 1:29 1:16 1:04 1:04 1:05

8 1:37 1:23 1:10 1:10 1:11

9 1:45 1:30 1:16 1:16 1:17

10 1:53 1:36 1:22 1:22 1:22

Roofs, nonloaded, 1?4 in. cover

3 2:35 2:33

4 3:08 3:06

5 3:39 3:37

6 4:08 4:06

7 4:35 4:33

8 5:01 4:58

9 5:26 5:23

10 5:50 5:47

1:27 1:30

1:45 1:50 2:02 2:09 2:18 2:26

2:33 2:43

2:47 2:59

3:01 3:15

3:14 3:30

Table 5. Estimated fire endurances of cellular concrete roofs These were derived from table 4, rounding off as described in the text.

Loaded

Nonloaded '

Thickness Time Limiting criterion Time Limiting s criterion

in. h : m h : m

3 0:35 Load 2:30 ST

4 0:45 Load. _ 3:10 ST

5 0:50 Load _ . 3:40 ST

6 1:00 Load _ 4:10 ST

7 1:05 Load 4:30 325

8 1:10 Load. - . 5:00 ST

9 1:15 Load _ 5:20 325

10 1:20 Load ._. - 5:50 ST

1 With ^4-in. cover, reinforcement appropriate for span, and load based on

design stress of 15,000 lb/in.2 for roofs.

2 With l|4-in. cover, reinforcement at least 60 percent of that design live load of 56 lb/ft 2.

3 Limiting conditions: Load = load failure, 250 = average surface temperature

rise, 325=maximum surface temperattue rise; ST=both 250 and 325 degree

temperature rises at same time.

Practical considerations made it obvious that

estimates on nonloaded floors were of no value,

and might be misleading. Only one floor specunen

was tested under load and this was not deemed an

adequate basis for estimates. Therefore, tables

4 and 5 were confined to estimates on roofs.

6. Conclusions

The results of the tests clearly indicate that

cellular concrete slabs, made up of precast planks

similar to those employed in this study, can be

designed to provide fu-e endiu-ances of 1 to 2 hr

and probably up to 4 hi".

' The use of greater cover for the reinforcing bars,

in the range of ji to Iji in., even without increasing

the total thickness wQl result in longer fire endiu-ance. In practice, this fact must be balanced

against the effect on the load-carrying capacity

under normal conditions. In normal practice, a slab is designed to have a particular loadcarrying capacity. Once a combination of percent reinforcement and effective depth have been

determined to satisfy this requu'ement, they would

be held constant and increased cover would be

achieved by increased overaU thickness. Such

procedure woiild lead to longer fire endurance

10
corresponding to the increased cover and overall

thickness.

Within the density range of 35 to 50 Ib/ft^ slabs

made of higher density concrete will provide

longer fire endurance than will those of lower

density, both in terms of heat transfer through

the slab and in terms of continued structural

stability.

Although slabs of greater thickness may be

expected to provide longer fire endurance than

those of lesser thickness on the same span, no

simple conclusion can be drawn from this study

for the case of greater thickness made necessary

by the provision of longer span members having

load-carrying capacities equal to those of shorter

spans.

The use of reinforcing bars of larger diameter

and consequently of greater ratio of reinforcement results in longer fii'e endmance than the

combination of smaller bars plus shear reinforcement employed in this study.

7. References

[1] R. C. Valore, Jr., Cellular concretes, J. Am. Concrete

Inst. 25 (May and June 1954).

[2] Steel Construction, Am. Inst. Steel Construction.

[3] Standard Methods of Fire Tests of Building Construction and Materials, ASTM, Designation E 119.

[4] J. V. Ryan and A. F. Robertson, Proposed criteria for defining load failure of beams, floors, and roof constructions during fire tests, J. Research NBS 63c,

(Eng. and Instr.) No. 2, 121-124 (1959).

[5] Lightweight Aggregate Concretes, HHFA, U.S.

Government Printing Office, Washington 25, D.C.

(1942).

[6] Fire resistance classifications of building constructions.

Appendix B, NBS BMS 92, U.S. Government

Printing Office, Washington 25, D.C. (1942).

8. Appendix

Information presented here includes the more

important observations from the test logs, statements of the various end points, and the times at

which they were reached. These tunes have been

corrected for deviation, if any, of the furnace timetemperature curves from that defined in ASTM

E 119 [3]. The correction formula is given in

the same standard. Representative plots of timetemperature and deflection data, plus tabulation

of end-point and other data, have been given in

the body of this report.

The planks were numbered from the south end

of the furnace in all but Test 380. In that test

they were numbered from the west. This was

done to facilitate the recording of observations

and has been carried over in the following smnmaries of the test logs.

Roof Test 357—At 11 min, furnace and specimen luminous; 17 min, light smoke issued from joints between slabs; 41 min, diagonal cracks in unexposed surface across four corners of specimen running across two or three

planks, also planks 1,4, 10, 11, 12were cracked longitudinally

;

48 min, cracks much wider, heavy smoke from crack in

plank 10; 53K niin, deflection about 7.7 in. and was

increasing at the rate of 67 in./hr, causing loading pressure

to fall off rapidly; 55 min, planks 7-10 broke 3 ft from

west end, but reinforcement prevented complete collapse;

load off; 1 hr, gas off. The initial limiting condition in this test was load

failure; it occurred at 57 min (corrected) when the deflec- tion and rate of deflection became excessive. The limiting

temperatures were not reached on the unexposed surface;

but 1,200 °F maximum was reached on the reinforcing

bars at 54 min and 1,000 °F average at 56 min.

Roof Test 371—At 9 min, fine crack across southwest

corner of plank 2, slight separation between filler grout

and furnace along south and east sides; 21 min, transverse cracks at west end of plank 1 and east end of planks 7, 8,

9, 10, longitudinal crack in center of plank 12, all in unex- posed surface; 39 min, crack in plank 12 extended full length of plank, sides of crack in west end of plank 2

offset % in.; 51 min, two parallel cracks 8 in. apart

extended almost full length of specimen about 3 to 4 ft from east side, heavy smoke rose from cracks; 57 min,

planks buckled along transverse cracks which extended

through to exposed surface; 1 hr 8 min, pressure in loading

system unstable, partial collapse of specimen, load off, gas off. The initial limiting condition in this test was load

failure at 53 min. The maximum one-point rise of 325 °F

on the unexposed surface occurred at 1 hr, and, by 1 hr

5 min, it was 700 °F. At 59 min, the 1,000 °F avg. on the lower reinforcing bars was reached. At the end

of the test it appeared that all of the planks except 1, 2, 11, 12 were broken 3 to 4 ft from the east end.

Roof Test 379—At 37 min, planks 1 and 12 cracked

diagonally 3 ft from each end and longitudinally near

center, all cracks in unexposed surface; 58 min, planks

2, 10, 11 had short fine cracks at ends; 1 hr 11 min,

planks 1 and 12 had fine but moderately long cracks,

plank 2 had fine cracks 3 ft from west end; 1 hr 22 min,

joints between planks 3 and 4, 4 and 5, and 9 and 10

cracked, smoke was seeping out, plank 3 cracked longitudinally, and considerable amount of smoke issued forth;

1 hr 31 min, maximum deflection 9.4 in., load off, gas off. The initial limiting condition in this test was the max- imum one-point temperature rise of 325 °F on the un- exposed surface. This occurred at 1 hr 29 min near the

crack in plank 3. By 1 hr 31 min, it was obvious that

load failure was imminent. At this time, the maximum

temperature on the unexposed surface was 650 °F. Cotton waste was placed over the crack in plank 3 at 1 hr

202 min. This waste did not ignite immediately. However, observations after the test showed that it had been

charred to a great extent. Roof Test 380—At 33 min, a few cracks 1 to 3 ft long

in joints and across planks near edge of unexposed surface, plank 4 cracked longitudinally 8,^2 to 9 ft long; 52

min, 15-ft-long crack in plank 4. Plank 8 had crack 10

ft long, plank 2 had crack 3 ft long in south end, maximum deflection 6.0 in.; 58 min, deflection 7.5 in.; 1 hr

12 min, sides of crack in plank 8 offset 4 in., joint between

planks 8 and 9 offset 6 in., planks 2 through 6 bowed

up at about 3 ft from south end, plank 9 cracked across exposed surface at center span; maximum deflection 11.4

in.; 1 hr 13 min, cotton waste placed over crack in plank

8 and ignited; 1 hr 16 min, load off, maximum deflection

exceeded 12 in.; 1 hr 17 min, gas off. The initial limiting condition in this test was the ignition of cotton waste over the crack in plank 8 at 1 hr

13 min. The 1,200 °F maximum temperature on the

reinforcing bars was reached at 1 hr 11 min; the 1,000

°F average temperature was reached at 1 hr 10 min.

Load failure was imminent at 1 hr 16 min, at which time

11
the center span came into contact with the furnace struc- ture 1 ft below the initial level of the specimen.

Roof Test 368, planks 5, 6, and 8 in. thick—At 12 min,

wisps of smoke issued from joint between planks 2 and 3; 52 min, separation between planks and furnace, ends of planks raised % in.; 1 hr 7 min, cracks at east end of planks

3, 4, and 8, and at west end of plank 4; 1 hr 15 min, 6-in. planks (5-8) bowed up % in. at ends, 5- and 6-in. planks

visibly dished; 1 hr 49 min, all planks had dished appearance, plank 4 had J-^-in.-wide transverse crack near east end;

planks 2, 3, 4, and 8 had fine transverse cracks near east end, planks 2, 3, 4, 9, 10 and 12 had fine transverse cracks

at west end, all cracks in unexposed surface; 2 hr 12 min,

longitudinal crack 2 ft long in west end of plank 4, plank

10 raised V/i in. at west end, plank 12 broken 3 to 4 ft from west end; 2 hr 29 min, plank 5 raised V/i in. at west

end, glow of furnace visible through crack in end of plank

10, 2 hr 55 min; transverse cracks in exposed surfaces of

three planks 3 ft from east end, also several short cracks

in planks 3 and 4; 3 hr 13 min, plank 4 cracked so that

cotton rag ignited after few seconds, planks 3 and 4

sagged at each side of center; 3 hr 15 min, several pieces

of concrete 1- to 4-in. diam fell from plank 4; 3 hr 40 min,

fiames passing through joint between planks 4 and 5, plank 4 offset 3 to 4 in. below plank 5 at centerspan; 3 hr

40 min, furnace glow visible from crack in plank 3 at 2 to 3 in. from east end; 3 hr 45 min, gas off. After the specimen had cooled, examination showed that

the exposed surface of the 5-in. planks had vitrified to a depth of 2 in., the 6-in. planks to a depth of % in., and the

8-in. planks only on a thin surface layer. The 1,000 °F

average occurred on the reinforcing bars of the 5-in. plank

at 2 hr 9 min, the 6-in. plank at 2 hr 6 min, and the 8-in. plank at 2 hr 53 min. The 1,200 °F maximum occurred

in the 5-in. planks at 2 hr 14 min, in the 6-in. planks at 2 hr 17 min and in the 8-in. planks at 3 hr 4 min. The

maximum one-point rise of 325 °F on the unexposed sur- face was reached on the 5-in. planks at 3 hr 37 min and

the average rise of 250 °F was reached at 3 hr 39 min.

These limits were not reached on the 6-in. planks nor on

the 8-in. planks. A cotton rag was ignited over a crack

through a 5-in. plank at 3 hr 13 min, and, at 3 hr 40 min,

flames were passing through the joint between planks 5 and 6. However, the times to limiting conditions resulting

from cracking should not be taken as representative of nor compared with those observed in the tests of specimens of uniform thickness under design load. Floor Test 372—At 25 min, crack along joint between

planks 1 and 2; 37 min, plank 1 cracked along south side

where supported by furnace frame, 55 min; blister of about

6 in. diam formed; 1 hr 8 min, plank 3 had 7-in. crack

along center, maximum deflection (at south quarter

point) 5.0 in.; 1 hr 21 min, sides of crack in plank 3 offset

in.; 1 hr 39 min, maximum deflection 7.5 in.; 1 hr 59 min,

maximum deflection exceeded 10 in. and was increasing

very rapidly; 2 hr 4}^ min, load off; 2 hr 12 min, gas off. The initial limiting condition in this test occurred at 1 hr 40 min when the maximum allowable one-point temperature rise of 325 °F on the unexposed surface was

reached. Load failure was reached at 2 hr 2 min. The | average temperature on the reinforcing bars reached

1,000 °F at 1 hr 24 min, and the maximum temperature I reached 1,200 °F at 1 hr 28 min. ; Test 373—Floor planks 5 in., 6 in., and 8 in. thick.

Note: the observations for the steel I-beam in this test are given separately. At 7 min, smoke issued from joints between planks 8 and 9 and between 9 and 10; 14 min, flames j (possibly from encasing material of thermocouples) issued ! into furnace from southeast corner, continuing until after 40 min; 1 hr, cracks in. wide between specimen I and furnace frame along east and west edges; 1 hr 15 min, I short cracks in extreme west ends of planks 3 and 4 on | unexposed surface; 2 hrs, two diagonal cracks across

' center of plank 12, 8-in. planks raised y2 to 1 in. at each

end; 2 hr 23 min, long crack in middle of plank 11, diagonal

cracks in southeast and southwest corners extending across planks 1 through 4; 3 hr 8 min, 8-in. planks raised 1 to IK in., 6-in. planks raised in., separations between

planks and furnace increased, some joints between planks

cracked; 3 hr 44 min, considerable smoke issued from joint

between planks 2 and 3; 3 hr 50 min, gas off. The 1,000 °F average on the lower reinforcing bars was ' reached at 2 hr 50 min for the 5-in. planks and at 3 hr 12 I min for the 6-in. planks. The maximum of 1,200 °F was I reached at 3 hr 6 min for the 5-in. planks and at 3 hr 40 min i for the 6-in. planks. Neither of these temperatures were , reached in the 8-in. planks, but it was estimated that 1,000 1 °F average would have been reached at 3 hr 52 min.

' Again, this test was not intended to represent a single | sample of construction and was not uniformly loaded. : Consequently, there were no definite criteria which could

j be used to determine the limiting conditions of the speci- men.

Test 373—Steel I-beam encased in blocks. At 37 min, I horizontal and vertical hairline cracks in several joints

[ between blocks, vertical cracks across 2 blocks near center i of east face; 1 hr, more cracks in joints, vertical cracks

across block in west face; 1 hr 41 min, one soffit block [ cracked through and was sagging; 2 hr 11 min, third soffit 1 block from south cracked and was sagging in 3-in. deep V, I side block on west face above this dropped about 1 in., ; 2 hr 15 min, cracked soffit block and three blocks from j each side fell; 2 hr 22 min, load off; 2 hr 23 min, soffit and

side blocks from half of beam fallen, 2 hr 26 min, separation i of to 1 in. between beam and floor planks; 2 hr 51 min, { only beam protection still in place were blocks in south

j V/i ft and blocks in north 2 to 3 ft, test continued to obtain ] data on floor planks.

j The limiting condition was load failure at 2 hr 22 min.

| The average temperature of 1,000 °F on the beam was j first reached on one section of the beam at 2 hr 1 min. i The maximum one-point temperature of 1,200 °F was

reached at 2 hr 9 min. By 2 hr 30 min, the average tem- ! peratures of all 4 sections of the beam were well above

1,000 °F; the highest section average was over 2,000 °F. The temperature of the steel does not constitute a limiting

factor in tests of beams carrying design loads. '

12 U.S. GOVERNMENT PRINTING OFFICE: I95S
U.S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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Atomic Physics. Spectroscopy. Infrared Spectroscopy. Solid State Physics. Electron Physics. Atomic Physics.

Instrumentation. Engineering Electronics. Electron Devices. Electronic Instrumentation . Mechanical Instru- ments. Basic Instrumentation.

Physical Chemistry. Thermochemistry. Surface Chemistry. Organic Chemistry. Molecular Spectroscopy.

Molecular Kinetics. Mass Spectrometry.

Office of Weights and Measures.

BOULDER, COLO.

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials . Cryogenic Technical Services.

Ionosphere Research and Propagation. Low Frequency and Very Low Frequency Research. Ionosphere Research.

Prediction Services. Sun-Earth Relationships. Field Engineering. Radio Warning Services. Vertical Soundings

Research.

Radio Propagation Engineering. Data Reduction Instriunentation. Radio Noise. Tropospheric Meastirements.

Tropospheric Analysis. Propagation-Terrain Efifects. Radio-Meteorology. Lower Atmosphere Physics.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. Radio and Microwave

Materials. Atomic Frequency and Time Interval Standards. Electronic Calibration Center. Millimeter-Wave

Research. Microwave Circuit Standards.

Radio Systems. Applied Electromagnetic Theory. High Frequency and Very High Frequency Research. Modulation Research. Antenna Research. Navigation Systems.

Upper Atmosphere and Space Physics. Upper Atmosphere and Plasma Physics. Ionosphere and Exosphere

Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.

Radio Physics.

Circuit Standards.
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