NATIONAL BUREAU OF STANDARDS REPORT

^59^

FIRE TEST

OF

STEEL JOIST ROOF DECK

by

J. V . Ryan

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1002-20-4820 May 11, 1956 4594

FIRE TEST

OF

STEEL JOIST ROOF DECK

by

J . V. Ryan

for

Department of Highways,

District of Columbia Government

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publicatic

unless permiss

25, D. C. Sue

cal ly preparei

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

le or in part, is prohibited

of Standards, Washington

1 a report has been specifilar report for its own use,

REPORT OF FIRE TEST OF STEEL JOIST ROOF DECK

for

Department of Highways, District of

Columbia Government

A fire endurance test was carried out on a roof deck whose

primary components were cement bonded shredded wood fiber planks

on open-web steel joists. The deck was tested under a total

load somewhat less than the design load. The results indicated

a fire endurance of 2 hr 22 min for the test specimen.

1. INTRODUCTION

A fire endurance test was carried out in general compliance

with the provisions of ASTM E119 except that the load was slightly

less than the design load. The specimen was a roof -deck-ceiling

assembly of open-web steel joists with a deck of tongue -and -groove

planks of shredded cemented wood fiber. The deck was topped with

four-ply asphalt rag felt roofing and slag. The ceiling consisted

of gypsum-perlite plaster on furred expanded metal lath. The

floor surface was approximately 18 ft long by 13 ft 6 in. wide,

and the ceiling 17 ft 3 in* by 12 ft 8 in.

2. TEST SPECIMEN

The materials employed and the methods of their assembly are

described in the following sections. Some materials were obtained

on the open market; others were obtained directly from the manufacturer. Certain parts of the assembly operation were performed

by outside contractors or others regularly engaged in such activities; the remainder by personnel of the Fire Protection Section.

2.1 Materials

The steel joists were of the open -web type, designated SJ102

by the Steel Joist Institute, and were manufactured by a membercompany of that Institute. They were 13 ft 4- in. long and 10 in.

deep. The top chords consisted of two steel angles, 1- by 1- by

1/8-in. The web consisted of a steel rod bent to a zig-zag shape.

Rods of 9/16 in. diameter were used near each end, and of 1/2 in.

diameter over the center 10 ft. The bottom chords consisted of

two steel rods of 1/2 in. diameter, each slightly over 11 ft

long. The components were welded together so that one bottom

chord rod extended to about 7 in. from one end of the joist, and

the other rod an equal distance from the other end. The web and

chords formed triangular panels of 12 in. long bases along the

center 10 ft, A bearing plate 6 in. long, 3 in. wide, and 3/16

in. thick was welded to each end. Each joist was finished with

a black bituminous material. The average weight of the joist was

58.5 lb.

2

Steel plates 1 in. thick, 5 in. wide and about 17 ft 11 in.

long were used as bearing surfaces for the joists.

Flat diamond mesh expanded metal lath, in 96- by 28- in.

sheets, was used as the base for the ceiling plaster. The lath,

a nominal 3.*+ lb/yd2

, weighed 2.88 lb/yd2 . Cold rolled steel channels, 3/4 in. size, were used as

furring for the ceiling lath. The furring channel weighed about

290 lb/1000 ft.

The plaster was fibered gypsum cement of a brand widely

available in this country, and marked "for use with lightweight

aggregates". It was obtained from a local contractor in bags

of 100 lb nominal content. An accelerator, provided by the

manufacturer of the plaster, was obtained at the same source.

The plaster aggregate was an expanded perlite, from a

local processor, in bags marked 4- ft3 contents. It weighed

an average of 7*9 lb/ftb.

The roof deck consisted of planks of shredded wood fiber

with binders represented by the manufacturer to have been magnesium oxysulfate and silicate cements, and faced on one side

with asphalt saturated felt. They were 2 in. thick, exclusive

of the felt facing, and 30-5/8 in. wide, including a 5/8 in.

tongue along one edge. The other edge was grooved to match.

The asphalt felt was carried to the base of the tongue and to

the near lip of the groove. The planks were weighed during

construction of the specimen and found to average 4.04 lb/ft2 . They were supplied directly by the manufacturer.

Self -tapping 3/16 -in. screws, 2-1/2 in. long with a 3/8

in. hexogonal head, were used to secure the roof planks to

the joists.

* Asphalt saturated rag felt, in rolls 36 in. wide, and

roofing asphalt were used for the roof.

Slag granules, prepared for the purpose, were used as

the surface finish of the roof.

2.2 Assembly

For the purpose of providing only moderate axial restraint

a 1- by 5~in. steel plate was secured to each 4- by 4-in. bearing angle along each side of the furnace by three bolts near

each end, A 1- by 1- by 3 -in. piece of steel was placed between

the plate and the furnace frame near each end as a spacer.
*
- 3

-

Figure 1 shows the spacer, plate, and bolt heads. Eight

steel joists were placed to span the short dimension of the

furnace opening and bear on the steel plates. The joists were

placed parallel to the transverse centerline of the furnace

opening, and spaced symmetrically therefrom. The spacings

between joists on each side of the centerline were 18 in.

,

36 in., 36 in., and 10 in. No joist was located at the centerline. The joist bearing plates were securely welded along

both edges to the 1- by 5 “in. plates across the entire 5 in.

width. Nine -sixteenths -inch rods were welded transversely to

the top and bottom chords of the joists as horizontal bridging.

The cemented fiber roof planks were laid on the joists and

transversely thereto. They were laid with the asphalt saturated felt face up and were aligned and forced together to form

tight, straight tongue -and -groove and butt joints. They were

cut to lengths such that the butt joints fell over the centers

of joists. Each piece was supported by either three or four

joists, including those under the butt joints. Holes were

drilled through the roof planks into the horizontal leg of one

of the top chord angles and the self -tapping screws, with

large washers under the heads, driven in until the tops of the

heads were flush with the top surface of the roof planks.

Screws were driven 5 in. from each edge and at the center of

the 30 in. wide planks, into each joist. The screws were

staggered from one to the other of the top chord angles. Figure

2 shows the specimen during this stage of the assembly.

Long length 6 in. wide strips of roofing felt were laid

over all the joints between the roof planks and were mopped

with hot asphalt. Full 38 in. width felt strips were laid on

with hot asphalt mopped on between successive strips. Each

strip lapped the preceeding one by 26- to 28-in., to produce

a four -ply roof. After all the roofing felt was down, the

entire surface was covered with a hot asphalt flood coat of

1/2 lb/ft^ and slag was spead on at the coverage of 3 lb/ft2.

The 3/4- in. steel furring channels were tied to the lower

chords of the joists, transversely thereto, at 12 in. on

centers. Each length of channel had been cut to provide about

1 in. clearance from the furnace wall at each end. The flat

expanded metal lath was tied to the furring channels. Six-inch

wide strips of metal lath were bent and attached to the ceiling

lath and to the furnace wall. Wooden ground strips 11/16-in.

thick were tied to the bottom of the metal lath along the center

of the furnace and near each side.

,The scratch coat plaster was mixed 1 bag of gypsum to

2 ft3 of perlite to 8 oz of accelerator, applied, and scratched.

The brown coat was mixed 1 bag of gypsum to 3 ft* of perlite to

8 oz of accelerator, and applied out to the grounds. The ground

strips were removed and their places filled with plaster. The

plaster was brought down the furnace wall about 4 to 6 in., beipg

carefully formed to curve from ceiling to wall with a 3 in. radius.
>
4

The ceiling was given a 1/16-in. white finish coat. The scratch

and brown coats were applied on successive days, and the white

coat a few days later.

3o TEST METHOD

The specimen was tested in general compliance with the

methods defined in the Standard Methods for Fire Tests of Building Construction and Materials, ASTM E119*

3.1 Furnace

The furnace was in the shape of a large, fire-brick lined

box with the specimen filling the otherwise open top. The

furnace was equipped with restraining and specimen support

frame, gas -air burners, thermocouples, loading apparatus, means

for measuring deflections, and windows for observing the exposed surface during test.

3.2 Aging

The specimen was aged 32 days from the day the brown coat

plaster was applied. During the latter 15 days of this period

the furnace was closed and the relative humidity was maintained

to about 40 percent.

3*3 Loading

The design load specified by the Steel Joist Institute for

SJ102 joists of 13 ft span at 3 ft on centers was 83 lb/ft2 uniformly distributed. The dead weight of the specimen was 17*3 lb/

ft2 ? as computed from the measured weights of the various component materials. In computing the dead load, the weights of

the two end .joists were not added in, neither were they counted

as supporting any of the live load. Therefore, the uniformly

distributed applied live load should have been 65.7 lb/ft^

,

However ? the loading apparatus applied the load through 36 steel

channels, each 5 in* wide and 2 ft long, in six rows of six each,

spaced 36 in. on centers along the length of the furnace opening and 27 in. on centers across the width. Each row of loading channels was centered over one of the joists, with the

channels at right angles to the joists. The design applied or

live load amounted to 422 lb on each loading channel. Since it

was felt that the cemented fiber roof planks might not distribute

the load uniformly along each joist, the applied live load was

reduced to 62.3 lb/ft^ or 400 lb on each of the 36 loading

channels • The load was applied from four hydraulic pistons

through a system of triangles.
-

,

‘
- 5

-

3.4- Temperatures

Temperatures were measured by means of thermocouples connected to self-balancing potentiometers calibrated to read in

degrees centigrade. The thermocouples in the furnace chamber

were in porcelain insulators and encased in black iron pipes,

the others were in glass fiber sleeving. Thermocouples were

placed in the furnace chamber, on the steel joists, on the unexposed surface, on one of the 1- by 5~in steel plates, on the

steel H-section restraining frame of the furnace, on the back

of the metal lath, in one of the roof planks, and on the heads

of two of the screws holding roof planks. The thermocouple

locations are indicated in figure 3. The furnace fires were

controlled to produce temperatures as close as feasible to

those defined by the Standard Time -Temperature Curve in ASTM

E119, which included: 1000°F at 5 min, 1300°F at 10 min, 1550°F

at 30 min, 1700°F at 1 hr, l850°F at 2 hr, and 192 5° F at 3 hr.

3*5 End-Point Criteria

The Standard Test Method, E119, required that: 1) the

specimen continued to sustain the applied load, 2) flames or

gases hot enough to ignite cotton waste, not have passed

through the specimen, 3) transmission of heat shall not have

been such that the average temperature of the unexposed surface increased 250 degrees F nor the one point temperature

increased 325 degrees F above their initial values. The

fire endurance was the time at which the first of these end

points was reached, with a correction to the time for variation, if any, of the furnace temperatures from those defined.

4. RESULTS

The fire test was conducted February 7 ? 1956. All times

are from the start of the test. At 26 min, cracks extended

the full length of the ceiling along the center and about 1 ft

from each side. By 38 min additional full length cracks had

formed and that along the centerline was 1/4- to 1/2-in. wide

in places. There was a slight sag along the centerline crack.

By 41 min the plaster along one side of this crack was offset

about 1 in. below that along the other side; the maximum deflection of any of the joists was 0.13 in., the average temperature of the unexposed surface had risen only 13 deg F, and

the average temperature of the bottom chords of the joists was

232°F. At 1 hr 4 min, flames issued into the furnace chamber

from a crack 1 ft East and 4 to 5 ft North of center, and

smoke came from others. By 1 hr 21 min, flames issued from

a few other ceiling cracks. By 1 hr 47 min the ceiling surface was moderately irregular. The maximum deflection had

increased to 0.8 in., the average temperature rise on the unexposed surface to 93 beg F, and the average temperature of

the bottom chords to 8l7°F. At 2 hrs 8 min, the asphalt under

the slag surface was melting. At 2 hr 20-1/2 min a thump was

- 6 -

heard, probably the sound as a weld broke, accompanied by a slight

momentary drop of the oil pressure in the loading system,, By

this time the deflection had reached 4.9 in. and was increasing

so rapidly that the specimen was no longer sustaining the applied

load satisfactorily and load failure was taken to have occurred.

The rise of the average temperature on the unexposed surface was

112 deg F and the average temperature of the bottom chords of the

joists was 1008°Fo By 2 hr 32 min the specimen had sagged until

the ceiling was touching the furnace thermocouple pipes near

the center. The live load was removed at 2 hr 3b- min. At 2 hr

40 min the asphalt was bubbling on the upper surface in some

places. At 2 hr 54 min, the limiting rise of 325 deg F was

reached at one point on the unexposed surface and a section of

the roof deck broke, permitting hot gases through from the furnace

The asphalt was ignited immediately. The furnace fires were shut

off and the blazing asphalt extinguished by a water spray.

The roofing was pulled off and it was found that the wood

fiber roof planks had been charred through about half their

thickness, leaving a gray ash. Much of the remaining portion

was glowing. The specimen was hosed down throughly with water

to extinguish this glow. Much of the plaster was knocked down

during the hosing.

Deflections were observed at the centers of the third, fourth

fifth, and sixth joists from the North end. The following is a

summary of the net deflections from the start of the fire exposure

Joist Number

Time 3 b- 5 6

Os 00 0.0 0.0 0.0 0.0

:15 0.1 ,0 .0 .1

*30 .1 .0 .0 ol

s 45 .2 0 0 .1 .2

1 5 00 • 3 .1 .2 *3

1:15 .b- ol o 3 o4

1;30 0 b- ol .4 »5

Is 45 .5 °3 .6 .8

2 s 00 e 6 .8 1.4 1.5

2 s 15 1-5 2.6 3*8 3*3

2:20 2.2 3.8 4.9 4.2

2:25 3.3 5.3 6.1 5.1

2:30 b-. 9 7.7 8.4 6.8
,* .
- 7

-

The fire endurance of the particular roof deck tested was

limited by load failure at 2 hr 21 min* The one -point maximum

temperature rise on the unexposed surface was reached at 2 hr

54 min followed in a few seconds by passage of gases hot enough

to ignite the asphalt . By the end of the test the average temperature of the bottom chords of the joists was over 1200°

F

and the maximum over 1325°

F

5 those of the upper chords were

essentially the same* The fire exposure severity, as determined by the ratio of the area under the average furnace temperature curve to the area under the standard time -temperature

curve, was 101*1$ and the correction to the fire endurance

limit was plus 1 min. Therefore, the fire endurance of the

roof -ceiling assembly tested was 2 hr 22 min.

Examination after cooling showed that most of the joists

were bent down several inches and some were bent transversely,

as shown in figure k , 5 ? and 6. Several welds in the joists

were broken but all between the joists and the bridging were

intact, as were all between the joist bearing plates and the

1- by 5-in. steel plates. The 1- by 5-in. steel plates were

bent laterally as much as 2-1/4- in. and bent up as much as

1-1/4 in.

5. DISCUSSION

5.1 Supporting System

The joists were secured to the 1- by 5-in, plates by heavy

welds. These welds proved stronger than some of the welds in

the joists, as evidenced by the fact that several of the latter

broke. Also, the 1- by 5“in. plates gave with the joists and

were deformed from their initial positions. These two facts

indicate that this particular support system for the joists was

very strong but sufficiently flexible to provide continuing

support as the joists bent. It Is possible, if not probable,

that some of the joists were acting as suspension members rather

than truss members by the end of the fire exposure period,

5.2 Loading

As was, mentioned earlier in this report, there was some

doubt that the roof deck was strong and rigid enough to insure

that the load as applied by the apparatus was distributed uniformly along the joists. On the basis of a preliminary stress

analysis, in which it was assumed that the applied live load

was concentrated at six points along each joist and that the

dead load was given by the nominal weights of materials the

applied load was reduced somewhat below the design live load.

A second analysis was carried out subsequent to the test, assuming that each joist was loaded along six evenly spaced lengths

each equal to the 5-in* width of the loading channels. The

stresses in the various members of the joists were computed and

found to have been well below the maximum of 18000 lb/in. 2

recommended by the Steel Joist Institute. The analysis also

was carried out on the basis of a live load of 4-22 lb per

loading channel, computed on the actual dead load of 17 • 3 lb/ft2

.
.

.

.
- 8 -

Had this live load been applied, coupled with the existing dead

load, the resulting stresses would have been significantly lower

than the allowable maximum

.

6. CONCLUSIONS

The foregoing sections indicate that two conditions existed

during the fire exposure test that may have been advantageous

departures from normal procedures. First, the joists were

secured in such a manner that they could deflect into relatively

deep curves without separating from their supports. Secondly,

the applied live load was reduced, so that the actual total load

was less than the design load recommended. It appears that

neither of these two conditions, nor their combination could

have the effect of decreasing the fire endurance of the specimen

and that they may have had the opposite effect. While it is not

possible to determine what the magnitude of this increase was,

except by another fire test, consideration of the results of

this test should be made only with an awareness of these conditions .
-

I
Fig. 1. Corner detail, showing steel

plate, -bolt heads, spacers,

and joists.

Fig. 2. Specimen during placement of

roof planks, also showing

joists with bridging.

EAST

x A

THERMOCOUPLE LOCATIONS

THERMOCOUPLE LOCATION PLAN

Fig. 3* Thermocouple locations

Fig. 4. Condition of joists after test.

Fig. 5» Condition of joists in South half.

Fig. 6. Condition of joists in North half.

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THE NATIONAL BUREAU OF STANDARDS

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