NATIONAL BUREAU OF STANDARDS REPORT

3792

TEST OF PRECAST PRESTRESSED

ROOF PANEL NO. 3

(Type B)

by

L. F. Skoda, J» 0. Bryson and D. Watstein

Report to

Bureau of Yards and Docks

Department of the Navy

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary NATIONAL BUREAU OF STANDARDS

A. Y

r

. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards is suggested in the following listing of

the divisions and sections engaged in technical work. In general, each section is engaged in specialized research, development, and engineering in the field indicated by its title. A brief description

of the activities, and of the resultant reports and publications, appears on the inside of the back

cover of this report.

Electricity. Resistance and Reactance Measurements. Electrical Instruments. Magnetic

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Standards. Microwave Standards.

^Office of Basic Instrumentation ^Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1001-10-4811 November 1, 1954 3792

TEST OP PRECAST PRESTRESSED

ROCE PANEL NO . 3

(Type B)

by

L. F. Skoda, J. 0, Bryson and D. Watstein

To

Bureau of Yards and Docks

Department of the Navy

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, ref

unless permission is

25, D. C. Such peri

cal ly prepared If tl

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

In part, Is prohibited

sndards, Washington

port has been speclfiiport for Its own use,

TEST OF PRECAST PRESTRESSED

ROOF PANEL NO . 3

(Type B)

by

L. F. Skoda, J. 0. Bryson and D. Watstein

Abstract

A prestressed thin-shell roof panel 5~ by

2L|_-ft In plan was tested under a uniformly

distributed load to determine its strength

and stiffness. The panel was subjected to

the load test twice. During the first test,

a load of 70 lb/sq ft was applied in several

increments. The longitudinal reinforcement

began to yield at 70 lb/sq ft and the center

deflection of the panel was 2.9 in. Upon

removal of the load, the panel had recovered

to within 0.19 in. of its initial position

during the recovery period of about 18 hr.

During the second load test, the specimen

was again loaded in increments of 10 lb/sq ft

until failure occurred at approximately 80

lb/sq ft.

1. Introduction

The study of precast - prestressed concrete roof panels

was initiated in the National Bureau of Standards at the request of the Bureau of Yards and Docks for the purpose of

establishing design data for prestressed thin-shell ribbed

panels of various cross-sections.

The following report presents the data obtained in the

test of one prestressed roof panel of type B.

2 „

2. Description of Test Specimen

Construction of the test specimen was accomplished in

accordance with the Bureau of Yards and Docks plans (Sketch

A- 2, May l8, 1953) and specifications. The 5~ by 2lp-ft panel

was essentially a thin slab stiffened by two edge beams and

two transverse ribs. The longitudinal edge beams were 8-in,

deep and 2-1/2 in, wide at the base. The outer surfaces of

the edge beams were plumb; the inner surfaces tapered up at

a slope of 1 l/2 in, in 8 in, and became an integral part of

the slab. At a distance of 9 in, in from either end, there

were two transverse ribs. The transverse ribs were 7 7/8 in,

deep, 2-in. wide at the base and tapered up at a slope of

1-

in. in 7 7/8 in, until they merged into the slab. The portion of the slab enclosed by the longitudinal beams and transverse ribs was 1-in. deep. At the two extremities of the

panel, the slab was 3-in, deep.

The slab and the stiffening members were reinforced with

2- by 2 in. by 12 ga welded wire fabric. Each transverse rib

also included two No. Ip deformed reinforcing bars, one at the

top and one at the bottom, bent 6 in. into the longitudinal

beams. These bars met the requirements of ASTM Specifications

A 305-50T and A 15-50T for deformed reinforcing bars of intermediate grade. Each longitudinal edge beam contained one

1/2 in. Stressteel prestressing bar housed by 3/)p~in. rigid

conduit and placed in a parabolic curve (See figure 1).

3. Fabrication of Specimen and

Description of Material

3.1 F orms

Since the prestressing bars were to be post-tensioned

and the specimen had to be supported along its entire length

prior to prestressing, the forms were constructed so as to

permit stripping of the side surfaces of the longitudinal and

transverse edge beams while continuing to support the base of

the longitudinal beams. Wood trestles were used to elevate

the forms to permit working beneath the specimen without moving

it. The trestles were joined with 2-in. by Ip- in. pieces of

lumber and leveled with an engineer's level. The 2- by Ip-in.

struts were then grouted with Hydrocal to assure even bearing

thereby eliminating deflections of the form under the weight

of the concrete. Plywood was used for all cast surfaces of

the specimen to assure smooth surfaces. The web of the specimen was supported with wedge shaped 2- by 6-in. pieces of

3.

lumber spaced lip in. on center. The wedges were cut so as to

permit complete support of the web and inner surfaces of the

edge beams. The outer surfaces of the edge beams were supported lip in. on center.

3.2 Prestressing units

The prestressing units consisted of a l/2 in. Stressteel

bar of high tensile alloy steel, end anchorage components

(bearing plates, nuts, washers) and the tensioning apparatus

consisting of a hydraulic jack, adapter bar and bridge. The

adapter bar and bridge were devised at the National Bureau

of Standards and fabricated in the Bureau’s shops from steel

conforming to ASTM Specification A7~50T„ A sample of the

Stressteel bars was tested in tension in a 60,000 lb testing

machine, and the ultimate tensile stress indicated was

167,000 psi.

3.3 "Welded wire fabric

Two inches by 2-in. by 12 ga welded wire fabric, obtained

from the American Steel and Wire Company was used to reinforce

the web and edge beams of the specimen. The wire fabric met

the requirements of ASTM Specification A185-37. One-half in.

slab bolsters were used as support for the wire fabric.

3. ip Concrete

Proportions of the cement mix used were 1 ° 2 „ Ip7 ° 2 . 02, by

weight. The cement factor was seven bags per cu yd White Marsh,

Maryland sand and pea gravel were used as fine and coarse aggregates. The pea gravel was passed through a 3/8 in. standard

sieve to obtain 3/8 in. top size aggregate. High-early strength

piortland cement was used with calcium chloride (2 percent by

weight of cement) to permit early stripping and prestressing.

Five cu ft batches of concrete were used with slumps varying

from 2 in, to 5 l/2 in. The specimen was cured under wet burlap for four days, then air dried for three days to permit attachment of bonded wire strain gages. Ten standard 6 in. by

12 in. cylinders were cast and cured in a curing chamber until

they were tested. Cylinder tests were performed during the time

of prestressing and during the time of testing, with two cylinders tested in each case, and the average compressive strengths

were 6,lpl0 psi and 7,210 psi, respectively.

3.5 Prestres sing technique

Prestressing was applied to the specimen 21 days after the

concrete was placed. First the Stressteel bars were slipped

into their respective conduits which were cast in each longitudinal edge beam of the specimen. The bars were then anchored

at the ends opposite to the jacking point, as shown in figure 2.

Dynamometers were inserted at the jacking points between the

bearing plates and the prestressing apparatus. The dynamometers

were used to control the load applied to the specimen. A close

up view of the dynamometers and prestressing apparatus in place

for prestressing is shown in figures 3 and 1|. The bars were

stressed in both edge beams simultaneously until a load of

18,500 lb was obtained. The jacks were released and the load

immediately dropped to 17? 200 lb. Twelve days later, the load

had dropped to 16,800 lb. The Stressteel bars were then tensioned to 19 s 500 lb and the tension dropped to 19., 200 lb immediately after the release of the jacks. This tension was

maintained within a reasonable tolerance up to the time of test.

4. Testing Procedure

4.1 Test set-up

Eight days after the prestressing was applied, the specimen

was placed on the testing piers by means of a lifting jig and a 3- ton bridge crane. Two by 2 l/2- by 3/4 -in. bearing plates

and 2 l/2-in. rollers 3/4~in ° in dia were used to provide simple

supports. The span length between supports was 22 ft 4 in. The

specimen was mounted on the testing piers by lowering the bridge

crane and positioning one end accurately in place with the other

end coming to rest on 5 ton hydraulic jacks and lowered gently

into place. Hydrocal was used between all bearing surfaces to

assure complete bearing of the specimen.

4.2 Method of loading

Water was used to apply the uniform load to the specimen

during test. Since large deflections were anticipated, it was

felt that the use of a single tank would cause too uneven a distribution of the water at large deflections. Therefore,

four individual 5- by 6-ft tanks 2 ft deep were used to minimize the concentration of load near the center of the panel.

The tanks were made of No. 8 treated duck and were coated with

a fatty acid pitch material to make them watertight. The tanks

were loaded simultaneously and at the same rate. The total
^ ,
5 . quantity of water entering the tanks was measured with individual water meters. Figure 5 shows the overall view of the test

set up with the water tanks in place.

4- . 3 Instrumentation

Deflections of the slab under load were measured with

0.001 in. micrometer dial gages and taut-wire mirror-scale devices. During the load test, the dial gages were located on

the underside of the slab at the same points at which measurements were taken on the top of the slab during the prestressing.

Four dial gages were placed across the center cross-section of

the span and four were placed at both ends over the supports.

Taut-wire mirror-scale gages were located on the outside of

the edge beams of the center section and quarter point sections

of the span. An overall view of the location of the gages is

shown in figure 5.

Bonded wire strain gages were placed longitudinally on

the specimen on top and bottom of the slab. A total of 16

gages were used to measure strain in the concrete. All strain

gages were located across the center section of the specimen.

The location of strain gages, dial gages and the mirrored scales

is shown in figure 6.

4.4 Description of test

The specimen was loaded in Increments of 10 lb/sq ft and

readings of all gages were recorded after each loading. The

first noted crack occurred in the center section of the east

edge beam at 50 lb/sq ft. The specimen was loaded to 70 lb/sq ft

and that load was maintained f or 1 l/2 hr at which time an additional set of readings of the gages was recorded. (See figure 7)

The load was then removed from the specimen by siphoning the

water from the tanks. The center section of the specimen recovered to within 0.2 in. of its original position and the tensile cracks closed up. Figure 9 shows the same crack as figure

8 with the load removed. The next day, the specimen was reloaded

and additional measurements were recorded.

It is noted here that the micrometer dial gages at the

center section had to be removed before the 70 lb/sq ft load

was reached due to excessive deflections of the slab and the

readings obtained from them was later eliminated because of

inconsistencies
/
6 .

5. Test Results

The observations made during the prestressing operations

included the measurement of tension in the prestressing units

by means of dynamometers, the strain on the surface of the

concrete at the transverse center section of the specimen and

the deflections at the center and quarter points of each edge

beam. The variation of the strain with the tension in the

prestressing units is shown in figures 10 and 11; the tensioning loads are shown plotted in figure 12 against the average

values of strains representing pairs of corresponding gages

arranged symmetrically about the longitudinal axis of the

panel. The maximum value of prestress observed at a point

1/2 in. above the bottom of the edge beam during the initial

prestressing was 150 x 10"^ in. /in., or about 750 psi, at a tension of 19 kips. The maximum stress at the outermost fiber

was estimated to be about 800 psi at the bottom of the edge

beam. and about 1/0 psi at the top.

The relationship between the tensioning force and the

strain in the edge beam was fairly linear up to about 12 kips.

Above that load, the strain at the bottom of the edge beam increased more rapidly with the tensioning force, while the strain

at the top reached its peak value at a load of lip kips and decreased from then on. It is noted that the relationship between

the tensioning force and the deflection at the center shows substantial linearity up to about 12 kips, and a sharp increase in

deflection at a tensioning force over l8 kips. (See figure 13).

As might be expected, at the quarter points the force - deflection relationship was linear up to about 17 kips but the same

sharp increase in deflections was observed at a tensioning force

of 18 kips.

The load test was conducted over a period of two days

(April 29 and 30, 1954). On the first day, at the design load

of 20 lb/sq ft, a deflection of 0.21 in. was observed at the

center section and at a load of IpO lb/sq ft a deflection of

0.1p8 in. was noted. At those loads, the tension in the prestressing units increased over the initial value by 0.5 and

2.9 percent, respectively. (See figures llq and l5) . No cracks

were observed until a load of 50 lb/sq ft was applied and the

deflection of the center section at that load was 0.89 in. At

70 lb/sq ft the steel began to yield with the center deflection being 2.9 in. The specimen continued to deflect under

this load and the deflection increased to 3*07 in. after 1 l/2

hr, at which time the load was removed. Prior to applying the

7.

load the following morning it was observed that the center

section of the panel had recovered to within 0.19 in. of

its initial position, and the tension in the prestressing

units was on the average 2000 lb less than at the outset

of the test.

The distribution of strain in the edge beams was observed during the first application of load. As can be

seen in figure 16 , the distribution was quite linear up to

a load of 30 lb/sq ft and was only slightly curvilinear up

to a load of Ip 0 lb/sq ft. Although the first crack was observed at a load of 50 lb/sq ft, it is probable that cracks

too fine to be seen formed soon after the application of

the load of 1|0 lb/sq ft. The maximum observed tensile

strain at that load was 380 x 10 ”® in. /in., and recalling

the fact that the prestressing strain was 150 x 10"° in,/in.,

we may conclude that the actual tensile strain at the

bottom of the edge beam at a load of IpO lb/sq ft was about

230 x 10" & in. /in., a value having an order of magnitude of

extensibility of concrete in flexure.

On the second day of test, the specimen was again

loaded in increments of 10 lb/sq ft until failure occurred

at approximately 80 lb/sq ft. The variation of the tension

in prestressing units with the applied load is shown in

figure 17 , and the load-deflection relationship is shown in

figure 18 . The deflections of the specimen at the center

section and the quarter points were substantially the same

during the second load test as in the first one, up to the

design load of 20 lb/sq ft. However, for higher loads the

deflections in the second load test were significantly

greater than those in the first load test. The increases in

the deflections during the second test were 115 percent,

55 percent and 26 percent at loads of IpO, 60 and 70 lb/sq ft,

respectively

The primary cause of failure was yielding of the longitudinal reinforcement accompanied by compressive failure at

the top of the edge beams and the adjacent portion of the

slab. The view of the panel at failure is shown in figure 20

and a close-up of the edge beam is shown in figure 21. The

same section of the edge beam after the load was removed is

shown in figure 22. It can be seen that fracture occurred in

the welded wire reinforcement at that section. An overall

view of the specimen after removal of the load is shown in

figure 23. The maximum observed deflection of the panel at
'

'
8 .

center was about 8.2 in., and at that point the panel was

temporarily blocked-up to prevent its complete collapse.

Upon removal of the load, the panel recovered to within

1.1+5 in. of its initial position.

The crack pattern observed during the test and after

removal of the load is shown in figure 21+

It will be seen

that tensile cracks developed at the edge beams within approximately the middle half of the span and that longitudinal

tensile cracks appeared on the top surface of the slab just

inside and parallel to the edge beams. A similar tensile

crack, not shown in figure 2I+, was observed along the longitudinal center line at the bottom of the slab.



I

i

.

t

ADDITIONAL MESH

f - 9" WIDE AT

SECTION 1^1 SECTION Z ^ Z

= i

’ - o" scale: 3" = i

1 - o"

GENERAL NOTES

CONCRETE CYLINDER STRENGTH

CEMENT FACTOR

MAX. SIZE AGGREGATE

INITIAL VKE STRESS FORCE

TER BAR

INITIAL PRESTRt.SS FORCE — RER WIRE

MIN COVER ON MESH

4,000 RSI

7 BAGS/CU.YP.

3/8 IN.

18,500 LB.

1,165 LB.

3/8 INPre stressed - Precast

Roof Panel -* Tyfe B

DATE: 5 IR-53 SKETCH - A2

DES. w. C. GREEN

DRAWN BY -

, krussell

scale: 3

FIG. 1
/ $
Close-up

view

of

anchorage

of

prestressing

bar.

Pig.

3

-

Close-up

of

prestressing

apparatus

and

dynamometer

in

olace

Arrangement

for

prestressing.
*

•/ 4 kI

6 A /
T72

LOCATION OF S R -4 STRAIN GAGES

4 MIRRORED SCALES

M

MIRRORED SCALES

GRADUATIONS

DIAL GAGES

SR - 4 ST R AIN GAGES

ON TOP OF SlAB

ON BOTTOM OF SLOB

X)H

WEST

DYNAMOfc c TPR

EAST

DYNAMOMETER

F l G. 6

Pig.

7

-

View

of

specimen

under

70

psf

loading.
I

*
Pig.

8

-

Crack

in

quarter-point

section

of

east

edge

beam

at

70

psf.

(First

crack

opened

at

center

section

at

approximately

50

psf)

„

Fig.

9

-

Same

crack

as

Figure

8

with

load

removed.

PRESTRESSED SLAB NO. 3

tf>l2

a.

STRAIN

FIG. 1 o

PRESTRESSED SLAB NO. 3

ENSIGN

SECTION

OOOOOO 00

flG. STRAIN

TENSION,

KIPS

PRESTRESSED SLAB NO. 3

20

18

16

14

10

8

6

4

2

0

IT VS. STRAIN

INTER

' L Q I (AXfFR ui- < OPP ryciTFTE

Li::: -L|-|4

L» 1 \

l -FrfnU U

: j-L , ur r

0 0 0 0 0

STRAIN

0 0

FIG. 12

PRESTRESSED

SLAB

FIG. 13

DEFLECTION

PRESTRESSED SLAB NO. 3

APPLIED LOAD VS. TENSION IN PRESTRESSING UNITS

TENSION IN PRESTRESSING UNITS, KIPS

FIG. 14

L

OAD.

PSF

PRESTRESSED SLAB NO. 3

APPLIED LOAD VS. DEFLECTION

DEFLECTION

00 ^

N I ‘lNV39 39Q3 30 dOl lAJOdd 30NV1SIQ

STRAIN,

I0'

b

IN/IN

APPLIED

LOAD,

PS

F

PRESTRESSED SLAB NO. 3

APPLIED LOAD VS. TENSION IN PRESTRESSING UNITS

APRIL

17 18 19 20 21 22 23 24 25 26 27

TENSION IN PRESTRESSING UNITS, KIPS FIG. 17

LOAD

,

PSF

PRESTRESSED SLAB NO. 3

APPLIED LOAD VS. DEFLECTION

FIG. 18

PRESTRESSED SLAB NO. 3

STRESS -STRAIN CURVE OF CONCRETE CYLINDERS

INDER

h :b

4-

I

0

STRAIN FIG. \9

Fig.

20

-

Overall

view

showing

failure

at

80

psf

.

Fig.

I4.

-

Arrangement

for

prestressing.
*

...

*
View

of

crack

in

east

edge

beam

at

failure.

(

Load

=

80

psf )

.
%

;

fc
ig.

22

-

View

of

crack

in

east

edge

beam

after

removal

of

1

oad

.



CRACK

PATTERN

to

MLD

CDO

o)

_/X

X

<

h- !

/ In

N

I 'lo

tT

cm

CM

MI !

1 In.

i LO

©

(D—

j

1 i

CO

FiG. 24

FLAM

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

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publications should be addressed to the Office of Scientific Publications, National Bureau of Standards, Washington 25, D. C.