NATIONAL BUREAU OF STANDARDS REPORT

6182

BEHAVIOR OP THIN- SHELL CONCRETE ROOP

PANELS 'UNDER SUSTAINED LOADS

By

L. F* Skoda and A. P. Kirstein

Report to

Bureau of Yards and Docks

Department of the Navy

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

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

3, 1901, as amended h) Congress in Public Law 619, 1950. These1 include the development and

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

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisor) services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of tin* Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particular!) tlu* Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the. back cover.

Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project

or program. Published papers appear either in the Bureau’s own series of publications or in the journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available .from the Go\ eminent Printing Ollioe: The Journal of Research, which presents

complete papers reporting technical investigations: the Technical News Bulletin, which presents

summai) and preliminary reports on work in progress: and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for radio communications throughout

the world. There are also live series of nonperiodical publications: The Applied Mathematics

Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards (SI.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office, Washington 25. D. C.

inquiries regarding the Bureau s reports should he addressed to the Office of Technical Information, National Bureau of Standards. Washington 25. D. C.
NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT MBS «t SPORT

1001-12-l|8ll October 13, 1958 6182

BEHAVIOR OP THIN-SHELL CONCRETE ROOF

PANELS UNDER SUSTAINED LOADS

By

L. P. Skoda and A. P. Kirstein

To

Bureau of Yards and Docks

Department of the Navy

NATIONAL BUREAU Of ST

intended for use within the

to additional evaluation and i listing of this Report, either

the OlHce of the Director, Nt however, l»y the Government

tc reproduce additional copk

IMPORTANT NOTICE

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

progress accounting documents

ormally published It is subjected

5, reproduction, or open-literature

Ission is obtatned Sn writing from

Such permission Is not needed,

y prepared if that agency wishes




U. S. DEPARTMENT OF COMMFRCE

NATIONAL BUREAU OF ST.iNDAKUS
-yg &'n*i '

. ::

;

'•
BEHAVIOR OF THIN-SHELL CONCRETE ROOF

PANELS UNDER SUSTAINED LOADS

By

L. F. Skoda and A. F. Kirstein

Sustained load tests were performed on four thin-shell

concrete roof panels to obtain information on their general

behavior and deflection with time. Periodic time-deflection

observations yielded data which indicated that the technique

of casting initial camber into panels of this type is a satisfactory method of eliminating the deflection due to dead

load. Comparisons of the time-deflection relationships of

the panels investigated also indicated that the history of

and transportation of these panels may cause initial differential displacements which in turn may cause distress in the

calked joints necessary in this type of construction.

1. INTRODUCTION

The object of these tests was to observe time-deflection

relationships of thin-shelled precast concrete roof panels

subjected to various loading conditions. A major factor to

be considered in the design of flat roof panels is the elimination of concave surfaces or pockets vhich would allow water

to accumulate and possibly seep through the joints. Since

thin-shelled concrete panels deflect under loads, the problem

of the designer is to compensate for this deflection. One

solution of this problem is to design panels having initial

positive camber so that expected loads cannot produce these

concave surfaces.

At the request of the Bureau of Yards and Docks, a cambered and a flat roof panel were cast and tested at the

Structural Engineering Laboratories of the National Bureau

of Standards for the purpose of studying this problem. In

addition to these panels the report describes the behavior

over an extended period of a previously cast prestressed and

and a non-pres t res s ed panel.
' ,* .
2. DESCRIPTION OF TEST SPECIMENS

Two types of roof panels designed by the Bureau of Yards

and Docks were constructed for this test. Type A was a prestressed panel and Type C was a non-prestressed panel. The

first panel to be put under test was a Type A panel. The

succeeding panels Nos. 2, 3* and 4 were of the Type C design.

2.1 Description of prestressed panel

The prestressed panel was channel shaped in cross section

with overall dimensions of 5 ft by 2I4

. ft by 8 in. with a web

thickness of one inch. The panel had transverse ribs at a distance of 9 in. from either end.

Two ten-wire Freyssinet prestressing assemblies were

used to apply a prestressing force of 30*300 lb on each longitudinal rib. Each assembly consisted of two end anchorage

cylinders, two end anchorage cones, one flexible steel conduit, and ten 0.196 in. diameter high yield point steel wires.

In order to accurately determine the amount of prestress applied to each longitudinal rib, a dynamometer was placed between the anchorage cylinder and concrete at one end of each

assembly.

In addition to the prestressing steel, 2- by 2-in. by

12/12 gage welded wire fabric was placed at the midplane of

the cross section of the panel.

A detailed description of the reinforcement, casting,

and prestressing of a duplicate specimen which was tested to

destruction under uniform loading was presented in NBS Report

No. 2295 entitled, "Test of Precast Prestressed Roof Panel

No. 1.” Report No. 2295 included the physical properties of

the prestressing steel as well as photographs of the dynamometers and tensioning devices used in the investigation presented herein.

2.2 Description of reinforced precast panels

The non-prestressed precast roof panels Nos. 2, 3, and

4 * were also essentially channel shaped in cross section with

overall dimensions of 5 ft by 24 ft by 10 in. Each panel had

six transverse ribs. Two of the ribs were located 9 in. from

each end and the four intermediate ribs were equally spaced

between them 4 ft 6 in. apart.

2 .

The major tensile reinforcement was located near the

bottom of each longitudinal rib and consisted of a single

No. 7 deformed reinforcing bar. All transverse ribs were

reinforced with a pair of No. 3 deformed reinforcing bars

located near the top and bottom of each rib. Welded wire

fabric 2- by 2-in. by 12/12 gage was placed at the midplane

of the cross section of the panels.

Panel No. 4 was similar to panels Nos. 2 and 3 in all

respects except that a positive camber was cast into this

panel such that the middle of the 2lp ft length was 0.75 in.

higher than the ends

.

The deformed reinforcing bars used in all panels were

of intermediate grade steel which met the requirements of

ASTM A15-54T Ton billet steel bars and the deformations of

these bars complied with the requirements of ASTM A305-53T.

2.3 Concrete mix

Type III cement, sand, and rounded pea gravel were proportioned 1:2.48:2.02, by weight, with a water-cement ratio

of 0.53* The average 7 day compressive strength of 6- by

12-in. control cylinders was 6800 psi for this concrete mix.

3. TEST SETUP

3.1 Loading conditions

After each panel was cast and moist, cured, it was transferred from the laboratory to the outdoor test site. The

first two panels were transferred to the test site in the

months of March and May 1953. The succeeding panels were

moved to the test site in September 1954 and February 1955.

All four panels were simply supported on a span of 22 ft 4 i n

A uniformly distributed live load of 45 and 20 psf was

applied to panels Nos. 1 and 2, respectively. Panel No. 1

was loaded on the day following its transfer to the test site

Panel No. 2 was placed under test supporting its own weight

for approximately one month before the required live load

of 20 psf was applied to its top surface. In both cases

the live load was applied with l/4 in. pea gravel carefully

screeded to the calculated height that would produce the required loads. Side boards were placed about the periphery

of the panels in order to retain the gravel.

3 .

The construction of an addition to the building adjoining the test site necessitated moving the panels to a new

location. In April 1954 both panels were unloaded and moved

to their present location. The panels had been subjected to

their particular live loads for approximately one year prior

to the unloading and moving. After the panels were unloaded

and moved to the new test site they remained unloaded for

the duration of the test.

Panels Nos. 3 and Ij. were only tested under dead load conditions. They were transferred to the test area directly from

the curing area and observed periodically to determine their

deflections

.

3.2 Instrumentation

Since deflection measurements were to be made over a period of years, the measuring technique had to be independent of temperature and movement of the supporting piers. A

taut wire system that enabled measurements to be taken with

an accuracy of about 0.02 in. was decided upon.

The taut wire system consisted of an 0.011 in. diameter

stainless steel wire and a stainless steel scale graduated in

hundredths of an inch. The wire was draped over pivot points

that were attached to the panel just over the panel supports.

Weights were attached to the ends of the wire so that a fairly constant tension would be maintained. The stainless

steel scale was attached to the edge beams of the panels and

it was intersected by the wire at mid-span. The stainless

steel scale and the pivot points were firmly anchored to the

panel with lead expansion shields. This system was applied

to each longitudinal rib of every panel.

Readings were also made with dynamometers that had been

used to determine the applied prestressing forces on panel

No. 1. These readings were taken for about 80 days; after

this period, the resistance to ground of the gage elements

was such that further readings were deemed unreliable.

4 .
-

'
4. TEST RESULTS

4.1 Load and time-deflection relationships

Figure 1 shows the variation of positive camber with

time and different loading conditions in panel No. 1. Initial measurements were made prior to the application of prestress. The amount of immediate positive camber as a result

of prestressing was 0.84 in. as indicated by point A of the

legend. The upward movement continued to increase and was

1.10 in. at point B just prior to the transfer of the panel

to the test site. As a result of lifting and moving the

panel to the test site an immediate increase in camber of

0.12 in. occurred. The total positive camber after moving

was then 1.22 in. as shown at point C. This upward movement

continued for 24 hr and became 1.34 in. just prior to loading (point D) . The reduction of positive camber due to the

application of the 4^ psf load was 0.60 in., leaving a net

camber of 0.74 in . shown by point E.

After being subjected to the live load for approximately

400 days, the panel was unloaded and moved to the new test

site. The camber just prior to unloading was 0.71 in. After

unloading and moving, the total upward displacement was 1.3 in

( point G)

.

After about two months of a rather steady increase in

upward displacement, the camber reached a value of 1.88 in.

From this time on the camber increased more slowly and fluctuated between maximum and minimum values of 2.03 and 1.63 in.

respectively. These fluctuations were apparently due to the

temperature and moisture content of the panel.

Figure 2 shows the time vs. deflection relationship for

panel No. 2. The deflection, in inches, is the downward movement of the panel at mid-span. An immediate deflection of

0.14 in. resulted when the panel was moved from the casting

table to the test site (point A). Prior to loading, the panel

carried its own weight for approximately 30 days. The deflection just prior to application of a superimposed load of 20

psf was O .36 in. (point B) and was 0.60 in. just after loading

(point C). After a recovery of 0.20 in. which took about

three weeks, the deflection progressed at a fairly linear

rate for a period of about a year. The deflection (point D)

was 0.36 in. just before the panel was unloaded. Immediately

after the panel was unloaded the deflection was 0.43 in.

(point E) . The panel remained unloaded at the old site for

two days before it was moved. During this time additional

recovery occurred and is indicated by point F at 0.40 in.

3 .
•

*
The deflection was measured just after the panel was moved and

no change was observed. From this point on the deflection increased slightly and fluctuated between maximum and minumum

values of 0.69 and 0 . Ip3 in., respectively.

Figure 3 illustrates the time-deflection relationships

for panels Nos. 3 and 4- These two panels were never subjected to live loads and the deflections were measured with respect to the original profiles of the panels as cast.

The deflection of panel No. 3 us it was placed on its outdoor piers was 0.58 in. Within the first ten days a recovery

of 0,13 in. occurred. The average total deflection for the

next 2l|.0 days was approximately 0.47 in. After 300 days the

deflection increased slightly and fluctuated between maximum

and minimum values of 0.72 and 0.56 in.

Panel No. 4 was cast with an initial camber of 0,75 in.

and figure 3 illustrates the downward movement at mid-span.

As the panel was placed under test a deflection of 0,28 in,

was observed. For the next 160 days the rate of increase

of deflection was rather rapid and the deflection reached

0.53 in. From this point on the deflection increased slightly

and fluctuated between maximum and minimum values of 0.65 und

0.50 in. The curve shows that a small amount of camber is

still present in the panel.

4*2 Variation of prestressing force

Figure 4 presents the variation of the amount of prestress

in Panel No. 1 with load and time. The initial prestress was

taken as 100 percent and the periodic dynamometer measurements

indicated the variation of the prestressing force. Observations were discontinued after 80 days due to the low resistance to ground of the sensing elements of the dynamometers.

5. DISCUSSION

It is obvious from the increase in camber with time shown

in figure 1, that the prestress applied to Panel No. 1 was In

excess of that required to eliminate dead load deflections.

Further observations indicate that the prestress was great

enough to maintain a camber of about 0,7 in. for about 400

days while the panel was subjected to a uniform live load of

45 psf.

6 .

The panels required different lengths of time to reach

a fairly constant rat e of deflection. This was due to the

fact that the loading conditions varied in each case and to

the fact that the initial deflections of panels Nos. 2 , 3r

and were different. These three panels were identical in

design, but when they were placed on their respective outdoor piers a maximum difference in initial deflection of

0.44 in. was observed between panels Nos. 2 and 3 . This

large difference in initial deflection can be attributed to

the manner in which each panel was handled and transported

to the test site. However, the important point to observe

is that all three panels reached equilibrium at approximately 0.6 in. of deflection regardless of their previous

history of handling, transportation, and loading condition.

Nevertheless, careful consideration should be given to the

methods used in handling and transporting panels of this

kind as they are easily damaged.

A comparative study of the time-deflection relationships of the various panels reveals that the deflection

characteristics of all panels are quite similar after the

initial deflections due to handling and loading. The variation in deflection after the uniform rate was reached can be

attributed to temperature and moisture conditions. However,

the magnitude of the changes due to the variations of temperature and moisture were not expected to be as great as they were. It is believed that in future tests of this kind

it will be necessary either to make careful observations of

the weather conditions at the site, or have a duplicate control specimen tested concurrently in the laboratory under controlled conditions.

The 0.75-in. camber that was cast into panel No. 4 adequately compensated for the dead load deflect ion, . as the

equilibrium deflection reached by this panel was approximately

0.65 in. This resulted in a mid-span elevation of 0*10 in,

which is not excessive considering the span length of 22 ft

4 in*

When considering a structure composed of panels of the

type investigated herein, it is also necessary to give consideration to the jointing material required to weatherproof

the structure. It is evident that any deflection or movement

of one panel with respect to the other will cause serious distress in the calked joints between them. Therefore, precision

casting is essential in the production of these panels.

7 *

6. CONCLUSIONS

l r Caution must be exercised when handling and transporting thin-shell prestressed and nori-prestressed panels

as they can be damaged through careless or improper handling.

2. The effect of temperature and moisture on the panels

was greater than anticipated indicating a need for careful

observations of the weather conditions at the test site.

3. The technique of casting initial camber into a nonprestressed panel appeared to be a very satisfactory method

of eliminating the deflection due to dead load.

8 .

2.2

<n><Q

Uis

FIG. I

VARIATION

OF

UPWARD

CAMBER

IN

PRESTRESSED

PANEL

WITH

TIME

AND

DIFFERENT

CONDITIONS

OF

LOADING

DEFLECTIONS

WERE

MEASURED

WITH

RESPECT

TO

THE

ORIGINAL

PROFILE

OF

PANEL

AS

CAST.

o

I

I

I S3HDNI N0I1331 330

FIG.

2

VARIATION

IN

DEFLECTION

AT

MIDSPAN

OF

FLAT

PANEL

NO.

2

WITH

DIFFERENT

CONDITIONS

OF

LOADING

DEFLECTIONS

WERE

MEASURED

WITH

RESPECT

TO

THE

ORIGINAL

PROFILE

OF

PANEL

AS

CAST,

CO

>-<

o

IaJ

hFIG.

3

DEFLECTION

AT

MIDSPAN

OF

CAMBERED

AND

FLAT

ROOF

PANELS.

DEFLECTIONS

WERE

MEASURED

WITH

RESPECT

TO

THE

ORIGINAL

PROFILE

OF

PANELS

AS

CAST.

o

o

rO

O

CM

OO

O

O

o

o

0>

o

o

00

c/> °o £<

N Q

o yy

o

o

in

o

o

o

o

W

o

o

CM

OO

O

S3H0NI N0I103U3C3 FIG.

3

DEFLECTION

AT

MIDSPAN

OF

CAMBERED

AND

FLAT

ROOF

PANELS

.

DEFLECTIONS

WERE

MEASURED

WITH

RESPECT

TO

THE

ORIGINAL

PROFILE

OF

PANELS

AS

CAST.

100^

if)

(/) UJ

QCH

if) UJ

0C

Cl

A - 0.1% INCREASE DUE TO MOVING.

B- 45 psf UNIFORM LIVE LOAD APPLIED.

C- 3% INCREASE DUE TO APPLICATION

OF LIVE LOAD.

<

hU.

O

if)

if)

UJ

tr

I—

if)

UJ

a:CL

UJ3

-J

$

90

20 40

TIME, DAYS

60 80

FIG. 4 VARIATION OF PRESTRESS WITH LOAD AND TIME.

U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Asiin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its headquarters in Washington

D. C., and its major laboratories in Boulder, Colo., is suggested in the following listing of the

divisions and sections engaged in technical work. In general, each section carries out specialized

research, development, and engineering in the held indicated by its title. A brief description of the activities, and of the resultant publications, appears on the inside front cover.

r WASHINGTON, D. C.

Electricity and Electronics. Resistance and Reactance. Electron Devices. Electrical Instru- ments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical instruments. Photographic

Technology. Length. Engineering Metrology.

Heat. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology. Engine

Fuels. Free Radicals Research.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics. Electron Physics. Atomic Physics. Neutron Physics. Nucleur Physics. Radioactivity. X-rays. Betatron. Nucleonic Instrumentation. Radiological Equipment.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry. Inorganic Chemistry. Electrodeposition. Molecular Structure and Properties of Gases. Physical Chemistry, Thermochemistry. Spectrochemistry, Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics.

Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather, Testing and Specifications. Polymer Structure. Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion,

Metal Physics,

Mineral Products. Engineering Ceramics. Glass. Refractories, Enameled Metals. Concreting Materials. Constitution and Microstructure,

Building Technology. Structural Engineering. Fire Protection. Air Conditioning, Heating,

and Refrigeration, Floor, Roof, and Wall Coverings. Codes and Safety Standards. Heat

Transfer.

Applied Mathematics, Numerical Analysis, Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques. Digitul

Circuitry. Digital Systems. Analog Systems, Application Engineering.

9 Office of Basic Instrumentation. ® Office of Weights and Measures. BOULDER, COLORADO

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

Gas Liquefaction.

Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular

Propagation Services. Sun-Earth Relationships. VHF Research.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Systems.

Navigation Systems. Radio Noise. Tropospheric Measurements, Tropospheric Analysis.

Radio Systems Application Engineering. Radio Meteorology.

Radio Standards, High Frequency Electrical Standards. Radio Broadcast Service. High.

Frequency Impedance .Standards. Calibration Center. Microwave Physics. Microwave Circuit

Standards.