National Bureau o1 Standards

Library, E-01 Admin. Bldg.

Referenc3 book nol to b-3 H

taken from the library.

BUILDING SCIENCE SERIES

31

U.S.

PARTMENT

t/35

J Hio

Cow 1

Flexural Behavior

of Prestressed Concrete

Composite Tee-Beams
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[See mailing list announcement on last page.]
Flexural Behavior of Prestressed

Concrete Composite Tee-Beams

J. O. Bryson and E. F. Carpenter

Building Research Division

Institute for Applied Technology

National Bureau of Standards Washington, D.C. 20234

on ^

Building Science Series 31

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 31, 14 pages (July 1970) CODEN: BSSNB

Issued July 1970

For sale by the Superintendent of Documents, U.S. Government Printing Office W.ishington, D.C. 20402 (Order by SD Catalog No. C 13.29/2:31), Price 25 cents
fiATIONAL BURLaU OF m]l

SEP 1 2 1970

mo

Contents

Page

1. Introduction 1

2. Test specimens and materials . 1

2.1. Beams 1 2.2. Concrete 4

2.3. Prestressing steel 4

3. Test procedure 5

4. Test results and analysis 5

4.1. Notations 5

4.2. Load response 5

4.3. Ultimate strength 7

5. Discussion 11

6. Conclusions

.

12

7. References 12

II
Flexural Behavior of Prestressed

Concrete Composite Tee-Beams

J. O. Bryson and E. F. Carpenter

Prestressed Tee-beams constructed by the split-beam method were tested to failure in flexure

to study the behavior and ultimate strength of these beams and to compare their flexural characteristics with those of prestressed beams of conventional construction. The compressive portion

of the cross section of the split-beam is cast after the web of the beam has been formed and pre- stressed. The variables in the study included the percentage of prestressing steel, strength of concrete in the compressive element of the composite split-beams, manner of prestressing and web

reinforcement.

Results showed that the composite split-beams behaved similarly to the monolithically con- structed beams on the basis of flexural response and ultimate load. The strength of the concrete

for the compressive element can be reduced within limits from that required for the prestressed element without sacrificing ultimate load capacity. The required percentage of reinforcing steel

is less for the split-beam compared with conventional beams.

Key words: Composite concrete construction; prestressed concrete beams; Tee-beams.

1. Introduction

A notable departure from the usual concept of composite prestressed concrete design was developed

several years ago by A. Amirikian [1] ^ of the U.S. Department of the Navy. The principal objective of

the Amirikian concept is to optimize the application

of prestressing for flexural concrete members by prestressing only the area of the cross section normally

subjected to tension under bending. This requires that the tensile and compressive areas of the beam's cross

section be constructed separately in order to restrict the precompression of the concrete to the tensile sec- tion. Therefore, this procedure can be considered as

a special case of composite construction in which the

interface of the two elements is set at the neutral axis

of the composite section instead of at the junction of

the flange and web as in normal composite construction. Beams constructed by this procedure are called "split-beams" and feature reduced prestressing forces

for the same working load capacity compared with

similar beams of conventional design.

A series of prestressed Tee-beams constructed by

the split-beam technique were tested to failure to study

the behavior and ultimate strength of these beams and

to compare their performances with those of conventional monolithic prestressed beams. The variables in the study included the percentage of prestressing steel, strength of concrete in the compressive element of

the split-beam, manner of prestressing, and web

reinforcement.

The work reported here is an extension of an earlier study [2] with split-beams of rectangular cross section. With the rectangular beams the principal difference between the split-beam and conventional beam was in the required prestressing force and location of the prestressing tendon in the cross section. The cross- sectional properties of Tee-beams lend themselves to an additional advantageous feature in the split-beam

technique. With Tee-beairs under flexural loading the

strain on the compressive surface ( top of the flange I is usually considerably less than that on the tensile surface (bottom of the stem) due to the position of the elastic neutral axis. This means that the strength

of the concrete in the flange section, provided to resist compressive stresses, need not be as high as that re- quired in the tensile section which is initially cast and

prestressed. This allows for savings in materials.

2. Test Specimens and Materials

2.1. Beams

The concept of split-beam design takes advantage of

the technique of composite construction for minimization of the prestress in the cross section. The design

' Figures in brackets indicate the literature reference at the end of this paper.

1
procedure is to determine the overall cross section for the beam as would be done for a conventional monolithic prestressed beam. From the properties of the

full cross section, the area that will experience tension

under loading is defined by the location of the elastic neutral axis. This area will be cast separately in the split-beam construction and is termed the tensile ele- ment. After prestressing the tensile element, the zone

of the split-beam that will resist compression is cast- on and is practically stress free prior to the application

of live load. The specimens in this investigation included beams

of conventional monolithic construction as well as the split-beams of composite construction. They were

all Tee shaped in cross section with a 3-in by 15-in

flange, an overall depth of 18 in, and were 19 ft long. The two monolithic beams were post-tensioned

while the split-beams included both post-tensioned

and pretensioned specimens.

Figure 1 shows the nominal dimensions of the beams with the location of the prestressing tendon in the cross section given for both the monolithic beam

and the split-beam. Also, the positions of support and

points of loading for tests are indicated.

fixed in position at the ends of the form and at the third-point and midspan locations. A single steel bar, threaded on both ends, was used

as the prestressing tendon in the post-tensioned beams.

In each case, the tendon was straight and located at a constant depth in the cross section throughout the length of the beam.

Figure 2 shows a tensile element setup for post- tensioning. The tensioning force in the tendon was measured with a steel dynamometer attached to the tendon at the end of the beam opposite to the jacking

end. This force was distributed over the ends of the prestressed element with 1-in thick bearing plates.

Heavy duty steel nuts bore against the dynamometer

on one end and the bearing plate on the other end to maintain the prestressing force in the element. The pretensioned beams were each prestressed with two strands of steel cable. In profile, the strands were

straight and parallel and spaced approximately 1-in center to center in a vertical plane throughout the

central 12-ft section of the span of the beam (6 ft to each side of the midspan section). From these points they spread apart at equal angles to about 3-in

4-6

P/2 9-0

P/2

18.0

ELASTIC N. A. 8 CONST JOINT

^STEEL TENDON ( MONOLITHIC ) STEEL TENDON (SPLIT-BEAM)

STIRRUP

CONST JOINT FOR SPLIT- BEAM

TENDON LOCATION FOR MONOLITHIC

TENDON LOCATION FOR SPLIT- BEAMS

WIDTH OF WEB FOR I FOOT

AT EACH END

Figure 1. Beam dimensions and loading arrangement.

The channel for the reinforcing tendons in each

post-tensioned beam was formed by placing a length

of thin-wall steel tubing, 1-in O.D., with 1/16-in

wall thickness, in the form in the position specified

for the tendon. The use of this tubing in prestressed

beams in an earlier study [2] had no apparent effect on the performance of the beams. The tubing was

center to center at the ends of the beam. Steel washers

with fl/o-in I.D. were used to accomplish the angle

change in the profile of the strands. The washers

encircled the strands and were located at the points

along the length of the strands where it was desired to change the angles. The two strands were tensioned

simultaneously against anchors which spaced them

2
r

Figure 2. Tensile element with jacking arrangement setup for post-tensiioning.

Figure 3. Jacking arrangement for pretensioning.

about 31/2 in apart. With the washers encircling the strands and located 4 ft away from the pretensioning

anchors, the strands, under tension, sloped down to the

location of the washers which restrained the spread

from these points inward to the 1-in spacing described

above.

A view of the jacking arrangement for pretensioning is presented in figure 3. The pretensioning wires were stressed between two stub columns which were

set 25 ft apart and fixed to a tie-down floor. A steel dynamometer was placed between standard chuck type strand grips and the anchoring column at the end

opposite to the jacking end to reflect the prestressing

force in the strands.

3
Stirrups were fabricated from mild steel No. 3 reinforcing bars having a yield strength of 50,000 psi.

They were U-shaped in the stem of the beam and placed

throughout the span length spaced 1 ft apart. The

stirrups looped under the prestressing tendon extended

up to the midheight of the flange and out 6 in horizontally on both sides. The tendon in the monolithic beam was located 6.1

in below the center of gravity for the Tee section and

the prestress force was 45,000 lb. The location of the tendon in the split-beam was 8.7 in below the center

of gravity of its composite Tee section and the prestress force was 27,000 lb. The essential difference here

is that the location of the tendon and the prestress for

the monolithic beam is determined from the properties

of the full Tee section while, due to the nature of the construction of the split-beam [2], the location of the tendon and the prestress is determined by the prop-

»ct fct = fcb/2

_J_i_

fcb -'cb

(0 > (b) MONOLITHIC CONSTRUCTION

(c)

'cb -fcb

(a) (b) SPLIT -SEAM CONSTRUCTION(c)

Figure 4. Stress conditions at midspan idealized, (a) after

prestress, (b) gradient produced by applied load causing stress at bottom fiber equal to prestress, ( c) resultant from combining

(a) and (b).

erties of the tensile element of the composite tee section. Figure 4 shows the idealized stress conditions for both types of beams for two stages of loading. The

stress conditions (a) show that for the monolithic

beam the stress block tapers from the maximum value

at the bottom fiber to zero at the top fiber of the beam.

The stress block for the split-beam, for this stage,

tapers from the maximum value at the bottom fiber to zero at the neutral axis of the composite section. This

difference in the stress blocks reflects the same pro- portional difference in the required prestressing force

for the two types of beams. The value of applied load

that will cause zero stress in the bottom fiber is represented by the stress condition (b) and is the

same for both the monolithic beam and the split-beam

The stress condition (c) results from the combinatior

of (a) and (b).

01

dill

\ 2.2. Concrete

The concrete used in the first 12 specimens was composed of Type III portland cement, siliceous sand

and pea gravel. This concrete was mixed in the labora-,i tei tory in a turbine-type mixer of 1/9 cu yd capacity.! la Concrete for the subsequent beams was obtained from

a local ready-mix company using the same materialsi except for the coarse aggregate which was a Maryland

No. 7 crushed lime stone (size No. 8, ASTM C 33-67).

The mix proportions were varied around a design mix

for 5000 psi concrete which was 1:3.2:2.6 by weight'

of cement, sand, and gravel. The water content varied' from 6 to 11 gal per sack of cement (94 lb) . The con-' Crete strengths at the time of beam testing are given'

in table 1. These strengths represent the average valuesJ determined from compressive tests of three 6-by-12-in' control cylinders.

2.3. Prestressing Steel

Two types of steel were used as prestressing tendons.

The stress-strain curves for the steels are shown in figure 5. For the post-tensioned beams, high strength

heat-treated, stress-relieved bars were used. Tensile

tests of these bars indicated a stress-strain relationship '' that is essentially linear up to a stress of 108,000 psi,

and an initial tangent modulus of approximately 29.8 X 10^ psi. The yield strength of the steel was 170,000

psi as determined by the 0.2 percent offset method.

The tensile strength was 190,000 psi. The prestressing

tendons for the pretensioned beams were 7-wire strands

of high strength steel. Tensile tests of the strands showed a linear stress-strain relationship up to a stress

260

240

220

200

180

I 60

14 0

120

I 00

8 0

60

40

20

0

1 1 1 1 1 1 1 1 7-WIRE STRAND

1 1

/ ' BAR

/ 0.002 _/

^

'

1 1

Figure 5. Stress-strain curves for prestressing steel tendons.

4
! of approximately 162,000 psi. The tangent modulus

for the strand was 28.1 x 10^ psi. The yield strength was defined at 0.2 percent offset and was 221,000 psi.

The ultimate strength of the strands, as could be

developed in the beams, was not determined precisely

I since in all cases the strands fractured in the grips in

J the tensile tests. However, indications from the tensile

• test and the beam tests suggest that the manufacturers

rating of 250,000 psi is valid.

f 3. Test Procedure

J All beams were tested as soon as the desired conj Crete strengths in the compressive element were

) reached. The ages of the beams at the time of testing

ranged from 10 to 16 days. The specimens were tested

J to failure with equal loads applied at the quarter

, points. In general, load increments of 2000 lb were

I applied except in the range of cracking where incre- ments of 500 lb were used. After the application of

j each load increment, the deflection of the beam, the

j force in the reinforcing tendon for unbonded beams,

j the strain on the concrete surface, and the extent of cracking were recorded. The time required for these

' readings varied from 3 to 5 min and the time for the

overall test varied from 60 to 90 min. For the prei tensioned beams, electrical resistance gages were

j applied to individual wires of the strands for strain

' measurements. However, readings from these gages

I beyond the cracking loads were erratic in all cases and

I were discarded.

4. Test Results and Analysis

4.1. Notations

As = area of main prestressing tensile steel

b' = width of flange of Tee section b = width of web of Tee section d = distance from extreme compressive fiber to the prestressing force

f'c = compressive strength of concrete at time of test

fcb = concrete stress in the bottom extreme fiber of

the cross section

fct — concrete stress in the top extreme fiber of the

cross section

/se = effective steel prestress after losses

/su = stress in prestressing steel at ultimate load

/sy = nominal yield point stress of prestressing steel kt = the ratio of average compressive stress to max- imum compressive stress k2 = coefficient determining position of internal compressive force (fig. 12)

ks = ratio of compressive strength of concrete in flexure to cylinder strength

kud — distance from extreme fiber in compression to neutral axis at ultimate load (fig. 12)

Mu = maximum beam moment at ultimate load

P = As/bd; ratio of prestressing steel qu — Pfsu/f'c; tension reinforcement index

Values of observed and computed characteristics of the beams are given in Table 1. The first order grouping of the specimens is by method of prestressing and

attachment of tendons. There are three classifications: (1) post-tensioned, unbonded; (2) post-tensioned,

grouted; and (3) pretensioned, bonded. The two beams Vv'ith grouted tendons, SG-1 and SG-2, experienced bond failures beyond the cracking loads. Since

there are no appreciable differences in the performance

of beams with bonded and unbonded tendons prior

to the onset of cracking, these beams were classed as unbonded for the purpose of comparison.

The beams in this investigation fall into one of five different steel ratio (p) groups. However, test results show that a better ordering of groups can be made

in terms of a moment index, Asfsi/d. AH beams, except

SU-11, SU-14, and SG-2, failed after the yield

strength of the reinforcement had been reached. Beams

SU-11 and SU-14 had 1900 psi and 2000 psi concretes

in the compressive zones, respectively, and failed by

compression of concrete with the reinforcement in the

elastic range. Beam SG-2 failed by interface separation in the shear span. The performances of the beams are compared in terms of load-deflection characteristics, ultimate

strengths, and crack patterns. The moment index

Asfsyd shows a direct correlation with both the loaddeflection relationship and ultimate strength. However,

the fact should not be overlooked that all beams in this study were of the same shape and size and were tested

in the same manner.

4.2. Load Response

A flexural load applied on a reinforced concrete beam of a given cross section will require a specific

T

X-Oenoles poinf of failure

1.0 2.0 3.0 MIOSPAN DEFLECTION, INCH

Figure 6. Load-deflection relationship for post-tensioned beams

icith unbounded solid bar tendons.

5
II

E 3

Anchor

nut

Tension Tension Tension Tension Tension Tension Tension Tension Tension Tension Tension

Compression Tension Tension

Compression Tension

bond

(interface

separation)

D a = c

o o o o

c a c c

o o r}- OO O Q O ^ CO CO 03

in m r- rssgs

20.0 21.5 27.0 28.9

CD

CO CO FH ^ CO FH CO

CO CO CO

CO d o

2 B is ;

Grouted s

"

1

0.0021 .0021 .0012 .0012 .0012 .0012 .0017 .0017 .0017 .0017 .0017 .0017 .0017 .0017 .0017 .0017

Post-Tensioned,

0.0012 .0017

1

0.0007 .0007 .0010 .0010

14.7 14.7 14.4 14.4 14.5

0.26 .370.16 .16 .22 .22

11 iiii nMUUtm 5100 4aoo mm

ii iiii mmimm 2100 2400 3600 5300

2500- 4600

None None None None None None None None None None None

dowels@12"c/c 3tirrups@12"c/c

stirrups

@12"c/c

3tirrups@12"c/c 8tirrups@12"c/

c a

o o 2 Z

u o o o "^^^^ ®@®®

£S.£.S. Iiii

Monolithic Monolithic Composite Composite Composite Composite Composite Composite Composite Composite Composite Composite Composite Composite Composite Composite Composite Composite Composite Composite Composite Composite

RU-1 RU-2 SU-2 SU-3 SU-4 SU-5 SU-7 SU-8 SU-9 SU-10 SU-11 SU-13 SU-14 SG-1 SG-2 SB-1 SB-2 SB-3 SB-4

I

i

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6
force to act at the level of the steel for equilibrium. In

a prestressed beam the strains at the level of the steel are a function of the moment of inertia of the full transformed cross section up to the cracking load. Within this range of loading, the effect of large differ- ences in steel areas on the straining rate at the level of the steel is relatively small. However, once the beam

has cracked, the amount of strain in the steel for a given increment of loading will vary inversely with the tendon area. Typical load-deflection relationships for post-ten- sioned beams with unbonded solid bar tendons are shown in figure 6 and for pretensioned beams with

bonded strand tendons in figure 7. The three curves

representing the post-tensioned beams in figure 6

clearly show the effect of the moment index {Asfsyd) on the performance of the beams. The same is true

for the two curves representing the pretensioned beams

in figure 7. The initial portion of the load deflection

for the two groups of beams. Typical crack patterns

for the beams are shown in figure 0. For the post- tensioned beams, a single crack first apijcared at or

very near midspan and was ftillowed shortly, as load- ing proceeded, by the development of two or three additional cracks on both sides of the crack at midspan. In beams without stirrups, the midspan crack

developed into a distinctive Y pattern with hori- zontal extensions just under the flange covering a large section of the constant moment zone. When

stirrups were used in the post-tensioned beams, the horizontal extensions of the central crack were elimi- nated. Views of a post-tensioned and a pretensioned

beam at ultimate load are presented in figures 10 and

11, respectively, to illustrate the difference in crack

distributions and their effect on deflection. In all post-tensioned unbonded beams the central crack

dominated the failure mode causing the maximum

compressive strain in the concrete, and consequently

1 ^ \ r30 X- Denotes point of failure

Asfsyd = 705in - KIPS

X SB -3

6.0 8.0 10.0 MIDSPAN DEFLECTION , INCH

12.0 14,0 16.0

Figure 7. Load-deflection relationship for pretensioned beams with bonded strand tendons.

curves in all cases was a straight line with practically the same slope, irrespective of the moment index. This

portion of the curve reflects the response of the beam

to loading prior to the onset of cracking. Subsequent

to cracking, however, the curves are ordered in accordance with the moment index. In figure 8 a basic difference is seen in the overall characteristics of the load-deflection curves between

the post-tensioned and the pretensioned beam groups. The pretensioned beams (SB-1, SB-3) showed con- siderably more ductility in their response to loading

than the post-tensioned group (SU-1, RU—2, SU—13).

Distinctively different crack patterns were observed

the maximum curvature of the beam to concentrate

at the midspan. For the pretensioned bonded beams,

10 to 12 equafly spaced cracks developed in the constant moment zone. These cracks propagated and

opened with equal magnitude as load increments were added and until failure occurred. This caused a more

uniform and greater overall curvature in the pre- tensioned beams than for the post-tensioned ones.

4.3. Ultimate Strength

Final failure in flexure of a reinforced concrete beam may be initiated by excessive elongation of the reinforcement, in which case it is called tension failure,

7
A f d

s sy su - 13 925 in -kips

RU - 2 755 in -kips

SB - 3 705 in -kips SU - 1 650 in -kips

SB - 1 510 in -kips

8 10 12

MIDSPAN DEFLECTION, inch

14 16

Figure 8. Combined curves of load-deflection relationship for all methods of prestressing and types and sizes of tendons.

Figure 9. Typical crack patterns for beams: (a) Post-tensioned unbonded without stirrups; (b) post-tensioned unbonded with

stirrups; (c) pretensioned bonded.

8
or crushing of the concrete may occur before yield of

the reinforcement which is termed a compression

failure. Other types of failures were of secondary con- cern in this study. In general, the beams in this investigation failed in tension. Two beams, R[J— 1 and

RU—2, failed prematurely and their results are not used for comparisons in the study. Beam RU—1 failed when the threads in an anchor nut on one end of its
post-tensioned bar were sheared off while the beam

was being loaded. Beam SG—2 failed by complete

separation of the interface of the tensile and compressive elements in the shear zone under load. The expression Asfsyd is here shown to be a valid index for an ordering of the overall load response

for the beams in this investigation. It can also be

seen in table 1 that the moment index (Asfsyd) values are in good general agreement with the respective

measured ultimate moments for all the beams failing

in tension. Therefore, the moment index is a means by

which a comparison can be made of the tendon sizes required to produce equal load response and capacity

for split-beams and monolithic beams. The moment

index for the monolithic beams (RU—1, RU—2) is 755 in-kips and the tendon size is 0.37 sq in. With

these values as references, the required tendon size for equal performance by the split beam is, ^ _ 755 in-kips

fsyd

With the type of steel used for the post-tensioned

beams, the tendon size for the split-beam is found

to be 0.30 sq in. This is a reduction of approximately

20 percent below the size for the monolithic beam.

An analysis of the principal properties of the

stress block was conducted to evaluate the performance

of the concrete at ultimate load. It has been demonstrated in laboratory tests [3] that the shape of the

stress block at the ultimate capacity of a beam varies with the strength of the concrete. The shape of the

stress block varies from nearly trapezoidal for low

strength concretes in the 2000 psi class to nearly

triangular for high strength concretes in the 7000 psi

class. The stress block has been found to be nearly

parabolic for 5000 psi concrete. However, in deter- mining the ultimate strength of beams, the exact shape

of the stress block is not as important as the magnitude

and location of the internal compressive force. This

force can be defined and located in terms of three

parameters [4], ki, k-i, and ks. The parameter ki is defined as the ratio of the average compressive stress to the maximum compressive stress of the concrete

in the compression zone of the beam at ultimate. The

parameter k-i is defined as the ratio of the depth to the line of action of the resultant compressive force

to the depth to the neutral axis. Parameter kz is defined as the ratio of the maximum compressive

strength of the concrete in flexure to the cylinder

strength.

The assumed stress conditions at ultimate load are shown in figure 12. The expression for the ultimate

resisting moment is: Mu = Asfsu id-kokud) = fsupbd'^ [l-kiku). (1)

An expression for ku is obtained from the equilibrium

of forces.

ku —

p/su

(2) i kikif'c

Substituting eq (2) into eq (1) and dividing both

sides by j'cbc^ gives

Mu ^ p/su

fcbd^ fc

Equation (3) is a convenient relationship for evaluating the expression

" by using the measured properties of a beam. The relationship shown in eq (3) was studied using the measured ultimate moments and

I

/

/

/ ^TT^ -"fru (1-0.52 "J-u)

V

/

/

/

• -RU-1,2 A -SU -1-4

O - SU -S-IO

-SU-ll-14

X - SG -1,2

-h - SB -1,2

.10 .20

Figure 12. Stress conditions at ultimate Inad. FlGfRE 1.3. Relationship hetueen ultimate moment and qu.

10
i measured steel stresses at failure. The results are

j the plotted points shown in figure 13. The curve

i| shown by dashed line in this figure was developed

i from the results of a study by Janny, Hognestad, and

3 McHenry[''] with rectangular beams covering five types

} of reinforcement. This curve represents the relation- ship in eq 3 for a value of ^-7^ equal to 0.52. The

I Kika

I plotted points in figure 13 are in close agreement with

the curve. This indicates that the basic characteristics of the stress block at ultimate load for the Tee beams

in this investigation are similiar to those for the rectangular beams studied by Janny et al. It also indi- cates that the unusually low steel ratios, tendon location, and initial stress gradient discontinuity in the split-beams had no adverse effect on the ultimate load

performances of these beams over a wide range of concrete strengths in the compressive elements (flange

sections)

As would be expected for beams failing in tension, the strength of the concrete in the compression zone had little if any effect on the ultimate capacity of the beams. In this study only a rather general trend may

be noted in a comparison of concrete strengths and

ultimate moments for beams in the 925 in-kip moment

index group (SU-5 thru SU-14). However, when the

results for the beams in the 650 in-kip index group

(SU—1 thru SU^) were considered together with

those of the 925 index group, no direct linear correlation of concrete strength and ultimate moment was

found.

5. Discussion

The principal differences between the split-beams

and the conventional monolithic beams in this study were in the initial prestress parameters allowed by

the methods of construction. However, it was found

that even with different initial prestress parameters,

beams having the same ultimate capacity showed essentially the same overall flexural response to loading.

The index Asfsyd, which was used to categorize the beams in relation to a scale of load response and

capacity is essentially a measure of the internal resist- ant moment capacity for under-reinforced beams. The

very close agreement between the index values and

the respective measured ultimate moments for the Tee beams can be explained by considering the factors

in the moment index expression. The factor d is approached within 10 percent by the actual moment

arm at ultimate and the actual stress in the steel is somewhat greater than the yield strength of the steel,

/sy, by a similar difference but opposite in direction

to that for the d factor. Consequently, the two departures from actual conditions conveniently compensate

for each other. Split-beams that had compressive elements with

concrete strengths around 2000 psi showed erratic performances under loading. The failure mode for these beams was unpredictable and included brittle failures in the constant moment region and in the shear span for different beams. In order to avoid the

risk of these type of failures, concrete strengths below

3000 psi should not be used in the compressive element

of split-beams.

Due to the manner of construclii.n and the design

of the precompressed section, the split-beam enjoys

the advantage of a reduced amount of reinforcing

steel for the same overall flexural characteristics as compared with the conventional monolithic beam.

However, it should be emphasized that the comparison

here is between a composite beam and a monolithic

beam. Also, no tensile stresses were allowed in the stages of construction. It may be better to evaluate

the split-beams in relation to other composite beams.

For example, it was stated earlier that the split-beam

is a special case of composite construction where the construction joint is designed to coincide with the neutral axis of the composite section. Figure 14

15"-

FicuRE 14. Cross section for conventional composite beam.

shows the cross section of a conventional composite

beam where the construction joint is located at the

intersection of flange and web. The dimensions of the cross section are the same as for the other beams in

this investigation. To include this section in a comparison with the other beam sections in this study, the

basic prestressing parameters for the three types of beam construction features were computed with respect

to the common moment index Asfsyd = 755 iii-kips. These values are presented in table 2. From the standpoint of performance, it is apparent

that nothing is gained, in comparison with the split- beam design, by locating the construction joint above

the neutral axis of the cross section. In fact, for the same flexural characteristics, the required area of the reinforcing steel will increase with the distance of the construction joint above the neutral axis. Con11
Table 2.

Computed prestressing denominators^ for post- tensioned tee-beams of different construction with a common moment index of 755 in-kips

Construction method d /«,

lb in.

Monolithic 45,000 12.1 0.37 0.71

Conventional 33,750 13.7 .33 .61 Composite

Split-Beam 27,000 14.7 .30 .53

a For the same cross section, initial conditions, and steel bars for this study. b Initial prestressing force.

versely, when the construction joint is located below

the neutral axis the required area of the reinforcement

will decrease as the distance of the construction joint

to the neutral axis increases. The limiting. distance of the construction joint below the neutral axis will be

affected by several practical considerations. Among

these considerations are: (1) the minimum cross sec- tion needed for prestressing to a desired value; (2) the degree to which tensile cracks will be tolerated in the zone between the construction joint and the neutral axis within the working load range. Although a strict economic evaluation for the

practical use of split-beams was not within the scope

of this study, the experience gained in preparing these specimens raises a serious question as to the balance

between materials savings and the added cost of form work and construction handling.

6. Conclusions

Placing the construction joint at the neutral axis of a prestressed composite beam allows for an efficient prestress distribution over the cross section with no

adverse effect on the performance of the beam.

Stirrups should be provided throughout the span

length for these beams to prevent the development of extensive horizontal cracking just above the neutral

axis in the region of maximum moment, and to serve

as reinforcement against possible interface separation.

The product of the factors Asfsyd was found to be

a satisfactory index and very close indication of the

ultimate moment for the beams in this investigation.

Also, the test data agreed extremely well with the more refined relationship [ 1 "'1, =qu (1 — 0.52(7u).

/ cod^ Concrete strengths in the compression zone can be

markedly reduced below that required for the pre- stressed element without significantly affecting the

flexural characteristics of under-reinforced members.

A practical lower limit would appear to be 3000 psi.

The use of lightweight concrete in the compressive

element should not be overlooked as an additional

benefit. The required amount of prestressing steel for the split-beam in this study was approximately 20 percent

less than that for a monolithic beam. However, when

compared with a conventional composite beam only a 9 percent reduction in steel was found in favor of the

split beam.

This study was carried out at NBS within the scope of a broad program sponsored by the U.S. Department of the Navy (NAVFACENGCOM) . The

major program, which extended over several years, was principally concerned with investigations of new

concepts for structural elements and systems in reinforced concrete.

7. References

[1] Amirkian, A., Split-beam prestressing, The Navy Civil Engineer 4, No. 11, 35 (1963).

[2] Bryson, J. 0., Skoda, L. F., and Watstein, D., Flexural

Ijehavior of prestressed split-beam composite concrete

sections, P.C.I. Journal 10, No. 3 (June 1965).

[3] Hognestad, E., Confirmation of inelastic stress distribution

in concrete. Journal of the Structural Division, ASCE,

Paper 1189, 83, No. ST-2 (March 1957).

[4] Billet, D. F., and Appleton, J. H., Flexural strength of prestressed concrete beams, A.C.I. Journal, June 1954, Proc. 50, pp. 837-854.

[5] Janney, J. R., Hognestad, E., and McHenry, D., Ultimate

Flexural Strength of Prestressed and Conventionally Reinforced Concrete Reams, A.C.f. Journal 27, No. 6 (Feb. 1956).

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