Static and Fatigue Tests of Composite T-Beams Containing Prestressed Concrete Tension Elements

peletonwhoopUrban and Civil

Nov 26, 2013 (3 years and 4 months ago)

91 views

Static and Fatigue Tests
of Composite T-Beams
Containing Prestressed
Concrete Tension
Elements
Paul Zia
J.
F. Mirza
Professor and Associate Head
Department of
Civil
Engineering
North
Carolina State
University
Raleigh,
North
Carolina
Associate Professor
Department of
Civi l
Engineering
North
Carolina
State University
Raleigh,
North Carolina
s.
H.
Rizkalla
Research Assistant
Department of Civil Engineering
North
Carolina State
University
Raleigh,
North
Carolina
Paul Zia
J.
F. Mi rza
s.
H.
Rizkalla
I
11
bridge design, the
advantages
of
using continuous spans over a seri es
of simpl e spans arc \VeH understood.
Continuous systems in composite con­
struction arc often achieved
by
joining
prefabricated members spanning be­
tween piers
wi th
a cast-in-place rein­
forced concrete deck providing inter­
span continuity.
Earlier
studies
l
,:!
have established
the feasibility
of
using long slender
pre·
76
stressed concrete pri sms
(tension ele·
mcnts) in pl ace
of
conventional
de·
formcd bars to develop continuity.
~I [ o r c
recently, using specimens con­
tai ning
a
single prestressed concrete
tension element, the
author s;~
h.lve
es­
tablished that the fatigue strength
of
such
speci men<;
under
repeated
loading
was
O.
7
Pen
where
Per
is the
cracking
load
of the
ten<;ion
elcment.
rn vestigat ion
by
Bishara.
~ I a son,
and
Synopsis
Thi s paper describes the static and fatigue
behavior of composite T-beams using a
combination of prestressed concrete tension
elements
and reinforcing bars as the tension
reinforcement.
Static tests indicated
similar
behavior of two
continuous beams, one being designed with
moment redistribution at
ultimate load
and the
ot her without moment redistribution. Repeated
load
tests were performed on six composite
beams.
Test
results
indicated that by augmenting
multiple
tension
elements
with
steel
reinforcing
bars, the endurance
limit
of the composite
beams was increased by
nearly
43 percent.
It
was
also
shown that, under repeated
load
tests, the maximum crack width of the beam
exclusively
reinforced with tension
elements
was
approximately one-half
of that of the
similar
beam reinforced with
conventional
bars
only.
Almeida<l
revealed that for a given
mo­
ment capacity. the use of a combination
of prestressed concrete tension elements
and conventional reinforcing bars (in
the ratio of 3 to 1) provided optimum
control of visible
crac ki n~
and
in­
creased overa]] flexural rigidity of
rein­
forced concrete beams.
ments as reinforcement behaved almost
like full y prestressed concrete beams, as
opposed to conventional reinforced
concrete beams, insofar as crack control
and deflection characteri stics were
con­
cerned.
A
similar
study carried out in Japan
6
also indicated that composite beams
us­
ing prestressed concrete tension ele-
In 1969, a three-span continuous
bridge
5
over a railway was constructed
in Burnaby, British Columbia, using
precast prestressed concrete
«rods"
as
continuity and crack control reinforce-
PCI
JOURNAL/November-December 1976 77
Table
1. Scope of test program.
Test
Specimen Spec.1rr.en
I
Tension
Date of Test
Type
of
M".
I
Date of
Series tlurrber
Reinforcement Element
Casting Setup
Loadl
ng
Mapolfed
Te
stin
9
Used"
0'
A1
Two
15
and
three
"
211/73 Continuous Static
..
5/10/73
A
AZ
tension elements II 4/5/73
SI~le
Reputed
1.63
10110173
AJ
7/16-in.
strand
V
5/25/73
SIq;Jle Repeated 0.95 10/24/73
A4
V
6/15/73
SI~le
Repeated 1.26
11/
9173
B1
Two lI'5
and three
I"
2/1/73
Continuous Static
..
5/16/73
B
B2 tension
elen;ents IV
4/5/73
Continuous
Repeated
0.77
1113/73
B3
3/8-ln. strand
III
5/25/73
Continuous Repeated
1.07 8/31/73
54
IV
6/15/73
Simple Repeated
1.11
11/16/73
C1
Two tension ele- I 12/18/13
Simple Repeoted
0.85
1{11/74
ments 3/8-fn.
and

two
tension
ele-
VI
C
ments
7/16-In.
strand
C2
Two i5 and
two
..
12/18/73
Slf11lle Repeated 0.85 2/15/74
17
Basee
on
Mcr
for
Cl
· See Table
2.
uMapplled
~
Maximum applied moment
under
repeated load,
including
dead
load effect.
'\r
~
Theoretical
III.lrr.ent corresponding
to
cracking
of tension
elerr.ent.
ment. Each of the 30-ft long rods
con­
sisted of
a
single
lh-in.
strand tensioned
to
25,000
lbs
and
encased in
a
3
x
21,6
in.,
6000-psi
concrete rectangle.
A
compression of
3000
psi was developed
in the
"rod."
As intended, the rods tended to close
lip
any
tension
cracks
in the deck slah
clue to li ve loadings, and the deck
showed no signs of tension cracking in
the negative moment areas.
Obj ective
This study is
an
extension of the
pre~
vious
work:
l
.
cl
to examine serviceabil ity
and post- cracking behavior under
re~
pea
ted cyclic loads
of
both simpl e
and
cont inuous members
using
combina~
tions of tension elements and
reinforc·
ing bars.
Specifically.
the study dealt with the
cracking behavior, endurance limit,
ul­
timate load capacity and amount of
moment redistri bution of continuolls
composite beams designed with and
without
20
percent moment
redistribu­
tion.
78
Scope
Ten specimens, classified into three
series, were tested under
t wo
different
types of loading as indicated in Table 1.
Each specimen was
designed as
a two­
span continuous composite T-beam.
Specimens of
Series
A were designed
without consideration of moment redis­
tributi on at ultimate load. while those
of
Series
B were designed
with
an
as­
sumed
20
percent moment redistribu­
tion.
Specimens
of
Series C were
designed
for an equal ultimate moment capacity
as
those of
Series
n,
but used
two cli.£­
ferent types of
continuity
reinforce­
ment.
Specimen
Cl
\Y.:S
reinforced
with
tension elements
excillsi\'(:·ly,
while
Specimen
C2
was reinforced
with
only
rei nforcing bars, so as to provide a
comparison of the
cffeC'th'cness
be­
tween the two types
of
reinforcement
for crack control.
In addition to the ten
specimens
for
static
and
repeated load tests, two long
prestressed tension elements were
fab­
ricated to demonstrate
the
practicality
of handling, transporting, and
section­
jng of the long elements.
A
descript ion
of
these
various
aspects is prcsented in
the Appendix.
cated from two precast prestressed
con­
crete
oS
x
IO-in.
steInS
with
continuity
over
the centra l support being developed
through
a combination
of
tension
clements
and
conventional
reinforcing bars
in a 5
x
42-in.
fl ange, as
shown in Fig. l (a).
Experimental
Program
Test specimens
In
addi tion to the natural bond
between
the stem
::md
the
fl ange, positive
shear
transfer
was
ach ieved through use of #3
hal'S
at 7.S-i n.
spacing, cxt<:'nding
out
of
the stem into the flange.
Each of the ten specimens was
des igned
as a two-span continuous composite

beam.
The composite beams were
Iabri-
A
5-in. thick
diaphragm
reinforced wit h
#4
bars was cast at
the
central support
section
connecting the two precast
pre­
stressed stems also shown in Fig. l (b).
6'
::f
L_- _- t--_-__
- _;_ ---,-~-:tn --== ~
---
EJ
t
-~- - -r._-- ~-- -- ~_+ --
I I
I I
I I
I
I
Tension
element
'5
1&
0
1
I I
~
___
!J
Precast
Prestressed Stem
8"
Fig. 1 (a). Cross section
01
composite T -beam.
(3 )Tension
-'r'7~6e,5~·(~sta~t-)'+-7 L-"'5o\· (=~
Ele~'";----6~
6"
-----
~:--
----- ------
1485" (static)
14B5"
(static)
15-0'
(repeated)
15-0"
(repeated)
SET UP
FOR CONTINUOUS
BEAM
~
(3)Tension
element
L
521
~
I
I
~
7
--c
------ ----

=>--
10'
10

Simple
Beam
SET UP
FOR SIMPLE
BEAM
Fig. 1(b). Test set-ups.
I
PCI
JOURNAL/November-December 1976
d
52i
"
5
79
Precast
Stem
1-,
7~6"
I
3
i~,
12(,) 7f "
II I
! ! ! ! ! ! ! ! ! !
I
J4~
<1'
I
-J
I
7-wire
str
4"0
4~'
.--;4"
~~rM}6
,.K,.
Precast
Sec
I
Tensbn
,
Element
11-----
12
(1)
270
k
Gra:le
str:
7
Sec
II
i ll
L n
Type TE1:
1-
Z
'q:,
7-wire
str
16
Type
TE2
· 1-
]."<1'
7-wire
sir.
. 8
Fig. 2. Details of precast prestressed stem and tension element.
Fabrication of prestressed stems
Details of the
precast
p restressed
con ­
crete stem arc shown in Fig. 2. The beam
stems we re fabrica ted
by
Arnold
Stone
Company, Greensboro, North
Carolina,
in
a
220-ft
prestressing bed.
Conti nuous wooden forms were used
with 8
x
10-in. end blocks at 15-£t
inter­
vals. The
lh-in.
strands were initial1 y
ten­
sioned to
28,000
lbs per tendon.
The properties of the strand as furnished
by the manufacturer are as foll ows:
Yield
strength at
1
percent extension ..
Ult imate strength .....
Modulus of
239
ksi
270
ksi
elasticity,
Ell
.......
29,000
ksi
Normal weight concrete was used for
the stems. The concrete mix proportions
per cubic yard were as follows:
Cement (Type III) ...... 705 1bs
Sand
................. 1254 Ibs
80
Coarse aggregate .......
1630
Jbs
Water
................
37\i,
gal
The
fine
aggregate was well graded
sand with a fineness modulus of
3.0.
The
concreto was vibrated externall y and the
top surface rough finished by wooden
Boat.
After normal air drying for 4
Ius,
the
cast·
ings were covered
with
heavy duty plastic
sheets and cured by steam with
tempera­
ture increased at the rate of
0.5
deg F j min
up to
150
deg and held for approximately
8
Ius
before being turned off.
Forms were removed and prestress
transferred about 45 hrs after casting,
when the concrete strength had reached at
least
5000
psi. However, the total curing
time for Casting
I
(see Table 2) was only
20
hrs, which is the nonnal product ion
cycle of the prestressing plant.
Fabrication of tension elements
Tension elements were of 2 x 3-in.
rec­
tangular section with a single 7-wire ten-
dOll.
Two types were made using
~ 6-ill.
and
%-in.
diameter strand tensioned in­
itiall y to
21,700
and
16,100
lbs, respec­
tively.
The tension elements were manufac­
tured simultaneously with the precast
stems using the same concrete mix, casting
and curing procedures.
A
continuous
wooden fonn was used with 2 x 3-in. end
blocks
at 12-ft intervals. Details arc shown
in
Fi g.
2.
Tendon force was determined by
gage
pressure of the hydraulic system and
checked
by strand elongation. Properties
of the strands were the same as given be­
fore for the stem.
In
addition
to 12-ft sections,
two
speci­
mens, 30
and
40
ft each, were cast to in­
\
estigate lifting,
handling and sectioning
(wit h saw) techniques.
For each of the first five castings listed
in
Tahle
2, eighteen 6-in. cylinders were
Ci!s t
ftnd
cured under the same condition as
the
prestressed stems
and tens ion elements.
The cylinders were tested at 3 days, 7
days, 28 days, 3 months, 6 months, and 1
year. Fig. 3 shows the variation of con­
crete strength with age.
1
t
is noted that after 6 months curing,
the concrete virtually reached its potential
strength. However, the concrete strength
of the first casting
was
considerabl y
lower
than those of other castings, due to the
shorter initial curing period as noted be­
fore.
~
£9
~
t7l6
.~
Vl
Vl
~3
8
ocasting
I
.,
casting
II
ocasting
III
~cast ing
IV
ocasting V
O ~~~L-~
__
L-~~
o
2 4 6
Fig. 3.
concrete
Age in
Months
Compressive strength of
used for tension elements
and stems.
Fabrication of composite
beams
Three series of beams were
cast.
All
were composite T -beams consisting of pre­
cast
stems
and laboratory cast flanges. Fol­
lowing erection of fonnwork, three tension
elements and two # 5 Crade reinforcing
bars were positioned and secured with
centroid
21'2
in. below the top of the
fl ange.
U-sti rmps
of #3 bars,
71h
in.
on cen­
ters, extended from the stem into the
Bange.
Eight L-shaped rods with threaded
end were appropriately spaced and secured
in pairs with threaded end protruding alit
Table
2. Casting
schedule.
Designation
Date of No. of
No.
of Tension Elements
of
Cas ting 8 x
10
in.
2 in. )( 3 in. 12 ft
Cas
ting
Stem
I
9/19/72
8
6 (7/16 i n. strand)
II
9/30/72
8
6 (7/16 in. strand)
III
10/
5/72
6
6 (3/8
in.
strand)
IV
10/10/72
---
6 (3/8 in.
strand)
V
10/15/72
---
7 (7/16 in. strand)
VI
10/20/72
---
4 (3/8 in. strand)
VII
10/25/72
---
2 (7/16 in. strand) *
*One
30 ft long and one
40
ft 10n9.
PCI
JOURNAL/November-December 1976
81
Fig. 4.
Static
test set-up.
of the flan ge
to
provide a means for lifting
and
moving.
In each beam, two
SR-4
strain gages
were attached to the reinforcing
bars
at a
location ncar the central support. Lead
wires were soldered and waterproofing
compound applied to encase and protect
the
strain
gage.
To determine periodically the total Joss
of prestress in the tension element, one
tension element from each casting, except
Castings VI
and VII,
was
instrumented
with mechanical strain gage points, and
strain
measurement was made at different
time intervals.
Ingredients per cubic yard of ready
mixed concrete used in the flange were:
Cement
Sand
Gravel
Water
Admixture
Bll ibs
......
1850
Ibs
.......
11071bs
34 gallons
40z
The concrete was vibrated internally
wit'h a needle vibrator and trowel finished
at the surface. Care was exercised not to
disturb the position of the tension elements
during
this process.
Six:
hrs after casting, the concrete was
covered with a heavy duty plast ic sheet
and
kept moist for 15 days. Forms we re
removed
after the concrete
had
cured for
21 days.
Eight concrete cylinders were prepared
simultaneously with the beams to monitor
compressive strength of tIle concrete. The
82
Fig. 5. Repeated load test set-up.
cyli nders were cured and tested under
eonditi ons identical to those of
the
beams.
Test procedure
Static
test- The setup for the static test
is shown in Figs. l (b)
~1Ilc1
4. Load was
applied by a pair of
60-ton
capacity hy­
draulic jacks located at 7.65 ft on each side
of the central support, which rested on a
semi -spherical head.
The two end supports
\\·ere
on roller
bea rings.
One lOO-kip
and two 25-kip ca­
pacity load
ceUs
were used to measure the
support reactions at the central and end
supports, respectively.
The strain in the reinforcing bars was
measured by a strain indicator through a
switch ing-balancing unit. The
lllich''P3n
de­
fl ections
of the
continuous
beam were mea­
sured hy two mechanical dial
gnges,
read­
ing to the nearest
0.001
in. After each
load
increment, the width of the major
crnck
was measured by a
50X
power mi­
crometer
mi croscope.
Foll owing
preli minaty
checks for proper
seating of the specimen and operation of
the instrumentati on, the load was applied
in increments of
10
kips up to failure; and
corresponding deflecti ons, strain
gage
readings, load cell readings, and
crack
widths were recorded. In addition, the
development and location of the
cracks
were traced.
Repeated load
tests-Repented
cyclic
load was applied by a
50-kip
capacity ram
of a hydro-electronic closed-loop testing
!,ystem
as;
shown
in Fig. 5.
For
Sp~cill1ens
B2
and
B3, which were
teskd as
continuous
beams,
a
steel
beam
vV
12
'\
81
was
used
to distribute the ram
load to the specimen
at
7%
ft on each
side of
the
central
sllpport
as shown in
Pig.
J(b).
The
load
was
applied
at a
frequency of
1
cycle
pel'
sec. Table
1
shows the
maxi·
mum
applied moment for each specimen.
The
minimum
applied
moment
in each
case
was 2.8 ft·kips for the continuous
beam and 2.5 ft·kips for the simple beam.
A
limit
switch was
mounted on the loading
ram
to
preven t
any
accidental
overload
and
to
control the
maximulll
stroke of the ram.
The repeated loading was periodicall y
stopped in order to conduct intermediate
static tests to determine the behavior of
the
specimen which included
measure·
ments of
strains,
crack widths, deflections
and
reactions. The
maximum
static
lond
was limited to the
peak
magnitude of the
repeated load.
At the concl usion of the repeated load·
ing,
if the specimen had not surfered a
fatiglle
failure
as evidenced by a vi rtuall y
complete loss of
stiffness,
it
was
tested to
destruction under static load. For eadl of
the static tests, the applied load and the
support
reactions were
plotted
by three

Y
recorders.
Likewise, the loacl·strain curves were
plotted
h}1
an
X· Y
recorder. The midspan
deflect ions were also
measured
hy dial
gages
as
des(;ribed previously.
For all other
specimens,
the above test
procedures were
followed,
except that the
specimens were tested as simple beams of
inverted T·section with the hydraulic ram
applying a single concentrated load at the
midspan,
as shown in Fig. l (b).
CASTING!!
.0-' -'
-0--
.
--0---0
~.
-: .. ...0--- --- -0--
---
00--<)
$;.0--
-~-
---0------<>---0--0
/'
--gage 1
-----gage
2
-.-gage3
°O:-~5~O~ ~-- 1=50=-~~2~5 =O
Time (days)
Fig. 6.
Change
in strain versus time
for Casting
II
Test
Results
Loss of prestress
As noted previously, one
t ension
e1e­
ment from each of the first
fi ve
castings
listed in Table
2
was select ed as
can·
troI
specimen
to
determine the total
Joss
of
prestress
clne
to elastic shorten·
ing, shrinkage and creep. Three sets of
strain gage
pOints
were embedded in
the concrete,
one
at
2
ft
from each
end
and one at
midpoint of
the tension cle­
ment.
A typical plot of
change
in strain as a
function of time is shown in Fig. 6. It
can
be seen
that the Joss of
prestress
occtuTed
mostly
in
the
first 100
days
after
casting.
Based on the average of three strain
Table
3. Prestress
loss.
f~
(psi)
Strand
Force (lbs) Percent
Casting
at transfer Initi a1 loss loss
I
4000 21,700
9795 45.1
II
5000
21,700
9500
43.8
III
5000
16,100
5710
35.5
IV
5000 16,100 4540
28.2
V
5600 21,700 7056
32.5
PCI
JOURNAL/November-December 1976
83
~
'"
c.
:8
!Jl
c
is
!U
'"
0::
c
.2
....
l;j
'"
0::
90

Center Reaction

South
Reac.
{.
70

North
&0
.,::;
/,s'$!
L,&"
.<;;-0
50
~0e-'
",. ACl
0'U. :;/ V/
30
.
"e.
/
cf)')V
.
'\Y'"
:/
./
~/
,.:::; "'iDeo.
.
.\a.
s
.
20 60 100
140
Applied Load (kips)
Fig. 7. Load versus reaction of
Specimen A 1 under
sIalic load
lest.
measurements, the total loss of prestress
was calculated for each group of
speci­
mens'
and arc summarized in Table 3.
It
should be noted that the total loss
of prestress
is
much higher t han the
val ue ordinarily enc.:ountercd in
pre­
stressed concrete designs. This was
ex­
pected since the tension element was
subjected to a fairly high initial
pre­
stress.
o
Center Reaction
90

South
o
North
.13.70
~0
6
/.-<Y'
.
<'
.
Aif
OJ
·i'!
~50
.,{,«'
,,<!9
·,.<00
0::
/'
/"
.
~30
<;]:/
"e¢'/
i
.
cf)'r
'Y'
.J7:.
0::
. .
"'iDe::
PJ
.¢>
20 60
100 140
Appl
ied Load (kips)
Fig. 8. Load versus reaction of
Specimen
61
under
sIalic load
lest.
Theoretical cracking and
ultimate strengths
\Vhen the composite beam is subject­
ed to bending, initial cracking would
develop in the concrete surrounding
the tension clements and reinforcing
bars when the
fl exural
tension exceeds
the rupture modulus of the concrete.
Under
increasing bending moment,
Table 4. Cracking and
ultimate
moments.
Tes
t
Specimen
Cracking
Moment, Ultimate
r-Ioment.
Series
Number
ft-kips ft-kips
A1
55
114.2
A
A2
55
116.7
A3
63.3
115
A4
63.3 115.2
Bl
47.3
93.6
8
82
50.8
95.2
83
47.3
"
"
50.8 94.3
C
Cl
54.3 94.8
C2
42.4
86.3
84
200

Appied
Load (kips)
Fig. 9. Redistribution of negative
moment in
Specimen
A 1.
the precompress ion
ill
the
tension cle­
ment is graduall y reduced
and,
finall y,
cracking
of the tension cl ements would
occur when the tensil e strength of the
concrete for the tension element is
ex­
ceeded. The response of the composite
bea m to bending
after
cracking is there­
fore similar to that of
.1
conventional
reinforced concrete
hellm.
To determine the
mOment
ca using
c r ac ki!l~
in the tension clement, the
elasti c theory with t ransformed
cracked
section can be used taking into account
the loss of prestress and the concret e
tensile strength of the tens ion element.
Likewise, the ult imate strength of a
composi te section
containin,g:
tension
clements
can
be determined
according
to t he well known
ultimate fl exure
strength theory.
Detail ed
anal yses have
been presented
el sewhere,'
and the re­
sults
are
summarized
in
Tull ie
4.
Moment redistribution
Specimens A 1 and 13
1.
were t-ested
under stat ic load
lip
to fa ilure
as
two­
span continuollS
beams. Measured
re-
200
l?160
li
§-120
tIl
L
<l!
~
c:
<l!
80
u
'"
"
<l!
E
40
0
2:
Computed
Moment
Applied
Load (kips)
Fig.
10.
Redistribution of negative
moment in Specimen 81.
actions corresponding to
the
appli ed
load are plotted
in
Figs.
7 and 8. The
deviation of the meas ured reacti on from
the
theoretical
value according to the
clastic theory indi cates cl earl y the
phenomenon of moment redistributi on.
Using
the average
value
of the two
measured end
reacti o!ls,
the negative
moment
at
the cent ral support
was
com­
puted from statics and plotted against
the applied load in
Figs.
9 and
10.
These load-moment rel ationships show
cl earl y the progress ive increase in the
moment redistrihuti on
as
the
load
was
increased.
At the instant of cracking of the ten­
sion element,
t
he computed cracking
moment at the
central
support based
on the measured reaction was 56.5 and
4.5 ft-kips for Speci mens
Al
and 81,
respecti vely.
These values compare closel y with
the theoreti cal results given above.
\Vhen
railure occurred, there was
a
maximum moment redistrihution of 34
percent for
Al
and 36 percent for
BI.
rt
should be noted t hat the moment
PCI
JOURNAL/November-December 1976
85
",12
0.
'2
0)
c
'5
8
m
0::
c
o
B4
&
O L-~ -L ~
__
L-~-L~~
010203040
Applied
Load (Kips)
Fig. 11. End reaction readings of
Specimen 82 during repeated
loading
test.
capacity
at
the central support for BI
was,
by
design, roughl y
20
percent less
than
that for Specimen AI. However,
the prestressed stem provided an ex­
cess moment capacity of the midspan
sect ion. Accordingly,
a
larger amount of
moment redistribution
was
obtained for
both specimens.
The computed load
carrying
capacity
of Specimen
Ai
was 117.5
kips,
assum­
ing the ultimate moment of the section
--1st,.1X10~
.25x1cf3
Cycles
Vl
------.5x1d! .75x1cf'
Cycles
212
50
Applied
Loads (kips)
Fig. 12. Measured end reaction of
Specimen 83 during repeated
loading
test.
86
\vas
fully developed at midspan as well
as
at the central support. This was
slightl y Jess than the actual failure load
of 124.6 kips carried by Specimen AI.
Similarly, the predicted load carrying
capaci ty of Specimen B1 was
109
kips
as compared to its actual failure load
of 117.4 kips.
Both Specimens B2 and B3 were sub­
jected to
a
repeated load of 37 kips and
50
kips, respectively. According to the
clastic theory, the maximum moment
at the central support due to the ap­
plied load plus the dead
load
would be
60.5 ft-kips for B2 and 78.8 ft-kips for
B3.
However, Figs.
11
and
12
clearly
show
the
effect of moment redistribu­
tion.
j\lfcasurcd
end reactions progressively
increased
in magnitude after the beams
were
subjectcd
to an increasing num­
ber of cycles of repeated load. Because
of the moment redistribution, the actual
applied moment at
the
central support
(including
the dead load effect) was
39.5 and
50.4 It-kips
for Specimen B2
and
B3,
respectively. These values are
only
0.77
and
1.07
timcs the respective
theoretical cracking moment of
the
sec­
tion for Specimens D2 and
B3
discussed
before.
Cracki ng behavior
The location and growth of cracks
were monitored during most tests.
In
the static tests, as increasing load was
applied, a transverse craek developed
on the tension
fnce
Oil
one side of the
diaphragm.
Other
cracks were observed
to develop as loading progressed; how­
ever, the first one was the widest and
was graphed as a function of load for
Specimens A2
::md
B L as shown in Fig.
13.
The use of higher tensioned elements
in Specimen A2 should account for the
lesser
rate of increase in crack width
compared to Specimen Bl. The de­
parture from linearity of the
CLU"ve
would indica te stiffness loss due to
cracking of
the
tension clements.
<il
"-
~
u
'"
0
--'
ci
"-
«
B1
120
60
20
-2
Crack Width (inches
x
10
)
Fig. 13. Crack width of Specimen A1
and 81 under static test.
The growth in
crack
widths due to
repeated cyclic loading is well
demon­
strated in
Figs,
14 and 15.
Specimens
Cl
and
C2
were subjected to cyclic
loading with peak load equivalent to
80
percent of
the cracking load of
Cl.
A better comparison is evidenced in
Fig. 16 which
gr~phs
crack width vari­
ation of
Specimens Cl
and
C2
with
number of cycles of repeated load. The
crack in
Specimen CJ,
which includes
tension elements, is much narrower
than lhat in
C2
which is designed with
conventional steel.
<il
.Q.
'"
u
'"
0
--'
u
.!1!
8:
«
8
4
nd

2 Cycle
.10
6
Cycles
6
o
2 x
10
Cycles
_8
Vl
.9-
'"
u
'"
0
--'4
u
.<!2
(i
"-
«
Crack Width

2
nd
Cycle


10
6
Cycles
2
10
6
Cycles
3 5
-2
(inches x
10
)
Fig. 14. Crack width of Specimen C1
during repeated
loading
test.
Load-deflection characteristics
Load-deflection curves are a useful
indicator of flexural stiffness of concrete
members. The curves shown in Fig. 17
plot deRection at midspan of Specimen
Al
as a two-span contin uous member
under static load.
Note that an increase in rate of
de­
flection at around 55 kips is indicative
of a decrease in
stiffness
due to crack­
ing of the tension dements. The
cor­
responding react ion is measured to be
10
kips. which indicates a support mo-
0
2 4
6
8
Crack Width
-2
( inches x
10
)
Fig. 15. Crack width curves of Specimen C2 during repeated
loading
test.
PCI
JOURNAL/November-December 1976
87
"'
a.
150
~
so
X1C?
.<::
A
Lload
=
.S
P-
u
§
60
Spec. C2
.<::
....
u
~
40
OSP
cr
-'"
AP
load=
.
u
Spec. C1
0
L
u
103 10
4
10
5
10
6
Number of Cycles
Fig. 16. Comparison between crack width of Specimens
C1
and
C2.
mcnt of
56.5
ft -kips.
6100
The theoretical cracking moment is
calculated to be 55 ft-kips
as
previously
mentioned.
Similarl y,
the experimental­
ly
obtained cracking moment
from ob­
servations of
t
he load-deflection test of
Specimen B
I
is
-15
ft-kips,
and
theoreti­
cal
value is 47.3 ft-kips.
~
jl
5 0
--<>-North
Span
--o-South
Span
~
R
6
Mid- Span
Def.,ction
(inches)
Fig. 17. Load-deflection characteristics
of Specimen A 1 under static load test.
40
--<>-1st
Cycle
6 6
I
-o-O.5X10 ,10
Cyc es
.....-...-1.5X10
6
cycles
4
Deflection (inches)
88
6
Typical load-deHection curves ob­
tained at the beginning of and
inter­
mittentl y durin g repeated loading are
shown in Fig. 18 for Specimen D2. The
peak load was 37 kips. This correspond­
ed to a moment of 1.23
Mer
for the
secti on over
the
central
support.
c al ~
clI!ated
according to
clastic theory.
In
realit y however,
due to moment
redistribution,
the moment
resulting
due
to this
load
was ca:culated
from
Fig. 18. Mi dspan deflection
of Specimen 82 during
repeated loading test.
measurements of reaction to be only
0.77 lIf
cr
.
Not surprisingly therefore,
there was no fat igue failure or much
evidence of its effects up to
1.5
million
cycl es, when the test was discontinued.
v; 120
"-
6
~
80
D
Fig.
19
shows
a
load-deflection curve
for the
final
static test on Specimen
£2
conducted to fai lure. The prestressing
tendons had not yet rupturcd as evi­
denced by an almost
50
percent recov­
ery.
~
40
~
S:xrth
Span
-0:
04
0 8 1-2
Several
be.:'1ITIS
wcre tested as simpl y
supported members in order to be able
to appl y hi gher bending moment. Typi­
cal load-deSection curves at midspan
obtained from an X-
Y
plotter are shown
in Fig.
20
where fatigue failure did not
occur and in Fig. 21 where fati gue fai l-
Deflection (inches)
Fig. 19. Midspan deflection of
Specimen B2 after 1.5 x
10'
cycles
of loading.
u
ro
o
--'
u
.~
'i'i

-0:
24
16
8
6
C
des
\0
st
Hf+-- 1
Cycle
1H--- 5)(10
4
Cy
U 11-.1---075)(
106cy.
Max.
App. Ld.=
0.9517:r
O UU~
__
-L __
~
__
L-~
__
-L __
~~
o
1-0 2·0 30
40
Mid
Span Deflection (inches)
Fig.
20. Load· deflection
characteristics of Specimen A3 during repeated
load test.
~
20
~
D
ro
o
.J
U
10
.~
'i'i
"-
-0:
12
4

200
C
10
20
3·0
Mid
Span Deflection (inches)
Fig. 21.
Load· deflection
characteristics of Specimen A4 during repeated
loading tes\.
1-6
Table
5.
Summary
of
test
results.
res
t
Specimen
M'
Nurrtler
of
Mode of
Series
NlIIrber
~
Cycles
of
Fal
lure
Loading
"
A1
..
'2
1.
63

.3
0.95
.,
1.26
Bl
..
B2
0.77
,
B3
1.09
B4
1.11
Cl
0.'
C
C2
0.8
{Sase d
on Mer
of
Sp.
Cll
*See
Table 1.
ure
was
experienced at
124,000
cycles
of repeated loading with peak value of
1.26
1'.
Details of load-deBection curves from
si milar tests on four other beams
are
given elsewhere.'
It
is observed from
these tests that load-deflection curves
are direct indicators of member fl exural
stiffness.
For repeated loadings of suffi cient
magnitude to produce fatigue failure
within a million cycl es, the loss of
stiffness was discernable earl y
in
the
loading history. However, for lower
levels of loading, loss of stiffness with
progressive repeated loading was
neg·
li,gible.
Endurance
limit
Test results are summari zed in Table
5.
Stiffness
variation as a functi on of
applied repeated load and crack width
90
Not
Collapse
Applicable
5,590
Fatigue
Failure
Over
No
,
,500,000
Failure
124,000
Fat!
gue
Failure
Not
Collapse
Applicable
Over
No
1,500 ,000
Failure
Ove r
No
750,000
Failure
1,41e,ooO
Fatigue
Fa ilure
Over
No
2,000 ,000
Failure
Over
No
2,000 ,DOD
Fallure
data are graphed with respect to the
number of cycles of load as shown in
Figs. 22 and 23, respectively.
Repeated load of peak magnitude
equal to the cracking load of the
speci·
men appears to be a criti cal value. Thus
when repeated loading is higher than
the cracking load, stiffness loss
in·
creases with number of cycl es of load
indicating progressive deterioration
of
the section.
This is
conflrmed
in Fi g. 24 which
pl ots the load versus number of icycles
curve. The curve is characteristically a
reverse
"S"
with the lower leg
asympto·
tically approaching a value equal to
Prl"
which may
be
takcn as
the
endur­
ance limit.
The results thus indicate that
by
using a combination of reinforcin'g steel
bars and prestressed tension elements,
the endurance limit is increased to
c
?
40
"-I""
s.
A3
(0·95 P"r )
V1
V1
84
(1·11
~r
"
<=
:;:i
20
<J1
E
ill
aJ
~
0
10' 10
4
10
5
10' 2x10
6
Number of Load Repetitions
Fig. 22. Stiffness variation with different range of
applied
repeated
load.
80
r
.c:
~
:Q
40
I-
3:
Sp
A4(126
~r)
So.B4 (1·11
Per)
o
103
-" A3(095 Per)
.1
10
6
Number of Load Repetitions
Fig. 23.
Crack
width variation with different range of
applied
repeated
load.
Number of Load Repetit ions
Fig. 24. Load versus number of repetitions relationship.
PCI
JOURNAL/November-December 1976
91
P
cr
as compared to
a
value of
0.7
l\.,.
determined in an earlier
investigationS
using specimens with single tensi on cle­
ments.
Conclusions
Tests report ed herein and other
availa ble data
indicate that prestressed
tension clemen ts
used
as a part of ten­
sil e reinforcement arc very effecti ve for
improved structural servi ceabiJi ty in
term'i
of cracking
and
deRection.
The prestressed tension elemcnts
should bc accompanied by an amount
of non-prestressed reinforcemcnt to ob­
t3in
optimum results for crack control
and fati gue
strengt h.
On
the
basis
of
this investi gation, the
following concl usions may
be
drawn:
1.
The use of prestressed concrete
tension cl ements as continuit y rein­
forceJllent in composite construction
creates
a
superi or sect ion. The pre­
stress ing strands tcnd to
dose
lip
the
cracks in the
slab,
thus providing better
protection of the reinforcement against
corrosion.
2.
Before failure, there was
a
con­
siderable amount of moment redistribu­
tion, up to
35
percent, in the continuous
beams tested under static load.
3.
The ultimate load carrying capa­
cities of the specimens were closely
p redicted
by
the theory.
4.
When the force induced in the
tension element
due
to
Rexure was
less
than
its cracki ng load, no
fat igue
failure
developed in the specimens tested un­
der repeated load for well over one
mil ­
lion cycl es of loading.
5.
The loss of stillness of a specimen
as measured by the slope
of
its load­
defl ection curves, was dependent on the
Il'wgni tude
of the repeated load. ] n gen­
eral, the hi gher the magnitude of the
repeated load, the greater the rate of
progressive loss of stiffness.
vVhen
the
induced force in the tension element
92
was less than the cracking load, the loss
of st iffness was negligible.
6. Under comparable loading,
i.e.)
80
percent
Pen
the beam reinforced
with tension elements exclusively
ex­
hihit ed Jess defl ection and almost
ODe
half
of
the
crack
width when compared
with members using reinforcing bars
only.
7.
The specimens subjected to re­
peated load tests developed onl y
a
ma­
jor
crack
at the
cri ti cal
section,
whereas
those subjected to static tests developed
many sma ller
<:racks
evenl y distrihuted.
References
1.
Burns, N. H., "Development of Conti­
nuity Between
Precast Prestressed
Con­
crete
Beams,"
per
JOUHNAL, V.
11,
No.3, June 1966,
pp.
23-26.
2.
Hanson,
:-.:.
\V.,
;'Prest ressed
Concrete
Prisms
as Rei nforccment for
Crack
r;ontroi,"
PCl
JOURNAL,
V.
14,
No ..
5,
October
1969, pp.
14-3l.
3.
~ f irza,
J.
F.,
Zia,
P.,
and
Bhargava,
J.
R.,
"Static
and Fat igue St rength of
Beams
Con taining
Prestressed
Concrete
Tension
Elements,"
Final
Heport,
NCSU
Project
EHD- J
JO-f9-2,
January
1970.
Also see
Hi[!.lw,;au Research
Record,
No.
3)~,
1971,
pp.
54-60.
I.
Bishara,
A. C.,
:'o.lason,
C.
E.,
;,l nd
1\1-
meicla, F.
~.,
"Conti nuous
Beams
with
Prp,;tressed
Hcinforcenlent,"
S/ructrlres
Didsion
Jormwl,
ASCE,
V.
97,
1\'0.
ST9.
Se-
t ~ rnber
J 971.
pp.
2261-2275.
.':. "1\
Pi oneering
First
for
Canada,"
Bridge
Blll/etill,
Pre"trcssed
Concrete 1nstitut e,
September-October
1969.
6.
Okamura,
11 ..
and MatS11l11oto, S., "ge­
h,lviors
of
Concretc
Composite
Stl'll C­
tures
RE.'inforced
by
Precast Prcstre<;sed
COJlcretc
~temb e r s,"
Collected
Papers,
V.
II,
Department of
Civil
Engineering,
University
nf
Tokyo,
1973.
7. Zia,
P.,
~ I irza,
J.
F., and
Hizkall n,
S.
H.,
''The
Efrecls of
Ht'peated
Loads
on
Serviceability
and
Ullimate
Strength
of Cont inuolls Bridge Cirders
Utilizing
Tension
Elements,"
ERSD-11O-7.1-2,
II
ighway
Research
Program,
NCSU
(pllblic<ltion
pending).
Acknowledgment
The research described herein was
spon­
sored
by
the
North
Carolina De partment
of Transportation and Highway
Safety,
Division of Highways, and the
U.S.
De­
partment of Transportation, Federal High­
way Administration through the Higbway
Research Program
of the Department of
Civil Engineering, North Carol ina
State
Univers ity at Raleigh.
Technical li;:uson
with the sponsoring
agencies was provided by Landis M. Tem­
ple, Bridge
Design Engineer, North
Caro­
lina Division of Highways, aod
Oevohn
D.
Hhame, Bridge Engineer, Federal Highway
Administra tion.
The opinions, findin gs, and concl usions
expressed herein are those of the authors
and not necessa rily those
of
the sponsors.
The authors arc indebted to Arnold
Stone
Company, Greensboro, North
Caro·
lina, who fabricated the test specimens.
Appendix
Handling
and Cutting of
Tension
Elements
One
of the practical questions
re­
ga rding prestressed concrete tension
elements is whether the cl ement could
bo mass produced and cut to desired
length. If so, what would be the
prac­
tical
length limitation of the element
that could be handled without special
care
at
job si te?
To provide some evidence to these
(lUcst-ions,
two long tension clements
were fabri cated, one being
30
ft and
the other
40
ft in length. The latter was
lifted easil y and convenientl y with
two­
point pickup by a mobil e crane jn the
casting yard and encountered no
diffi­
culty in handling as shown in Fig. A.
The
40-ft
long tension element was
then shipped
70
miles to the laboratory
and withstood well in transit without
special care. along with other tension
clements and beams.
The 3D-ft long tension clement was
Fig. A.
Handling
of
40-11 long
pre­
stressed tension element
by
two·
pOint
pickup.
easily cut in the casting yard using a
masonry saw into one 15-ft section and
two
7Jh·ft
sections. Due to bond
trans­
fer, the prestressing strand withdrew
approximately
lA6·in.
at each eut end.
However. there was no apparent
dis·
tress or loss of bond due to the cutting
operation.
Discussion of this paper is invited.
Please
forward your discussion
to
PCI
Headquarters by
April 30,
1977.
PCI
JOURNAL/November-December 1976
93