Experimental Study on Seismic Performance of Prestressed Concrete Beams under Low Reversed Cyclic Loading

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Nov 15, 2013 (3 years and 7 months ago)

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Experimental
Study
o
n Seismic Performance
o
f Prestressed Concrete Beams

under Low Reversed Cyclic Loading


Jian

SU
1

and
Zong
-
guang

S
UN
1


1

Institute of Road and Bridge Engineering
,

Dalian Mar
i
time

University,
116026
,
Dalian
,

PH (
860
)
1
5524582525
; email:
su
jian
@
dlmu
.edu
.cn


ABSTRACT

Tests are conducted on four pairs of bonded and unbonded prestressed concrete
beams under low reversed cyclic loa
ding to investigate
the
seismic per
formance,
such as ductility, energy dissipation capacity, displacement restoring
capacity, failure
patterns, etc. Studies have shown that: bonded

prestressed concrete beams
have

a
better carrying capacity and energy dissipation capacity,
however

unbonded
prestressed concrete beams have more superior crack resistance and displacement
re
storing capacity
. This paper

would provide a reference for different projects

on the
usage of
different

types of

prestressed beams.

K
EYWORDS

U
nbonded; bonded; prestressed concrete beam; low reversed cyclic loading;
seismic performance

1 I
NTRODUCTION

The
f
ast
development
in

China's construction industry
widen the usage of
prestressed technology in concrete structures. There are two ways of prestressed
concrete technology.
Pretensioning technology is prestressed reinforcement before
the pouring of concrete.
Post
-
tensioned technology is prestressed reinforcement after
the poured of concrete.

People often use post
-
tensioned prestressed technology

because

it provides

better seismic performance

since the
pouring
is done
in the field.
At the same time, post
-

tens
ioned prestressed concrete structures have bonded and
unbonded prestressed concrete beam
s
. Bonded prestressed concrete beams refers to
the beams where
the prestressed steel and concrete are bonded together.
In contrast
,
in the
unbonded prestressed concrete

beams
the prestressed steel and concrete are

separat
ed
. Which technology is better? Which technology should be used in different
kinds of projects?
Due to the lack of

experimental studies. Based on this reason, the
paper taked tests on unbonded and bonded

prestressed concrete beams, compared
the seismic performance difference under low reversed cyclic loading to investigate
seismic performance, such as ductility, energy dissipation capacity, displacement
restoring capacity, failure patterns, etc
. This
comp
arison

would provide scientific
basis for the
promotion and application
prestressed beams structure
.

2 E
XPERIMENTAL STUDY

2.1 Specimen description

In this experiment, test beams were divided into two groups, namely, bonded
prestressed concrete beams (HY1,
HY2, HY3, HY4) and unbonded prestressed
concrete beams (HW1, HW2, HW3, HW4). In the same group, in addition to
differences with the same kinds of materials, the four test beams are almost identical.
Test beams have the same length, section size, concrete m
ix as well as ordinary steel
as shown
in Table 1.

Table 1. T
he

D
imensions of

Specimen

Specimen

Prestressing
Tendon

Upper Ordinary
Steel Bar

Bbottom Ordinary
Steel Bar

4000×200×300

2Φ15.2

2Φ14

3Φ18

In the test beams, the concrete cube compressive strength is 41.28MPa, the
concrete elastic modulus is 3.29×10
4
Mpa. Bonded prestressing tendon is a high
-
strength and low
-
relaxation steel strand. Unbonded prestressing tendon constituted
by

a high
-
strength and low
-
relaxation steel strand
internal
ly

and a polyethylene pipe
externally
.

At both ends of
of each
beam

there is

five
-
layer steel fabric, distance is
75mm

as

shown in Figure 1. The mechanical properties of steel bars

are listed

in
Tab
le 2.

Table
2
.
The Mechanical Properties of Steel

Type

Elastic Modulus

Y
ield
S
trength

U
ltimate
S
trength

Prestressing Tendon

Ordinary Steel Bar

1.95×105

2.0×105

1395.0

375.2

1860.0

571.6


Fig.1 Detailed drawings of specimen

2.2 Measuring points

A d
isplac
ement meter
is used to
measure
the
changes in the deflection.
F
ive
displacement meters

are used in the experiment
, a displacement meter at each side, a
displacement meter at the quarter
-
point, a displacement meter at the midpoint, a
displacement meter at t
he three
-
quarters point.

Strain gauge
is
used to measure the stress changes in the real
-
time data.
Reinforced strain gauge measures real
-
time data of the steel bars strain. Each
reinforcement has 15 strain gauges in the five equidistant cross
-
section. Each

cross
-
section has three strain gauges which standard size is one millimeter. Concrete strain
gauge measures real
-
time data of the concrete strain. Each of test beams has 6 strain
gages which arranged equidistant and vertical at the midpoint of the side.

T
he measuring point arrangement shown in Figure 2.


Fig.2 The measuring point arrangement

Fig
.3

Loading setup

2.3 Testing setup

Loading setup shown in Figure 3. Test beam(charpy) fixed on the reaction
frame,
the c
oncentration is provided by hydraulic

jacks. Test data collection devices
are mainly static strain collection instrument, strain dynamic collection instrument
,
force sensors, displacement sensors and computer terminals processing system.

2.4 Loading sequence

The lateral loading history presen
ted in Fig
ure
4 is applied to all the specimens.
The loading cycles are divided into two phases: load control and displacement
control.

The load control phase is used to define the piers’ experimental yield
displacement Δy, steps are as follows. First, calculate the piers’ experimental yield
load Fy. Load value when the first root yield of longitudinal reinforcement is defined
as the piers
’ experimental yield load Fy.
The controlled loading history includes
three complete cycles each for 0.7Fy.

The arithmetic average of the maximum
displacement of Δ
+

and Δ
-
. According to the following formulas to calculate the
piers’ experimental yield disp
lacement Δy:

2
2
1





y
;
7
.
0
1




;
7
.
0
2





;

Besides, a displacement control 1oading sequence is used. The displacement
controlled loading history includes three complete cycles each for Δy, 2Δy,
3Δy, ……, until t
he shear capacity of the piers declined and to 85


of the peak
loads. Here, Δy, 2Δy, 3Δy, ……, is the ratio of the applied lateral specimens.
Loading direction provides that: vertically downward is positive, on the contrary is
negative.


a
D
efinition of

Δ
y



b Loading sequence

Fig.4 Lateral loading sequence for pier specimens

3
OBSERVED DAMAGE STATES AND FAILURE PATTERNS

Two groups of test beams have similar approximately regular patterns in the
crack, crack development and failure mode. The
destructive process is listed as
follows:

1) Two groups of test beams have a good crack closure properties and
deformation recovery. Unbonded test beams cracks have better performance than
bonded test beams. Two groups of test beams of the crack closure
ar
e

good when
uninstalling. Flexural cracks first appeared in the pure bending section.

2) As the load increases, bending cracks grow and run through from top to
bottom, with the winding and shear cracks.

3) When the deformation reaches a certain degree, the

new cracks are no longer
appear, the length and width continue to develop on the basis of the original cracks,
some concrete in pure bending part of the test beams was crushed and peeled off,
ordinary steel is bent and pressed outward bulge
.

4) Test beam
failure patterns are all bending destroyed. In the large deformation
stage, the two groups of test beams are able to maintain a high carrying capacity,
showing good ductility and high energy dissipation capacity.

Fig
ure
5 shows the failure process of the s
pecimens at the tests.


a.Cracks appeared. b.Cracks growing


c.Steel yield d.Specimen crush

Fig.5 The failure process of the specimens

4
EXPERIMENTAL RESULTS AND ANALYSIS

Figure 6 shows the hysteresis curves of two gr
oups of test beams. Figure 7
shows the skeleton curves of two groups of test beams.


Fig.6 The hysteresis curves.


Fig.7 The skeleton curves

4.1 Ductility

The displacement ductility coefficient is used to describe the ductility of the test
beams,

denoted as
y
u




/

.

From the table 3, we could find that the ductility of unbonded prestressed
concrete beams is superior to the bonded ones. The deflection when bearing capacity
of the test beams started to decline is the limit displaceme
nt Δu.

Table 3 The
D
isplacement
D
uctility
C
oefficient

Specimen

y

/mm

u

/mm




HY1

HY2

HY3

HY4

HW1

HW2

HW3

HW4

9.19

9.08

9.15

9.26

9.33

8.93

9.29

9.04

41.38

41.13

39.65

42.63

49.51

49.66

49.59

49.13

4.50

4.53

4.33

4.60

5.31

5.56

5.33

5.43

4.2 Dissipated energy

Dissipated energy is a very important indicator to evaluate seismic performance
under earthquake loading. In this study, the equivalent viscous damping coefficient

(h
e
) suggested by Jaco
bson (Jacobson,1930) is used to judge dissipated energy of the
specimens.


Fig.8 Calculate the equivalent viscous damping coefficient

Shown in Figure 8,
OBD
ABC
e
S
S
h

2
1

, the results in table 4:

Table 4 Energy
P
arameter

Specimen

h
e

Specimen

h
e

HY
1

HY2

HY3

HY4

0.170

0.166

0.165

0.171

HW1

HW2

HW3

HW4

0.130

0.126

0.135

0.129

As the results showed that both of the two test beams had manifested favorable
energy consumption ability. Energy consumption ability of bonded prestressed
concrete beams is sup
erior to the unbonded ones.

4.3 Displacement recovery and bearing capacity

Displacement recovery ability of unbonded prestressed concrete beams is better
to the bonded ones, especially in the small displacement stage

and

unbonded test
beam to restore intact unloaded crack
.

Bearing capacity of bonded prestressed concrete beams is superior to the
unbonded ones
as shown
in table 4.

Table 5 Bearing
C
apacity

Specimen

Bearing capacity

Specimen

Bearing capacity

HY1

HY2

HY3

HY4

207.55

210.00

200.32

206.23

HW1

HW2

HW3

HW4

192.99

192.72

190.23

191.23

5 C
ONCLUSIONS

1) Unbonded prestressed concrete beams have a better ductility and deformation
recovery than bonded prestressed concrete beams.

2) Bonded prestressed concrete beam
has

better energy dissipation capacity than
unbonded prestressed concrete beams.
Therefore, b
onded prestressing technology
should be

used in the prestressed concrete beams of centralized reinforced.

3) Bearing capacity of bonded prestressed concrete beam is hi
gher than the
unbonded ones
by

7.42%.

REFERENCES

GB50010
-
2002 Code for design of concrete structures.

(
2002
).

Ministry of
Construction of the People
′s

Republic of China
,

Beijing
(In Chinese)
.

Priestley MJN and Park R (1987)
.

“Strength and Ductility of Concrete Bridge
Columns Under Seismic Loading
.


ACI Structural Journal,
84(1):51
-
75.

Sun Zhiguo

and

Si Bingjun

(2008).

Experimental research and finite
element
analysis of bridge piers failed in flexure
-
shear modes.


Earthquake
Engineering and Engineering Vibration
, 7
(
4
)
: 403
-
414.

Tanchan P

(
2001
)
.

Flexural Behavior of High Strength Concrete Beams Prestressed
with Unbonded Tendons.


New Jersey: New Bruns
wick Rutgers.

Zhang Li
-
mei

and

Zhao Shun
-
bo

(
2005
)
.

Experimental study of ductilily of
prestressed high
-
strength concrete beams.


Engineering Mechanics,

22(3):166
-
171(In Chinese).

Zheng Wen
-
zhong

and

Xie Heng
-
yan

(2007).


Analysis and calculation of factors on
curvature ductility of unbonded prestressed concrete beams.


Journal of
Harbin Institute of Technology (New Series),
14
(
1
)
:18
-
22.