CONCRETE LIBRARY OF JSCE NO. 37, JUNE 2001
QUANTITATIVE EVALUATION OF EARTHQUAKE-DAMAGED RC PIERS
(Translation from Proceedings of JSCE, No. 648/V—-47, May 2000)
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Kenji KOSA
Hideki SONE
Tsunekazu NAKATA
Mikio TASAKA
To evaluate the relationship between the damage level and the ultimate bearing capacity of piers, a quantitative
investigation was conducted on the RC single piers along the Hanshin Expressway Kobe Route, which were
severely affected by the 1995 Hyogo-ken Nambu Earthquake. According to the results of the investigation, 72
piers ranked As and A (severest damage and severe damage, respectively) and had termination of
reinforcement at the column mid-height were all damaged at the termination point. Their average shear strength
index was smaller than the index of piers ranked Bl or below. There were also ll piers ranked As and A that
were damaged at the column mid-height, even though their reinforcement was not terminated at that point.
These piers were found to have ashear strength vs. flexural strength ratio of below 1 and an extremely small
shear strength index.
g
Keywords: seismic resistance, ductility, failure mechanism, termination of reinforcement
Kenji KOSA, a member of the JSCE and the ASCE, is an associate professor in the Department of Civil
Engineering at the Kyushu Institute of Technology. Before joining the faculty in I999, he was a senior
highway engineer at the Hanshin Expressway Public Corporation. He obtained his Ph.D. from the University of
Michigan in 1988. His main research interests are the design and analysis of concrete structures.
Hideki SONE is a Director of the Kyoto Construction Division at the Hanshin Expressway Public Corporation.
After 30 years since his work for the original construction of the Kobe Route fi'om 1968 to 1970, Sone was
again assigned to work for the restoration and reconstruction of the earthquake-damaged Kobe Route from
1995 to 1998. His research interest is the design of steel and concrete structures. He is a member of the JSCE.
Tsunekazu NAKATA is a section manager at Yachiyo Engineering Co., Ltd. He obtained his MS in civil
engineering from Kyoto University in 1977. His research interest is the seismic design of concrete structures.
He is a registered consulting engineer and a member of the JSCE.
Mikio TASAKA is a deputy manager at the Obayashi Corporation in Tokyo. He obtained his BS in civil
engineering from Hokkaido University in 1984. He is a registered PE and a member of the JSCE.
-———1I7—-~
1. INTRODUCTION
Tremendous damage was caused to the bridge structures on the Hanshin Expressway Kobe Route when the
Hyogo-ken Nambu Earthquake hit the region in 1995. The damage included the falling of girders at several
locations. The route, immediately closed to traffic, was reopened 20 months later after the necessary repairs,
strengthening, and reconstruction were made.
In a quick survey after the earthquake, the damaged RC piers were grouped into the following damage ranks:
As (severest) , A, B, C, and D (minor or no damage). The piers were grouped based on the results of visual
inspections and analyses of photographs
[ 1 ]. Of the 943 RC piers inspected, 142 piers or 15% were ranked
As and A, severest and severe damage, respectively, and 327 piers or 35% were given ranks of B and C, which
represent relatively light damage. Piers ranked As and A were dismantled and new piers reconstructed in their
places. Piers ranked B through D were reused after necessary repairs or strengthening. All foundations were
reused because they were found to have been virtually unaffected by the earthquake.
Visual inspections may be adequate to assess the damage level of piers and judge the need for repairs,
strengthening, or reconstruction in an urgent situation after the earthquake. But to better evaluate the cause of
damage and the damage level of those piers, a quantitative investigation
should be carried out using detailed
damage information on concrete cracking, reinforcement buckling, etc. This is because visual inspections have
the following limitations:
1 ) Most damage to the concrete and significant damage to the reinforcement can be identified from visual
inspection of the cracking and falling of the concrete and the exposed main reinforcement below the fallen
cover concrete, respectively.
But, visual inspections cannot be used to evaluate B- and C- rank buckling
damage to the main reinforcement which is not so evident.
2) Visual inspections cannot evaluate damage to a pier below the ground simply because that section is
invisible.
In this research, the detailed results of the investigation
of the RC single piers ranked B, C, and D were
compared with the results of the visual inspection and then used to perform a quantitative evaluation. First, the
level of damage level to the concrete and the pier reinforcements, including the quantity of cracked concrete
and the length of the buckled portion of the main reinforcement, were measured. Next, the effects of the
damage type, the reinforcement termination, and the point of damage on the damage level of the piers were
evaluated. As it was found that the damage level of the piers was significantly
influenced by the reinforcement
termination and the ground conditions, an analysis was performed using parameters such as the position of
reinforcement termination and the ground spring.
2. DETAILED DAMAGEASSESSMENT
Piers ranked B, C, and D in the initial visual inspection right after the earthquake
with a focus on the spalling and cracking of the cover concrete and the buckling
reinforcement. Based on the results obtained, the piers were recategorized into ranks
replacement ratio of the outermost main reinforcement due to buckling, and ranks
severity of spalling and cracking of the concrete:
Bl:
B2:
B3:
Cl:
C2:
D:
were reassessed in detail,
and bulging of the main
of Bl. B2, and B3 by the
of Cl, C2, and D by the
nearly the entire outer main reinforcement needs replacement because of bulging around the entire
circumference of the column.
about 1/2 of the outer main reinforcement needs replacement.
about 1/4 of the outer main reinforcement needs replacement.
part of the main reinforcement is exposed, but there is no bulging of the main reinforcement
main reinforcement is not exposed, but major cracks observed
minorcracks orno damage
A detailed damage assessment was not performed on piers ranked B or below, which had been removed due to
significant residual tilting. Piers ranked As and A, and hence immediately covered with steel jacketing for
safety and then dismantled for reconstruction, were also not included in the assessment.
To minimize the effect of the pier type, only RC single-type
piers were selected
for investigation.
Figure
1
118
compares the damage ranks of 443 piers as determined by visual inspection and detailed damage assessment.
The figure shows that not a few piers previously given a C or D rank were assigned new ranks of Bl, B2, or
B3. Figure 2 shows the number of the piers whose initial damage ranks were changed to higher ranks as a
result of the detailed assessment. 46 C-ranked and 1 1 D-ranked piers were assigned new ranks of B3 or above.
This is primarily because predominant damage in the underground section of the column, which could not be
detected by visual inspection, was discovered during the detailed assessment.
60
58
49
Positions of damage
D Column mid-height
D Near ground surface
à"Underground
20
As-A
B
C
D
10
19
10
Damage ranking by visual inspection
Fig.l
Comparison
of damage levels
0
Detailed investigation
Visual inspection
3.
QUANTITATIVE
3. 1 Assessment
ANALYSIS
OF DAMAGE
of main reinforcement
Fig.2
Positions
of pier damage
damage
When designing a retrofit for a column, the lengths of the buckling and the bulging of the main reinforcement
wereused to determine the need to replace the main reinforcement. Here, the length of the buckling means the
vertical length of the column where the main reinforcement is bent, and the length of the bulging means the
horizontal length from the original position to the apex of the horizontal projection of the main reinforcement.
The main reinforcement at the point on the column where the concrete spalled were examined on all piers to
determine the need for replacement.
3.2
Length of bulging of the main reinforcement
Table 1 shows good agreement between the
damage ranks determined
by the detailed
assessment and the actual replacement ratio of the
main reinforcement, and that the increase in the
length of the bulging remained small, from 10 cm
to 14 cm, even though the damage rank was
increased from B3 to Bl. This tendency was
0
The length of the bulging
of the main
reinforcement was measured on about 20 piers
selected at random from each pier group for ranks
Bl, B2, and B3. Table 1 shows the relationship
between the damage ranks of the piers and the
damage levels of the main reinforcement. Figure 3
shows the relationship
between the length of the
buckling and the length of the bulging of the main
reinforcem ent.
1
Fig.3
0
15
20
Bulging length <5 (cm)
Relationship
between bulging length and buckling
main reinforcement
25
length of
11
probably caused by the process - when the
main reinforcement buckled due to loading
and an approximately
10 cm of bulge
occurred, the resistivity
of the main
reinforcement
to further loading
was
declined,
the load then began to be
transferred to the side, and the buckling of
the main reinforcement also progressed to
that direction. Some piers of Bl rank, the
reinforcements of which bulged over 20
cm, were rectangular-shaped
piers. The
load was probably not as easily transferred
to the side on that type of pier as with the
circular type.
250
O
à"
A
A
D
à"
X
200
fft
150
wj
100
Table 1 Damage levels of actual bridge piers affected
R e p la c e m e n t ra tio
C o n c r e te o f o u te r m o s t m a in
c h ip p e d
re i n fo rc e m e n t
D am age
ra n k
by the earthquake
B u lg in g le n g th
o f m a in
re in fo rc e m e n t
B u c k lin g le n g th
B l
6 .7 m 3
86 %
1 4 c m (4 .Q D )
8 8 c m (0 .3 5 W )
B 2
2 .3 m j
39 %
1 2 c m (3 .4 D )
7 7 c m (0 .3 0 W )
B 3
2 .0 m 3
18 %
1 0 c m (2 .9 D )
8 4 cm
C l
1 .O m 3
C 2
0 .0 8 m 3
D
0 .0 m 3
(0 .3 3 W )
D : D35 is assumed for the main reinforcement
W: width of the cross section
Bl(Circular)
Bl(Square)
B2(Circular)
B2(Square)
B3(Circular)
B3(Square)
Specimen
Regression analysis
50
0
0
PQ
50
Fig. 4 Relationship
3.3
Length of buckling
100
between buckling
150
250
200
300
350
400
Size of cross section D (cm)
length of main reinforcement and size of column cross section
of the main reinforcement
Figure 3 indicates that no clear relationship
exists between the length of the buckling, which varied widely,
from 40 cm to 140 cm, and the assigned damage rank. Figure 4 shows the relationship
between the size of the
pier cross-section and the length of the buckling in the main reinforcement as measured on specimens and
actual bridge piers of ranks Bl, B2, and B3. Here, the length of the buckling of the main reinforcement is
assumed to be the length of the plastic hinge since the difference between them is relatively
small, though the
former is, in the strict sense, different from the latter, which is used for calculating the ultimate displacement
for the seismic design.
Two lines are described in the figure: one is the line which assumes a length of 0.5D for the plastic hinge, as
recommended by past studies; the other line assumes a length of 0.5D (Do/D) 03" which is calculated by the
"least square" method by taking into account the cross-sectional size of the actual piers. Do is assumed to be 55
cm,the average cross-section of the column specimens.
According to Fig. 4, the length of the plastic hinge on an actual bridge pier with a cross-section of over 200
cm tends to be below 0.5D because the length of the buckling of the main reinforcement of this big pier does
not increase, even though size of the cross-section is larger. In the case of small piers with a cross-section no
greater than 100 cm, the length of the buckling of the main reinforcement increases in proportion to the
increase in the size of the cross section [2].
3.4
Analysis of concrete damage
Table 1 shows the amount of the concrete that had chipped from the piers in each damage rank. Here, the
chipping range means the range at which the cover concrete fell, or the cracked concrete came apart, as a result
of chipping. The amount of chipping from piers of ranks C2 and D was very limited because their damage was
simply cracking. In contrast, the chipping amount from piers of ranks Bl, B2, and B3 was greater and in
proportion to the increase in main reinforcement replacement. For example, in the case of Pier 153 with a
720
-
square cross-section (3.2 x 3.2 m) and a rank of B3, the ratio of reinforcement replacement was 24%, the
length of the buckling was 97.6 cm, the length of the bulging was 10.4 cm, and the amount of chipped
concrete was 1.6 m3 (surface area 9.9 m2; average depth 0.16 m) , indicating that the concrete was chipped up
to a level near the main reinforcement.
3.5
Quantitative
assessment of damage
The damage levels obtained from 1/6 scale pier specimens and from actual earthquake-damaged bridges on the
Kobe Route were compared. The experiment, conducted under reversed loading conditions,
was intended to
confirm that earthquake-damaged concrete piers could be reused if necessary repairs or strengthening
was
applied. The details of the experiment are given in Reference 3. The cross-section of the specimens was a 60
x 60 cm square. D10 reinforcing bars were arranged with a spacing of 80 mmas the main reinforcement, and
D6 reinforcing bars with a spacing of 200 mmas the hoop ties. The cover concrete was applied to a depth of
40mm.
The specimens were loaded until they reach damage levels of A, B, and C ranks. The specimen intended to
simulate A-rank damage was loaded to a displacement of 7 S y (a displacement far exceeding the ultimate
displacement)
at which all the reinforcements on all four sides buckled. The specimen simulating B-rank
damage was loaded to 5 S y (nearly equal to the ultimate displacement)
at which the main reinforcements
on two sides began to bulge and the core concrete began to fail. Specimen C was loaded to 3 <5 y , a
displacement at which no buckling of the main reinforcement is expected.
Table 2 shows the damage levels of the specimens. As to the specimen loaded to an A-rank damage level, the
ratio of reinforcement replacement was 100 % and the length of the bulging was 4.0D. For the specimen of
B-rank damage, the former ratio was 70% and the latter length was 2.5D. In contrast, the average
reinforcement replacement ratio for actual piers of ranks Bl, B2, and B3 was 47% and the average length of
the bulging was 3.4D, as seen in Table 1. In other words, in terms of the replacement ratio of the main
reinforcement, the average value of actual piers of ranks Bl, B2, and B3 roughly equals the value of specimens
of B rank or below. And, in terms of the length of the bulging, the average value for actual piers of the same
ranks falls somewhere in between the value of specimens of ranks A and B. Therefore, it can be said that
B-rank damage to actual piers is roughly in agreement with B-rank damage to the specimens, which means that
actual piers were subjected to earthquake loading that caused an ultimate displacement
( approximately
5 8
y ). As actual piers with C-rank damage showed slight damage to the concrete and no buckling of the
reinforcement, it roughly corresponds to C-rank damage to the specimens ( a displacement of 3 S y ).
Table 2 Damage levels of specimens
D am ag e
rank
R eplacem ent ratio
o f o uterm ost m ain
reinfo rcem en t
C on crete
ch ipp ed
Table 3 Typical
B u lging length of
m ain
rein forc em en t
P ie r N o .
cracking damages of piers
In c lin a tio n a n g le
W id th o f c ra c k s
S p a c in g b e tw e e n
o f c ra c k s ( -)
(m m )
c ra c k s (c m )
K o b e P -8 2
65
0 .4
1 7 .1
A
0 .24 m j
10 0 %
40 m m (4 .OD )
K o b e P -2 1 6
70
0 .9 5
1 6 .0
B
0.10 m 3
70 %
25 m m (2.5D )
K o b e P -2 6 9
5 9
0 .6 1
1 7 .1
0
c
4. ANALYSIS OF DAMAGETYPES
4.1
Characteristics
of shear damage
The number of piers that were damaged by shearing during the earthquake was relatively
limited, just 20
(4.5%).
Of this number, 17 were assigned an As or A rank, having suffered a quick brittle fracture. Three
other piers were given a rank of Bl or below and had visible damage characteristics;
these are shown in Table
3. The main characteristic of these piers was that their crack inclination angle was steeper than 45 °. This is
because their hoop-tie ratio was smaller than the main reinforcement ratio, as indicated in Reference 4.
The cracking of the three piers was grouped into two types: one was the type in which cracking propagated to
a degree of about 70 ° after originating
from the intersection
of a beam and a column, as shown by the
left-hand sketch in Fig. 5. When this cracking continued, the beam and the main reinforcement in the column
12
were torn, as seen in the right-hand sketch
in the figure. One reason for this damage
was that the number of hoop ties, which
were arranged
around
the main
reinforcements
in the beam, was not
sufficient in quantity. The other cracking
type was shear cracking, which propagated
at a degree of about 60 ° after starting
from somewhere between the bottom and
the mid-height of the column, as shown by
the left-hand sketch in Fig. 6. Characteristic
of this cracking is that it started near the
ground surface of the column. When this
cracking continued, it resulted in typical
shear damage at the column's mid-height,
as seen on the right in Fig. 6. Both of these
types of damage are considered due to an
insufficient number of hoop ties around the
main reinforcement.
4.2
Effect of reinforcement termination
Fig.5
Damage pattern
(left : cracking
right : fracture)
Fig.6
Damage pattern (left : cracking
right : fracture)
The positions
of reinforcement termination in the actual piers were confirmed utilizing
detailed damage
sketches, photos, and the original bar arrangement drawings used at the time of bridge construction. Table 4
shows the relationship
between the presence or absence of reinforcement termination and the damage types. Of
the 278 piers without reinforcement termination, 234 ( 84%) were damaged at the pier bottom by flexure or
flexural shearing. The remaining 44 were damaged by shearing or flexural shearing at the column's mid-height.
This is because the resistivity
of the column's cross-section to an acting shear force is sometimes smaller at the
column's mid-height than at the column's bottom because of the effect of a/d (shear span ratio).
Table 5 shows the relationship
between the presence or absence of reinforcement termination and the damage
levels. Of the 95 piers without reinforcement termination that suffered damage of Bl level or higher, 13 were
damaged at the mid-height of the column. Actually, 1 1 of them were of As and A damage rank. They were the
piers with a large square cross-section that were located at the crossings of ground-level roads. Of the 287 piers
without reinforcement termination that suffered damage of various ranks, 234 were damaged by flexure at the
column bottom. Of the 82 piers ranked Bl or above, 52 were damaged by flexural shearing.
Table 4 Termination
of reinforcement
M id R e in fo r c e m
P o sitio n s
ent
te rm in a te d
o f da m age
h e ic h t
B o tto m
T o ta l
19
41
R e in f o rc em
P o s itio n s
en t no t
te r m in a te d
of dam a ge
B o tto m
T o ta l
83
4
87
A - A s -B l
3
0
3
105
45
14
44
15 8
73
3
2 34
15 piers without
17
R ein fo rc e m
P o s itio n s
ent
te rm i n a te d
of d am ag e
2 78
M id h ei e h t
71
B o t to m
R e in f o rc e m P o s itio n s
o f da m ag e
e nt no t
B 2 -B 3
T o ta l
C l -C 2 4
36
Il l
2
9
34
45
70
156
73
13
M id h e ie h t
13
2
38
53
B o tto m
82
37
115
23 4
95
39
15 3
2 87
T o ta l
15 0
30
103
and damege ranking of piers
T o ta l
0
15 8
of reinforcement
D a m a g e ra n k
D a m a g e ty p e
F le x u ra l
F le x u ra l
sh e a r
shear
60
M id h e ie h t
Table 5 Termination
and types of pier damage
te rm in a te d
T o ta l
damage were excluded
Figure 7 shows the distribution of damage to piers with reinforcement termination. Figure 8 shows the same
distribution,
but to piers without reinforcement termination. With regard to the former type of piers, about half
of them were of As or A damage rank and the remaining half were mostly C-rank or below, with a few piers
suffering intermediate level of damage of Bl, B2, or B3 rank. This means that piers with reinforcement
termination tend to cause shear-type damage at the termination point, and once damage is triggered, the damage
722
quickly develops into major damage. In contrast, the damage distribution of piers without reinforcement
termination is scattered, as seen from Fig. 8. This is because they tended to suffer flexural damage at the
bottom of the column. Therefore, it can be said that the presence or absence of reinforcement termination had a
significant effect on the damage level, the damage type, and the location of the damage to piers.
With reinforcement
14.1%(22)jD
Cl
0.6%(1)
3.2%(5)
Fig.7 Distribution
Bl
B2
à"As-A
à"Bl
à"B2
OB3
DC1
DC2
DD
1 0 .1 % (2 9 )
B l
7 .3 % (2 1 )
B 2
27.5%(79)
Fig.10
U
Column mid-height
Near5raund
surface
On the road
Damage ranking and damage positions
rein forcement termination
Locations
nderground
6.3%(18)
s -A
l
2
3
l
D C 2
D D
B3
of damage to piers without
IA
IB
B
D B
D C
reinforcement.termination
termination
Fig.9
nderground
Cl
Fig.8 Distribution
(a) Column mid-height
U
[g lG 3 E ^ B a
C2
of damage to piers with reinforcement
Nearground
surface
D
17.8%(51^
A_
5.1%(8)B3
4.3
Without reinforcement termination
8.0%(23)
46.2%(72)
12.8%(20)
C2
17.9%(28)
termination
(b) Near ground surface
Positions of pier damage
U
Column mid-height
Other locations
of piers with
Nearground
surface
Fig.ll
nderground
(c) Underground
Column mid-height
Underground
Nearground
su rface
On the road
Damage ranking and damage positions
rei nforcement term ination
Column mid-height
Other locations
of piers without
of damage
To investigate the effect of the locations of the damage, the locations of the damage to the piers were grouped
into three types: underground, near the ground surface, and at the column's mid-height. These groups are shown
in Fig. 9. Here, the position "underground" means predominant damage occurred in the underground section of
the column and the damage was not observable from the surface. The position "near the ground surface" means
the dominant damage was near the ground surface of the column. The backfill depth at the column bottom
averaged
1.5 m.
Figures 10 and ll show the locations of damage to piers with and without reinforcement termination,
respectively,
and grouped by pier location, either on a ground-level road or other locations. Figure 10 shows
that major damage to the piers occurred mostly at the mid-height of the columns, and that damage near the
12
ground surface was very limited. The location of the damage and the damage rank did not differ much in this
type of pier, regardless of their locations (either on a ground-level road or other locations).
In contrast, piers
without reinforcement termination suffered relatively low-level damage when the piers were at other locations,
but when piers were on a ground-level road, many of them suffered damage to the column near the ground
surface, and their damage levels tended to be higher, even though their reinforcement was not terminated, as
seen from Fig. ll. This is probably because seismic motion was very severe locally and the column bottom
was confined due to the effect of ground conditions, such as pavement. The effect of these ground conditions
will be examined in detail in Section 5.
4.4
Strength of piers ranked As and A
a) Piers with reinforcement termination
The cause of the damage to As- and A- ranked piers on the Kobe Route were analyzed. Table 6 is a
breakdown of the damage to the piers. Piers with reinforcement termination totaled 72 and those without
reinforcement termination totaled 66. Of the 72 with reinforcement termination, 70 were damaged at the
mid-height of the column. Comparing the damaged piers with their bar arrangement drawings, it was found that
the locations of the damage corresponded to the points of reinforcement termination. With regard to the two
piers damaged near the ground surface, their damage also corresponded to the reinforcement termination points,
as drawings indicated that the reinforcement was terminated 2 m from the column bottoms. The relationship
between the shear strength index ( a *) and the yield flexural strength index ( a my) for the 72 piers is
shown in Fig. 12. The values of a * and a myare expressed by the following equations:
a su = 980à" Su/ (Wu+Wp)
/ (Wu+O.SWp)
a my = 980 (My-Mo)
(1)
ia
where ,
a su:
shear strength
index (gal)
Su: shear strength derived fromthe 1990 Specifications
for Highway Bridges
Wu: weight of the superstructure carried by the pier (tf or kN)
Wp: weight of the pier (tf or kN)
( tf)
: yield flexural strength index (gal)
bending momentat the time when the outermost reinforcement reaches the yield stress on the
cross-section to which constant axial force acts (tf à" m or kN à"m) , which is derived by the 1990
Specifications for Highway Bridges
la : distance from cross-section of the focus to the point at which the inertial force is imposed by the
superstructure ( m )
Mo: Eccentric bending moment (tfor kN)
OC my
My:
Average of piers ranked
Bl or below amy'=293.5gal
F
b
500
A *
to piers with As and A ranks
R ein forcem L oca tions
R ein fo rcem
ent
te rm in ated
R ein fo rcem
ent no t
term in ated
of p iers
O n the
R o ad
Positio ns o f dam age
M id -h eigh t
0
U n dergro un d
0
M id -h eigh t
N ea r gro un d su rface
400
ked
- ABver
L or
agf
hkl
rofpi
fi y ertfs-suM
ranked
SZ
g acal'l
*¥- x[_｣iQ]-
59
N ea r g ro und su rfac e
M id -h eigh t
O ther
loc ations N ear g rou nd su rfac e
U n derg ro un d
O n the
R oa d
N u m ber o f p iers
59
300
2
D ¥LLFO-1.
I
72
ll
a 200
13
U n dergrou n d
Co lum n m id-heig hl(shear)
100
ll
44
M id -h eigh t
O ther
N ea r gro un d su rface
lo cations
U nde rgro un d
^ Ground surface (a m y'=232 .8. a sul=455.0)
!
57
2
66
100
0
5
m m ¥
n Colum n m id- heigtil( a m y'= 283.5, a su'=30 1.8)
0
0
e nt
:
2
9
Fig.12
4
Relationship
flexural index
00
300
400
The average shear strength index of the 72 piers, or 306 gal, was much smaller than the 353 gal derived
the piers of Bl rank or below. Two piers that suffered damage, near the ground surface had a relatively
124
-
500
a my'(gal)
between shear index and yield
from
large
shear strength index of 450 gal because their reinforcement reduction from 2 layers to 1.5 layers did not cause
any significant decline in their shear strength, but their yield flexural strength index was as small as 200 ~
250 gal. Therefore, it is assumed that the flexural cracks at the point of reinforcement termination near the
column bottom continued to propagate, leading to severe damage from flexural shearing [ 5 ].
All the piers with reinforcement termination that were ranked As and A were damaged at the point of
termination. Though the damage levels differed slightly, the severe damage was caused because the flexural
and shear strengths at the point of reinforcement termination were comparatively smaller than at other points.
b ) Piers without reinforcement termination
Of the 66 piers without reinforcement termination, 1 1 were damaged at the mid-height of the columns and 55
at the bottom of the columns (near the ground surface and underground). The ll were all large piers with a
square cross-section constructed at the crossings of ground level roads. To support their large dead loads, the
cross-sectional width is large and the a/d comparatively small. Figures 13 and 14 show the relationship between
the flexural strength and shear strength of piers without reinforcement termination, and the relationship between
the yield flexural strength index and shear strength index, respectively.
As seen from the figures, the ratio of shear strength to flexural strength of the piers that suffered damage at the
mid-height of the columns was below 1, and the average shear strength index of those piers, 327 gal, was
smaller than the average shear strength index of the piers of Bl rank or below, which was 397 gal. This
indicates that piers with a small shear strength/flexural
strength ratio and piers with extremely low shear
strength tend to be damaged by shearing at the mid-height of the column rather than at the bottom. These
results suggest that sufficient shear strength, and an additional margin for safety, should be provided not only at
the bottom area of the column but also at mid-height. Piers without reinforcement termination that were
damaged near the bottom and underground had an average shear strength of 360 gal and 387 gal, respectively,
which was slightly smaller than the 397 gal for piers of Bl rank or below. The yield flexural strength index of
the piers was widely scattered and no qualitative tendency was found.
25000
20000
I
- 1
C o lu m n m id -h e ig h t
A
U n d e rg ro u n d
O
N e a r g ro u n d su rfa c e
S u - la / 1 .2 M u -M o = 1 .0
'S
s
500
X
400
X
A v ei r
0
o
(
5000
D
Q C o l u m nm i d - h e i g h t ( a m y = 3 0 2 . 2 ,a s u = 3 2 6 . 5 )
^ U n d e r g r o u n d( a m y = 3 0 2 8. , a s u = 3 8 7 6. )
1 0 0
O N e a rg o u r n d s u r f a c e ( o r m y = 2 6 06. , a s u = 3 5 9 . 7 )
10000
15000
20000
25000
1 . 2 M u - M o ( tmf )-
Fig.13
A
e 200
I
.#
oD
Do
0
5000
Sid
a g os fp i e r sr a n k e d
Blor
0 3 0 0
I
15000
10000
A v e r a og fpe i e r sr a n k e d
B l o rb e l o wa m y = 2 7 7 . 3 g a l
/ '/ '
R e l a t i o n s h i pb e t w e es hn e a cr a p a c i t ya n d
f l e x u r ac la p a c i t y
0
100
200
300
400
500
am y ( g a l )
F i g . 1 4 R e l a t i o n s h i p b e t w e esnh e a ir n d e xa n d
yield flexural index
5 . A N A L Y S IOS F G R O U NRDE S I S T A N C E
5 . 1 O u t l i n eo f a n a l y s i s
a) Piers f o r a n a l y s i s
A s a l r e a d ys e e nf r o mT a b l e s4 a n d5 , m a n py i e r sw e r fe o u n dt o b e d a m a g ende a rt h e b o t t o m( n e a r t h e g r o u n d
s u r f a caen d u n d e r g r o u n dT )h. e e f f e c to f g r o u n dr e s i s t a n c oe n t h e d a m a g lee v e l o f t h e p i e r s w a sa n a l y z e bd y
t w o - d i m e n s i o n na lo n l i n e a rF E Ma n a l y s i s .D e t a i l s o f t h e a n a l y t i c a lp r o c e d u raer eg i v e ni n R e f e r e n 6c .eS e l e c t e d
f o r a n a l y s i sw a sa n a c t u a Tl - s h a p e Rd C p i e r , t h e c o n f i g u r a t i oonf, w h i c hi s s h o w ni n F i g . 1 5 . T h e p i e r h a s a
7
2
circular cross-section, 3.5 m in diameter, a height of 12.66 m, and supports a steel I-shaped simple girder that
has spans of 60 m and 39 m. The 1/5 of the reinforcements is terminated at a height 6.45 m from the upper
face of the footing. The backfill depth is 1.5 m. Of this length, 0.5 m is concrete pavement and 1.0 m is earth
with an N value of 15.
19 5 0 0
6847
* 3500 ,
9153
1 9 50 0
6850
I
fO
o
fｻ
IlI l Ill l
.H o o p tie :D 1 6@ 30 0
l!
Te rm in a tio n o f m a in re in fo rce m e ntl"
o
o 00
r-.
= == = = =
iｫ
:: :
H o op tie :D 1 6
(tru ss e le m e n t)^
M a in re in fo rc e m e n t
1s t la ye r:9 0 - D 3 5
2 n d la y er :9 0 - D 3 5
3 rd la y e r:4 5 - D 3 5
R e in fo rce m e n t
te rm in a tio n :u p p e r p o sitio n
M a in re in fo rc e m e n t:D 3 5
(tru ss e le m e n t)x
K ^Q ^ ^ ^n
o
3500
10 1
R e in fo rc em e n t
te rm in a tio n :lo w e r po s itio n
C o n c re te
(p la n e s tre ss e le m e n t)
E
o
U T
Q J
L IT
D J
u
Fig.17
Fig.15
Analysis
model
Configurations
of a pier for analysis
and its reinforcement
arrangement
T a b le 7 P h y s ic a l p r o p e r t ie s
Element
used
f
or analysis\
Hoop tie
Y o u n g 's m o d u lu s (N / m m " )
P o is s o n 's ra tio
Fig.16
Element
for analysis
N
r<E-rfSK
*M
C o n c re te
2 .5 9 X 1 0 4
R e in fo r c e m e n t
2 .5 9 X 1 0 4
0 .1 6 7
0 .3 0 0
C o m p re s s iv e s t re n g th (N /m m '
3 5 .2
T e n s ile s tr e n g th ( N / m m "
2 .5
3 4 9 .1
Y ie ld s tr e n g th ( N / m n r
D e te rm i n e d b a s e d o n th e S p e c ific a t io n fo r H ig h w a y B r id g e s [3 ]
used
as a hoop tie
b) Analytical model
The section of the pier above the footing's upper face was selected for analysis. Focus was on the direction
perpendicular to the bridge axis. The bottom of the column was assumed to be fixed. The pier was modeled
two dimensionally. The concrete of the pier was assumed to be a plane stress element and the reinforcement a
truss element. As parameters, the point of reinforcement termination and the ground resistance were selected.
The model was constructed based on the following conditions:
0) Two positions were assumed as the reinforcement termination points:
< Upper termination point >
The upper reinforcement termination point was set at 5.65 m from the upper face of the footing by deducting
the anchoring length of 80 cm specified by the Concrete Specifications
from 6.45 m, which is the
reinforcement termination point of the actual piers.
< Lower termination point >
The lower reinforcement termination point was set at 2.4 m from the upper face of the footing by deducting the
anchoring
length
of 80 cm from 3.2 m, which is 1/2 of 6.45
m.
0) Hoopties
As the model was two dimensional, the effect of the hoop ties was taken into consideration
hoop tie ratio into the area of the inplane element, as shown in Fig. 16.
by converting the
(D Ground resistance by backfill
This was considered to be an elastic spring, and the spring value was determined based on the Specifications
for Highway Bridges.
The derived analysis model, constructed
shown in Fig.
of two dimensional
finite elements based on the above conditions,
17.
126
-
is
c) Physical properties of analysis model
The physical properties of the analysis model were determined as described in Table 7 and based on the results
of a laboratory test conducted on samples taken from actual piers. The following models were used to account
for the nonlinear characteristics
of the analysis model.
0) Stress-strain relationship
under compressive force : Saenz model
0) Strain softening characteristic
under compressive force : Darwin-Pecknold model
0) Shear transfer after cracking : Al-Mahaidi model
0) Reinforcement : bilinear model
d) Loading condition
Loading was applied to the nodal points of the main girders,
analysis under the dead weight condition was conducted.
5.2
Analytical
producing
displacements
up to 50 cm after
results
a)
Case 1: ground resistance is not considered
The load-displacement
relationship
is shown in Fig. 18. The maximumload was 8.96 x lO^N when the main
reinforcement was terminated on the upper side of the column, and 8.81 x 10*N when it was terminated on
the lower side of the column. The analytical load-displacement
relationship
is roughly in agreement with the
flexural strength derived from the Specifications
for Highway Bridges, which is also shown in the- figure.
Figure 19 shows the distribution
of vertical strain and shear strain at the time of 7 S y ( 6 y : displacement
when the main reinforcement within the 45 ° of the tensile side of the cross section buckled).
When the main reinforcement was terminated on the upper side, the piers failed at
as the vertical and shear strain areas were enlarged at those locations. In contrast,
was on the lower side, the piers failed at both the bottom and at the reinforcement
vertical strain area was enlarged at those two locations and the shear strain
reinforcement termination point toward the bottom of the column.
the bottom of the columns
when the termination point
termination point after the
area propagated from the
b)
Case 2 : ground resistance is considered
The load-displacement relationship
for Case 2 is shown in Fig. 20. The maximumload was 9.82 x 10*N when
the reinforcement was terminated on the upper side of the column, and 9.43 x 10*N when it was terminated
on the lower side of the column. Both values are greater than those of Case 1. Figure 21 shows the
distributions
of vertical strain and shear strain at the time of the displacement of 7 <5 y. When the
reinforcement termination point was on the upper side, the vertical strain area was enlarged on the column near
the surface of the backfill and the shear strain area was concentrated in the area between the surface of the
backfill and the reinforcement termination point. Though the flexural capacity was increased by about 10%
because the flexural span was shortened due to ground resistance, the shear failure mode was apparently
enhanced on the pier by the dominating area of shear strain near the ground surface.
5.3
Discussion
When designing a pier, the ground resistance around the pier is usually excluded from consideration so as to
increase the safety of the pier. However, if the physical condition near the ground surface is. rocky due to the
presence of concrete, the shear fracture mode will be enhanced on the pier and will likely cause a brittle
fracture. Because of this, the deformation capacity of the pier will decline, even though flexural strength is
increased. This is probably why shear damage and flexural shear damage are often found near the ground
surface of the column. Therefore, sufficient shear strength should be provided in this area of the column in
order to avoid shear failure.
6. DAMAGETYPES AT REINFORCEMENT TERMINATION POINTS
As stated earlier, damage to the piers varied widely, depending on the presence or absence of reinforcement
termination. Damage analysis was conducted focusing on piers with reinforcement termination. Figure 22
shows the relationship
between the yield flexural strength index ( 0 my) and the actual damage to the piers.
As seen from the figure, those piers which were damaged at the bottom of the column all had a considerably
greater a myvalue at the reinforcement termination point than at the bottom of the column and, therefore, the
727
Reinforcement terminatiormiDDer Dosjtion
′一■ヽ
2
Reinforcement termination:uDDer position
1､〇
⊂)
<-I
Distribution of vert仙n
X
Distribution of sheor st〔如
ヽー
●1コ
3
10 20 30 40 50 60
ユ Horizon:ミミ慧 !n¥vサr悪
Re㈱o山owe｢ Distribution
DOSlt蜘
of shear stro担
Distribution of vertical strain
′ ヽ
Z
lく〉
⊂)
▼･一l
X
ヽ-′
｢コ
ぐq
3
Fig.19 Distribution of strain(Cosel)
10 20 30 40 50 60
Horizontal displacement(cm)
* Spec, of Highway Bridges
Fig.18 Load-displacement relationship(Casel)
尽einforcement termmation:uDDer oosition
Reinforcement termmation:uDDer oosition
Distribution of shear strain
′-■ヽ
Distribution of vertical strc〕in
Z
＼○
⊂)
rH
X
)
｢⊃
ct)
3
10 20 30 40 50 60
Holizontal displacement(cm)
Reinforcement terminc】tiordower position
Distribution of vertical strain
Distribution of shecコr strain
′ ､
2
1■)
⊂)
T-I
X
)
■｢コ
cg
3
Fig.21 Distribution of strain(Case2)
10 20 30 40 50 60
Horizontal displacement(cm)
rela吉ase.2)
Fig.20 Load-displacement ｡nship
128 -
damage preceded from the latter position. There were also many other piers which suffered damage at the
termination point, even though the value of a myat that position was greater than at the bottom of the column.
The a myvalue at the termination point of these piers was relatively small compared to that of piers that had
failed at the bottom of the columns. Also, most piers damaged at the bottom had failed due to flexure, but piers
damaged at the reinforcement termination point mostly failed due to flexural shearing. The a myvalue, or yield
flexural strength index, is an index that causes a yield bending moment on the cross-section of the focus. It is
calculated by Equation (2).
amy
=My/(Wà"la)
(2)
where,
la : distance fromthe cross-section of the focus to the point at which the inertial force is imposed from the
superstructure ( m )
W: weight contributing to flexure of the column above the cross-section of the focus (tf)
Figure 23 shows the values of a myand a my,which are yield flexural strength indices (gal)
at the bottom
of the column and the reinforcement termination point, respectively. The marks in the figure are gathered into
two groups: ranks A and B, and ranks B2, B3, C, and D. As seen from the figure, the damage level is higher
for small piers with a gal of below 350 at both the bottom of the column and the reinforcement termination
point. Also, a correlation is seen between the damage level, the shear strength index, and the yield flexural
strength index. It is also seen that piers with a relatively large a 0' / <x myratio at the reinforcement
termination point and in the bottom area around the column suffered comparatively little damage. This is
probably due to the fact that the reinforcement termination in those piers did not become a vulnerable point and
hence the bottoms of the columns suffered minor flexural damage.
~700
à"g. 700
"c3
0
1 600
Q.
C
o
I
500
0u 400
u
u1 300
O column bottom -flexural
D column bottom -flexural shear
Acolumn bottom-shear
à"mid-height* flexural
à"mid-height-flexural
shear
A mid-height-shear
200
100 J
100
Fig.22
7.
200
300
400
500
600
200
700
amyat the column bottom (gal)
Relationship
between yield flexural index and
damage positions
300
400
500
600
700
amy at the column bottom (gal)
Fig.23
Relationship
between yield flexural
damage ranks
index and
CONCLUSIONS
A detailed damage assessment was conducted on the RC single piers on the Hanshin Expressway Kobe Route,
which suffered damage in the 1995 Hyogoken-Nambu Earthquake. The following conclusions were drawn from
the results:
CD Based on the results of a detailed damage assessment, many piers ranked C and D in the first-round
visual inspection were given a B rank, which required their reinforcements to be replaced. This is primarily
because damage to the columns below the ground was newly found.
(D It was found by the detailed investigation
of the damage levels that the length of the buckling of the main
reinforcement was slightly smaller than the width of the column cross-section if the cross-section was over 200
cm,as was the case with the actual piers.
(D
Piers with reinforcement termination mostly suffered damage of As and A ranks and few suffered B rank
damage. This is because the damage at the reinforcement termination point propagated quickly and ended in
129
severe damage.
@ Of the 138 piers ranked As and A, 72 had reinforcement termination and 66 did not. The 72 piers with
reinforcement termination failed at the termination point. Their shear strength index, 306 gal, was significantly
smaller than that of piers of Bl rank or below, which had a shear strength index of 353 gal.
<D The ll piers of As and A ranks, which suffered damage at the mid-height
of the column even though
they did not have reinforcement termination at that point had a shear strength/flexural
strength ratio of below 1 ,
and their average shear strength index, 327 gal, was smaller than that for piers below Bl rank, which was 397
gal. The 55 piers which suffered damage near the ground surface and underground had a relatively small shear
strength index of 363 gal.
® If the shear strength index of the pier was below 400 gal, the damage level tended to be more severe
regardless of the presence or absence of reinforcement termination. This suggests that piers with a small shear
strength index value need to be strengthened.
0)
Piers without reinforcement termination and situated on a ground level road were mostly damaged near
the ground surface, and their damage level tended to be higher. According to the two-dimensional nonlinear
FEM analysis, if the ground surface is very hard, the shear fracture mode is enhanced and damage occurs at the
bottom of the column. Therefore, sufficient shear strength should be provided to the bottom of the column near
the ground surface.
REFERENCES
[ 1 ] Road Association of Japan, "Handbook for Roads Damaged by Earthquakes: Retrofit Procedure", 1988.2
(in Japanese)
[2]
Shimpo, H., Murayama, Y., and Suda, K., "Experimental Study on Buckling of Main Reinforcement in
RC Columns", Proc. of 49th Annual Conference of JSCE, V-5, pp. 966-967, 1994.9 (in Japanese)
[3 ] Yokoi, K., Fujii, M., Yasuda. F., and Kosa, K., "Experimental Study on Seismic Strengthening
Effect of
Damaged RC Piers", Proc. of JCI, Vol. 18, No. 2, pp.137-142,
1996 (in Japanese)
[4]
Mizuta, T., Kobayashi, K., Kosa, K., and Yasuda, F., "Evaluation of Shear Strength of RC Piers in
Existing Bridges Using Modified Compression Field Theory", Proc. of JCI, Vol. 19, No. 1, pp.387-392,
1997
(in Japanese)
[5]
Kosa, K., Fujii, M., Hayashi, H., and Nakata, T., "Evaluation of Earthquake-Damaged
RC Piers by the
Ultimate
Bearing Capacity Procedure", Proc. of JSCE, No.592/V-39,
pp.73-82,
1998.5
( in Japanese)
[6]
Ouchi, H., Hayashi, H., Kosa, K, and Tasaka, M., "A Numerical Study on Bridge Column Damage due to
Seismic Actions", Proc. of JCI, Vol. 19, No. 2, pp.411-416,
1997 (in Japanese)
[7]
Ishibashi,
T., "Damage to RC Viaducts for Railway and Their Design Earthquake
Resistance",
Concrete
Journal, JCI, Vol.34, No.ll,
pp.56-58,
1996.ll
(in Japanese)
[8]
Abe, S., Fujino,
Y., Abe, M., "An Analysis
of Damage to Hanshin
Expressway
During
1995
Hyogoken-Nambu Earthquake"
Proc. of JSCE, No.612/1-46,
pp.181-199,
1999.1
(in Japanese)
130