Capacity and demand of retrofitted bridges with RC jacketing
CONEJO, W.M., JARA, J.M., OLMOS, B.A. AND JARA, M.
Graduate Division, Civil Engineering School
Universidad Michoacana de San Nicolás de Hidalgo
Ciudad Universitaria, Morelia, Michoacán
MÉXICO
Abstract: - Medium span length bridges are spread all over the world. Most of them are RC structures built
several decades ago and designed for very low seismic forces. The code regulation changes and the age of the
bridges require the assessment of their vulnerability. According to current seismic regulations, many of them
have to be rehabilitated to increase their seismic capacity. One way to reduce the vulnerability of the bridges is
by using retrofitting techniques that increase the strength of the structure or incorporating control devices to
reduce the seismic demand. RC jacketing is a very common retrofit technique used to reduce the seismic
vulnerability of the bridge substructures. In this paper, we assess expected demands of seismically deficient
medium length highway bridges retrofitted with reinforced concrete jacketing, by conducting a parametric
study. We select a suite of twenty accelerograms of subduction earthquakes recorded close to the Pacific Coast
in Mexico. The bridges are simple supported structures with five 30 m long spans. We consider five pier
heights of 5 m, 10 m, 15 m 20 and 25 m and the analyses include three jacket thickness and three steel ratios.
Pushover analyses were conducted to evaluate the performance point using the family of accelerograms. The
results allow determining the influence of each parameter of the reinforced concrete jacketing on the expected
seismic behavior of the bridge models and the parametric study shows the best parameters of the jacketing
system to be used for improving the seismic behavior of bridges subjected to ground motions originated in a
subduction seismic source.
Key-Words: - RC pier jacketing, Seismic behavior, Seismic capacity, Seismic demand, Pushover analysis
1 Introduction Many countries of the world have an important
number of bridges built with reinforced (RC) and
prestressed (PC) concrete. During the bridges´
serviceability life, they have to support several
changes of the design parameters. Dead loads have
usually changes in amplitude because of
maintenance works that increase the asphalt
thickness. Live loads can be increased due to new
type of trucks circulating on the highways and
because of an increment of the loads carried on the
vehicles. Finally, accidental loads as earthquakes or
wind loads modify their intensity when the state of
knowledge change and the code regulations are
actualized.
Any load increase requires a structural
assessment of the bridges. It means that during the
useful life of the bridges they can be subjected to
structural interventions in order to improve their
stiffness and strength or because the materials
present degradation when the bridges are exposed to
aggressive environments.
Among the structural components that conform a
bridge, the piers are the elements more vulnerable
under the action of seismic loads; moreover, in case
of individual failure they put in risk the full bridge
system. In order to improve the flexure, shear and
ductility capacities of piers, many bridges have been
retrofitted through different techniques (Espeche,
2007; Giménez, 2007; Maralapalle, 2014; Shuenn
2014) as RC jackets, steel jackets, CFRP jackets,
external prestressed cables or even by the use of
passive energy devices.
Giménez (2007) reported that the number of
research works published since 1945 to 2007 related
to RC piers retrofitted by RC jackets is very small in
comparison to the number of studies associated to
beams and piers retrofitted with steel, CFRP or
other materials. Espeche et al. (2007) studied
experimentally rectangular and square RC columns
reinforced with micro-concrete jacketing that was
located along the pier high, confined by alternated
stirrups with an U or L shape. The rehabilitation
showed an improvement on the column strength and
durability. Another alternative for retrofitting is the
steel jacketing that could be considered as a very
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common technique implemented by practitioner
engineers. The steel jackets are used as external
reinforcement and situated where a plastic hinge
formation is expected or along the height of the pier.
Some studies used steel jacketing to retrofit piers
modifying the original transverse cross section of
the columns to an elliptical shape via the
implementation of steel plates.
In spite of the large number of bridges in zones
of high seismicity in México, there is a lack of
studies to determine the effectiveness of RC
jacketing to improve the seismic behavior.
Moreover, most of the studies currently published
deal with particular structures, without determining
the specific contribution of the jacket thickness and
the additional steel reinforcement on the expected
seismic behavior of the bridges. In this study, we
analyze the capacity and demand of one of the most
common typologies of RC bridges in Mexico, which
are usual structures in other countries as well,
retrofitted by RC jacketing, and we also determine
the contribution of the jacket thickness and steel
reinforcement on the seismic response of the
structures,
2 Reinforced concrete jackets The use of RC jackets to retrofit piers is a very
common technique that increases the shear and
flexure capacity of the columns. The jacket can also
be used to solve the problem of columns with
insufficient lap spice length. Longitudinal and
transverse reinforcement are appended around the
original section (Fig. 1).
Figure 1. Columns retrofitted with RC jackets
The seismic retrofit by using RC jacketing must
be aware of the capacity increase at the column
base. The stiffening and flexural enhancement in
this zone can increase the seismic demands in the
foundation. If the foundation is not capable to take
the additional forces, the jacket should be
interrupted, leaving a gap between the jacket and the
footing.
Circular columns are the cross sections more
appropriate to be retrofitted with this technique. For
a better behavior of RC jacketing in rectangular
cross sections, an ellipse cross section jacket can be
more suitable.
3 Response spectra A family of seismic records to assess the seismic
demands were selected from the Mexican base of
strong motion data that contains earthquakes
recorded in the period of 1960-1999. The database
provides information of 527 strong motion stations
in Mexico. We chose a suite of 20 seismic records,
all of them from earthquakes originated in
subduction zone, which is one of the most important
seismic sources in the country. Table 1 presents the
earthquake date, location, Focal depth and
magnitude of each event.
Table 1. Earthquake characteristics
Date Lat.
N
Long.
W
Focal
depth
(km)
Magnitude
(Mw)
09/19/1985 18.08 102.94 15 8
09/21/1985 17.62 101.82 22 7.5
02/08/1988 17.49 101.16 19 5.8
04/25/1989 16.60 99.4 19 6.9
05/15/1993 16.47 98.72 15 6
10/24/1993 16.54 98.98 19 6.6
09/14/1995 16.31 98.88 22 7.3
The seismic demand is assessed by using the
response spectra of 20 seismic accelerograms
recorded in hard soil sites. Table 2 displays the
location of the seismic stations, the peak ground
acceleration (PGA) and the distance between the
station and the epicenter.
The first column in Table 2 gives the occurrence
date with six numbers: the first two are year, the
next two the month and the last two the day of the
earthquake occurrence. The seismic records have
PGA in the range of 69 gals to 625 gals and the
distance between the seismic station and the
earthquake epicentre is in the range of 62 to 331 km.
In order of having similar intensities of the
seismic excitations, we scaled the seismic records
for a return period of 2500 years, with the use of
uniform hazard spectra for a site close to the
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subduction source in Mexico. The scale factor
applied to each accelerogram was the required value
to attain the maximum amplitude of the response
spectrum with the maximum amplitude of the
uniform hazard spectrum for the selected return
period.
Fig. 2 displays the uniform hazard spectra used
in this study. The maximum amplitude of the
spectra is located in the low period region, which is
a typical characteristic of hard soil sites. The
maximum expected amplitudes are in the range of
0.39 g to 1.8 g for return periods in the range of 30
years to 2500 years, respectively. For periods
greater than 1.5 s, the amplitudes of the uniform
hazard spectra are reduced more smoothly than the
changes observed in the zone of periods smaller
than 1.5 s. This is the result of considering
attenuation laws of hard soil sites in the seismic
hazard assessment of the zone.
Table 2. Description of the seismic records
Earth.
date Station ID
Lat.
N
Long.
W
PGA
(gals)
Dist.
(km)
850016
AZIH8509.191 17.60 101.46 153.93 166
FICA8509.191 17.65 99.84 69.18 332
PAPN8509.191 17.33 101.04 154.95 218
PARS8509.191 17.34 100.21 109.82 300
SUCH8509.191 17.23 100.64 103.12 262
UNIO8509.191 17.98 101.81 165.29 121
VILE8509.191 17.65 99.84 69.18 332
850018
ATYC8509.211 17.21 100.43 79.66 154
PAPN8509.211 17.33 101.04 242.69 89
PARS8509.211 17.34 100.21 625.78 173
SUCH8509.211 17.23 100.64 85.98 133
880004 MAGY8802.081 17.38 100.58 102.09 63
PARS8802.081 17.34 100.21 246.91 102
890024
ACAP8904.251 16.84 99.91 104.39 61
COYC8904.251 16.97 100.08 85.08 83
OCTT8904.251 17.25 99.51 201.16 73
PARS8904.251 17.34 100.21 117.11 120
930005 VIGA9305.152 16.76 99.234 67.31 64
930009 MSAS9310.241 17.01 99.46 119.05 73
950001 VIGA9509.141 16.76 99.24 100.35 63
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Sa
/g
Period (s)
Tr = 30 yearsTr=100 yearsTr=500 yearsTr=1000 yearsTr=2500 years
Figure 2. Uniform response spectra for a site close
to the subduction source in Mexico
4 Bridge description Medium span length bridges are the most common
structures in a highway road system. These bridges
usually have similar superstructures with a deck
supported in AASHTO type beams or box type
girders. Frame type piers with one or more columns
in each pier frequently compose the substructure.
4.1 Bridges’ types The bridges are composed of five simple supported
30 m long spans. Five structures with different pier
height are analyzed: 5 m, 10 m, 15 m, 20 m and 25
m. Most of the bridges designed all over the world
in the 70’s have piers with small longitudinal
reinforcement ratios product of gravitational
designs. We consider this type of bridges in the
study with circular RC columns and a longitudinal
reinforcement ratio of 0.5%.
The bridge models are composed by reinforced
concrete slab resting on prestressed concrete
AASTHO type IV girders (Fig. 3). At each span end
and at intermediate span length, there are
diaphragms to provide lateral stiffness to the
superstructure. The compressive concrete strength
of the girders was of 34.3 MPa and of 24.5 MPa for
the rest of the structural elements.
Figure 3. Deck cross section of the bridge models
The substructure (Fig. 4), consists of frame type
piers with four circular columns of constant
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transverse section and wall type RC abutments. The
girders are supported on elastomeric bearings
located on top of the bent cap and over the
abutments.
AASHTO GIRDERS
SLAB
ELASTOMERIC BEARINGS
PIER
CAP BEAM
DIAPHRAGM
KERB
WEARING SURFACE
Figure 4. Deck cross section of the bridges
The circular columns were strengthened by using
three possible jacket thicknesses of 0.10 m, 0.15 m
and 0.20 m and three reinforcement ratios of 0.5%,
1% and 1.5%. The parameter combination (heights,
jacket thickness and reinforcement ratio) produced
50 bridge models, five original structures and 45
retrofitted models.
4 Pushover analyses The analytical model was built with the SAP2000
software. Frame type elements were used to model
girders, columns, diaphragms and bent caps. Linear
links and shell finite elements idealize the bearings
and the slab elements, respectively. Fig. 5 shows the
3D analytical model created.
Figure 5. Three-dimensional model of the bridges
The nonlinear behavior of the columns was
assumed by using a concentrated plasticity model.
We assigned plastic hinges at both column ends.
The hinge properties were determined with moment-
curvature relationships. Fig. 6 displays one of the
typical moment-rotation relationship used to
characterize the bridge columns. The vertical axis is
the moment normalized by the column yield
strength. The figure presents the real curve and a
three-linear idealized model.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 0.05 0.1 0.15
M /
My
Rotation (rad)
Real behavior
Idealized behavior
Figure 6. Moment-rotation relationships
The capacity of the bridges was determined by
conducting pushover analyses according to the
ATC-40 (ATC-40) and using the SAP2000 program
(CSI, 2011). We analyzed the group of non-
retrofitted bridges and the retrofitted models.
Figs. 7 and 8 display the pushover curves of the
five meter high bridges in longitudinal and
transverse direction, respectively. The frame type
behavior in transverse direction increase the seismic
capacity of the models.
Non retrofitted
Shea
rfo
rcé
(kN
)
Displacement (m)
4500
4000
3500
3000
2500
2000
1500
1000
500
0
Figure 7. Pushover curves in longitudinal direction
of the 5 m high bridges
Non retrofitted
She
arfo
rcé
(kN
)
Displacement (m)
11700
10400
9100
7800
6500
5200
3900
2600
1300
0
Figure 8. Pushover curves in transverse direction of
the 5 m high bridges
The curves showed strength and stiffness
increases with the RC jacketing. In the figures, the
jacket thickness (in centimeters) is identified with
the letter t. Each curve has different jacket thickness
and reinforcement ratio.
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Figs. 9 and 10 present the results of the pushover
analyses for the highest bridge models analyzed.
The increase of the column diameters in these
models reduced the influence of the height on the
seismic capacity of the bridges.
Non retrofitted
Shea
rfo
rcé
(kN
)
Displacement (m)
3150
2800
2450
2100
1750
1400
1050
700
350
0
Figure 9. Pushover curves in longitudinal direction
of the 25 m high bridges
Non retrofitted
Shea
rfo
rcé
(kN
)
Displacement (m)
5850
5200
4550
3900
3250
2600
1950
1300
650
0
Figure 10. Pushover curves in transverse direction
of the 25 m high bridges
To observe the results of the pushover analyses
of the bridge models in one graph, Figs. 11 and 12
show a tridimensional image of the shear force
demands in both directions of analyses. The bridge
height is displayed in one of the horizontal axis, the
reinforcement ratio in the other and the vertical axis
presents the shear force demands.
Shear
force
(kN)
Figure 11. Shear force demands of the non-
retrofitted and retrofitted bridge models in
longitudinal direction
The case S/E corresponds to the non-retrofitted
bridge, the first three rows of the reinforcement ratio
(0.5%, 1% and 1.5%) corresponds to the 0.10 m,
thick jacket; the next three reinforcement ratios are
the results for 0.15 m thick jacket and the last three
are the shear forces of the 0.20 m thick jacket.
The capacity spectrum joins the pushover curves
and the seismic demands in a single graph. Fig. 13
shows, as an example of these results, the capacity
spectrum of the 15 m high bridge model, retrofitted
with a 0.10 m thick jacket and 0.015 of
reinforcement ratio. The horizontal axis is the
spectral displacement (Sd) and the vertical one the
seudoacceleracion (Sa). There are 20 response
spectra of the seismic records previously described
and one capacity curve.
She
arfo
rce(kN
)
Figure 12. Shear force demands of the non-
retrofitted and retrofitted bridge models in
transverse direction
Figure 13. Capacity spectrum of the 15 m high
bridge retrofitted with a 0.10 m thick jacket and
0.015 reinforcement ratio
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The performance point, which provides the
expected Sa(g) and Sd demands, is the intersection
of the capacity and demand curves. Each response
spectrum crosses the capacity curve in different sites
giving the bridge response for that seismic record.
The use of a suite accelerograms produced a
family of displacement demands. The mean
displacement demand in each case of analysis was
calculated by averaging the 20 displacement
demands of each performance point for the different
jacket thickness. Figs. 14 and 15 display, in the
vertical axis, the mean displacement demands of all
the analyses in longitudinal and transverse
directions, respectively. The horizontal axis in both
figures are the same parameters of the Figs. 11 and
12.
Mean
disp
lacemen
td
eman
ds
(m)
Figure 14. Mean displacement demands of the
bridges in longitudinal direction
Mean
disp
lacemen
td
eman
ds
(m)
Figure 15. Mean displacement demands of the
bridges in transverse direction
The single curvature deformed shape in
longitudinal direction produced larger displacement
demands than those of the transverse direction of the
bridge, which is characterized by a frame type
behavior with a double curvature deformed shape.
5 Conclusions We presented a parametric study of non-retrofitted
and retrofitted bridge models with RC jacketing.
Bridge typologies of medium-length structures with
five possible pier heights, three jacket thickness and
three reinforcement ratio were analyzed. The
seismic capacity was determined with pushover
analyses and the seismic demands were assessed by
using a family of 20 seismic records, scaled for the
maximum amplitude of a uniform hazard spectrum,
corresponding to a return period of 2500 years. The
interpretation of the results can be summarized as
follows.
We found that the 0.15 m and 0.20 m thick jacket
with a 0.015 reinforcement ratio gave the best
expected behavior as compared with the other
retrofitted cases and the non-retrofitted bridges.
Even though the 0.10 m thick jacket produced
slightly larger demands, it can be considered as a
plausible jacket as well. The conclusion is based on
the shear force demands and displacement demands
of the bridges.
The seismic behavior was considerable different
in both directions of analyses, mainly because of the
different deformed shapes observed in longitudinal
and transverse directions. The importance of this
parameter basically depends on the pier height and
the reinforcement ratio.
References:
[1] ATC 40, Seismic evaluation and retrofit of
concrete buildings, Applied Technology
Council, Report ATC 40, USA, 1996.
[2] CSI, SAP2000 V14.0, Integrated software for
structural analysis and design, Computers and
Structures Inc., 2009, USA
[3] Espeche A., Column retrofitting with RC
jacketing subjected to axial loads, Grupo
Hormigón Estructural, Politechnique
University of Madrid, España (in spanish),
2007.
[4] Giménez E., Experimental and numerical study
of axially loaded RC columns strengthened by
steel jacketing, PhD Thesis, University of
Valencia, Spain, 2007, pp. 271–350.
[5] Marlapalle V C, Salunke P J and Gore N G
2014 Analysis & design of RCC jacketing for
Recent Researches in Mechanical and Transportation Systems
ISBN: 978-1-61804-316-0 225
buildings, International Journal of Recent
Technology and Engineering, 3, pp 62-63.
[6] Shuenn C, Tin C, Ngoc T and Wen L 2014
Seismic retrofitting of RC columns with RC
jackets and wing walls with differente
structural details, Earthquake Engineering and
Engineering Vibration, 13, pp 279-292
Recent Researches in Mechanical and Transportation Systems
ISBN: 978-1-61804-316-0 226
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