Capacity and demand of retrofitted bridges with RC · PDF fileCapacity and demand of...

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

[email protected]

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

Recent Researches in Mechanical and Transportation Systems

ISBN: 978-1-61804-316-0 220

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


ISBN: 978-1-61804-316-0 221

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


ISBN: 978-1-61804-316-0 222

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.


ISBN: 978-1-61804-316-0 223

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


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


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


ISBN: 978-1-61804-316-0 224

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


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


ISBN: 978-1-61804-316-0 226

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