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H.R. Anajafi, A.K. Ghorbani-Tanha and M. Rahimian

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COMPDYN 2011

3rd ECCOMAS Thematic Conference on

Computational Methods in Structural Dynamics and Earthquake Engineering

M. Papadrakakis, M. Fragiadakis, V. Plevris (eds.)

Corfu, Greece, 25-28 May 2011

SEISMIC VIBRATION CONTROL OF IZADKHAST BRIDGE USING

VISCOUS DAMPERS

H.R. Anajafi1, A.K. Ghorbani-Tanha1, and M. Rahimian1

1School of Civil Engineering, University of Tehran

P.O. Box 11155-4563, Tehran, Iran

E-mail: hamidanajafi@Tazand.com, {ghtanha, rahimian}@ut.ac.ir

Keywords: Vibration control, Izadkhast Bridge, Viscous damper, Passive control, Earthquake

Abstract. Present study addresses the effectiveness of viscous dampers (VDs) in reducing the

response of Izadkhast Bridge under earthquake ground motions. With the length of 485 m,

Izadkhast Bridge is the longest box girder bridge in Iran and is located in Isfahan-Shiraz

railway. The bridge is installed with VDs at the two ends. The Finite element models of the

bridge are developed. Five pairs of representative earthquake records are selected and

scaled using the earthquake code and applied to the models. Nonlinear seismic analyses of

the structure without and with VDs are performed and the results are reported. Comparison

of the results clarifies VDs effectiveness on seismic response reduction of the bridge.

Sensitivity analyses are performed to demonstrate the effects of damper parameters on

structural response.

H.R. Anajafi, A.K. Ghorbani-Tanha and M. Rahimian

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1 INTRODUCTION

The basic function of passive energy dissipation devices when added to a structure is to

absorb and dissipate a portion of the input energy, thereby reducing energy dissipation

demand on primary structural members and minimizing possible structural damage. Serious

efforts have been devoted to the development and utilization of passive energy dissipation

devices [1]. Viscous dampers (VDs) are a kind of these devices which significant efforts have

been directed toward their application for structural vibration control. In VDs, energy

dissipation occurs via conversion of mechanical energy to heat as a piston deforms a thick,

highly viscous substance [1]. In present study, the effectiveness of VDs on response

reduction of Izadkhast Bridge under earthquake ground motions is investigated.

With the length of 485 m, Izadkhast Bridge is the longest box girder bridge in Iran. This

bridge is located in the central part of Iran, spanning Izadkhast valley on the railway line

between Isfahan and Shiraz. The bridge is composed of five 77 m spans in the middle, two

side spans having the length of 60 m and 40 m, six piers and two abutments, as shown in Fig.

1. The cross section of piers is shown in Fig. 2. With the width of 6.6 m, the deck is

composed of two 4.5 m high box girders (Fig. 3). Seismic considerations were considered in

the design of the bridge due to its location. The bridge is equipped with four 1000 KN VDs

(stroke 250 mm) longitudinally directed at both ends (Fig. 4). According to the manufacturer

catalogs and design documents, the governing equation of the dampers is a

cvf , where f is

force, c is the damping coefficient, v is the velocity, and 15.0

a [2, 3]. Four 800×800×219

mm rubber bearings are placed at the top of each pier and two 800×800×357 mm rubber

bearings at the top of each abutment (Fig. 5).

The Finite element model of the bridge is developed. Five pairs of representative

earthquake records are selected and scaled using earthquake code and applied to the model.

Nonlinear seismic analyses of the structure without and with VDs are performed and the

results are reported. Comparison of the results clarifies VDs effectiveness on seismic

response reduction of the structures.

44

Figure 1: Izadkhast Bridge and its support conditions on the piers and abutments [2]

H.R. Anajafi, A.K. Ghorbani-Tanha and M. Rahimian

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11.9 to 44 m2.5 m

Figure 2: View and Cross section of piers [2]

Figure 3: Cross section of bridge deck at piers[2]

H.R. Anajafi, A.K. Ghorbani-Tanha and M. Rahimian

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Figure 4: VDs placement [2]

(a) (b)

Fi

g

ure 5: Details of rubber bearin

g

s (a) at the top of piers, (b) at the top of abutments [2]

2 MODELLING AND ANALYSES

2.1 Finite element model of the bridge

A finite element model of the bridge is developed in PERFORM-3D [4]. Constraints are

applied to restrict the deck from moving horizontally at Piers 2, 3, 4, and 5 and laterally at all

piers. The displacement capacity of the rubber bearings are calculated as following

TtgVx

(1a)

a

T

tg

a

T

tg

a

T 9.0:2.0;7.0:2.0

(1b)

Where T is the effective thickness of the rubber bearing which according to the

manufacturer catalogs is 144 mm for piers and 252 mm for abutments; and a is the minimum

H.R. Anajafi, A.K. Ghorbani-Tanha and M. Rahimian

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dimension of the cross section of rubber bearings. Under seismic loading, an increase of 50%

is applied to displacement capacity. As a result, the displacement capacity for the abutment

bearings is 220 mm while this value for the pier bearings is 150 mm. The rubber bearing are

modeled as non-linear springs whose initial stiffness are TGAKs

, where G is shear

modulus, A is the area and T is the effective thickness of bearings. Under seismic loading, the

shear module of bearings is considered two times greater than its common value. For the

definition of plastic hinge in piers, the famous available models are employed. The length of

plastic hinge, Priestley relation is used [5,6]

),(044.0022.008.0 MPammdfdfLL blyeblyep

(2)

Where LP is the distance between critical section and inflection point of the member; db is the

diameter of longitudinal bars and fy is the yield stress. According to design documents, the

expansion joint between deck and abutments has a width of 250 mm which is taken into

account in the model. If the longitudinal displacement of the deck is greater than this value,

the deck knocks the abutments.

2.2 Earthquake records used

For the excitation of the bridge, five pairs of earthquake records are chosen (Table 1). These

records are scaled according to UBC [7] and then applied to the structure.

Table 1: Earthquake records used

No. Record Name Date PGA(g)

1 IZMIT1 17/8/1999 0.2195

2 IZMIT2 17/8/1999 0.1521

3 ELCENTRO1 19/5/1940 0.2148

4 ELCENTRO2 19/5/1940 0.3129

5 K0BE1 16/1/1995 0.5985

6 K0BE2 16/1/1995 0.8213

7 NORTHRIDGE1 17/1/1994 0.493

8 NORTHRIDGE2 17/1/1994 0.8283

9 SANFRANCISCO1 18/10/1989 0.056

10 SANFRANCISCO1 18/10/1989 0.105

3 RESULTS

3.1 Uncontrolled bridge

Dynamic analyses of the bridge without VDs under scaled earthquake time histories are

conducted. The first natural period of the bridge is 2.5 sec. The results show that the

maximum longitudinal displacement of the deck occurs under scaled Izmit earthquake

(PGA=0.546g) which is 510 mm. The time histories of the longitudinal displacement

responses under two components of Izmit (Izmit 1 & 2) are shown in Fig. 6. The moment-

H.R. Anajafi, A.K. Ghorbani-Tanha and M. Rahimian

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curvature diagram for pier P6, as critical pier, is shown in Fig. 7. It is clear that the piers will

exhibit nonlinear behavior. Fig. 8 shows the energy response of the bridge. A significant

portion of the energy input to the structure is dissipated with both inelastic hysteretic

mechanisms and inherent viscous damping. In this case, the expansion joint will be closed

and the deck will knock the abutments which can cause serious damages.

DIS P. R E S P ONSE

-30

-20

-10

0

10

20

30

0 2 4 6 8 10121416

Tim e( s ec )

Dis

p

.

(

cm

)

Figure 6: Longitudinal displacement response of the uncontrolled bridge under Izmit earthquake records

Mome nt-C u rva tur e

-100000

-80000

-60000

-40000

-20000

0

20000

40000

60000

80000

100000

-0.01 -0.005 0 0.005 0.01 0.015

(rad/m)

M(KN.M)

Figure 7: Moment-curvature diagram for pier P6 (shortest pier with the height of 11.9 m) for the uncontrolled

bridge

WI THOUT DAMPER

0

5000

10000

15000

20000

25000

30000

0 2 4 6 8 10 12 14 16

Tim e( s ec)

Energy(KN .M)

dissipated inelastic energy

beta-K viscous energy

alpha-M viscous energy

strain energy

kinetic energy

total energy

DIS P .R E S P ONS E

-30

-20

-10

0

10

20

30

40

50

60

0 2 4 6 8 10121416

Tim e(s e c)

Disp.

(

cm

)

H.R. Anajafi, A.K. Ghorbani-Tanha and M. Rahimian

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Figure 8: Energy response of the bridge under scaled Izmit earthquake

3.2 Controlled bridge

As mentioned before, the bridge is equipped with four 1000 KN VDs at both ends.

Dynamic time-history analyses of the controlled bridge show that the maximum displacement

of the bridge reduces to 380 mm but it is still greater than the width of expansion joints and

the deck knocks the abutments (Fig. 9). However, a significant portion of the energy input is

absorbed and dissipated by the dampers which reduce the nonlinear deformation of the

structure. The moment-curvature diagram for pier P6 is shown in Fig. 10.

DIS P .R E S P ONS E

-30

-20

-10

0

10

20

30

40

0 2 4 6 8 10 12 14 16

Tim e (s e c )

Disp.(cm)

Figure 9: Longitudinal displacement response of the bridge fitted with four 1000 KN VDs under Izmit

earthquake

Moment-Curvature

-100000

-80000

-60000

-40000

-20000

0

20000

40000

60000

80000

100000

-0.003 -0.002 -0.001 0 0.001 0. 002 0.003 0.004 0.005 0.006 0. 007

(rad/ m)

M(KN.M)

Figure 10: Moment-curvature diagram for pier P6 for the case that the bridge is equipped with four 1000 KN

VDs

H.R. Anajafi, A.K. Ghorbani-Tanha and M. Rahimian

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DIS P .R E S P ONS E

-2 0

-1 0

0

10

20

30

0 2 4 6 8 10121416

Tim e (s e c )

Disp.(cm)

Figure 11: Longitudinal displacement response of the bridge fitted with four 2500 KN VDs under Izmit

earthquake

Moment-Curvature

-60000

-40000

-20000

0

20000

40000

60000

80000

100000

-0. 0015 -0.001 -0.0005 0 0.0005 0. 001 0. 0 015 0. 002 0. 0025

(rad/m)

M(KN.M)

Figure 12: Moment-curvature diagram for pier P6 for the case that the bridge is equipped with four 2500 KN

VDs

Proper VDs will prevent structural damages and does not let the deck knock the

abutments. A trial and error procedure employed and finally it was concluded that if 1000 KN

dampers are replaced by 2500 KN dampers, then the maximum longitudinal displacement

reduces to 240 mm and piers will remain elastic (Figs. 11 and 12). These dampers are more

expensive and off course apply higher values of reaction forces to the abutments which

should be taken into consideration in design procedure.

4 SUMMARY AND CONCLUSIONS

H.R. Anajafi, A.K. Ghorbani-Tanha and M. Rahimian

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The inclusion of VDs enhances the seismic behavior of the bridge and the dampers

dissipate a significant portion of the energy input to the bridge. This reduces the hysteretic

energy dissipated by the sub-structural members and decreases damage to the structure and

is favorable for earthquake resistant design.

For the case that the bridge is fitted by four 1000 KN VDs, the maximum longitudinal

displacement under design earthquake is 380 mm which is more than the displacement

capacity of the damper and expansion joint width and the deck knocks the abutments.

To overcome the above-mentioned problems and improve seismic performance of the

bridge, 2500 KN dampers are recommended to be used. In this case the maximum

displacement reduces to 240 mm.

ACKNOWLEDGEMENT

The authors are grateful to Tazand Co. for providing them with the bridge design documents

and data.

REFERENCES

[1] T.T. Soong, G.F. Dargush, Passive energy dissipation systems in structural engineering,

John Wiley & Sons, 1997

[2] Tazand Consulting Engineers, Design documents of Izadkhast Bridge, 2006 (In Persian).

[3] http://www.fip-group.it

[4] CSI PERFORM-3D, V4.0.1, Computers and Structures Inc, Berkeley, California, Release

2006, Components and Elements.

[5] Caltrans, Seismic Design Criteria, V1.4, June 2006.

[6] T. Paulay, M.J.N. Priestley, Seismic design of reinforced concrete and masonry buildings,

John Wiley & Sons, 1992.

[7] Unified Building Code, 2006.