98-Application of Double Spherical Aseismic Bearing in Concrete-filled Steel Tubular Arch Bridge

8/12/2019 98-Application of Double Spherical Aseismic Bearing in Concrete-filled Steel Tubular Arch Bridge

1/7

1 INTRODUCTION

For arch bridges, concrete filled steel tube (CFST) have many advantages compared to steel orconcrete ones. The infilled concrete delays the local buckling of steel tubes, which reinforce theconcrete against tension, bending moments and shear forces. The tube also serves as a formworkfor the concrete during the construction of an arch bridge, thus saving major construction costs(Yoshimura M et al.2006, Zong Z. H. et al 2005, Wu Q. X. et al 2006). Moreover, thecomposite structural action between the infilled concrete and the steel tube improves theload-carrying capacities of components. As a result, CFT construction for arch bridges has beenwidely employed during the recent decades (Clawson W. C.1999, Nakamura S.2000). In China,more than one hundred CFT arch bridges have been constructed since 1990 (Zhen Z et al 2000).

Because of the great mass of the CFT arch bridges, the substructures of such bridges are morevulnerable to earthquakes. One of the most effective methods to reduce the earthquake effectson the substructure is the base isolation. The most common isolators used for the base isolationinclude high-damping rubber bearings, lead rubber bearings and sliding friction steel bearings.Sliding friction steel bearings have been wildly used due to their simple structures, highcarrying capacity, satisfactory durability and self-healing capacity. In order to make moreeffective use of sliding friction steel bearings, the Department of Bridge Engineering of TongjiUniversity has developed a new type of steel isolator, named the double spherical aseismic

bearing, based on the mechanics of friction pendulum sliding isolators (Peng T. B. et al 2007).In this paper, the configuration and working mechanism of the double spherical aseismic

bearing are presented first. Then a CFT arch bridge, the Yigangchi Bridge, is introduced. Themodels with and without the installation of the double spherical aseismic bearing is analyzed by

the nonlinear and linear time history method respectively. Finally, with the comparison of theanalyzing results, the isolation performance of the double spherical aseismic bearing isevaluated.

2 THE DOUBLE SPHERICAL ASEISMIC BEARING

As shown in Fig.1, the double spherical aseismic bearing consists of a top bearing board with asliding concave spherical surface, a mid bearing board with a protruding spherical surface on

both sides and a bottom bearing board with a rotational concave spherical surface. The slidingand rotational surfaces are both made of stainless steel plates and PTFE (polyfluortetraethylene)

plates(Peng T. B. et al 2007).

Application of double spherical aseismic bearing inconcrete-filled steel tubular arch bridge

Hua Zhang, Jianzhong Li and Tianbo PengDepartment of Bridge Engineering, Tongji University, Shanghai, China

ABSTRACT: The configuration and working mechanism of a double spherical aseismic bearingdeveloped recently are presented. Then two models of an actual concrete-filled steel tubulararch bridge with and without the installation of the double spherical aseismic bearing wereanalyzed by time history method. The analysis results from the two models were detailedlycompared each other, in including the force response and displacement responses of thestructure. The comparison demonstrates that the double spherical aseismic bearing is applicableto the concrete-filled steel tubular arch bridge and has a distinct advantage in seismic isolation.


2/7

776 ARCH10 6th International Conference on Arch Bridges

Figure 1 : Configuration of the double spherical aseismic bearing

The working mechanism of the double spherical aseismic bearing is decribed as follows: thedeck is supported on the top bearing board and the restraining bolts limit the deck movementunder regular load; when the earthquake comes, the restraining bolts will be snipped and the top

bearing board will slide and rotate relative to the bottom bearing board; because of the sphericalsurface, the movement can make the elevation of the deck, which can provide resilience underthe action of gravity. The relative movement can prolong the basic period of the structure,which is the aim of the base isolation. The friction slide of the bearing can also providehysteretic damping.

If the supporting load of the bearing is W, the largest sliding displacement is D and the

friction coefficient is , then the resilience can be defined as:

+= )(sgnDWD

H

WF & (1)

where Hdenotes the distance between the centres of the two spherical surfaces.The first part on the right of equation (1) is the horizontal resilience from the elevation of the

deck, from which the horizontal stiffness can be derived as:

h

WK

H= (2)

If the basic period of the original structure is T1, the basic period with the installation of thedouble spherical aseismic bearing will be:

1/2

2 2

14

HT

gT

= +

(3)

Equation (3) indicates that the basic period will change with different H. Therefore, idealisolation effects can be achieved by the double spherical aseismic bearing.

The mechanism of the double spherical aseismic bearing can be modelled by equivalentlinearization, as shown in Fig.2. The equivalent stiffness is:

0eff

W W

K H D= + (4)

And the equivalent damping ratio is:

0

200

0 0

4 2 1

2 12eff

eff

WDarea of the hysteresis loop

DWK DD W D HH

= = = ++

(5)


3/7

Hua Zhang, Jianzhong Li and Tianbo Peng 777

Figure 2 : Lateral Hysteretic Loop of Double Spherical Aseismic Bearing

3 ANALYTICAL MODEL

3.1 Brief Description of The Yigangchi Bridge

The Yigangchi Bridge carries the Tangshan Caofeidian Xitong road over the Yigangchi Riverand is a long span bridge having an entire span length of 1057 m. As shown in Fig.3, the mainstructure of the Yigangchi Bridge comprises two spans of 138m+138m. The bridge width is29.5m. The arch ribs consist of there concrete filled steel tubes spatially arranged. The stiffeninggirder of the bridge, which is a composite beam, is supported by the suspenders.

Figure 3 : Elevation of the Yigangchi Bridge

3.2 Analytical Model of the Yigangchi Bridge

Fig.4 shows a three-dimensional FE model of the Yigangchi Bridge. The coordinate system ofthe bridge is set to be the longitudinal direction X, the transverse direction Y and the verticaldirection Z. The arch ribs, the stiffening girder, the lateral bracings and the piers are allmodelled using three-dimensional beam elements based on their actual cross-sectional

properties. The suspenders are modelled using three-dimensional beam elements releasing bothmoment and torsional degrees of freedoms. The pile caps are modeled as the mass pointsconstrained by the springs. The constraint conditions of different positions are shown in Table1.

Table 2 gives the material properties of this CFT arch bridge (the cross-sectional area Aandthe elastic modulus E). Since the cross sections of the arch ribs, the stiffening girder etc. are

different depending upon their positions, the basic material properties are measured at thecenters of those structures.


4/7


The nonlinear model with the installation of the double spherical aseismic bearing is the sameas the linear model described above, except the constraint conditions between the stiffeninggirder and the piers. The bearings are modeled as equivalent bilinear models, which aredescribed as Plastic (Wen) in SAP2000. The parameters of the bearings areH=5m and =0.02.

Table 1 : Constraint Conditions of the Model

DOFPosition

x y y x y z

Between Girder and Ribs 1 1 1 1 1 1

Between Girder and Side Piers 0 1 1 1 0 1

Between Girder and Mid Pier 1 1 1 1 0 1

Bottom of the Piers s s s s s s

where 0 denotes no constraints, 1 denotes the constraint without the relative movement ands denotes the constraint by the spring.

Figure 4 : Three-Dimensional FE model of the Yigangchi Bridge

Table 2: Basic Cross-Sectional AreaAand Elastic ModulusEof the Model

Part A(m2) E(kN/m2)

Arch rib 2.0534 3.45107

Stiffening girder 2.0625 2.10108

suspender 3.271E-03 2.10108

4 ANALYSIS RESULTS DISCUSSIONS

4.1 Natural Dynamic Characteristics of the Yigangchi Bridge

The natural dynamic characteristics of a structure determine the earthquake resistanceperformance of the structure. Table 3 and Table 4 show the first five in-plane and out-planenatural vibrations, respectively. It can be observed from Table 3 that the first in-plane naturalvibration having the frequency of 0.4931Hz is a longitudinal pendulum-like movement of thestiffening girder. Table 4 shows that the first out-plane natural vibration having a frequency of0.3104Hz is the swing movement of the arch ribs. By comparing Table 3 to Table 4, it can befound that the out-plane frequencies are smaller than the in-plane ones, indicating that the

bridge has more problems in the out-plane stability.


5/7


Table 3 : In-Plane Natural Vibrations

No.Freq.

(Hz)Model Shape

1 0.4931

2 1.1523

3 1.1884

4 1.7305

5 1.7403

Table 4 : Out-Plane Natural Vibrations

No.Freq.

(Hz)Model Shape

1 0.31042 0.3116

3 0.8637

4 0.8953

5 0.9159

4.2 Comparison of the Time-History Analysis Results

In order to make comparison between the models without and with the double spherical

aseismic bearing, both linear and nonlinear time-history analysis was conducted. The timehistory used in the analysis is El-centro, which has been wildly used in the relevant literature.The PGA was adjusted to 0.2 g, where g represents the acceleration of gravity.

Fig.5 illustrates the time histories of the relative displacement between the stiffening girderand the side pier of the models with and without the double spherical bearings. It can be foundthat with the installation of the double spherical aseismic bearings, the relative displacement can

be reduced to some extent. The constraint condition of the model without the aseismic bearingsis free along the longitudinal direction, so the side piers almost have no restraints on thestiffening girder. On the other hand, since the aseismic bearings have post-yield stiffness, thelongitudinal movement of the stiffening girder can be partly restrained by the side piers. It helpsto avoid the longitudinal unseating.

The time histories of the moments at the bottom of the mid pier are shown in Fig.6.

Compared to the time histories of the moments of the model without the double spherical


6/7


aseismic bearings, those of the model with the double spherical aseismic bearings areconsiderably reduced. The largest moment of the former model is 469153kNm but that of thelatter model is only 117149 kNm. The reduction of the seismic requirement should partlyattribute to the resistant contribution of the side piers and mostly to the isolation effect of the

bearings.

Figure 5 : Time Histories of the Relative Displacements Between the Stiffening Girder and Side pier

Figure 6 : Time Histories of the Moments at the Bottom of the Mid Pier

The realistic lateral hysteretic loop of the bearing in the nonlinear model, which is shown inFig.7, indicates that the double spherical aseismic bearing performs well on dissipating theseismic energy. The largest displacement is 0.13m, than the equivalent damping ratio can becalculated from the following equation:

0

2 1 2 127.68%

0.13 11 (0.02 5)

eff DH

= = =

++

(6)


7/7


Figure 7 : Lateral Hysteretic Loop of the Bearing in the Nonlinear Model

5 CONCLUSIONS

This paper presents a novel seismic isolator named double spherical aseismic bearing, whichis developed by the Department of Bridge Engineering of Tongji University. A CFST arch

bridge was used to verify the performance of that bearing, and also the natural dynamiccharacteristics were studied. Because of heavy deadweight in a CFST arch bridge, it is a

practical way to use the base isolator to improve the aseismic performance of that kind ofbridge. By comparing the time-history analysis results of the two models with and without thedouble spherical bearing installed, the advantage of employing above bearing is distinct.

REFERENCES

Yoshimura M., Wu Q. X. and Takahashi K., etc.2006. Vibration analysis of the Second Saikai Bridge-aconcrete filled tubular (CFT) arch bridge. Journal of Sound and Vibration, 290 (2006) p.388-409.

Zong Z. H., Jaishi B. and Ge J. P., etc.2005. Dynamic analysis of a half-through concrete-filled steeltubular arch bridge. Engineering Structures, 27 (2005) p.3-15.

Wu Q. X., Yoshimura M. and Takahashi K., etc.2006. Nonlinear seismic properties of the Second SaikaiBridge A concrete filled tubular (CFT) arch bridge. Engineering Structures, 28 (2006) p.163-182.

Clawson W. C.1999. Bridge applications of composite construction in the US. Structural Engineering inthe 21st Century, Proceedings of the Structures Congress, 1999, p.544-547.

Nakamura S.2000. New structural forms for steel/concrete composite bridges, Structural EngineeringInternational 1 (2000) p.45-50.

Zhen Z., Chen B. and Wu Q.2000. Recent development of CFST arch bridge in China. Proceeding of 6thASCCS Conference, USA, 2000, p.205-212.

Peng T. B., Li J. Z. and Fan L. C.2007. Development and application of double spherical aseismic

bearing. Journal of Tongji University, 2 (2007) p.176-180 (in Chinese).Peng T. B., Li J. Z. and Fan L. C.2007. Analysis of vertical displacement of double spherical aseismic

bearing. Journal of Tongji University, 9 (2007) p.1181-1185 (in Chinese).

98-Application of Double Spherical Aseismic Bearing in Concrete-filled Steel Tubular Arch Bridge

Documents

Transcript of 98-Application of Double Spherical Aseismic Bearing in Concrete-filled Steel Tubular Arch Bridge