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Seismic Protection of Cable-stayed Bridges with LRB Isolation devicesB. Asgari, S. A. Osman and A. AdnanThe Pacific Structural Steel Conference (PSSC 2013)Singapore, 8 11 October 2013

Cable-stayed bridges have become increasingly popular in the past decade due to their remarkable economic efficiency and aesthetic appearance.

Seismic isolation has become a promising alternative to traditional design methods for controlling the seismic responses of cable-stayed bridges in last few years. Seismic isolation system consists of isolation devices that lengthen the natural period of the superstructure and dissipating devices which increase the energy dissipating capability.

LRB (Lead Rubber Bearing) has a good load support ability, restoring force and damping (energy dispersion). It is also easy to construct and maintain and cost-effective. LRB would be a promising seismic isolator.

Introduction

The cable-stayed bridge description

The Tatara bridge in Japan with a total length of 1 480 m with 890 m centre span is considered for studies.The main girder is a 3-cell steel box section which consists of three spans, 270 m, 890 m, and 320 m with 2.70m deep. The stay cables are arranged in 21 levels and two planes.Towers have inverted Y shape with 220 m length.

FE modelling of the bridge Modeling of the Deck The bridge deck is modelled using a single central spine with offset rigid links (fishbone model). The BEAM4 elements from ANSYS element library were used to model the central spine. The MPC184 elements were applied to model the rigid links.

Modeling of the Towers The towers are modelled as BEAM4 elements. The rigid links (MPC184 elements) were extended from the axial center of the tower to cable anchor points.

Modeling of the Cables Modelling the cables is conducted by employing LINK10 elements and utilizing stress stiffening capability.

Dynamic Characteristic of the BridgeThe dynamic behaviour of cable-stayed bridge can be identified through modal analysis. To evaluate the seismic response of the uncontrolled bridge, the bridge deck is assumed to be rigidly connected to the towers.

1st Transverse mode (0.1326 Hz) The inherent damping of the structure is assumed 2% in present study.

Characterize the behavior of LRB bearingsThe experimental tests and analytical studies confirm that the force-displacement hysteretic relation of LRB devices can be reasonably described by equivalent bilinear models.

The experimental investigations show strong nonlinearities and stiffening behavior of rubber-based materials.

According to all experimental and analytical studies characterizing the nonlinear behavior of rubber-based isolation devices, the selection of the analytical model for control devices affects the dynamic responses of the isolated structures significantly.

Characterize the behavior of LRB bearingsTo define the bilinear model for seismic isolation systems, the initial elastic stiffness, the post-yield stiffness, the characteristic strength, and the yield displacement are required parameters. The equivalent stiffness and damping of LRB in bilinear model can be describes as (AASHTOO 2000):

Characterize the behavior of LRB bearings

The primary design of the LRB devices for seismic protection of the modelled cable-stayed bridge are performed based on existing codes and manuals for general bridges (AASHTO 1999, AASHTO 1996, and the Caltrans Seismic Design Criteria V1.3 (Caltrans 2004)).The maximum displacement of the bearings specified in the design manuals corresponds to approximately 250% shear strain of the minimum height bearings available for a specific diameter size. The bearings selected for bridge were defined with a larger height and therefore the specified correspond to 160% shear strain, respectively.

For this study, the maximum displacement of the bearings corresponding to failure was defined at 300% shear strain.

Characterize the behavior of LRB bearings

The number of isolation bearings in each connection is determined according to the dead load. Three isolators are considered in each deck-to-tower connection .The mass of the superstructure (deck) of the bridge to be isolated is considered equal to the first mode effective mass .ParameterIsolator diameter (circular bearing)1000(mm)Total isolator height508(mm)Total rubber thickness355(mm)Rubber characteristic stiffness1751 (KN/m)Maximum displacement660(mm)Shear yield strength8.27(Mpa)Lead plug diameter254(mm)Isolator characteristic strength444.8 (KN)

The maximum bearing capacity and the compressive stress of the bearings are checked.

Numerical studiesThe seismic response of bridge is investigated under Kobe, 1995, earthquake.

The peak ground acceleration (PGA) of Kobe earthquake in longitudinal direction is 0.821g.

In the numerical studies of the seismic responses, 30 (s) of the record accelerations is used in the analysis.

The 0.02 s time step is used for time history analysis.

Numerical studies LRB bearings are applied in deck-to-tower connections to reduce seismic forces and to absorb large seismic energy.

For avoiding large bearing force, which makes the energy-absorbing device do not work efficiently, bearing stiffness with 1.7 times the original main period (T) is chosen (based on the study on a simplified model of the bridge under seismic motion).

The FE model is analysed in ANSYS commercial program through time-history analysis, Using Newmarks constant average acceleration (=1/4) integration of the equations of motion.

Evaluation of results

Acceleration of deck in isolated and non-isolated bridge

Base shear force in isolated and non-isolated bridge

Evaluation of results

Base moment in isolated and non-isolated bridgeDisplacement of deck in isolated and non-isolated bridge

Conclusions

The results of investigations show that LRB bearings are efficient devices in decreasing seismic forces of cable-stayed bridges. However, the application of LRB bearings increases the displacement response of the deck.

Applying energy dissipating devices in parallel with LRB bearings would be an effective solution to overcome the mentioned problem which can be considered in future studies.

It can also be found from the results of this study that increasing the damping ratio of LRB bearings would improve their efficiency to control the seismic forces in cable-stayed bridges. This fact would be helpful in manufacturing process of LRB bearings for cable-stayed bridges.

Thank you for your attention