Seismic Isolation in Bridge Design #5: Analysis with Friction Pendulum Bearings

Updated: Apr 25

Transitioning from traditional seismic design approaches, this post delves into the innovative realm of seismic isolation techniques, with a focus on the application and analysis of Friction Pendulum Bearings (FPB) in bridge design. By introducing these isolation devices, engineers aim to enhance the seismic performance of bridges, significantly reducing the forces transmitted during an earthquake. We'll explore the principles behind friction pendulum bearings, their design parameters, and the results of seismic analysis using these devices.

For a background on this study review Post #1 here.

The Principle of Friction Pendulum Bearings

Friction Pendulum Bearings are a type of seismic isolation device that allows a structure to move horizontally during an earthquake, significantly reducing the seismic energy transferred to the structure. The key components of an FPB include a spherical sliding surface and a slider that moves along this surface during seismic events. This movement allows the bridge to undergo a pendulum-like motion, which can be precisely controlled through the geometry of the sliding surface and the characteristics of the slider material.

Working Mechanism: During an earthquake, the FPB allows horizontal displacement, with the restoring force being provided by gravity. This mechanism increases the structure's fundamental period, reducing the seismic forces acting upon it.
Energy Dissipation: The sliding motion between the slider and the spherical surface generates friction, which dissipates seismic energy, further protecting the structure.

Figure 1 : Schematic Arrangement of FRPB and Friction-Force Displacement Behavior

Design Parameters and Analysis Method

The design of FPB systems is guided by specific parameters, including the radius of curvature of the sliding surface and the coefficient of friction. These parameters are crucial for tailoring the seismic response of the structure to meet design requirements.

Upper and Lower Bound Design Properties: Analysis considers both the upper bound (UBDP) and lower bound (LBDP) design properties of the isolation system. UBDP is used for obtaining design forces for the substructure, while LBDP analysis aims to determine the maximum displacement of the isolating system.
Elastic Seismic Acceleration Method: The analysis employs this method, modified for the Elastic Response Spectrum given in the design standards. It involves considering the bridge as a single degree of freedom (SDOF) system to evaluate the seismic response, taking into account the effective stiffness and damping of the isolation devices.

Figure 2 : Bilinear approximation of hysteretic force-displacement

Seismic Analysis

The seismic analysis with FPBs is conducted using MIDAS Civil software, which allows for detailed modeling of the isolation system's effects on the bridge's seismic response. This analysis provides insights into the displacement demands and the seismic forces that the bridge is subjected to when equipped with FPBs.

Modeling Isolator Stiffness: In MIDAS, the effective stiffness of each bearing/isolator is modeled using elastic links, which accurately simulate the behavior of FPBs under seismic loading as shown in Table 1 below.

Table 1: Effective Stiffness of Isolation System

Effective Damping: The analysis incorporates effective damping values calculated based on the FPB design properties, further refining the seismic response predictions.

Figure 3: Effective Damping Ratio provided for UBDP Case in MIDAS Model

Results of Seismic Analysis with FPBs

The application of FPBs significantly alters the seismic response of the bridge, as evidenced by the analysis results:

Reduced Seismic Forces: The analysis demonstrates a substantial reduction in seismic forces transmitted to the bridge, highlighting the effectiveness of FPBs in isolating the structure from ground motion.
Displacement Demands: While FPBs reduce seismic forces, they also result in increased displacement demands. The analysis provides critical data on these displacements, ensuring that the design accommodates the expected movements without compromising safety or functionality.

Table-2: Comparison of Forces Manual Vs MIDAS analysis — Table-2: Comparison of Forces Manual Vs Seismic Analysis

Restoring Capability and Displacement Demand Checks

An essential aspect of the analysis is verifying the self-restoring capability of the isolation system and ensuring that the displacement demands can be adequately met:

Self-Restoring Capability: The FPB system must be capable of returning to its original position post-earthquake, a feature confirmed through the analysis.

Displacement Capacity: The maximum allowable displacement of the isolators is checked against the design displacements to ensure that the system's capacity is not exceeded during seismic events.

Table-4: Check for Displacement Demand with LBDP

Conclusion

The analysis with Friction Pendulum Bearings represents a significant advancement in seismic design strategies for bridges. By effectively decoupling the superstructure from ground motion, FPBs offer a promising solution for enhancing the seismic resilience of bridge structures. This post has outlined the principles, design considerations, and analysis results of applying FPB technology, demonstrating its potential to reduce seismic forces and improve overall seismic performance. As we continue our series, we'll delve into the comparative analysis of conventional bearings and FPBs, shedding light on the practical benefits and considerations of adopting seismic isolation techniques in bridge design.

This series is brought to you by Varun Garg, based on a paper he co-authored with Mr. Rajiv Ahuja for the Structural Engineering Digest, Quarterly Journal for the Indian Association of Structural Engineers in March 2021. The paper can be downloaded by clicking the link below.