Although Friction Pendulum Bearings (FPB) appear simple in structure, every component and design detail is precisely engineered in line with mechanical principles. Understanding their structure and working mechanism allows one to fully grasp why they are regarded as one of the optimal solutions for seismic isolation.
Standard FPB Structure: Four Core Components with Distinct Functions
A standard friction pendulum bearing consists of four key components, which work together to achieve seismic isolation, energy dissipation and automatic recentering.
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Upper Bearing Plate
Rigidly connected to the superstructure, such as beams, floor slabs and bridge piers, the upper bearing plate has a precision-machined concave spherical surface as its base. It serves as the main track for oscillating motion, and undertakes vertical load transfer and horizontal guidance.
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Sliding Block (Spherical Cap Liner)
Positioned between the upper and lower bearing plates, the sliding block is the core moving component. Its surface is inlaid with low-friction and wear-resistant materials such as polytetrafluoroethylene (PTFE), forming a friction pair with the spherical stainless steel surface. This ensures smooth sliding while dissipating energy through friction.
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Lower Bearing Plate
Fixed to the foundation or pier, the lower bearing plate has a flat or matching concave spherical top surface. It provides a stable base, restricts the swing range and maintains the overall stability of the bearing.
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Sealing and Limiting Assembly
This assembly includes dust-proof seals, limit pins, guide keys and other parts. It prevents dust and moisture from entering the sliding interface to avoid abrasion. Limit pins control displacement under normal service conditions and automatically unlock during earthquakes to allow sufficient swinging space.
Working Principle of FPB: Three-Stage Seismic Protection
Friction pendulum bearings operate entirely on physical laws without external power. They activate automatically during earthquakes and recenter spontaneously after the event, ensuring high efficiency and reliability throughout the process.
(1) Initiation and Decoupling: Interrupting Seismic Energy Transmission
When the horizontal seismic force exceeds the static friction threshold between the sliding block and the spherical surface, the rigid connection of the bearing is broken. Relative sliding occurs between the superstructure and the foundation, completely cutting off the path of seismic energy transfer to the superstructure and preventing direct seismic impact.
(2) Oscillation and Energy Dissipation: Converting and Consuming Seismic Energy
The sliding block performs a pendulum-like motion along the concave spherical surface, slightly lifting the superstructure and converting seismic kinetic energy into gravitational potential energy. Meanwhile, continuous friction at the sliding interface generates resistance, turning remaining seismic energy into heat and greatly reducing structural vibration amplitude.
(3) Gravity Recentering: Automatic Reset After Earthquakes
Once the earthquake ceases, gravity acting on the superstructure pulls the sliding block back to the central position along the spherical surface, achieving unpowered automatic reset with nearly zero residual displacement. This ensures the structure returns to its original position without affecting subsequent use.
Key Design Parameters: Core Indicators Determining FPB Performance
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Spherical Curvature Radius
The curvature radius determines the isolation period. A larger radius results in a longer isolation period, helping avoid the dominant seismic period of the site and preventing resonance.
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Friction Coefficient
It controls the activation force and energy dissipation efficiency, with a typical range of 0.03–0.12. This balances structural stability under minor earthquakes and wind loads, as well as energy dissipation capacity under major earthquakes.
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Ultimate Displacement
Designed to accommodate the maximum swing amplitude under rare earthquakes, it ensures the bearing does not pull out or fail under extreme conditions.