Selecting Between Lead-Rubber Bearings (LRB) and Friction Pendulum Systems (FPS): Applications, Differences, and Seismic Isolation Design Guide

Lead-Rubber Bearings (LRB)

LRB is a traditional elastomeric seismic isolation bearing, composed of alternating layers of high-damping rubber and steel plates, with a solid lead core at the center.

• Function: The rubber layers provide flexibility to extend the building's natural period beyond earthquake energy peaks, while the lead core dissipates seismic energy through plastic deformation.

• Key Performance: Typical effective damping ratio ranges from 15–20%, with an isolation period of 1.5–2.5 seconds, suitable for standard structural applications.

Friction Pendulum Systems (FPS)

FPS is a sliding-type seismic isolation bearing, consisting of a concave steel surface, a low-friction slider, and a articulated upper plate.

• Function: Energy is dissipated through friction between the slider and the curved surface, while gravity and the spherical geometry create a self-centering effect.

• Key Performance: The isolation period depends only on the radius of curvature (independent of building mass), and damping ratios exceeding 30% are achievable with different friction materials and surface treatments, as validated in numerous seismic isolation test reports.

Choosing between LRB and FPS largely depends on your project's structural type, load conditions, environmental factors, and performance objectives. Below is a breakdown of their most common applications and critical differences:

1. Structural Type and Building Height

2. Load Conditions and Sensitivity

• LRB Limitations:

￮ Sensitive to variations in vertical load, which can alter stiffness and isolation period.

￮ Limited tensile capacity (typically <1 MPa), making them unsuitable for structures with significant uplift forces (e.g., tall towers, offshore structures).

￮ Damping performance may degrade under extreme cyclic loading or high temperatures.

• FPS Advantages:

￮ Load-independent isolation period: The system's period is determined solely by the radius of curvature, ensuring consistent performance regardless of building mass or load changes.

￮ Ultra-high vertical load capacity, with individual bearings supporting thousands of tons, making them ideal for heavy industrial and critical infrastructure projects.

￮ Low torsional sensitivity, which is beneficial for irregularly shaped buildings or structures with uneven load distribution.

3. Environmental Durability and Temperature Stability

• LRB Considerations:

￮ Temperature-sensitive rubber components, with optimal performance between -20°C and 40°C. Extreme cold or heat can harden or soften the rubber, reducing isolation efficiency.

￮ Vulnerable to UV radiation, ozone, and chemical corrosion, requiring protective coatings and regular maintenance inspections.

• FPS Advantages:

￮ Wide operating temperature range (-40°C to 60°C), making them suitable for extreme climates, from cold northern regions to hot desert environments.

￮ Resistant to aging, corrosion, and environmental degradation, with minimal maintenance requirements over a 50+ year service life. This makes FPS a top choice for coastal structures, industrial facilities, and remote infrastructure projects.

4. Post-Earthquake Resilience and Damping Performance

• LRB Performance:

￮ Residual displacements may occur due to plastic deformation of the lead core, potentially requiring post-earthquake realignment.

￮ Standard damping ratios (15–20%) are sufficient for most low-to-moderate seismic zone projects.

• FPS Performance:

￮ Near-zero residual displacement: The self-centering mechanism ensures the structure returns to its original position after an earthquake, minimizing downtime and repair costs-critical for resilience-focused projects like hospitals, data centers, and emergency response facilities.

￮ Tunable high damping: By adjusting the friction coefficient and slider material, FPS can achieve damping ratios over 30%, which is highly effective in controlling seismic displacements in high-seismic zones.

The choice between LRB and FPS is not about which system is "better," but which aligns best with your project's specific requirements. Use this step-by-step framework to make your decision:

Step 1: Define Your Structural and Performance Goals

• Building height and span:

￮ Choose LRB for mid-rise, regular buildings with standard spans.

￮ Choose FPS for super-tall, large-span, or irregular structures requiring long isolation periods.

• Resilience requirements:

￮ If your project demands rapid re-occupancy post-earthquake (e.g., critical facilities), prioritize FPS for its self-centering and low residual displacement.

￮ For standard residential or commercial buildings with moderate resilience goals, LRB is a cost-effective option.

Step 2: Evaluate Load and Environmental Conditions

• Load characteristics:

￮ Select LRB for buildings with uniform, stable vertical loads.

￮ Select FPS for heavy, variable, or concentrated loads (e.g., LNG tanks, stadium roofs) and structures with uplift risks (with additional anti-uplift devices).

• Climate and site conditions:

￮ Use FPS for projects in extreme temperatures, coastal areas, or industrial environments with high corrosion risks.

￮ LRB is suitable for mild climates with controlled environmental exposure.

Step 3: Ensure Code Compliance and Test Validation

According to EN 15129, the design properties of seismic isolators must be confirmed by test results. A common challenge in FPS design is discrepancies between assumed and tested friction coefficients:

• If design assumptions use μ = 8–12% but test results show lower values (e.g., μ ≈ 5.2–5.7%), you must either revise the design parameters or provide additional justifications to support the use of the original range.

• For LRB, ensure full-scale tests validate the effective period, damping ratio, and seismic force demands before finalizing the design.

Step 4: Balance Initial Cost and Lifecycle Value

• Upfront cost: LRB typically has a lower initial cost, making it attractive for budget-sensitive standard projects.

• Long-term cost: While FPS may have a higher upfront investment, its durability, low maintenance, and resilience often lead to lower lifecycle costs, especially for critical infrastructure and projects in high-seismic zones.