Seismic Isolation

Seismic isolators are placed between the foundation and the building. They allow the ground to move while the building remains relatively still, dramatically reducing transmitted forces (these mechanisms are referred to as base isolation in the foundations section).

IMPORTANT CONCEPTS

Seismic isolators balance three competing properties:

Property	What it Does	Too Little	Too Much
Flexibility	Lengthens the building’s period, reducing transmitted force	The building still feels strong shaking (ineffective isolation)	Building sways excessively; it may hit the moat walls
Damping	Dissipates earthquake energy as heat, reducing displacement	Isolator displacement grows too large; the building may pound adjacent structures	Can increase transmitted forces at some frequencies; reduces recentering
Recentering	Returns the building to its original position after the earthquake	Residual displacement; building remains tilted; unusable after the quake	Not possible to have “too much” recentering

The goal: Find an isolator that is flexible enough to reduce forces, has enough damping to control displacement, and provides reliable recentering so the building is immediately usable after the earthquake.

WHY DAMPING MATTERS

Damping is the isolator’s ability to absorb and dissipate earthquake energy. Without damping, a flexible isolator would act like a perfect spring—the building would continue swaying back and forth long after the ground stopped moving, potentially hitting moat walls or adjacent buildings.

How different isolators provide damping:

Isolator Type	Damping Source	Typical Damping Level
Natural Rubber (NRB)	Internal rubber friction	Very Low (2-3%)
High Damping Rubber (HDRB)	Carbon black fillers in rubber	Moderate (10-15%)
Lead-Rubber (LRB)	Lead plug deforming plastically	High (20-30%)
Friction Pendulum (FPS)	Sliding friction	Moderate to High (adjustable)
Flat Sliding	Sliding friction	Moderate (but no recentering)

THE TRADE-OFF: DAMPING VS. RECENTERING

High damping can sometimes reduce recentering. A perfectly elastic isolator (like pure rubber) always returns to the center. Adding damping dissipates energy but can slightly resist the return to center.

The best isolators — LRB and FPS — achieve high damping while maintaining excellent recentering through different mechanisms:

LRB: Rubber provides recentering; lead provides damping
FPS: Curved surface provides recentering; friction provides damping

This is why these two systems dominate seismic isolation for buildings requiring immediate occupancy.

WHAT THEY ARE AND HOW THEY WORK

Instead of anchoring a building rigidly to the ground, seismic isolation places flexible bearings or sliding devices between the foundation and the structure above. During an earthquake, the ground moves, but the building stays relatively still.

Seismic isolation works through two mechanisms:

Period elongation: The isolators lengthen the building’s natural vibration period, shifting it away from the dominant frequencies of earthquake ground motion
Energy dissipation: The isolators absorb seismic energy through friction or material deformation

The key advantage: A fixed-base building experiences the full force of ground acceleration. An isolated building experiences only a fraction — typically 25-50% less force. This means less damage to both structure and non-structural components.

ELASTOMERIC BEARINGS (RUBBER-BASED)

Elastomeric bearings are layers of rubber with thin steel plates inside. They hold up the building really well, but let it sway side to side during an earthquake.

How forces transmit: When the building’s weight pushes down, the rubber squishes a little, but the steel keeps it from spreading out. When an earthquake shakes the building sideways, the rubber stretches sideways, allowing the building to move rather than crack.

Pros:

Very good at holding heavy weights
Predictable behavior under design-level earthquakes
Well-understood with decades of use

Cons:

Rubber can wear out over time
Performance varies with temperature
Requires careful quality control in manufacturing

NATURAL RUBBER BEARINGS (NRB)

These are a type of elastomeric bearing. The rubber is unmodified natural rubber. These are very flexible, but don’t absorb much earthquake energy on their own (typically 2-3% of critical damping).

Pros:

Let the building move easily
Cost-effective for low-to-moderate seismic zones
Well-understood material properties

Cons:

Doesn’t reduce shaking much (needs extra devices)
Can crack or age over time
Large displacements in major earthquakes

Best for: Low-to-moderate seismic zones with supplemental damping, or bridges with displacement capacity.

HIGH DAMPING RUBBER BEARINGS (HDRB)

The rubber in high-damping rubber bearings uses specially formulated rubber compounds with fillers (carbon black, resins, or oils) that have the same flexibility as normal rubber but also absorb shaking energy (providing damping of 10-15% without separate dampers).

Pros:

Doesn’t need extra dampers
More compact (takes less space)
Proven performance in buildings and bridges

Cons:

Performance changes with temperature and movement
More expensive than basic rubber
Can stiffen at large displacements

Best for: Buildings and bridges in moderate to high seismic zones where space constraints favor compact isolators.

LEAD-RUBBER BEARINGS (LRB)

Lead-rubber bearings are NRB with a chunk of lead inserted through the center. The lead plug deforms plastically during earthquakes, providing high energy dissipation (20-30% damping) while the rubber provides flexibility and recentering.

How it works: During shaking, the lead deforms and absorbs energy. After the earthquake, the rubber’s elasticity returns the building to its original position.

Pros:

Very good at reducing shaking
Bring the building back to its original position
Widely used and proven
Compact, single-unit design

Cons:

Lead is environmentally problematic
Performance degrades after multiple large earthquakes (lead work-hardens)
Requires replacement after major events
Higher cost than NRB

Advanced version (FSLRB – Fail-Safe LRB): A recent innovation adds a friction slider in series with the LRB. Under design-level earthquakes, it behaves as a standard LRB. Under extreme earthquakes beyond design expectations, the slider activates, preventing the LRB from reaching damaging shear strains and protecting the building from collapse.

Best for: High seismic zones where both energy dissipation and recentering are required; the most common isolator for buildings and bridges.

SLIDING/FRICTION BEARINGS

Sliding bearings work on the principle of Coulomb friction. The building rests on a flat or curved surface with a low-friction interface (typically PTFE on polished stainless steel). When ground motion generates enough horizontal force to overcome static friction, the building slides.

How forces transmit: When the building experiences small shaking, it stays still. When large levels of shaking occur, the building starts to slide, but the friction slows it down and reduces energy.

Pros:

Insensitive to temperature variations
No aging concerns (unlike rubber)
Predictable force-displacement behavior
Can be combined with recentering systems

Cons:

Requires separate recentering mechanism (unless curved surface)
Can move in a jerky way (“stick-slip”)
Friction properties depend on velocity and bearing pressure
Requires careful detailing of sliding interfaces

FLAT SLIDING BEARINGS

Flat sliding bearings consist of a flat PTFE surface sliding against polished stainless steel. They provide no inherent recentering force — the building can remain displaced after an earthquake.

Pros:

Simplest and least expensive sliding bearing
Very high displacement capacity
No vertical stiffness change with displacement

Cons:

No recentering capability (building may not return to original position)
Requires separate recentering system (springs or rubber)
Large residual displacements after earthquakes
Not suitable as a sole isolator for buildings

Best for: Bridges with expansion joints, or as part of a hybrid system with rubber bearings for recentering.

FRICTION PENDULUM SYSTEM (FPS)

The Friction Pendulum System uses a curved surface like a bowl. The building slides on the curved surface and naturally rolls back to the center like a pendulum. The curved surface provides both recentering force (like a pendulum returning to its lowest point) and period elongation independent of building mass.

How it works: The earthquake causes the building to slide up the curve, gravity pulls the building back down to the center, and friction helps absorb energy.

Pros:

Built-in recentering (no separate system needed)
Period independent of building mass (unlike rubber bearings)
Very high displacement capacity
Excellent performance in moderate to high seismic zones
Proven in major projects (Benicia-Martinez Bridge)

Cons:

Higher cost than elastomeric bearings
Requires precision manufacturing of curved surfaces
Can experience “curvature effects” at large displacements (coupling between horizontal and vertical motion)
Underestimating vertical response by ignoring curvature effects can lead to errors of 20-40% in severe earthquakes

Advanced considerations: Under extreme earthquakes with large displacements, the slope of the curved surface creates coupling between horizontal and vertical motion, generating additional vertical acceleration and bearing axial load. Research shows that ignoring these curvature effects can underestimate vertical response by 20% or more.

Best for: High seismic zones, bridges, buildings where displacement capacity and predictable restoring force are priorities. The most widely used sliding isolator globally.

ORIGINS

Ancient Origins (6th Century BCE): The earliest known seismic isolation concept dates to the 6th century BCE in Greece, where builders placed structures on layers of sand and gravel to allow sliding during earthquakes. Similar techniques appeared in ancient China and Japan, where wooden buildings were placed on stone foundations without rigid connections.

Modern Rediscovery — Late 19th Century: In 1891, a British engineer named John Milne proposed isolating buildings by placing them on rollers or balls — an idea ahead of its time. However, the concept remained theoretical for decades.

The Breakthrough — 1960s-1970s: Modern seismic isolation began with the development of elastomeric bearings in the 1960s. Researchers discovered that layering rubber with steel plates created a bearing that was stiff vertically but flexible horizontally — perfect for isolating buildings.

1974: The first seismically isolated building was constructed in Skopje, Yugoslavia (now North Macedonia) — a school using natural rubber bearings.

Widespread Adoption — 1980s-1990s: 1985: The Friction Pendulum System (FPS) was invented by Dr. Victor Zayas and patented by Earthquake Protection Systems. The curved sliding surface concept revolutionized isolation by providing inherent recentering.

1990s: Seismic isolation gained global recognition after performing successfully in several earthquakes. The 1994 Northridge and 1995 Kobe earthquakes validated the technology, with isolated buildings experiencing far less damage than conventional structures.

Modern Era — 2000s-Present: Seismic isolation is now standard for critical facilities (hospitals, emergency centers, data centers) and is increasingly used for residential and commercial buildings.

2019: Beijing Daxing International Airport opened as the world’s largest single isolated building, using 1,152 bearings and 160 dampers.

2024: The Benicia-Martinez Bridge in California demonstrated significant cost savings by adopting friction pendulum bearings instead of traditional expansion joints.

PERFORMANCE EVIDENCE

Kobe Earthquake (1995, M6.9): The Western Japan Postal Savings Computer Center (an isolated building) remained fully operational after the earthquake while surrounding buildings suffered severe damage. This became a landmark case demonstrating isolation effectiveness.

Northridge Earthquake (1994, M6.7): The University of Southern California Hospital (under construction with isolation) suffered no structural damage, while other hospitals in the region were damaged and closed. The experience led California to require seismic isolation or equivalent protection for all hospital buildings.

Tōhoku Earthquake (2011, M9.0): Japan’s isolated buildings, including the Sendai City Hall and numerous hospitals, performed exceptionally well. While the magnitude 9.0 quake caused widespread damage, isolated buildings experienced minimal shaking and remained operational — critical for emergency response.

WINNER OF THIS CATEGORY

For a mid-rise building on San Francisco clay soil designed for immediate occupancy after the earthquake, Lead-Rubber Bearings (LRB) and Friction Pendulum System (FPS) are both very good options.

Performance Factor	LRB	FPS
Displacement on soft soils	Can increase significantly; displacements grow over time	Better controlled and more predictable
Accelerations	Low to moderate	Very low
Energy Dissipation	Very high (lead core)	Moderate to high (friction)
Recentering	Excellent (rubber elasticity)	Excellent (gravity pendulum)
Long duration/soft soil	Lead core can heat up and lose efficiency	Not sensitive to duration; maintains performance

Based on this comparison, FPS is the better option. Additionally, according to studies in California and Japan, LRBs perform well in typical earthquakes but can experience larger displacements in long-period motions, whereas FPS maintain stable response regardless of duration and are better at limiting displacement on soft soils.

LRB behavior is material-dependent (rubber and lead properties), while FPS behavior is geometry-controlled (curved surface). Geometry-based systems (FPS) are more predictable and less sensitive to changing conditions.

For detailed scoring comparisons, see the Shock Absorption Mechanisms spreadsheet below.