Separating the building from the ground: how seismic isolation works

Not resisting an earthquake, nor absorbing it, but not letting it in. Seismic isolation does this by slipping a soft layer between building and ground. From the workings of laminated rubber bearings to Japan’s first isolated house, and the project that lifted a century-old station from below to isolate it.

Resist, absorb, isolate

There are three broad ways to prepare a building for earthquakes. Earthquake resistance withstands the shaking, damping absorbs it, and isolation keeps it out. An ordinary building stands directly on the ground, so the ground’s motion passes straight into it. Resistance makes columns, beams and walls strong enough not to give way. Damping places dampers inside the building to soak up the energy of the shaking.

Isolation thinks differently. An “isolation layer” is placed between building and ground, so that when the ground shakes, little of the motion reaches the building. Instead of making the building stronger, it narrows the door through which the shaking enters.

Soft sideways, stiff upright

The leading part in an isolation layer is the laminated rubber bearing. Stack many layers of soft rubber sheet and thin steel plate, and you get a curious combination: stiff vertically, but very soft horizontally. It carries the full weight of the building while slipping sideways shaking with a slow, wide movement.

Softness alone, though, would let the swaying go on and on, so dampers are added to bring it to rest: oil dampers, steel dampers, lead dampers. A “lead-plug” bearing, with a lead core in the rubber, does both jobs at once, supporting and damping. As the lead deforms, it turns the energy of the motion into heat.

• Isolators (support): laminated rubber bearings, sliding bearings, rolling bearings
• Dampers (stop): oil, steel, lead
• Types of laminated rubber: natural rubber, lead-plug, high-damping rubber

A gap for slow swaying

In an earthquake, an isolated building moves slowly and widely relative to the ground. When the city hall of Iwaki, Fukushima, was isolated, its period of sway was about 4.3 seconds, and the clearance around it was set at 55 cm plus a 5 cm margin. A sample notice from the Japan Society of Seismic Isolation warns that, in an earthquake, the floor panels around a building may move by about 50 cm.

Keeping this gap clear is a promise that comes with using an isolated building. Movable floor panels around it, and the moving joints at its entrances, exist for this reason.

Isolation in Japan

The lead-plug rubber bearing was developed in New Zealand in 1975. Japan’s first isolated building is said to be the Yachiyodai isolated house in Yachiyo, Chiba, completed in 1983 (some sources give 1982, when it was built). Its clearance was 35 cm.

The turning point was the 1995 Great Hanshin-Awaji Earthquake. In the decade before it, a little over 80 isolated buildings had been built; afterwards numbers surged, with 110 to 150 a year from 1997. Today there are about 5,300 excluding detached houses, and more than 10,000 including them.

Isolating a building that already stands

Isolation is not only for new buildings. Building an isolation layer beneath an existing building is called an isolation retrofit. Because it barely touches the building above, it suits the preservation of historic buildings. The first such retrofit in Japan was at the main building of the National Museum of Western Art in Ueno (1959, based on a design by Le Corbusier), completed in March 1998.

Tokyo Station: lifting a century-old station from below

In the preservation and restoration of Tokyo Station’s Marunouchi building, completed in 2012, an isolation layer was built beneath the station. Conventional strengthening would have meant altering about half of its interior walls; with isolation, almost none were needed. Not having to cut into the brick walls was the reason for choosing isolation.

The station, weighing about 70,000 tonnes, was held up on temporary piles while the ground beneath was dug out. Between the basement and the first-floor structure sit 352 isolators, including lead-plug bearings, and 158 oil dampers. The layer is designed to move no more than 20 cm in a major earthquake, against around 50 cm for a typical isolated building.

To protect a century-old building, put the newest technology in the place no one will ever see.

Base isolation and mid-storey isolation

There are different places to put the isolation layer. Base isolation sets it below the foundations and isolates the whole building. Mid-storey isolation sets it partway up, leaving the floors below it unisolated. Shiodome Sumitomo Building in Tokyo has its isolation layer beneath the 12th floor; Nakanoshima Festival Tower in Osaka, beneath the 7th.

Find the isolation when you stay

Isolated buildings must display a notice saying so at their entrances or another visible place. Look for it at your hotel’s entrance. Then look down: at the line between building and ground, you may find a metal joint or floor panels that seem to float slightly. That is the gap left for the building to move in an earthquake.

Where to see this technique

• The Tokyo Station Hotel (Tokyo, Kingo Tatsuno): stay in the Marunouchi building, isolated in 2012

Sources

• Japan Society of Seismic Isolation (JSSI), “What is seismic isolation” and notices for isolated buildings
• National Institute for Land and Infrastructure Management annual report (2006); Hokkaido University materials (laminated rubber)
• Building Research Institute publication No. 162; Building Research Institute, Epistula No. 102
• Mineo Takayama (Fukuoka University), history of seismic isolation; MENSHIN No. 118 (2022)
• Iwaki City, city hall isolation retrofit; Japan Meteorological Agency, long-period ground motion materials; City of Kawasaki (mid-storey isolation)
• MLIT Kanto Regional Development Bureau, National Museum of Western Art
• JR East Technical Review No. 52 and No. 44; Highlighting Japan (October 2012); Japan Construction Machinery and Construction Association journal (October 2007)
• Osaka City Central Public Hall official site