Stronger, without changing a thing: how seismic retrofitting works

You can stay in an old hotel because the building has been prepared for today’s earthquakes. How have the standards changed, and how are buildings strengthened? Braces, shear walls, wrapped columns, base isolation, and the hard choice faced by historic buildings: to become stronger while changing nothing you can see.

Which standard was the building built to?

Japan has revised how its buildings prepare for earthquakes after almost every major quake. The Building Standards Act came into force in 1950, and after the 1968 Tokachi-oki earthquake caused heavy damage to reinforced concrete buildings, the rules for columns were tightened in 1971.

The biggest turning point was the “new seismic standard”, in force from 1 June 1981. It checks buildings at two levels, a moderate earthquake and a major one, with the aim that a building suffers almost no damage at around seismic intensity 5-upper, and does not collapse in a way that endangers life at intensity 6-upper to 7.

In the Great Hanshin-Awaji Earthquake of 1995, more than 80 percent of the deaths were caused by collapsing buildings, and the damage was concentrated in buildings from before 1981. That year, a law was passed to promote the seismic retrofitting of older buildings.

An amendment in 2013 made seismic diagnosis compulsory for buildings used by many people, such as hotels, inns, hospitals and shops, that have three or more storeys and a total floor area of at least 5,000 ㎡ and were built to the old standard. Local governments publish the results. Of about 10,600 buildings published nationwide, about 1,800, or 17 percent, were judged insufficiently resistant to a major earthquake.

Measuring strength: the Is value

Seismic diagnosis expresses a building’s strength as an “Is value” (seismic index of structure), calculated from its stiffness, ductility, shape and age. At 0.6 or above, the risk of collapse in an earthquake is judged low; from 0.3 to below 0.6, there is a risk; below 0.3, the risk is high.

The 0.6 line is backed by past earthquakes. In the Tokachi-oki and Miyagi-oki earthquakes, buildings with an Is value of 0.6 or more suffered almost no damage, and in the 1995 Hyogo-ken Nanbu earthquake their damage was generally slight. Buildings at 0.4 or below were in many cases destroyed or badly damaged.

There is more than one way to strengthen

Once a diagnosis has shown where a building is weak, it is strengthened. There are many methods, each with strengths and drawbacks. Making a building stronger can mean closing windows or changing how rooms look.

• Steel braces: diagonals inside the frame of columns and beams. Very effective, but you see the diagonals across windows and rooms.
• More shear walls: openings filled with concrete walls to stiffen the building, at the cost of light and views.
• Wrapped columns: steel plate or carbon fibre around columns to make them tougher. Columns get slightly thicker, but rooms are used the same way.
• External frames: a steel frame added outside the building, sparing the walls and window frames inside.
• Damping and isolation: devices that absorb the shaking, or a layer that separates the building from the ground. Isolation barely changes how the building above looks.

For historic buildings there is one more condition: no open flames during the work. Welding sparks are a danger to old timber and interiors. Methods have been developed that strengthen columns with segmented steel plates and fibre sheets, without heavy machinery or welding.

Hotel New Grand: unchanged, and open throughout

The main building of Hotel New Grand in Yokohama (1927, Jin Watanabe) was among those required to be diagnosed after the 2013 amendment. It has six storeys above ground, in steel-reinforced and reinforced concrete, with a floor area of 5,006 ㎡. Shimizu Corporation designed and carried out the retrofit.

Because the original drawings were incomplete, the work began with four years of surveying the whole building and redrawing it as it stood. The policy was to change neither the exterior nor the interior. For the columns, an interlocking steel-plate wrapping method that needs no open flame was chosen. The plaster and gypsum-relief ceilings were taken down, their bases strengthened, and the ornament returned to its place.

You cannot see that it has been made stronger. That was the greatest achievement of the work.

And the work was carried out while the hotel stayed open. Methods were chosen to disturb guests as little as possible, and working hours were adjusted. The second phase cost about 2.3 billion yen, and the hotel celebrated its renewal in October 2016. The project received the BELCA Award, which honours long-lived buildings, and an award for excellent seismic retrofitting.

Choosing to close and restore

Other hotels chose to close for major works. The Fujiya Hotel in Hakone closed in April 2018 and began seismic strengthening of its main building and six other buildings. The reinforcing members were placed inside floors, walls and ceilings so they could not be seen, giving top priority to preserving the design. With about four years of surveys and design and about two years of construction, it reopened in July 2020.

At Tokyo Station’s Marunouchi building, too, the Tokyo Station Hotel closed from 2006 to 2012 for major works that created an isolation layer beneath the station. Stay open or close and restore: the choice depends on the building’s size, its condition and what it most needs to protect.

Look for the strengthening when you stay

The better a retrofit is done, the less you notice it. Still, if you look closely at the corners and thickness of columns, or where ceiling meets wall, you may find traces of new work. If you find none, that too is proof of how well the craftsmen and designers did their job.

Where to see this technique

• Hotel New Grand (Yokohama, 1927, Jin Watanabe): retrofitted while open, with exterior and interior unchanged

Sources

• MLIT Shikoku Regional Development Bureau, “Housing safety Q&A”; Tokyo Metropolitan Government, condominium retrofit guide
• Building Research Institute lecture materials; Cabinet Office, White Paper on Disaster Management 2007
• MLIT, materials on the 2013 amendment and published diagnosis results
• Kyoto Prefecture, Is value criteria; MEXT, school seismic retrofit materials
• MLIT technology development report (flame-free column strengthening)
• Japan Federation of Construction Contractors, seismic retrofit case (Hotel New Grand); Kanagawa Shimbun, 4 October 2016
• Fujiya Hotel press release (2020); BELCA Award materials; Kanagawa Shimbun, 3 April 2018
• JR East Technical Review; JR East press release, 6 September 2011