Story. 03

For a long time, only "horizontal forces" were considered in earthquake-resistant design

When earthquake shaking occurs, a building receives complex forces. Today, we can apply seismic wave data observed with an actual earthquake to a building model to perform a detailed analysis of direction and strength of various forces that would be exerted on the various parts of a building as the time elapses, using the results in designing a building. It is a four-dimensional analysis which considers the time axis in relation to the three-dimensional axes.

Earthquake-resistant design in the pre-computer era...

The most widely used approach to practical earthquake-resistant design was to establish the "design seismic coefficient" adequately, based on the rule of thumb. A design seismic coefficient is a ratio that indicates the relations between the forces exerted by earthquake shaking and building's own weight. After the Great Kanto earthquake of 1923, a regulation was enacted and a design seismic coefficient of 0.1 was adopted. Then, in the revised building standard law of 1950, it was changed to 0.2. Even now, the concept of design coefficient method is used in earthquake-resistant design of many buildings.

"Vertical shake increases damage" - A lesson learned from the Mid Niigata Prefecture Earthquake

In the Mid Niigata Prefecture Earthquake of 23 October 2004, major damage was caused by multiple simultaneous occurrences of the large-scale slope failures. Along with numerous landslide sites, the "Myoken-Zeki" (Myoken Weir) was especially notorious. It attracted people's attention because of the dramatic rescue of an infant who had been buried alive took place there. But among the disaster prevention specialists, it was known for a different reason. The maximum acceleration in this earthquake was recorded at the Myoken-Zeki, by a seismometer that was installed there by the Ministry of Land, Infrastructure, Transport and Tourism.

The Mid Niigata Prefecture Earthquake was a near-field earthquake with the epicenter depth of only 13 km. The maximum acceleration at the Myoken-Zeki, which is located about 7 km from the epicenter, exceeded 1500 gal. This numerical value was almost twice as large as the numerical value measured at the Kobe JMA Observatory at the time of the Great Hanshin Awaji Earthquake. Moreover, it should be noted that its vertical component reached the largest ever numerical value of about 800 gal, which is equal to the gravitational acceleration.

This vertical shaking brought about damages beyond expectations. Some National Treasure Straw-Rope Pattern Potteries that were on display at the neighboring city of Tokamachi fell down and broke. All the potteries were supposed to be well protected against an earthquake. They were placed on base isolation tables that were capable of reducing horizontal shaking by 90%, but it was not enough after all.

At the core of the aforementioned concept of the seismic coefficient method lays an idea that "the seismic force can be represented by the horizontal force". This notion has become a central dogma of structural design, and it might have kept designers' thoughts back in the world of two-dimension without them knowing it. This introspection has led to a stronger cautiousness against "vertical shaking" in a near-field earthquake.

It is difficult to reduce "vertical" shaking with laminated rubber alone

A "base isolation building" in where horizontal shaking is reduced by supporting the frame of a building with "laminated rubber" - rubber-steel sandwich structure - is no longer a rarity. But any attempt to reduce vertical shaking altogether with horizontal shaking faces a very high hurdle.

"We wanted a building that will float in a stable position, like Laputa, the castle in the sky, in the anime film series by Hayao Miyazaki. However, if we just keep it afloat, the building would be a quite noisy place, like Howl's Moving Castle. We needed to find the ways to control axial rotational motion. " (Chief Engineer, Osamu Takahashi)

The "Chisui-Kan" is the world's first building that has a three-dimensional earthquake-proof system. It controls 3-axis 6 degrees of freedom motion of a building by combining 1) horizontal (lateral vibration) isolation with "laminated rubber", 2) vertical (vertical vibration) isolation with "air spring", and 3) vertical rocking motion (like a motion of a top that spins, tilts and falls down) restraint.

Can we verify by simply enlarging the size of the experimental device? That is, how many tons would the building weigh? How can we make this world's first attempt in compliance with the building standards act? How about the maintenance of the device? ・・・ By clearing all these hurdles, we advanced the earthquake isolation technology from the two-dimensional world to three-dimensional world.

The first step into the new era of earthquake isolation technology by adding 3-dimensional perspective

KKE was founded in 1959. It was the first time when structural design experts began to attract public attention. We adapted computers into structural design field when they were still novelty. In contrast to the traditional world of structural design where only horizontal forces were considered, we introduced a new dimension of time axis, and added dynamic analysis that looked at the behavior of a building using the actual seismic waves. KKE has always been at the forefront of these innovations, which is why we worked diligently on yet another innovation of adding a new dimension to the earthquake isolation technology.

"The 3-dimensional seismic isolation has long been studied and technically established by professor emeritus Fujita of Tokyo University. However it was not yet made practical. Although it's easy to claim to have achieved 'the world's first', we still faced many challenges, and honestly speaking we were not absolutely sure whether or not we could do it. Nevertheless we did achieve our goal because of the cooperation and support provided by the joint development partners Shimizu Corporation and Kayaba System Machinery Co. Ltd., and passion of all parties concerned including our staffs." (Tetsuya Tomizawa, Structural Design Department)

"KKE was in a unique position where we could coordinate all aspects - hardware, laws and regulations, fund, and technology. It was a project that we had to do. Once the system is produced in large quantities, we can offer it at a reasonable price and implement it in many buildings. I want to convey this good news to as many people as possible, that a new technology that can reduce shaking in any earthquake is about to be realized." (Tomizawa)

The project started in 2005 and completed in 2011. We issued a completion news release on March 3. Unexpectedly 8 days after the news release, the Great East Japan Earthquake occurred, and the building's seismic isolation capacity was demonstrated. We hope that many more people would find out about the "Chisui-Kan". It is the first step into the new world of 3-dimensional seismic isolation technology.