The Great Hanshin earthquake which struck Kobe on January 17, 1995 was a wake up call for Japan’s national preparedness authorities.
Over 6,300 people lost their lives, 300,000 were left homeless, and 150,000 buildings were destroyed. Roughly 80% of the victims were crushed to death by collapsed houses and fallen furniture. Twenty percent of the buildings in the worst-hit areas were completely destroyed or uninhabitable as a result, and a 1 kilometer section of the Hanshin Expressway collapsed. Structures built after 1981, when earthquake building codes were overhauled, suffered less damage.
However, as Tokyo architect Alan Townsend points out: “some of the collapsed structures were engineered to meet those updated regulations and calculated to be safe. These surprising failures exposed an unsettling truth: calculations alone could not fully predict the complex movements that brings crashing down in a major seismic event.
This article summarizes, with permission, Townsend’s in-depth description and analysis of the E-Defense earthquake simulator that was born as a result of the 1995 Kobe quake.
E-Defense Shake Table
In the aftermath of the Great Hanshin disaster, Japan’s National Institute for Earth Science and Disaster Prevention (NIED) began planning an earthquake simulator that could subject all types of structures to simulated earthquakes with magnitudes as great as 7.0.
The result is the E-Defense (“E” stands for Earth) shake table, measuring 20 by 15 meters, the world’s largest earthquake simulator, located about 30km north of Kobe. The simulator is a basically a kinetic platform with 24 pneumatic pistons attached to the underside, controlled by a massive hydraulic mechanism. Test structures weighing up to 1,200 tons, such as wooden houses and even concrete office buildings, are built in an adjacent warehouse, lifted over by crane and placed on top of the table.
The video below shows the E-Defence in action!
Engineers in a control room above the shake table program it to gyrate along all three axes.
As Townsend explains, in an earthquake, the ground doesn’t just thrust from one side as waves move outward from the quake’s epicenter: “Displacement occurs in all three directions, so seismographs record ground motion along the X, Y, and Z axes. The 3D forces make structural behaviour more difficult to predict and prepare for. An unique feature of the E-Defense is that it can faithfully replay any recorded earthquake in all three of its vectors.”
Cameras record the shaking inside and outside the structure being test, capturing its ability to sway and absorb major shocks. Some experiments are designed to identify weak points that can lead to the structure’s collapse. Other tests look at two houses side-by-side, an unaltered house and one that has been retrofitted to check on the efficacy of the earthquake engineering.
Structural Safety a Major Factor in Homebuying
In Japan, where tremors are almost a daily fact of life, and especially in the aftermath of the 1995 Kobe quake and 2011 Fukushima disaster, structural safety is one of the most important factors that home buyers consider, and in practice, all new wooden houses must be inspected to meet earthquake codes.
In a 2013 survey by Recruit on homebuyer preferences, for example, people were asked to indicate the most important factors when buying a home. Respondents were given a choice of 45 factors and only one answer was allowed. “Price” was chosen as the number one factor by 86.3% of respondents, while 74.8% of people chose “earthquake resistance” as the most important factor.
E-Defence has also been used to test old homes to study methods of retrofitting them for earthquake safety. Many of these older houses, built in Japan’s post-WWII boom have tiled roofs supported by frail lathes and plaster walls. However, as Townsend points out, “As Kobe demonstrated, these homes — most lacking sufficient cross bracing, steel ties, or plywood sheathing — still present a widespread risk, particularly for the country’s disproportionate geriatric population, who have lived in them since they were built.”
Japan is a country that has had to rebuild time and again from natural disasters. Its geographic position means that it will continue to face earthquakes, tsunamis, and mudslides on whatever scale that the forces of nature determine. E-Defence may be the world’s biggest and most sophisticated earthquake simulator, but it is still only meant to help minimize structural earthquake damage, and not to do the impossible of eliminating the human and economic cost all together.
E-Defence offers tours of its facilities. Reservation information can be found here, but is only in Japanese.