Japan’s Earthquake Buildings – a History
With all the devastation in Japan caused by the earthquake and subsequent tsunami, we thought we’d take a look into one of the more positive stories to emerge from Japan over the last few days; the success of their taller buildings to withstand the regions highest magnitude earthquake since records began.
Japan is an island nation, with a population of about 127,350,000 with an area of only 145,925 sq mi, giving a population density of 873.1/sq mi, making Japan the 36th most densely populated country on the planet. Of the total 145,925 sq mi of Japan, between 70-80% of this land is forested and mountainous, and unsuitable for industrial, agricultural and residential development. This area of unsuitable land makes the habitable areas in Japan even more highly populated than the numbers would suggest.
If we look at Tokyo, Japan’s largest, and one of the country’s most densely populated cities, it becomes clearer why the technology of seismic control is so vital to the people and cities of Japan. The population of the Tokyo Metropolis as of April 2010 was 13,010,279, with a population density of 15,143.7/sq mi. To have this many people living in such a relatively small area means structures tend to be taller.
With so many people squeezed into such a relatively small place, it is no wonder Tokyo (as a city, not a municipality) goes up, rather than out. In Tokyo city there are 39 buildings that rise above 180m. Though these would be considered as tall, they are by no means amongst the tallest structures in the world. Japan has a very colourful building regulation history over the last 100 years, dictated by the natural (and military) disasters that it has encountered. To understand how Japan has developed its urban environment, we must introduce the Japanese Building Standard Laws.
For nearly 100 years they have been around dictating Japanese building laws in response to the devastation bought about by natural disasters and war. Much like British building regulations, originally created in London as a response to the great fire of 1666, Japanese regulations are designed to protect buildings. They were often created after a catastrophic event caused unpredictable damage. It would be too simplistic to suggest that all of Japanese building standards have a direct link to natural disasters. Japan underwent incredibly fast modernisation, and plenty of laws were introduced to mitigate the factors born out of ultra fast urban expansion. For the purpose of this piece though, we aren’t interested in these, and will instead focus on the Japanese response to natural disasters.
Over the past 120 years, major events have shaped Japanese Building Laws
The 1891 Nobi Earthquake
This earthquake struck prior to the development of the Richter scale in 1935, but subsequent research has put the magnitude at around the 8.0 scale – making this the largest known inland earthquake in Japan’s history. The quake struck with such strength that the visible fault line post shock created a land height difference of 6m on either side of it.
In 1893, seismologist John Milne and engineer W.K. Burton worked together in co writing and co photographing a book, and recorded the resulting devastation of the earthquake. Milne made the following observations:
“… buildings on soft ground … suffer more than those on the hard ground.”
“… we must construct, not simply to resist vertically applied stresses, but carefully consider effects due to movements applied more or less in horizontal directions.”
During his time in Japan, Milne is credited with creating the horizontal pendulum seismograph, designed to measure and record earthquake waves and velocities. His work in the field of seismology in Japan was honoured by the Emperor with one of the country’s highest accolades – the Order of the Rising Sun.
1919 Urban Building Standards Act
In response to the incredible urban expansion of Japan, the government enacted the City Planning Act and the Urban Building Standard Act in 1919. These acts contained specific regulations dictating and regulating building in 6 major urban centres in Japan. They can be divided under 2 acts: the 1920 Law Enforcement Order and the 1920 Law Enforcement Regulations. Here are the relevant laws from within these 2 acts:
1920 Law Enforcement Order
• Height limit: 100 feet
• Structural design for timber, masonry, brick, reinforced concrete and steel constructions.
1920 Law Enforcement Regulations
• Structural design specifications
• Allowable stress design
• Quality of materials
• Dead and live loads
• No seismic requirements
The 1923 Kanto Earthquake
Only 4 years after the acts were published, Japan was struck by another quake, believed to have registered around 7.9 on the Richter scale. The quake led to 4% of Tokyo’s buildings being levelled or damaged in some way. Many thousands of people lost their lives as fires spread across the cities, aided in no small part by the winds whipped up by a typhoon that was battering the coast of Japan at the same time.
This earthquake led to the 1924 Revision of Law Enforcement Regulations.
Revision of Law Enforcement Regulations in 1924
Introduction of seismic design forces
• Maximum ground acceleration at University of Tokyo = 0.3 G (a method of measuring an earthquakes acceleration on the ground, in gravity)
• Safety factor in allowable stress design = 3.0 (the measure of which a building materials’ elasticity can stretch under pressure)
• Seismic Coefficient = 0.3 / 3.0 = 0.1 (represents the (maximum) earthquake acceleration as a fraction of the acceleration due to gravity).
These measures were a direct response to the damage and destruction of the 1924 earthquake.
Post WW2 Japan, and building regulations
At the end of the Second World War, Japan was a country that was in a state of destruction, in great need of rejuvenation and rebuilding. The regulatory bodies took this moment of change to further update the regulations with the following:
• Minimum quality of buildings, safety, health and usage
• Protection of built properties, construction contracts and quality of construction
• Conformation to legal requirements, use of land and enforcement of code
• Qualification of design engineers
• Respect for human rights, right of construction and right of property
Further to this, 3 new laws were introduced in 1950 to enable the construction of a better urban fabric in Japan:
1 Building Standard Law (1950)
To safeguard the life, health, and property of people by providing minimum standards concerning the site, structure, equipment, and use of buildings.
2 Architect Law (1950)
To define the qualification of engineers who can design buildings and supervise construction work.
3 Construction Trade Law (1949)
To improve the quality of those engaged in the construction trade and to promote fair construction contracts.
Over the next decade, further earthquakes and subsequent damage led to many more changes to the regulations controlling construction in Japan. In 1971, the Emergency Revision of Building Standard Law reduced the spacing of steel ties in concrete columns to 100mm after the failure of many columns in previous earthquakes.
Further to this, measures related to the ‘protection of society’, specifically related to seismic technology were created:
• Development of seismic design codes
• Vulnerability assessment of existing construction
• Retrofit technology of vulnerable construction
• Evaluation of damage to affected construction,
• Repair and strengthening of damaged construction
These measures marked the beginning of an ecosystem that would help to protect Japan from the worse effects of earthquakes. Over the next few decades, and more earthquakes, research into the effects of vibrations on construction as well as the development of more advanced materials and construction methods led to clearer definitions in seismic construction:
• designs were to be specified based upon -
• story shear rather than horizontal floor forces
• age of the structure
• serviceability and safety levels
• research into a storys’ shear resistance capacity with the inclusion of a collapse mechanism, under safety lateral forces
• penalties for irregular construction resulting in unequal stiffness along height and eccentricity in plan between centres of mass and stiffness
• limit of story drift angles under serviceable earthquake forces to help protect architectural elements
These new measures led to a massive increase in the number of buildings that suffered only light damage during the 1995 Kobe Earthquake, built post 1982. As you look at the buildings age, the older they were, the higher chance of severe damage.
The 1998 revision introduced performance based design requirements:
• Foreign Demand to Open Construction Markets (prior to this, trade laws limited the ability for foreign companies to market their technologies in Japan)
• Fire-resistance and Fire-prevention Requirements (the spread of fire post earthquake is a very real consideration).
Japan, due to its proximity to fault lines has experienced first hand on many occasions the awesome destruction power of an earthquake, and over the past 100 years has not only improved its built environment, but created an internationally recognised industry dedicated to improving a buildings ability to resist the energy released by an earthquake. In particular, Japan has excelled in the creation and installation of Mass Dampers (both passive and hybrid). A mass damper (in the context of a high rise building) acts to reduce the effects caused by an earthquakes vibrations on the building. They can effectively reduce the discomfort of shake on the buildings occupants, and in many cases stop total building failure.
The mass damper is one aspect of a wave of technology introduced into buildings that acts to work with the earthquake, instead to against it. Before, buildings within fault areas were solid, built to withstand a quake through solid construction. This proved ineffectual as an earthquake can quite literally tear a building apart. To counter this, it is better to allow an earthquake to have some effect on the construction, allowing it to move with the earth, rather than trying to resist it.
This technology falls under the banner of ‘vibration control’. The other major tool in an engineers arsenal when designing buildings to be earthquake proof are base isolation technologies. In a nut shell (as this is quite a complex area), base isolation or seismic isolation is the decoupling of a structure from its sub structure to enable it to better protect the buildings integrity.
Base isolation system consists of isolation units with or without isolation components, where:
• Isolation units are the basic elements of base isolation system which are intended to provide the mentioned decoupling effect to a building or non-building structure.
• Isolation components are the connections between isolation units and their parts having no decoupling effect of their own.
It is worth noting that this technology, whilst incredibly effective at reducing the impact an earthquake has on a structure, will not make a building earthquake proof – this unfortunately is a commonly peddled idea.
One other area of earthquake and seismic prevention in construction worth noting is a buildings ability to withstand lateral forces. The lateral force on a structure is often the force that will cause the building to buckle at lower levels. Whilst seismic isolation technologies allow the building to sway under force, lateral bracing means that the structure will remain intact, acting under the forces as a unit. It is vital that a building can withstand the seismic forces acting upon it, but it is also equally important that it does so as a whole; the buildings innate strength comes from its complete structure acting as a single unit. If this were to not be the case, it would greatly reduce its ability to withstand the quake, or even its own weight afterwards.
Earthquake resistant buildings are a modern marvel, a true marker of how far structural engineering in architecture has come. Our ability to design buildings that can withstand the very shaking of the ground it sits on is a testament to man’s capacity to create. One shining light of the Earthquake at Sendai is the survival of much of the built environment post quake. Its a real tragedy that the resulting tsunami has caused such destruction and damage, but one thing is certain: it will become a goal of many engineers around the world to develop systems that can be implemented into buildings of the future that will limit the damage caused by future tsunamis, this we can be confident of.