Issue 01: Seismic Design in Japan

Although our mission as engineers the world over is the same, the way we solve engineering problems at a more granular level can vary. Interstices between physical phenomena are filled with local histories and quirks that can at times result in altogether different pictures of how we tackle similar problems. This societal influence is something I have become increasingly interested in as I have practised in differing cultures across my career.

Having worked in Japan and the UK, I learned structural design staples that were developed in response to local disasters and the damage they caused. Both staples are essential to design in each country. Both are also concerned with providing structural robustness to improve the life safety in the event of disaster.  I focus on these two principles primarily because they were new to me in each country and had immediacy as I needed to understand them for live projects. Buildings need to be designed against the criteria outlined for each principle, and the response to these, can affect the feasibility of a project and the final structural expression.

The disaster the Japanese tackle is a geological phenomenon. The country is located at the confluence of 4 major tectonic plates and is frequently affected by earthquakes. Several hundred quakes occur each year, and they have caused some of the biggest crises in the country’s history. Seismic design is therefore an integral part of a structural designer’s life.

In the UK, the disaster reacted to in the design codes is man-made. The Ronan point collapse in 1968 highlighted the importance of robustness in a building’s ability to survive accidental events. The collapse was initiated by a gas explosion in one of the apartments which eventually led to the collapse of a corner of the building. This event led to the codification of disproportionate collapse guidance. Events like the Grenfell Tower fire later followed and made their own impact on the way designers work. The incident has caused fire performance, and its role on disproportionate collapse considerations, to become prominent in construction conversations. The impact of the legislative changes is still percolating through the construction industry.

This first part discusses the lessons about seismic design that became the totem of the start of my career.

PART 1: Seismic Design in Japan

Earthquakes have been part of Japanese life and psyche for centuries. Before the development of seismology as a scientific field or the idea of design standards, the Japanese lived with the impact of earthquakes and tried to rationalise their occurrence.

This desire to make sense of the physical trauma attributed to natural phenomena like earthquakes is evident in geomyths created across the world. In Māori tradition Rūaumoko is the god of earthquakes who in some versions of the myth causes quakes by moving in his mother’s womb. Europe is peppered with stories of its own earthshakers - Poseidon for the ancient Greeks, Drebkhuls in Latvia. Some Middle Eastern and North African cultures tell of the restless shifting of the bull on whose horns the world rests. The Japanese have Namazu.

Namazu is a giant catfish that was said to live underground, and its thrashing tail was believed to be the cause of earthquakes. The catfish was controlled by the god Kashima who used a capstone to restrain it, but when he was distracted and Namazu could move, earthquakes would occur. As with other cultures, natural disasters like these were sometimes associated with divine punishment, but over time, myths are often dispelled with improved understanding of natural systems. While earthquakes are now better understood, Namazu remains their symbol in Japan, and its image can still be seen across the country on signs with earthquake emergency information.

Figure 1: Woodblock print showing people attacking Namazu in the wake of the 1855 Edo earthquake

My experience of earthquakes in Japan started with the surreal experience of being woken up by one in the middle of the night a few months after moving there. I could immediately empathise with the visceral the-world-is-definitely-ending feeling they can trigger and though I (eventually) recovered, I did not understand then how pervasive their spectre would be. While apartment hunting, I was encouraged to check whether any prospects were designed to the most current seismic codes and when I moved into my first place, the landlord explained where our local emergency assembly points were after he gave me my bike sticker and keys. In time, I learned how to prepare a disaster preparedness kit and practised numerous earthquake drills. Even after nearly a decade, I never did shake the anxious anticipation of the next 'big one' that hummed just underneath the surface of everyday life.

An early conversation when starting my first job sought to impress on me that the primary goal was to create structures that protect people and give them a fighting chance for survival when disasters strike, when nature happens. This was further amplified by learning accounts of the Tōhoku disaster survivors in Higashimatsushima where my first school project was located. The school was part of the rehabilitation efforts in the region following the 2011 earthquake and tsunami. The project was impactful for me because it was my first timber project, and the first on which I had significant responsibility after having worked on my technical Japanese and adapted to local design.

The survivors’ stories left me feeling impassioned, and I was acutely aware of the urgent need to provide spaces that would help people return to some kind of normalcy. By the end, I was proud of the work I had done, and the restorative power of good design was reaffirmed for me.

However, sandwiched between these emotional project bookends was a crucial period of grappling with the technical, construction and regulatory requirements of the project. Much of my time was spent poring over the design codes – a practice emblematic of the rest of my time designing in the country. Seismic codes are unavoidable in Japan, so you best get cosy with them.

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History & Overview

The Japanese seismic design code is unique to the country and was developed over several decades. Empirical data collected during surveys of domestic and foreign disaster-struck regions has been used alongside extensive research to revise and edit the codes ever since their first iteration. This process of feeding back real-life lessons into the codes is still considered essential work. There have been some key quakes since the start of seismological study and the observations from these have assisted the development of modern codes as shown below.

Earthquake	Magnitude (Mw)	Observations, Key Lessons
1855 Edo	6.9 - 7.4	· Following this, quakes were seen as punishment for human greed
1880 Yokohama	5.5-6	· Inspires the founding of the Japanese Seismology Society in 1881
1906 San Francisco	7.9	· Professor Sano Toshikata visits California to investigate incurred damage which later informs his seminal 1916 report; this becomes the basis of Japanese seismic design standards · US Concrete industry uses survival of Bekins Warehouse to heavily promote concrete construction touting it as durable, earthquake- and fire-resistant; this foments the widespread use of concrete in America
1923 Great Kantō Earthquake	7.9-8.2	· Event noted for the significant casualties and loss of infrastructure caused by outbreaks of fires, and their subsequent spread exacerbated by the high winds from a concurrent typhoon · Failure of brick and stone buildings not designed for seismic performance led to gradual disappearance of these materials from country's construction repertoire · Reinforced concrete (RC) buildings with sufficient amounts of walls survived well, as did structural steel frames · Survival of The Industrial Bank of Japan made of steel reinforced concrete (SRC) drew international interest and popularised SRC buildings in the country · Importance of ground conditions on structural performance made clear as timber houses on alluvial deposits suffered more damage than those on firmer soils · Seismic shear coefficient which determines design lateral force is developed (and remains key design feature in all iterations of code)
1964 Niigata	7.6	· Liquefaction of saturated sandy soils and damage caused by this draws attention to importance of foundation design
1968 Tokachi-oki	8.3	· Damage of 'short' RC columns kickstarts a variety of research into the seismic performance of RC buildings and focus is placed on understanding and preventing shear failure of structures · Redundancy identified as a crucial factor for preservation of structures in the event of an earthquake · RC design standards updated 3 years later considering damage incurred in this quake
1978 Miyagiken-oki	7.7 (Ms)	· Compression damage to piles tops on buildings with no basements observed (Basements would have provided lateral and rotational resistance under seismic loading) · Damage to buildings with only columns and no shear walls at ground floor highlights importance of additional considerations required for buildings with so-called ‘piloti’ columns · Collapse of secondary structures like concrete block walls/fences which caused loss of life illustrates importance of considering safety as a more total issue · Research stems from the observed damage to buildings with large eccentricity ratios and too-variable elevational stiffness distribution (see Route 2 design below) which later leads to 1981 New Earthquake Resistance Standard
1995 Great Hanshin earthquake	6.9 Mw 7.3 MJMA	· Severe damage to buildings constructed prior to 1981 Building Standard Law including whole floor collapse (pancake collapse) observed · Severe damage to bridges and railways triggers first iteration of seismic design guidance of civil engineering structures · Severe damage to infrastructure systems like sewers, embankments, water supply lines etc. leads to a series of strengthening works for infrastructure projects · Vital information gained about national and local emergency and evacuation systems
2011 Tōhoku earthquake & tsunami	9.0-9.1	· Largest ever recorded quake in Japan and disaster considered the country's biggest crisis since WW2 · Triggers a major nuclear incident at the Fukushima Daiichi Nuclear Power Plant which calls to attention culture of corruption and collusion on public projects · Reinforces importance of understanding and utilising up-to-date information on (natural) hazards and how they affect design of critical structures · Analysis conducted on unanticipated liquefaction, effect of long-period quakes on tall buildings and bridges

Like many other standards, the currently used seismic codes attempt to simplify the behaviour of a complex and capricious natural occurrence to practical methods that help designers to create safe structures. The seismic design concept is split into 2 overarching steps: Level 1 design and Level 2 design.

Level 1 design is the core of design for all buildings. Its purpose is to ensure main structural members do not incur damage and can continue providing support under everyday loads like self-weight and occupancy. Loads that may act frequently like heavy wind and snow are also included. Level 1 design also includes considerations for seismic loads from moderate earthquakes. Horizontal forces, which are based on ground accelerations, are determined to simulate the effect of seismic ground motions on buildings.

The maximum ground acceleration for moderate earthquakes is 80-100gal which equates to a design acceleration of 200gal at minimum (~2.5 - 3 times the ground acceleration). Gravitational acceleration, 1G (9.81m/s²), is approximately 980 gal, so for moderate earthquakes, a force of ~0.2 times the building superstructure's weight is applied to the base (ground floor) as the design seismic force.

Level 2 design is additionally required for larger and more complex structures to ensure the building's robustness during large earthquakes (maximum ground acceleration 300-400gal with a design base seismic force of 1G). This level of design aims to ensure that structures have adequate capacity to avoid collapse in the event of rare earthquakes.

To avoid in-depth seismic analysis for every building, a system of design routes was developed and codified in the Japanese Building Standard Law. Broadly speaking, the number and complexity of checks increases with the scale and complexity of the structure. The design routes are organised by the material of the members resolving seismic forces, namely: steel, reinforced concrete (RC), steel-reinforced concrete (SRC), and timber. The routes within each material category are ranked from 1 to 3 and are defined based on the size and geometry of the structure.

The design of small buildings such as small-scale timber structures may at times be simplified and exempted from route design. Conversely, large buildings (those over 60m in height) or structures of any size that are deemed to be highly complex also sit outside the route system and are subject to detailed specialist review. However, many buildings will rely on the route system for their design, and it is important to understand this as the route can impact the final structural expression, design time and costs, and the complexity of the review process to grant approval. From an engineer's perspective it is often one of the first things to establish at the start of a project, and most designers will discuss and agree the applicable routes with approval authorities during schematic design. These discussions are particularly important for hybrid structures where the resolution of seismic forces using 2 or more materials may be complex.

The image below shows a simplified chart for steel design routes. The design checks stipulated for each route are considered a minimum specification to ensure safety. Small structures may still be complex, so it is the engineer's responsibility to account for this complexity in design, even when the applicable design route is low. In my experience, this has also been a critical part of the building review process where experienced third-party reviewers not only check that the design complies with Building Law, but also that structural issues have been thoroughly considered regardless of route.

Figure 2: Simplified version of design routes for steel structures excluding April 2025 updates

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Design Routes

Route 1 design is reserved for small- and medium-scale buildings. Complex checks are abbreviated by increasing the seismic force applied to the structure.

Route 2 design is for buildings which do not exceed 31m in height. Considerations for inelastic behaviour can be exempted if checks are conducted to ensure the stiffness is adequately distributed. Members and their connections are also checked for strength and ductility. Storey drift checks are also required. They are a serviceability measure which ensure the building is sufficiently stiff to prevent excessive lateral deflection which may affect non-structural elements or adjacent buildings. This is a critical check where brittle finishes like glazed curtain walls are used.

The eccentricity and stiffness ratio checks are to verify that stiffness is adequately distributed in the horizontal and vertical planes, respectively. Members along the building's perimeter, particularly the corners, can be affected by torsional effects if the horizontal stiffness distribution is imbalanced, and this can cause damage.

If the relative stiffness between floors varies greatly, stresses can concentrate in less stiff floors. This can lead to severe damage to load-bearing members or complete collapse of the floor, known as pancake collapse (Figure 4). As is easily imaginable, pancake collapse poses significant risk to the life and property in the affected floor and the sudden loss of a floor can compromise evacuation efforts.

Figure 3: Example of pancake collapse suffered by the Ministry of Telecommunications building in the 1985 Mexico earthquake

Route 2 design also includes checks to ensure members do not fail locally which can lead to sudden strength loss. Connections are designed to maximise ductility which is critical for the structure's robustness. Enhanced ductility allows for a structure to absorb energy and deform plastically without losing strength or collapsing, which is a key mechanism for seismic resistance.

It also allows structures to have more redundancy as load can travel along alternative load paths. This increases insensitivity to progressive and brittle collapse. Increasing a structure’s ductility can be critical in increasing this insensitivity. (The importance of ductility will be revisited in Part 2 where disproportionate collapse is discussed.) For steel design, there is an intermediate route, Route 1-2, which includes some of the Route 2 checks. In the April 2025 update of the seismic code, Route 1-3 was also added.

Route 3 design is required for all buildings between 31m and 60m in height. The main aim is to ascertain whether the structure has sufficient lateral resistance to prevent collapse in the event of a large earthquake. This relies on the structure's ability to absorb seismic energy, and while damage may be difficult to avoid, collapse should be prevented.

The structure's inelastic behaviour is critical to understanding the building's energy absorption capacity. Incremental pushover analysis is typically used to assess inelastic behaviour and is often supplemented by the assessment of the presenting collapse mechanisms. The analysis involves determining a structure’s force-deflection relationship, also known as the capacity or pushover curve. This is done by gradually applying lateral loads while gravity loads simultaneously act. The resulting curve is used to assess the structure against the damage conditions deemed acceptable for the required performance. The damage condition can include the threat to life safety, the building’s post-earthquake serviceability, and the extent of damage within the building. In Japanese design, limits are placed on each floor’s lateral capacity and sometimes also on the final story drift.

For complex structures or those taller than 60m, time history analyses may be conducted. The design and review of this analysis can be complex and time-consuming which can affect the design fees and programme of a project.

Figure 4: Examples of classifications of hybrid structures (may be subject to interpretation)

The impact of hybrid design was evident on a new-build house I worked on. Basements are treated separately from the superstructure as their seismic force is calculated differently. Where the number of floors is used as a route assessment criterion, only superstructure floors are counted. For this structure, whether the lowermost floor was designated a basement (seismically separate substructure) or a ground floor (part of the superstructure) impacted both the architectural and structural design.

Basements were required to have at least 75% of their perimeter walls embedded in the earth and in contact with soil. This requirement was not met in the initial design, so the building would be designed as a 3-storey, Route 2, hybrid timber-RC structure. If the lowermost floor met the requirement, the building would be designed as a 2-storey Route 2 timber with an RC basement – essentially, it would not be considered as a hybrid building.

The difference in definition is subtle but for the hybrid structure, the seismic force on the upper timber portion would be 1.5 times larger than that in the non-hybrid version. This amplification is due to the difference in stiffness between the RC and timber portions that are required to act compositely. This increase would have a significant impact on the structural planning of any building, but it was even more impactful for this building with its design snow depth of 4.0-4.5m which contributed to the building's overall weight and therefore seismic load. The hybrid version would require many more braces or shear walls to resist the increased load which would have compromised the architectural proposal with its large open internal spaces and freer facades.

Ultimately, the non-hybrid version was chosen which simplified the seismic considerations.

Figure 5: Difference in structural requirements on perimeter between hybrid and non-hybrid structures

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初心を和するべからず (Don't forget the original intention)

As I mentioned at the start, Japanese seismic design is unique to the country and most of the key literature is still only in Japanese. Language learning was therefore an inextricable part of my journey to becoming an engineer. I found myself becoming reacquainted with a rather obvious point: speaking other languages expands your world.

Engineering is a response to social need

At a basic level, learning Japanese meant I could pay my bills and travel more easily. At a deeper level, the concepts I learned through the language are foundational to how I see design and my attitude towards engineering. I speak engineer in Japanese first. What feels instinctual is what I learned over long nights poring over design codes with a dictionary to help decode. It is what I was taught by colleagues who spent their dinners patiently helping me to parse how history turned into modern design.

The most powerful influence for which language was a conduit was understanding what structures meant to people. After spending much time with my thoughts, it was listening to how people's lives changed when structures failed; how they lost their memories, their livelihoods, their sense of anchoring with their communities, their refuges.

As I have heard it once put, engineering is a response to social need.  And in the rhythm of everyday design with its flowcharts and tick boxes, it can be easy to lose sight of this. Nourishing the connection to this commitment chases away the cobwebs that collect around familiarity. It encourages us to look beyond what has been systematised to understand deeper and pursue better solutions.

In my experience, looking to your neighbours to see how they are fighting the same fight has helped me resuscitate the sense of shared focus by providing fresh perspectives. Tokyo gave me a jolt when I needed to learn seismic design to meet local challenges, the same was true of learning to design in the UK. Here, we have our own norms but even within that there were echoes of the familiar. A principle I had to grapple with early on was disproportionate collapse and its impact. Over time I could see the parallels between that and the points discussed above and we shall discuss that re-entry into UK design in the next issue.

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Selected Bibliography & Further Reading

Earthquake Hazards Program. (n.d.). Earthquakes . Retrieved from U.S. Geological Survey: https://www.usgs.gov/programs/earthquake-hazards/earthquakes

Piccardi, L. &. (2007). Myth and Geology. Geological Society, London, Special Publications 273.

西山功. (2011). 東日本大震災を踏まえた建築分野の研究の展開. 国土技術政策総合研究所 (NILIM).

Image Credits

Thumbnail image: ©Yuta Niwa

Figure 1: 国際日本文化研究センター (n.d.).[額に「江戸」「信州」とある2匹の大鯰が、人々に襲われている。]. Retrieved from https://www.nichibun.ac.jp/YoukaiGazouCard/U426_nichibunken_0207_0001_0000.html

Figures 2, 4, 5: © analogs | this~that 2025

Figure 3: U.S. Geological Survey (1985). [Mexico City- Collapsed and damaged upper floors of the Ministry of Telecommunications building] Retrieved from https://ja.wikipedia.org/wiki/%E3%83%91%E3%83%B3%E3%82%B1%E3%83%BC%E3%82%AD%E3%82%AF%E3%83%A9%E3%83%83%E3%82%B7%E3%83%A5 

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