the Carpentry Way: Shaken, not Stirred                                                          

Shaken, not Stirred

    
The Great Hanshin earthquake, or Kobe earthquake, occurred on Tuesday, January 17, 1995. It made rather a mess of things - more than 6400 people lost their lives, and about ten trillion yen ($100 billion) in damage was caused, 2.5% of Japan's GDP at the time.

The strength of this earthquake moved the ground 7 inches horizontally and 4 inches vertically. Buildings and structures of all types suffered tremendous damage, as the following collection of photos illustrate:  

 





Kobe, an industrial manufacturing center, was a part of Japan firebombed into a wasteland by the US in WW II, and had been rebuilt in the post war period fairly hurriedly - you might say that corners were cut in certain instances. Despite the normal Japanese fastidiousness, and their vast and sophisticated use of concrete generally speaking, the expressway support columns had lacked certain key pieces of rebar reinforcement:

 

Seems like it has plenty of rebar, judging from the photos, but I think not enough horizontal banding was the issue.

Older buildings, constructed before the 1980 promulgation of a national building code for Japan, suffered from insect damage in the timbers, lack of shear bracing, and in some cases had been retrofitted with heavier tile roofs that exceeded the design for the structure as originally configured. In a report entitled Lessons in the Strengthening and Reinforcement of Historical Buildings from Rescue Projects Following the Great Hanshin Earthquake, Professor Nobuo Ito said:

"In the case of shrines, the collapse of the one-bay Nagare style Yakujin Honden of the Rokko Hachiman Shrine is worthy of note. The way in which it had collapsed, just as if its legs had been kicked from under it, makes one think that it was thrown off balance in an instant and the body of the building was crushed by the weight of the roof. There can be little doubt that, as Dr. Kuroda has reported, the main cause was that the roof covering had been changed from cypress bark roofing to heavy classic roof-tile, but it is thought that the rather unbalanced Nagare roof form was also a contributory factor."

Few owners chose to retrofit older structures with uprated bracing systems, regardless of code changes, as there is a cultural preference in Japan to demolish and build anew when it comes to most houses, rather than do retrofits. And these things do cost money after all, and severe earthquakes in the region were relatively uncommon.


While houses of all sorts of construction types, traditional and modern, and varying materials - concrete, brick, stone, 2x and ply, and timber-frame - were trashed along the primary lines of seismic activity, the Pre-fab and 2x construction industries were quick to take advantage of the sudden need for new buildings in the greater Kobe area - and elsewhere - and advertised their systems in part by showing pictures of failed traditional structures. As traditional structures are considerably more costly to build anyhow, a usual disincentive in any market, this marketing campaign turned out to be one of the proverbial nails in the coffin of traditional building in Japan. The industrialized building approaches thereby gained a greater foothold in the Japanese market, a trend that has continued to this day. 

 

This earthquake demolished homes of 200,000 people. Professor Ito, in the aforementioned report, noted that damage was most devastating in wooden buildings with:

  1.  poor foundations and sills
  2.  insufficient diagonal bracing
  3.  inadequate connecting elements
  4.  insufficient areas of solid wall
  5.  heavy roofs.
Looking specifically at two kinds of traditional Japanese architecture, Ito first considered thatched roof farmhouses:
"Among other things, in the earth-floored area (doma) of farmhouses, posts are close together and the surrounding walls are more or less continuous, making this part of the house comparatively strong, whereas the reception rooms (zashiki) at the other end of the house are more open, with fewer walls and posts. For this reason, it is reported, there was a characteristic trend for damage to be concentrated in the reception rooms.
Another point that has been noted with respect to farmhouses is the strength of thatched vernacular houses. The Kosaka house, in Ashiya, was one of the few thatched farmhouses still surviving in the area of severe shocks, and it suffered only slight damage. The center of gravity of a thatched vernacular house as a whole is low. Moreover the whole roof frame is a flexible network held together with rope, and as a result the roof frame can evade earthquake forces, making it highly earthquake resistant."
And then he looked at Japanese traditional detached houses characteristic of Kobe:
"In the Kansai region, where there had been no earthquakes for a long period, countermeasures against typhoons had been given priority. The use of comparatively heavy tiled roofs, despite the light eaves detailing and the wide south facing openings, seems to have been an adaptation to cope with such local climatic conditions. Sukiya houses of the kind referred to as modern Japanese-style houses were built in large numbers in Hanshin area until the last war. These houses developed as if each were competing to be the most delicate and the least enclosed, but it cannot be denied that, with their lack of solid walls and ill-balanced layouts, they were ill-conceived for an encounter with an earthquake on this scale."
A final point Ito makes in respect to traditional architecture, a plus and a minus at the same time, concerns relative repairability:
"With the kind of vernacular houses mentioned above, there is a need to inform widely that one of the advantages of the traditional method of building is that, even in the case of severe damage such as cracking of the walls and inclination of the posts, it is possible to repair the structure. Craftsmen known as house pulling carpenters (hikiya daiku) make a profession of moving buildings, but they can also undertake this kind of straightening of the frame of a building. Since they can easily correct a degree of inclination of the structural frame that ordinary carpenters would regard as hopeless, we have come to appreciate their true worth in the aftermath of this earthquake. In the Hanshin area there are no longer any such craftsmen, and it is deeply regret that there are not more hikiya daiku."
The Japanese have taken steps to thoroughly analyse buildings and their performance in seismic events. In 2003, a massive structure, the Hyogo Earthquake Engineering Research Center, aka 'E-defense', was completed:

 

This structure is purpose-built as a giant test bed for entire structures. And when I say 'bed', I mean a huge mobile bed is inside this building, atop of which structures of various types are built, and then evaluated by subjecting them to movements approximating seismic events. 

Here's a video clip, or three, to show what goes on when the shake test occurs:





Another traditional frame, completely bare:



Here's a pair of completely traditional, fully-detailed houses, and here the test is at 100% of the seismic force of the Hanshin Earthquake:


There are several approaches being taken to making structures more resistant to earthquakes. One approach is to make the structure as rigid as possible. Another involves sophisticated rollers or springs in the foundation to dampen uplift and sideways shifting loads. The traditional Japanese approach has evolved over a long time to produce structures which behave flexibly in earthquakes, like the proverbial willow shedding the load of snow by bending while the oak has its branches snap off under a similar loading. The difficulty in making design decisions in this area is balancing the cost of making a structure more resistant to earthquakes against the likely frequency of the event. Few would want a structure to withstand the worst earthquake likely in 1000 years, if the building is unlikely to last more than 150 years otherwise. And given that many parts of Japan deal with typhoons on a more frequent basis, trying to design structures to perform well in those conditions only complicates the picture. Then there are tsunami - how would one make a structure, other than some sort of bunker, that could resist the sort of wave seen in the recent disaster in Japan?

Culturally, the Japanese have developed a sort of fatalism about such things. Expressions such as 'shigata ga nai' (nothing can be done") and 'shoganai' ("it can't be helped") are common in everyday conversation, and, some would say, more-or-less representative of philosophical outlook. Even 'sayōnara' literally means, "well, if it must be so." There are many other examples in the Japanese language where fatalism is expressed, far too numerous to mention here.

I think it is intriguing though to see the 'E-Defense' structure and the willingness to put building systems under reality testing. While it can't be cheap to do this sort of thing, hopefully some highly useful data will come out of those tests perhaps showing new ways forward in building design and construction, and potentially saving lives in the process.

Thanks for coming by the Carpentry Way. Comments always welcome.