BY AN extraordinary irony, the day the Kobe earthquake struck, the Fourth Japan-US Workshop on Urban Earthquake Hazard Reduction was due to start in Osaka, just across the bay. One of the Americans who had arrived for the meeting, Charles Kircher, of the civil engineering firm Kircher Associates in Mountain View, California, recalls: 鈥淗ere we are in our hotel rooms, and we鈥檙e all thrown out of our beds at 5.45 in the morning, and all we could say was, 鈥楪ee, they鈥檝e really planned this one well鈥�.鈥�
The workshop was immediately cancelled as the 100-odd experts rushed off to visit the afflicted area in buses, taxis and 鈥� in Kircher鈥檚 case 鈥� a helicopter. The extent of the devastation astonished even veteran earthquake engineers from California. 鈥淲e were amazed at the number of buildings that were collapsed, or partially collapsed, leaning into the street,鈥� says Kircher. 鈥淭o walk past block after block and see maybe 20 out of 100 buildings that have collapsed a floor or worse is really shocking.鈥�
Kircher says that unusually strong ground motions are what caused such a high percentage of buildings to fail. He believes that ground motions can be much stronger than building codes assume, so he was less surprised by the damage than some of his colleagues. 鈥淎 lot of engineers have been saying that, even though you might have stronger forces, we think we鈥檙e doing enough. What we haven鈥檛 seen before is this level of ground motion so close to such a dense urban area.鈥�
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In the Californian earthquakes at Loma Prieta in 1989 and at Northridge last year, the strongest shaking was up in the mountains, says Kircher. 鈥淏ut this time, it was right underneath a very long stretch of very densely built area.鈥�
The buildings that were destroyed were mainly traditional houses with wooden frames, a heavy tiled roof and weak walls. 鈥淢ost of the failures in mid-rise/high-rise commercial buildings that I observed appear to be pre-1980 construction, and had steel reinforcement that would likely not be compliant with current seismic codes.鈥�
In addition to highlighting the need for better reinforcements, the Kobe quake will undoubtedly have other lessons for the future. Engineers will be encouraged, for example, by the performance of the modern high-rise buildings 鈥� including hotels and apartment blocks 鈥� located on Port Island in Kobe Bay.
Because the artificial island consists of very soft landfill, the structures are built on piles, which in some cases reach down 36 metres to the seabed beneath. While the ground liquefied during the quake, the piles supported the buildings, preventing them from sinking. In fact, as Kircher points out, 鈥渢he ground liquefying probably mitigated the shaking rather than enhancing it鈥�.