The Earthquake design codes of Japan, one of the best prepared countries in the world for natural disasters, were thrown into the spotlight on 17 January 1995 when the 6.8 magnitude Great Hanshin earthquake devastated Kobe, a city of 1.5 million people.
Around 6,400 people died and over 200,000 homes were destroyed. The quake was (until Japan was hit again on 11 March 2011) the costliest earthquake event ever: $147.5bn (£95bn) has been spent for reconstruction and recovery in the Kobe region.
Primary effects included the collapse of 200,000 buildings, the collapse of 1km of the Hanshin Expressway, the destruction of 120 of the 150 quays in the port of Kobe, and fires which raged over large portions of the city. Secondary effects included disruption of the electricity and gas supply.
The biggest cause of death transpired to be the country’s 1981 building codes for medium to low rise buildings. Damage surveys in the worst affected area showed that the collapse ratio of mid-rise pre-1981 reinforced concrete and steel frame buildings was unexpectedly high. In central Kobe as much as 15% of pre-1981 reinforced concrete buildings collapsed
According to Japan’s 1981 codes, buildings not exceeding a height of 31m were designed elastically to withstand moderate earthquakes at serviceability limit state (SLS), with no checks made at ultimate limitability state (ULS).
The Institution of Structural Engineers’ led Earthquake Engineering Investigation Field Team (EEFIT) concluded that it would be expected that buildings so designed would have performed very poorly.
“The generally better performance of buildings exceeding a height of 31m calls into question the ULS design exemptions for lower buildings in, terms of the omission of checks at ULS,” it said.
Fires also caused widespread damage, with emergency services unable to reach the affected areas of the city quickly because many of the major roads and most of the freeway system had been badly damaged. Once in the region of the fires, access was further restricted as many of the smaller streets were completely blocked with rubble.
The main water supply lines were also badly damaged during the earthquake, further reducing the ability of the fire services to control the outbreaks.
As a result many of the small fires that started after the earthquake quickly became large fire storms, with the little space between the buildings meaning there were no effective fire breaks that could stop the spread of fire. It has been estimated that over 1.3km2 of the city was destroyed by fire and most of the areas destroyed were residential.
“The Kobe Earthquake has potentially provided structural engineers with more valuable information than any previous earthquake,” said EEFIT at the time. This has proved to be the case, with more recent quakes in Japan and New Zealand all causing less loss of life because of lessons learned from Kobe.