Why do buildings collapse in earthquakes? Building for safety in seismic areas. Robin Spence

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Why do buildings collapse in earthquakes? Building for safety in seismic areas - Robin Spence


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href="#ulink_5dba3a22-0fae-563e-aecf-4b37468ecca9">Figure 2.18 Typical damage to masonry construction in urban areas of Nepal.

      Source: EERI. Reproduced with permission.

      The descriptions also tell us that in cases where collapse of buildings is the predominant cause of death, most of the buildings which collapsed were masonry buildings. Many different methods of masonry construction were tested in these earthquakes, and not all were found equally vulnerable. Relatively good performance was noted in buildings of well‐constructed confined masonry, and also those of brick or block masonry where the walls were tied together with RC floors and roof slabs. Particularly, poor performance was noted in buildings of adobe masonry (2003 Bam earthquake), and buildings using rubble stone masonry, in which the walls were prone to disintegration, bringing down heavy roofs (Nepal, Kashmir and Bhuj). But brick and block masonry buildings also performed poorly (in the Yogyakarta and Wenchuan earthquakes, for example) when walls and roofs were not adequately connected. The Haiti earthquake tested a type of masonry/RC frame mixture, which has some similarity with confined masonry, but did not meet the essential requirements of that form of construction, and many occupants were killed by the collapse of such buildings. In Christchurch too, much of the damage was related to the older masonry buildings in the old city centre, though nearly three quarters of the 181 deaths happened because of the collapse of two mid‐rise RC office buildings.

      In the Tohoku (Japan) and the Christchurch (New Zealand) earthquakes, the residential buildings were mostly single‐family one or two‐storeyed timber‐frames, and the evidence showed that these buildings survived the severe ground shaking well. There was damage to brick elements, like chimneys and cladding panels, but few buildings collapsed.

      In most of these events, a part of the building stock, particularly in the cities, was built using RC frame construction, for multistorey apartment buildings and for commercial or mixed commercial/residential buildings. These were tested in the Bhuj, Kashmir, Nepal and Wenchuan earthquakes, and many such buildings collapsed in each of those events. In all of these countries, there are codes of practice which define how to design RC buildings to withstand seismic action, but it was evident to the reconnaissance teams that such codes were often not being implemented, and that the failings noted in previous earthquakes (inappropriate building form, poor materials, poor detailing of reinforcement) were being repeated. The Bhuj and Kashmir earthquakes also provided further evidence that tall RC buildings can be at risk at a considerable distance from the earthquake's epicentre. Hundreds of tall buildings collapsed in Ahmedabad situated more than 200 km from the epicentral areas of the Bhuj event. The Margalla Towers apartment block collapsed in Islamabad situated 100 km from the epicentral areas of the Kashmir event. RC buildings built before current codes of practice were introduced were also heavily damaged or collapsed in the Tohoku and Christchurch events.

      Some of the events provided tests for retrofitting programmes carried out in previous years. In Nepal, the retrofitting programme for school buildings carried out in the previous 10 years was found to be very effective as explained in Chapter 7. In New Zealand, however, the effectiveness of retrofitting carried out under the earthquake risk buildings programme was mixed, and numerous previously retrofitted buildings were damaged in the earthquake and subsequently demolished (though not necessarily as a direct result of earthquake damage).

      In general, the events demonstrated that where buildings are built in accordance with codes of practice adopted internationally since the 1980s, they performed well, even in places (Wenchuan and Christchurch) where the level of ground shaking was extreme and considerably higher than the code allowed for. But they also showed that implementation of the codes is often patchy, either because they are not required by law or because they are poorly implemented or because many buildings are constructed informally. This applied in particular to school buildings which collapsed in large numbers in several earthquakes (Kashmir, Wenchuan, Haiti and Nepal) confirming already growing evidence of the high vulnerability of this vital public service.

      In some cases (Bam, Kashmir and Haiti), the number of casualties was reported to have been magnified as a result of very limited or delayed emergency response, because of infrastructure damage, difficult terrain or simply because the local emergency response infrastructure and personnel were also directly affected. And, in both the events in which large tsunamis were triggered, a lack of public awareness and a lack of an adequate warning system was considered to have increased the death toll, although in the case of Japan it is considered that warnings and pre‐event evacuation plans which did exist (though based on lower tsunami inundation level) did contribute significantly in the reduction of loss of life.

      More detailed accounts of the performance of buildings are given in the EEFIT and EERI reports referenced in relation to each event. In addition, these reports contain detailed information on the performance of geotechnical structures, of building foundations and of infrastructure, particularly roads and bridges, dams and ports. They also contain some recommendations for reconstruction and for future building. The earthquake performance of buildings of different forms of construction is discussed in more detail in Chapter 5, and ways in which buildings can be improved are discussed in Chapter 7.

      Table 2.1 lists 23 significant earthquakes which have occurred since the year 2000, causing a total of 699 000 deaths and US$488bn of damage costs. Using data on national income given by the World Bank, we can allocate these totals by the four country income categories defined by the World Bank, as shown in Table 2.2.

      Source: World Bank (2019).

World Bank Country Group GNI/capita (2018 US$) Deaths (% of total) Damage
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