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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construction; in other cases, the walls were infill within a previously built frame, and some investigators found that the ‘wall‐first’ buildings performed better in the earthquake. But for both types of construction, the materials used, the amount and detailing of the reinforcement and the process of construction were generally inadequate, resulting in buildings which were totally unable to resist the earthquake forces they were subjected to. Failures of the concrete frame leading to the overturning of masonry walls were common (Figure 2.13). Engineered commercial buildings mostly performed better, and most of the few traditional timber‐framed buildings remaining in the city suffered only moderate damage (EEFIT 2010).

Photo depicts typical damage to low-rise informal building in Port au-Prince.

      Source: EEFIT. Reproduced with permission.

      The vulnerability of Haiti's building stock had a number of underlying reasons. There had been no significant earthquakes in the area since the eighteenth century, so earthquake awareness was low. Haiti is the poorest country in the Western hemisphere, and lacks effective government institutions, so there was no effective building code. Topography and ground conditions probably contributed to the scale of the damage, with some soils causing ground motion amplification, and failure of buildings on steep slopes resulting from foundation failure (EEFIT 2010).

      2.2.8 The 22.2.2011 Christchurch New Zealand Earthquake: Mw6.1, 181 Deaths

      The earthquake occurred at 12.51 p.m. local time, causing severe shaking throughout the City of Christchurch, the largest city in New Zealand's South Island. The Mw6.1 earthquake was an aftershock of the Mw7.1 Darfield earthquake which took place five months earlier, and had also caused shaking and damage in Christchurch, but the epicentre of the 22 February 2011 event was much closer to the city, and caused ground shaking well in excess of the design level which the New Zealand Code specifies for Christchurch (EEFIT 2011a). The ground shaking destroyed hundreds of buildings in the city, including many old URM structures as well as two large office buildings, and 181 people were killed, nearly three quarters of them in the two collapsed office buildings. The earthquake also caused extensive liquefaction of soft alluvial ground both in the Central Business District and the eastern suburbs which added to the extent of building damage. The estimated total losses were US$15bn, of which around 80% was insured (King et al. 2014).

      New Zealand has had programmes of strengthening URM buildings since 1968 (see Chapter 8), and there were a significant number of URM buildings in Christchurch which had been retrofitted. In general, these performed better than the non‐retrofitted buildings, but many of these too were damaged, and have had to be demolished. By 2013, more than 90% of the non‐retrofitted URM buildings, and over 70% of the retrofitted URM buildings in the Central Business District (CBD) had been demolished (Moon et al. 2014). The earthquake was therefore an important test for the efficacy of previously used retrofitting techniques.

Photo depicts typical damage to pre-1930s masonry buildings in Christchurch.

      Many important lessons have been learned from this well‐studied event, about building performance, retrofitting, insurance, damage assessment and recovery procedures, as well as the limitations of life‐safety‐based codes of practice. These lessons are now beginning to be implemented, not just in New Zealand, but worldwide (Chapter 8).

      2.2.9 The 11.3.2011 Great Tohoku Japan Earthquake: Mw9.1, Over 18000 Deaths and Missing

      Direct losses from the event are thought to be in excess


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