Читать книгу Why do buildings collapse in earthquakes? Building for safety in seismic areas - Robin Spence - Страница 18

This earthquake occurred at 5.26 a.m. local time, on a hitherto unidentified fault passing under the historic city of Bam (Berberian 2005). Surface ruptures were identified along this fault south of Bam, and extending northwards towards the centre of the city. The area as a whole is one with a well‐known history of active seismicity (nine earthquakes have been felt in Bam since the beginning of twentieth century). The earthquake was catastrophic in the city of Bam itself (far more than would be expected for an earthquake of this magnitude), as well as in the nearby town of Bharavat and neighbouring villages of Kerman Province. The intensity map, produced by IEES, indicated heavily damaging shaking intensity over an area of about 1000 km². The degree of damage in the city was clearly visible in high‐resolution satellite images (Figure 2.5).

The massive death toll in Bam has been attributed to the extreme weakness of the adobe houses which are inhabited by the majority of the population. This method of building has been documented in the World Housing Encyclopedia (Maheri et al. 2005).

Adobe construction is an appropriate response to the climate of Southern Iran, given high day–night temperature swings, and also the lack of timber available for construction. At the time of the earthquake, this was still the predominant way of building in Bam. But in the event of an earthquake its weakness is extreme. The problems include (Maheri et al. 2005):

 Thick heavy walls, which attract large lateral seismic forces.

 Lack of connections between perpendicular walls.

 Heavy domed or vaulted mud roofs, exerting lateral pressure on walls.

 Poor quality of the adobe units (local sun‐dried mud).

 Poor quality of mortar and bonding.

 Lack of foundations.

 Limited maintenance.

Architectural conservator Randolph Langenbach who made his own study in Bam following the earthquake (Langenbach 2015) has suggested that the use of straw reinforcement in adobe construction may have allowed termite attack, which could have reduced the inherent cohesion of the material. Certainly, many of Bam's adobe buildings simply disintegrated as a result of the ground shaking, leaving only heaps of dried mud brick rubble (Figure 2.6). The danger to occupants was increased by their close spacing, leaving little opportunity for escape, and this also inhibited search and rescue. Since the earthquake attempts have been made to develop a way of building dwellings which conforms to the climatic and space requirements, and uses local materials, but which is able to resist earthquakes (Maheri et al. 2005).

The huge death toll of nearly 27 000 was about 25% of the population of Bam at that time. It was undoubtedly the result of the collapse of very large numbers of adobe dwellings, coupled with the early morning time of day, when most people were sleeping. It has been reported that only 2% of those who died were in buildings which did not collapse (Ghafory‐Ashtiany and Mousavi 2005). Of the 23 600 injuries, 9477 were serious, and had to be treated in hospitals in Kerman and elsewhere as all the hospitals in Bam were severely damaged. Building collapse‐related traumas constituted most emergency surgery cases. However, it has been suggested that a further very significant contribution to the death toll was the lack of immediate response capability (Movahedi 2005). The local emergency response capability was totally destroyed by the earthquake, and for the crucial first 24 hours the only rescue was being carried out by the local survivors using their bare hands. The loss of electricity meant that rescue stopped at nightfall, and freezing temperatures reduced the chances of overnight survival under the rubble. Asphyxiation resulting from the huge amount of dust was suggested as a further cause of many deaths (Movahedi 2005).

Figure 2.5 High‐resolution satellite imagery of the centre of Bam taken (left) before and (right) after earthquake, clearly indicating the extent of the damage.

Source: DigitalGlobe.

Figure 2.6 Failure of adobe dwelling in the Bam earthquake.

Source: World Housing Encyclopedia. Reproduced with permission of EERI.