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Pancake collapses tell story of poor construction

Each major earthquake disaster has its own patterns of structural failure. NCE reports on what has been learned so far in Gujarat.

LOWER FLOORS crushed pancake-like on top of one another while upper storeys remain intact are a common site in Bhuj city.

The 'pancake' effect of multistorey structures is the result of open plan ground floors with the upper storeys supported on exposed concrete columns.

'Soft storeys' - as these open plan areas are known - created a handy parking space for residents but the unbraced columns took most of the horizontal stress and failed almost instantly. In many cases the impact of the fall has overloaded the second and third floor columns, creating three layers of crushed concrete.

Professor Rakesh Goel of the Earthquake Engineering Research Institute in California has created a computer simulation of the structural failure. This shows that many second floor columns also buckled but most of the upper columns remain rigid and do not share horizontal stress, he says. Internal walls on these ground floors would have added shear capacity and spread the stresses more evenly to the higher floors.

In cases of complete collapse walls would not have helped because the connections of columns and beams gave way.

While buildings took the pounding of vertical forces in the first minute of the earthquake, column/beam connections were critically weakened and some columns were uprooted from their pad foundations.

During the second horizontal wave of seismic forces beams and columns came away from each other. Structural experts want to know why column and beam reinforcement was apparently not overlapped and no extra joint reinforcement seems to have been present.

Asymmetrical loading then caused many buildings to twist as they swayed from side to side and increased the torsional effect. Large dead loads such as water tanks on one side of buildings were common.

The collapse of one side of the nine year old Mansi Tower in Ahmedebad is being blamed on a private swimming pool installed on the top storey.

Substandard concrete crumbled as these forces did their worst. The pale colour of the chunks of concrete lying strewn around Bhuj testifies to the low cement content and high water/ cement ratio of the mix.

Concrete on multi-storeys under construction would often not be given the proper time to gain strength before the next floor was added, leading to cracking and deformities in the structure. Local architect Heman Twala tells of multistorey buildings completed in Ahmedebad in just two months.

Heavy concrete blocks of up to 40kg used in infill walls behaved independently under dynamic force, says Twala. 'This heavy masonry made structures non-monolithic so that when some blocks started falling the others would reject them.'

Skimping on column reinforcement also slashed safety margins. Locals tell of column rebars being removed after inspection and approval and sold on before concreting began.

Rogue builders also had a hand in the collapse of many of the better designed older structures. Some traditional earthquake proofing had been carried out on buildings built before the age of concrete but in many cases the coarse rock masonry came crashing down because modern floors were built on top without any investigation into the ability of the original structure to take the extra loads.

As the faults become clear arguments are raging as to whether more stringent design regulations promised by the authorities should be introduced (News last week). City engineer from nearby Surat, BM Desai, says that multi-storey buildings should be restricted to three storeys and a maximum of 12m.

Soft storeys should be outlawed.

Superintending engineer from Ahmedebad MM Jivani counters that buildings that collapsed would have stayed up had existing design codes been followed.

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