The Hindu 13.01.2011
29% of buildings in Chennai vulnerable to quake: survey
Study recommends “ground truth” verification of select buildings
A seismic hazard map for Chennai. — Photo: R. Shivaji Rao
A survey to assess seismic vulnerability of the built environment in
Chennai has classified 29 per cent of the buildings in the city as
highly vulnerable in an earthquake event.
The survey undertaken by the Centre for Disaster
Mitigation and Management (CDMM) at Anna University aerially mapped
22,758 medium-to-tall buildings in the city deploying the Rapid Visual
Screening technique.
The surveyed buildings included 11,224 three-storeyed
structures, 7,500 four-storeyed buildings and 2,053 buildings with five
floors or more.
Launched as part of a Government of India-funded project
to evolve a GIS information based e-governance model for vulnerability
assessment of Chennai, the study also recommended “ground truth”
(physical) verification of select buildings to confirm the extent of
vulnerability.
The study that also analysed soil types in the city
found that the clay and unconsolidated sediments were likely to cause
higher local site amplification of earthquake energy effects.
Such amplification areas were identified at Vyasarpadi,
Siruvallur, Otteri, Purasawalkam , Vepery, Egmore, Kolathur,
Villivakkam, Anna Nagar, Aynavaram, Koyambedu, Arumbakkam, Kodambakkam,
parts of T. Nagar, Taramani and parts of Velachery.
Recommendation
The soil liquefiable areas were mapped in Vyasarpadi,
Purusawakkam, Koyambedu, Villivakkam, Saligramam, T. Nagar and West
Mambalam. The study recommended soil improvement in these areas for
mitigation of seismic damage.
Buildings coming up in future could also incorporate the
amplification factor to realistically design appropriate earthquake
resistant features relevant to the varying levels of seismic hazards
across the city instead of uniformly following the coefficient for
Seismic Zone III, the study has recommended.
It is pointed out that while Chennai was re-designated
from Hazard II zone to Hazard III zone in 2001, several buildings that
came up prior to that year remain ill-designed to withstand earthquake
events. “As a result, even relatively moderate earthquake events can be
the source of huge socio-economic impact,” said S. Rajarathnam,
Director, CDMM, Anna University.
While the draft seismic hazard map for the city is ready
from the perspective of borehole data analysis and engineering
seismograph survey, the GIS-based e-governance model will be put
together after gathering inputs from various user departments, said A.
R. Santhakumar, CDMM consultant.
Surveyed buildings also include 11,224 three-storeyed structures, 7,500 four-storeyed buildings