www.sefindia.org

[rni]

A critique of hazard estimation as perpetrated by IS-1893

The way the seismic zones have changed for the country and the PGA values
are fixed by the code has been beyond logic and scientific thinking.  I
can understand, initially driven by the myth of stable peninsular India
and good geology being misunderstood for seismology, some mistakes had
been done.  But in later years when, earthquake engineering became an
independent discipline and engineering seismology was given its due place,
why our code makers have publicly exhibited their ignorance of the subject
and made only knee jerk reactions in mapping the seismic hazard of the
country?  In these days of globalization is code-harmonization only a
subject for workshops and seminars and not for practice within the
country?  Structural engineers should wake up from their slumber about
seismic zones of IS-1893-2002. Please see the three previous code maps
for sake of comparison. How is it after an earthquake only, the zones
change? SE's would agree, their designs depend sensitively on the force
estimation, which in turn depends on the basic PGA and/or Spectral
acceleration levels. If these are underestimated by the code the design
would be unsafe and if over estimated the owner pays through his nose.  
Hearing too often, that India has an international level building code,
the SE community has been lulled to a level of complacency.  But, try to
see how the present four zones can be made to compare with the zones given
in UBC-97.  At the level of the definition itself, IS-1893 breaks down.  
Why are we not using return period concepts, which are used
internationally (IBC-2003)?  What about sites that are very near active
faults ?  Does a strong earthquake cause only ground vibration, so that
SRSS is good enough always?  Should not the present code be overhauled to
address the above and many other questions including issues about soil
amplification?  I will add some more after seeing the response.
(note: I had attached the previous zone maps. But the econf.site did not
accept the attachments).
RN Iyengar

Posted via Email

[Rudra Nevatia]

Prof. Iyengar has raised a very pertinent issue. We all are moving towards
a regime of accountability. Just this fact that the Supreme Court is reviewing
its ruling on criminal liabilty of medicos should jolt us out of stupor.

Codes of practice have a legal status and as such need to be drafted very
carefully. Each word has to be weighed so that there is no room for factual
error or misinterpretation.


Rudra Nevatia

--- rni@civil.iisc.ernet.in wrote: