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Response reduction factor R

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JMSSNL at ltecd.ltindi...

PostPosted: Wed Oct 15, 2003 10:37 am    Post subject: Response reduction factor R Reply with quote

Dear All,

While as I agree with Prof. Chandrasekaran's point of allowing larger
'R' value for steel structures, I will like to draw attention to
mismatch between IS:800 and IS:1893 in R values.

1. Ordinary moment frames (OMF) per IS:1893 (Table 7 item (1)  i) is
3.0. Please read with foot note 20 which states frames detailed per
IS:456 or IS:800 , there by meaning this is applicable for steel frames

'R' value for similar frame is provided as 4.0 in Table 12.1 Sr no. 2
a) in IS:800 draft.

While as it is understandable that for steel frame 4.0 is more logical
value ,per reason cited by Prof. Chandrasekaran below, the appropriate
place to define it is IS:1893 using which an engineer determines seismic

Since, IS:1893 is already published , it makes sense to eliminate table
12.1 from IS:800 draft



for_prof_arc@hotmail.com 10/06/03 06:45PM >>>
SEFI e-Conference on draft revision of  IS:800, October 06 to 18, 2003

Dear Prof. Kalyanraman,

First of all, I would like to congratulate you for opening discussions
on draft code IS:800 via web. I do hope such procedure would be followed
for other draft codes also which are of interest to members of this
forum. Such efforts are all the more necessary as BIS does not respond
to requests for drafts meant for wide circulation even to persons like
me who were associated with BIS for a very long time.

I feel that IS:800 should have explicit provisions for earthquake
effects as IS:1893 is primarily meant for RCC structures. In particular,
the punitive clauses of IS:1893 should not be applied to steel
structures. The empirical formulae for fundamental period [which is used
for defining minimum base shear] is NOT applicable to steel
structures.The modelling of steel structures could be far more accurate
than RCC structure as the properties of section [EA,EI,GJ]can be
accurately determined. Further, it is likely that steel structures would
be "an engineered construction" and therefore let the forces as obtained
from analysis be taken for design without imposing arbitrary penalties.

There is also a case for larger reduction factors for steel structures
as compared to concrete construction !!
The reduction factor of 5 is mostly unachievable in RCC whereas it
could be achieved in steel. Hence to have overall
effective seismic factor [a combination of Z,I,R,T] of the same order,
R for steel structure could be larger, say, 5 & 6
instead of 4 & 5.

Some attention may also be given in composite construction in which the
vertical elements are mainly steel or steel encased with concrete.

A R Chandrasekaran

The views, opinions, analyses and assessments contained herein do not
necessarily reflect the views of SEFI. Also SEFI does not guarantee the
accuracy of the data included in the proceedings of this e-conference
and accepts no responsibility for any consequences of their use.
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