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Implications of Drastic Upgradations in Code

 
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for_prof_arc at hotmai...
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PostPosted: Fri Dec 24, 2004 10:28 am    Post subject: Implications of Drastic Upgradations in Code Reply with quote

Dear Sefians,

Very recently I went through draft proposals for elevated water tanks which is described in EQ-08 of IITK-GSDMA.
First of all, I would like to thank and also express my appreciation to Prof. S K Jain for the excellent report and for making it available freely for discussion and comments.

To start with, I would quote one example - may be it is an extreme one from that report. [some more analysis may follow in my subsequent messages]. Please refer to example 3 on "elevated tank supported on RC Shaft". Art 3.5 gives Design Seismic Coefficient as 0.30. According to IS:1893-1984, this value would be 0.075 only. Thus a 400% increase is now recommended.

If indeed such very high values are finally adopted by BIS for implementation [ right now since BIS has not yet revised the code for elevated water tanks, it must be a free for all regarding how designers are implementing. may be a combination of 1893-1984 with modification of spectra of 1893-2002 Part I and using R=5], I feel it is INCUMBENT on the code to state that all their earlier recommendations are grossly in error and most such existing structures would now require RETRO-FITTING.

I would also like to appeal to the code writers to have a serious rethink before settling on very high values. I would like to point out that in case of Buildings, the revision kept this fact in mind and for RC Structures having ductility provisions, there was no change in Zone III and marginal changes were made in Zone IV & V which was more to correct the earlier imbalance. This was achieved by assigning a value of R=5. [ basically, R=5 was worked backwards in order to ensure seismic coefficient remains same as 0.04]
In case of elevated tanks also, the end result should be taken as a package rather than looking at its part like "I", "R", factor of "2", etc. This could result in a higher value of "R"  than presently proposed.

It would be a good idea to re-examine the behavior of such structures during actual events since 1962 since the code was first published. Art. 8.6 of the Japanese Research Report on Natural Disasters states" Many of the water tanks, which are elevated, did not suffer any substantial damage although severe damage could be observed at nearby buildings and structures". Photo 8-15 prominently shows a non-damage of elevated shaft tank among all other ruins surrounding it.  However, Chap. 15 of Earthquake Spectra edited by Prof. SK Jain indicates damages mainly to tanks on RC shaft. There is not much description of non-damages to water towers, though there must have been very many in places like Ahmedabad.

I feel that RC Shaft water tanks are more vulnerable than those with staging. The code should therefore discourage such constructions. Long long ago, Housner proposed bracing of staging. In fact in Chile earthquake of 1960 a number of water tanks with braces performed well. In the sixties, we at Roorkee proposed diagonal steel bracing for RC elevated water tanks and this was implemented by BHEL, Haridwar and IDPL, Rishikesh. There is no doubt that methods of improving ductility like provision of braces should be encouraged rather than increasing the seismic coefficient by several orders of magnitude.

If, however, it is felt that forces should indeed be increased by several orders of magnitude, we must warn all those, particularly where retro-fit work is currently being actively pursued about GSDMA EQ-08 provisions. In particular, Sefians at Delhi may alert the Delhi Jal Nigam who are currently engaged on retro-fitting more than 100 elevated water tanks.

Sincerely,

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