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Proper finite element model for R.C elevated water tanks on shaft staging

 
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Pronoy Roy Chowdhury
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Joined: 06 Feb 2010
Posts: 3

PostPosted: Tue Jan 15, 2019 9:02 am    Post subject: Proper finite element model for R.C elevated water tanks on shaft staging Reply with quote

My fellow Engineers at SEFI, I work as an Executive Engineer at the Public Health Engineering Department Govt. of W.B. During my tenure I have designed and executed many water elevated water tanks as per prevailing BIS codes of practice. However It has always appeared to me that the codal provisions of IS-11682 is quiet adhoc regarding design of shafts staging. The formula of thickness of shaft, 150+(D-6000)/120, does not include any expression for load neither takes into account the r/t ratio into consideration. The formula was loosely adopted from older version of R.C chimney code. The stress calculation also uses the formula associated with the chimney. But essentially  the elevated water tank is an inverted pendulum structure heavily loaded at the top not much like chimney. These structures are mostly designed as per lump sum tenders and the drawings are vetted by third party. So the country has wide range of population and the consultants have very varied range of ideas. Sometimes only buckling check is done as per shell code, while at other cases the shaft is designed as cantilever subjected to axial compression and lateral load due to wind and earthquakes. Thus is the shaft a column or shell. Should it be modelled as a hollow cantilever beam element under flexural action due to later load or is it a shell which is only subjected to membrane forces, which should be proper. The formula that we have for initial thickness should it be revisited and a proper empirical formula for initial thickness of shaft should be devised. I have read papers by Sir U.H Varyani, who has opined that the shaft is a column, the minimum reinforcement in column is 0.8%, but the code recommends only 0.25%, but is the shaft not a column supporting the tank container. The famous book learning from failures by Sir, R.N Raikar indicates that the shafts are very prone to collapse so it must be properly designed. in 2001 bhuj earthquake many shaft supported tanks failed so such tanks are very less in use now a days but the structure is sleek and easily constructed with slipform and faster than beam column staging. thus my request to all kindly enlighten me as which shall be the proper design model and if is possible to generate a rational formula for the thickness of the shaft.
The book, theory of elastic stability by Timoshenko gives some input on the behaviour of thin shells under axial compression, but the results are fro metallic shells not much result is available on concrete shells which are again cast at the site. So is there any available data which could taken into account, regarding the behaviour of concrete thin shells under axial compression, then the same could act as a guide. kindly help me out in this regard. Thanks to all
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uhvaryani
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Joined: 21 May 2008
Posts: 250

PostPosted: Sat Jan 19, 2019 7:30 am    Post subject: Proper finite element model for R.C elevated water tanks on shaft staging Reply with quote

Dear Er.Pronoy Roy Chowdhury,    The above subject is of my liking.There is a need for a more rational formula for the thickness of the staging shaft.It should take into account not only the global
buckling of the structure but also the local buckling of the thin wall of the staging shaft.For steel chimneys,IS code gives formula for the thickness of chimney circular wall.On the same lines,one can develop formulae for concrete thin/thick
wall of staging shafts.
    With the use of computer programs based on finite element method,we should be able to design these structures safely.I request senior members of the FORUM
to help clarify this matter.
with best wishes and regards,
uhvaryani
    

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