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Considerably high seismic demands of ESRs supported on shaft

 
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vmscons
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Joined: 26 Jan 2003
Posts: 16

PostPosted: Thu Aug 19, 2004 11:58 am    Post subject: Considerably high seismic demands of ESRs supported on shaft Reply with quote

----- Original Message -----
Message From  Sudhir K Jain <skjain@iitk.ac.in>
To: rushikesh <rushikesh@vmsconsultants.com>
Cc: Omprakash Jaiswal <ojaiswalvnit@yahoo.co.in>; <codes@iitk.ac.in>
Sent: Wednesday, August 18, 2004 5:47 PM
Subject: Re: Considerably high seismic demands of ESRs supported on shaft
asper proposed draft


Quote:
Rushikesh:

Are you aware that shaft supported tanks routinely fail even without
earthquakes!! Even recently, there seems to be one such failure in
Kutch district of a newly constructed shaft supoprted tank.

What would be the ductility in such cases?

And, if there are a lot of tanks that were done wrongly, should we
continue to make those mistakes for years to come?

Regards,

SKJ

--------------------------------------------------------------------------
Sudhir K. Jain
Professor
Department of Civil Engineering
Indian Institute of Technology Kanpur
Kanpur 208 016, India

Phone: 91-512-2597867 (off), 2598367/ 2590583(home)
Fax: 91-512-2597866 (preferred), 2597395
email: skjain@iitk.ac.in
--------------------------------------------------------------------------
-
Quote:



On Wed, 18 Aug 2004, rushikesh wrote:

Dear Sir,

I really appreciate the effort undertaken by Jaiswal et al (2003,2004)
for preparaion of
Quote:
exhaustive review of seismic codes on tanks and for proposal of draft
for IS 1893 (Part - II).
Quote:

I have attached the results of seismic analysis of an ESR (200 m3 cap)
proposed to be
Quote:
constructed at Bhuj with a shaft type staging.

In nutshell, the configuration of shaft/tank that satisfactorily worked
before as per IS 1893: 1984
Quote:
doesnt pass Proposed Draft criteria..

Kindly go through it and forward your comments.

Are there any methods / solutions available for making shaft type
staging ductile enough
Quote:
so as to reduce the level of forces for design purpose.

Cant we have more confined rings to the vertical steel in the bottom
region where there might be a
Quote:
plastic hinge formation during severe shaking.

We had a discussion on this issue and are currently working on comparing
many alternatives.
Quote:
The increased base moment has a great significance in proportioning of
foundation for shaft..
Quote:

Let me tell you, is it really rational to just decide R values based on
level of seismic forces used
Quote:
by other seismic codes...

We accept that tanks are less ductile..have little or no toughness.. and
poor energy dissipation characteristics..
Quote:
But the level of forces has treamendously increased by penalizing
shafts...
Quote:

If all the shafts have been designed to IS 1893 : 1984, all of them may
have been converted into debris.. if the real
Quote:
seismic demand of such structures would have been even near to that
specified by Proposed Draft...
Quote:
We do not (can not)  design structures to respond elastically..come on.

So as compared to ductile building frames (where we reduce by 10 ),
shafts seem to be criminals..(where it is
Quote:
proposed to reduce seismic forces to the best by only a factor of 3 )
... (i.e. 2*R)
Quote:

We really love shaft type stagings and cant digest this value of R... as
the forces may increase by several times...
Quote:
Rather than (in this manner) depopularize shaft supported tanks... dont
we need to develop good detailing
Quote:
practices and suitable configurations along with reduced (some what )
forces for shaft type stagings...
Quote:

Waiting for your feedback on this issue.

Thanking You,
With Best Regards,

Rushikesh
VMS Engineering & Design Services (P) Ltd.
Ahmedabad





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vmscons
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Joined: 26 Jan 2003
Posts: 16

PostPosted: Thu Aug 19, 2004 11:58 am    Post subject: Considerably high seismic demands of ESRs supported on shaft Reply with quote

----- Original Message -----
Message From  Omprakash Jaiswal
To: rushikesh ; codes@iitk.ac.in
Cc: Sudhir K Jain ; Omprakash Jaiswal
Sent: Thursday, August 19, 2004 3:43 PM
Subject: Re: Considerably high seismic demands of ESRs supported on shaft as per proposed draft


Dear Rushikesh,

We do not consider fereeboard in the seismic aanlysis. What I mean by partially filled tank is that at the time of Bhuj earthquake most of the tanks were either half filled or even less or were empty. Obvisously for a half filled tank, seismic forc es will be much less.
Shaft supported tanks are used in USA and other countries. ACI 371 (1998) is dedicated to pedestal (or shaft) supported tanks only!. This code also deals with seismic design aspects. It suggests value of R = 2 for these tanks as against a value of R = 8 for the most ductile building !!.
This implies that design seismic force is 4 times higher for these tanks; if importance factor is taken same.

The basis for arriving at R values is explained in Figure 25 of our review document.
The proposed values of R for shaft supported tank is less than that for tanks with frame type staging. This is quite logical, since shaft has less redundancy (in fact it statically determinate!!) and less ductility.
Even in IBC, for tanks with frame type staging R = 3.0 and that for shafts R = 2. We have also retained same ratio of R values in our code (2.25 Vs. 1.5).
Also think of irregularities in center-line of shaft due to its step-wise construction. Unless slip-form is used (like in chimney), it is difficult to ensure no eccentricity in the center-line. Moreover, this eccentricity varies with height in +ve and -ve direction. Some designers feel that this is the most dangerous aspect of shaft type tanks.

Regards,

Jaiswal

rushikesh <vmscons@eth.net> wrote:
Dear Sir,

No tank which serves us can be completely filled with water.
Each Tank must have some free board and it really has throughout its lifetime.
Hence, SLOSHING is definitely going to occur and we have no doubt about that.

Introduction of Two Mass Model is really a step towards realistic seismic analysis
of liquid storage tanks. But to decide about Response Reduction Factors, I can
not digest the methodology proposed in Jaiswal et. al. (2003, 2004) that IS 1893
(Part - II) should provide for level of forces which are at par with IBC.

As noted by you in same publication that very few references are available on
elevated tanks... which are most common in our region. Many codes donot
have elaborate specifications on ESR supported on shaft.

The tanks which have collapsed were really of poor quality/ workmanship.
The tank that recently collapsed as referred to by you, did not fail in earthquake.

If shafts are really bad structural members for earthquake loads, many tanks could not
function in Ahmedabad too. And for the example sited for one such tank, failure
of single or few tanks can not justify lowest response reduction factors proposed.

Many tanks are still functioning in Gujarat and other parts of the country.
Even in the regions affected by Bhuj Earthquake, Many SHAFT SUPPORTED Tanks
continue to function.Reasonable increase in design seismic forces should be adopted
for shaft supported tanks.

Response Reduction Factors marginally affect the economy of projects.. and much
detailed study, investigation need to be presented before deciding their values.

May I have background for decision of R values for buildings in IS 1893 (Part - I) : 2002 ?

I suggest you to include design of foundation (atleast proportioning) for shaft supported
tank which is given in explainatory handbook published along with proposed draft code.

Regards,
Rushikesh

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