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Considerably high seismic demands of ESRs supported on

 
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ojaiswalvnit
SEFI Member
SEFI Member


Joined: 26 Jan 2003
Posts: 9

PostPosted: Fri Aug 20, 2004 9:15 am    Post subject: Considerably high seismic demands of ESRs supported on Reply with quote

Dear Rushikesh,

You have rightly pointed out that any structure

is expected to be designed for REASONABLE horizontal

seismic force.

Now question is what is this REASONABLE force

for shaft supports ?

Its not that we are penalizing shaft supports!,

rather their structural characteristic is deciding

the value of R.



The value of R essentially depends on ductility,

redundancy and over strength. The energy absorbing

capacity of a structure decides its value of R.

In buildings, non-structural components like brick

walls, significantly contribute to energy absorbing

capacity. In tanks with frame type staging we do not

have walls and hence it is reasonable to expect that

their energy absorbing capacity will be less than that

of buildings. Moreover, in frame type staging of

tanks, braces are generally of nominal sizes

(their sizes are smaller than sizes of beams in

buildings) and then we do not have any slab at brace

level. Thus, ductility characteristics of brace-column

joint of tank will be different than beam-column-slab

joint of building.

Based on these considerations, it is quite logical to

have lesser value of R for frame type tanks than

buildings.

The shaft supported tank looses the battle of R on two

additional fronts: Firstly it has much lower redundancy

(Say zero) and secondly its thickness is of the order

of 200 to 250 mm which can not provide much ductility.

Next, what is the meaning of

“Shafts have shown years of performance ?”.

We do not have the data to show that under design

seismic loads (Water full condition) shaft supported

tanks have been tested.

Seismic codes world over, specify 3 to 3.5 times

higher seismic force for shaft supports than for

ductile buildings.  

Yes, a 4 column staging has less redundancy that

a 8 column staging; and you expect that this should

get reflected in the value of R !!.

Now, question is how significantly R depends on

redundancy ? We do not expect R to be a linear

function of redundancy.

There are other contributors to R.

My feeling is that if redundancy changes from

zero to one then there will be more effect on R;

however, if redundancy changes from 10 to 15,

then effect on R may not be significant.




Jaiswal




rushikeshtrivedi@yahoo.com wrote:Dear Dr. Jaiswal,

As evident to all.. Let me make my view point clear.

Any structure can be expected and is accepted to be
designed for a reasonable horizontal earthquake force
that can be expressed in some percentage of its mass.

All seismic codes (for all structures)use
fundamentally,this same philosophy for arriving at
suitable seismic coefficient..

This coefficient.. I think is dependent on three
things
Importance
Location
Structural Properties..

We have for most ductile building, max value of this
coefficient as 13.5 % for R=5;I=1.5;Z=0.36 and Sa/g =
2.5... As per proposed draft for shaft supported tank,
we get Ah equal to 45 % of seismic mass... i.e. 3.33
times higher percentage of seismic mass..as compared
to the former one.

So.. in nutshell, u can make a code saying that dont
construct shaft supported tanks.. which have shown
years of performance.

May we expect the similar level of R values for
chimneys too.. in the future code ???

I do not know whether at the time of deciding R values
for buildings, the commitee had considered them to be
relative to other standards or relative to some other
structural configurations...

I design a tank supported on 4 columns or on 6 or 8
columns for same capacity.
Will all have same redundancy..?? No...

Still we have same R value for all tanks supported on
framed staging..any comments??

May I request Dr. S.K.Jain & Dr. C.V.R. Murty to
explain the basis
and procedure that was used to determine R values
for buildings that are used currently as per
IS 1893 (Part - I): 2002..

Were they really inspired from IBC/ACI/FEMA ???

Rushikesh
rushikesh@vmsconsultants.com


--- vmscons@eth.net wrote:

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