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8/3/2019 Query on Dynamic Load as Per is 1893
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Fwd: [SEFI] Re: Query on dynamic load as
per IS 1893
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prasad barve [email protected] to me show details Aug
27
---------- Forwarded message ----------
From: ishacon
Date: Tue, Jul 5, 2011 at 8:34 AM
Subject: [SEFI] Re: Query on dynamic load as per IS 1893
To:[email protected]
Dear All,
This query was posted as early as March 2003 by me and it was
suitably replied by
Mr. Rajeev Sharma of CSI on 16 March 2003, Ms Alpa after due
discussions with Dr A.S.
Arya
on 16 March 2003 also , who all agreed as under :
The specific issue to be noted is that research has shown that
appendages do
attract very high seismic shear whuich can not be computed by
conventional
Equivalent lateral load methods since the buiklding part above
terrace level has much
smaller
stiffness than that of the base which gives rise to vertical
irregularity.
Remember thatonly the support system of the appendages needs to
be analysed & designed
for
stability for 5 times the seismic coefficient.However the main
building it is sufficient to model
the weight of appendage at roof level andtake height only upto
terracelevel.
However there was one dissenting statement from Prof. A.R.
Chandrasekaran who opinedon 18 march 2003 that this punitive clause
was introduced in earlier codes to discourage
such projections.
He poiinted out that since most MS buildings are now analysed
using sophisticated computer
models, these
appendages should be modelled with the main building and then no
additional forces are
needed to be considered.
My take is that if the appendage constitutes only 10 to 15% of
the roof area, create a separate
model
for this above roof level and design the columns or stub columns
alongwith connecting beams
above terracefor 5 times the lumped mass. Ensure that main
building columhns reinf required extends up to
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8/3/2019 Query on Dynamic Load as Per is 1893
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midheight of the
floor below roof.
I also agree with Vipul Ahuja about UBC and present IBC
requiremennts are identical :
"Having said that for high-rise buildings, the base shear coeff
turns out to be less than even
0.02; and 5 times that is only 0.1.I know that long back as per
UBC the minimum lateral force for non-structural components,
the lateral force used to be 0.3."
I hope all respected professors would agree with stetements made
above.
If not, kindly elucidate.
Regards,
V.P. Agarwal
ISHA CONSULTANTS (P) LTDCHHATTARPUR,
NEW DELHI 110 074
PH : 011-26301158 ;
093 1345 2180 ; 098 6826 2759
Quote:
-- Original Message --
From: Kumar_Abhishek_Singh ([email protected])
To:[email protected] ([email protected])
Sent: Saturday, July 02, 2011 1:57 PM
Subject: [SEFI] Query on dynamic load as per IS 1893
Dear all,
As per clause 7.12.2.2 of IS 1893 which says "Tower, tank,
parapets, smoke stacks
and other vertical cantilever projections attached to buildings
and projecting above
the roof, shall be designed and checked for stability for five
times the design
horizontal seismic coefficient Ah specified in 6.4.2. In
analysis of the building, the
weight of these projecting elements will be lumped with the roof
weight."
Pertaining to this clause my queries are mentioned below
seriatim:
1. How do we interpret and apply this clause to STAAD/ETABs
model?
2. If we are modelling the entire building with full accuracy,
that is upto terrace as
well as all the projecting elements above terrace are exactly
modelled in softwarethen do we need to lump the weight of the
projecting elements (above terrace),
along with the terrace?
3. In case of high rise buildings these projections, inclusive
are part of a framed
structure in which the columns erect from shear walls coming
from the foundation.
Since these water tanks, elevation features etc. all are framed
and have considerable
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8/3/2019 Query on Dynamic Load as Per is 1893
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stiffness (for eg, water tank walls provide a lot of stiffness)
can they actually be
considered as "cantilevered" and thereby moving independently
and actually "fixed"
to the terrace?
4. Suppose that we are somehow able to model the earthquake for
this part as 5
times, then what about it's effect onto the building portion
below? Suppose we havea 30 storey building, then practically the
plans, sections and other drawings of the
cantilevered projections above terrace are finalised by the time
we reach 25th floor.
Now if I increase the earthquake of this part (by exactly
modelling it in software),
what about it's effect on the lower part of the building, as the
lower part has been
casted?
5. Suppose, as per cluase, we lump this mass with the terrace
floor, then do we need
to increase the seismic coefficient by 5 times for terrace
only?
6. Finally, for such a clause to apply, what is the height of
the building to be taken
for calculation of time period- height upto terrace or height
upto the top of the
cantilevered projection?
Kindly advise...
Thank you,
Kumar Abhishek Singh
Deputy Manager-Structures, DLF Ltd.
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