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Daily Digest Thu Dec 23 23:00:03 2004

 
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adutt
Progressive Member
Progressive Member


Joined: 26 Jan 2003
Posts: 43

PostPosted: Mon Dec 27, 2004 5:13 pm    Post subject: Daily Digest Thu Dec 23 23:00:03 2004 Reply with quote

Subject_Wind Induced Oscillations

Dear Sirish,
Please refer clause 7.1 of IS 875 (Part -3-Wind Code),you
will find that this clause gives a height/width criteria & also giving
that if the  natural frequency of the structure is below 1 Hz (My
interpretation is that if  the structure is tall & slender )wind induced
oscillations will have to be taken into consideration.

With Regards
Amit Dutt-Structural Engineer


-----Original Message-----
Message From  general@sefindia.org [mailto:general@sefindia.org]
Sent: Friday, December 24, 2004 4:30 AM
To: Amit Dutt
Subject: Daily Digest Thu Dec 23 23:00:03 2004

----------------------------------------------------------
Message From  sirish@lntecc.com
Subject: IS:875-Part-3 Wind Code - Dynamic Analysis
Date: 23/12/04
Time: 09:22:20
----------------------------------------------------------
Dear All,

In IS:875(Part-3)-1987 under Dynamic Effects (p.47), while describing
the applicability of the dynamic wind analysis, it has been mentioned in
Note:2  that
If preliminary studies indicate that wind-induced oscillations are
likely to be significant, investigations should be persuade with the aid
of analytical methods or, if necessary, by means of wind tunnel tests on
models

But, my question is how can we prove that wind-induced oscillations
are significant.. Are there any procedures to establish that???  I
mean, by any chance, IS:875 (or any other IS code) has made any
mentioning about that?  Like, one can check for some parameters and, if
they cross some limits, wind-induced oscillations are significant...

And also, all the major international wind codes say that their methods
are not applicable if the buildings are suceptible to dynamic
excitation.

Is it must to do some local (on-site) analysis to find out the same?  In
specific, if I want to construct, a 50-story building in Banglore, what
precautions, I must take to ensure that all dynamic effects due to wind
are properly taken care... (My assumption is that Wind Tunnel study is
very costly to afford)

Regards
Sirish, Bellana


----------------------------------------------------------

----------------------------------------------------------
Message From  rudra_nevatia@yahoo.com
Subject: IS:875-Part-3 Wind Code - Dynamic Analysis
Date: 23/12/04
Time: 09:46:36
----------------------------------------------------------
Dear Sirish,

Susceptibility of buildings to dynamic loads arises on
account of cross wind vibrations caused by vortex
shedding and along wind vibrations caused by gust.

Cross wind vibrations will dominate if the frequency
of  vortex shedding given in Cl. 7.2.1 is close to the
natural frequency of the building.

Along wind load due to gust is calculated by Cl. 8.3
as Fz and should be compared with the static wind
force given by Section 6.

Hope that helps.

Regards,
Rudra Nevatia



Hope that
--- sirish@lntecc.com wrote:

Dear All,

In IS:875(Part-3)-1987 under Dynamic Effects (p.47),
while describing the applicability of the dynamic
wind analysis, it has been mentioned in Note:2  that
If preliminary studies indicate that wind-induced
oscillations are likely to be significant,
investigations should be persuade with the aid of
analytical methods or, if necessary, by means of
wind tunnel tests on models

But, my question is how can we prove that
wind-induced oscillations are significant.. Are
there any procedures to establish that???  I mean,
by any chance, IS:875 (or any other IS code) has
made any mentioning about that?  Like, one can check
for some parameters and, if they cross some limits,
wind-induced oscillations are significant...

And also, all the major international wind codes say
that their methods are not applicable if the
buildings are suceptible to dynamic excitation.

Is it must to do some local (on-site) analysis to
find out the same?  In specific, if I want to
construct, a 50-story building in Banglore, what
precautions, I must take to ensure that all dynamic
effects due to wind are properly taken care... (My
assumption is that Wind Tunnel study is very costly
to afford)

Regards

Sirish, Bellana




Structural Engineers
Forum of India










          
__________________________________

Send holiday email and support a worthy cause. Do good.
http://celebrity.mail.yahoo.com


----------------------------------------------------------

----------------------------------------------------------
Message From  kalgal_mr@yahoo.com
Subject: Daily Digest Wed Dec 22 23:00:02 2004
Date: 23/12/04
Time: 10:21:49
----------------------------------------------------------
Dear SEFIANS
This has reference to the questions posed by Mr. Singh for budding
engineers and the subsequent rplied Mr. James et. al. Divergent
answers can be expected from even experienced engineers.
There is a mention in the question  that the concrete is of grade
M30/M40 and that the slump specified ; is 125mm +/- 25mm. If an
upper limit on slump was specified it might not be without reason. If
the mix is not cohesive enough, higher slump might cause segregation or
bleeding or both. If higher slump is due to excessive water then
strength will also be less.
So, if site engineer is to strictly follow the specifications he has to
reject the truck. The engineer might be held responsible for NOT
following specifications and the consequent problems that might arise.
Since the RMC people are paid for sticking to specifications, there is
no need to follow the practice of thoda adjust karo!

Merry Christmas to all in advance!

regards
Dr. M. R. Kalgal
General Manager - Tech.
BBR(India) Ltd.

          
---------------------------------

Yahoo! Mail - Find what you need with new enhanced search. Learn more.
----------------------------------------------------------

----------------------------------------------------------
Message From  sirish@lntecc.com
Subject: IS:875-Part-3 Wind Code - Dynamic
Analysis
Date: 23/12/04
Time: 10:46:50
----------------------------------------------------------
Dear Mr. Rudra,

The hitch that we are facing is, how to calculate the frequncy of vortex
shedding with the available information in code.  We have very raw
parameters that the building is in Banglore (or some other city).  We
have 100% control on the structural behavior of the building, like its
natural frequencies, mode shapes etc etc.,  

Whether the shedding frequency (Cl 7.2.1 p.4Cool can be helpful?? Can we
conclude that the structure is safe, if the building natural frequency
is greater than the shedding frequency??

We have done the wind load calculations both in static and dynamic cases
also with in-house developed specialized software.  We like to ensure
that the building will perform well under wind, if the building is
shifted from Banglore (Vb=33m/s) to Chennai (Vb=50m/s).

Regards,
Sirish

-----Original Message-----
Message From  rudra_nevatia@yahoo.com [mailto:rudra_nevatia@yahoo.com]
Sent: Thursday, December 23, 2004 3:17 PM
To: Sirish Bellana
Subject: IS:875-Part-3 Wind Code - Dynamic Analysis


Dear Sirish,

Susceptibility of buildings to dynamic loads arises on
account of cross wind vibrations caused by vortex
shedding and along wind vibrations caused by gust.

Cross wind vibrations will dominate if the frequency
of  vortex shedding given in Cl. 7.2.1 is close to the
natural frequency of the building.

Along wind load due to gust is calculated by Cl. 8.3
as Fz and should be compared with the static wind
force given by Section 6.

Hope that helps.

Regards,
Rudra Nevatia


----------------------------------------------------------

----------------------------------------------------------
Message From  rudra_nevatia@yahoo.com
Subject: IS:875-Part-3 Wind Code -
Dynamic    Analysis
Date: 23/12/04
Time: 13:46:40
----------------------------------------------------------
Dear Sirish,

My understanding of the subject differs from that
implied by the equation given in Cl. 7.2.1. Rather
than calculating vortex shedding frequency, one has to
calculate the critical velocity Vcr corresponding to
the first and in rare cases to the second mode of
vibration of the building. If the critical velocity
turns out to be less than design wind velocity,
cross-wind vibrations are possible. Again, wheather
resonance will occur at Vcr depends on Scruton number.

I think this is a very important topic and should be
discussed threadbare.

For those on the list not familiar with Indian codes,
may I point out that IS:875 being discussed here is
very similar to CP3.

Regards,
Rudra Nevatia

--- sirish@lntecc.com wrote:

Dear Mr. Rudra,

The hitch that we are facing is, how to calculate
the frequncy of vortex shedding with the available
information in code.  We have very raw parameters
that the building is in Banglore (or some other
city).  We have 100% control on the structural
behavior of the building, like its natural
frequencies, mode shapes etc etc.,  

Whether the shedding frequency (Cl 7.2.1 p.4Cool can
be helpful?? Can we conclude that the structure is
safe, if the building natural frequency is greater
than the shedding frequency??

We have done the wind load calculations both in
static and dynamic cases also with in-house
developed specialized software.  We like to ensure
that the building will perform well under wind, if
the building is shifted from Banglore (Vb=33m/s) to
Chennai (Vb=50m/s).

Regards,
Sirish

-----Original Message-----
Message From  rudra_nevatia@yahoo.com
[mailto:rudra_nevatia@yahoo.com]
Sent: Thursday, December 23, 2004 3:17 PM
To: Sirish Bellana
Subject: IS:875-Part-3 Wind Code -
Dynamic Analysis


Dear Sirish,

Susceptibility of buildings to dynamic loads arises
on
account of cross wind vibrations caused by vortex
shedding and along wind vibrations caused by gust.

Cross wind vibrations will dominate if the frequency
of  vortex shedding given in Cl. 7.2.1 is close to
the
natural frequency of the building.

Along wind load due to gust is calculated by Cl. 8.3
as Fz and should be compared with the static wind
force given by Section 6.

Hope that helps.

Regards,
Rudra Nevatia




Structural Engineers
Forum of India










          
__________________________________

Send holiday email and support a worthy cause. Do good.
http://celebrity.mail.yahoo.com


----------------------------------------------------------

----------------------------------------------------------
Message From  sirish@lntecc.com
Subject: IS:875-Part-3 Wind Code - Dynamic Analysis
Date: 23/12/04
Time: 14:57:02
----------------------------------------------------------
For the dynamic analysis portion, IS-875 strictly followed Australian
Code (AS 1170.2).  When we are checking the equations given in the
australian code, the results are almost similar to IS-875.  Our checking
is still going on..

Sirish

-----Original Message-----
Message From  rudra_nevatia@yahoo.com [mailto:rudra_nevatia@yahoo.com]
Sent: Thursday, December 23, 2004 7:17 PM
To: Sirish Bellana
Subject: IS:875-Part-3 Wind Code
- Dynamic Analysis


Dear Sirish,

My understanding of the subject differs from that
implied by the equation given in Cl. 7.2.1. Rather
than calculating vortex shedding frequency, one has to
calculate the critical velocity Vcr corresponding to
the first and in rare cases to the second mode of
vibration of the building. If the critical velocity
turns out to be less than design wind velocity,
cross-wind vibrations are possible. Again, wheather
resonance will occur at Vcr depends on Scruton number.

I think this is a very important topic and should be
discussed threadbare.

For those on the list not familiar with Indian codes,
may I point out that IS:875 being discussed here is
very similar to CP3.

Regards,
Rudra Nevatia


----------------------------------------------------------

----------------------------------------------------------
Message From  aneesh@vsnl.net
Subject: Daily Digest Wed Dec 22 23:00:02 2004
Date: 23/12/04
Time: 15:11:11

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