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IS:875-Part-3 Wind Code - Dynamic Analysis

 
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sirish
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Joined: 26 Jan 2003
Posts: 17

PostPosted: Wed Dec 22, 2004 1:04 pm    Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis Reply with quote

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

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sirish
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Joined: 26 Jan 2003
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PostPosted: Thu Dec 23, 2004 3:52 am    Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis Reply with quote

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

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Rudra Nevatia
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...


Joined: 26 Jan 2003
Posts: 207
Location: Mumbai

PostPosted: Thu Dec 23, 2004 4:16 am    Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis Reply with quote

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:

Quote:
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














          
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sirish
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Joined: 26 Jan 2003
Posts: 17

PostPosted: Thu Dec 23, 2004 5:16 am    Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis Reply with quote

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

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sirish
SEFI Member
SEFI Member


Joined: 26 Jan 2003
Posts: 17

PostPosted: Thu Dec 23, 2004 9:27 am    Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis Reply with quote

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

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James_Cohen
SEFI Regulars
SEFI Regulars


Joined: 26 Jan 2003
Posts: 31

PostPosted: Fri Dec 24, 2004 2:46 pm    Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis Reply with quote

Before deciding that a wind tunnel study is cost-prohibitive, check with one
of the wind tunnel testing firms. Cermak Peterka Petersen is a good firm in
the United States. Their website is found at www.cppwind.com

You may also wish to post your question to wind_haz_mit@yahoogroups.com
which I moderate. You will first need to subscribe to the group which can be
done at wind_haz_mit-subscribe@yahoogroups.com. Please include an
identifying message in your request.

I hope this helps.

James Cohen, PE
James Cohen Consulting, PC
http://expertpages.com/jccpc

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acpl_sefi
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...


Joined: 26 Jan 2003
Posts: 63

PostPosted: Mon Dec 27, 2004 11:34 am    Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis Reply with quote

IIT Delhi & SERC Chennai also do wind tunnel testing. A small wind tunnel test recently cost us about Rs. 3.5 Lakh ($7000).

Vipul Ahuja  


On Fri, 24 Dec 2004 jccpc@msn.com wrote :
[quote]Before deciding that a wind tunnel study is cost-prohibitive, check with one
of the wind tunnel testing firms. Cermak Peterka Petersen is a good firm in
the United States. Their website is found at www.cppwind.com

You may also wish to post your question to wind_haz_mit@yahoogroups.com
which I moderate. You will first need to subscribe to the group which can be
done at wind_haz_mit-subscribe@yahoogroups.com. Please include an
identifying message in your request.

I hope this helps.

James Cohen, PE
James Cohen Consulting, PC
http://expertpages.com/jccpc

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sdec.in
Silver Sponsor
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Joined: 26 Jan 2003
Posts: 473

PostPosted: Mon Dec 27, 2004 1:06 pm    Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis Reply with quote

IITK also has extensive facilities for wind tunnel tests and more details
can be obtained from Dr Sudhir Jain.
Sangeeta Wij
----- Original Message -----
Message From  <jccpc@msn.com>
To: <sdec@bol.net.in>
Sent: Friday, December 24, 2004 8:20 PM
Subject: IS:875-Part-3 Wind Code - Dynamic Analysis


[quote]Before deciding that a wind tunnel study is cost-prohibitive, check with
one
of the wind tunnel testing firms. Cermak Peterka Petersen is a good firm
in
the United States. Their website is found at www.cppwind.com

You may also wish to post your question to wind_haz_mit@yahoogroups.com
which I moderate. You will first need to subscribe to the group which can
be
done at wind_haz_mit-subscribe@yahoogroups.com. Please include an
identifying message in your request.

I hope this helps.

James Cohen, PE
James Cohen Consulting, PC
http://expertpages.com/jccpc

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