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sirish SEFI Member
Joined: 26 Jan 2003 Posts: 17
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Posted: Wed Dec 22, 2004 1:04 pm Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis |
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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 SEFI Member
Joined: 26 Jan 2003 Posts: 17
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Posted: Thu Dec 23, 2004 3:52 am Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis |
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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 ...
Joined: 26 Jan 2003 Posts: 215
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Posted: Thu Dec 23, 2004 4:16 am Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis |
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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 SEFI Member
Joined: 26 Jan 2003 Posts: 17
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Posted: Thu Dec 23, 2004 5:16 am Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis |
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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.4 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
Joined: 26 Jan 2003 Posts: 17
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Posted: Thu Dec 23, 2004 9:27 am Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis |
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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
Joined: 26 Jan 2003 Posts: 32
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Posted: Fri Dec 24, 2004 2:46 pm Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis |
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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 ...
Joined: 26 Jan 2003 Posts: 63
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Posted: Mon Dec 27, 2004 11:34 am Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis |
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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
Joined: 26 Jan 2003 Posts: 473
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Posted: Mon Dec 27, 2004 1:06 pm Post subject: IS:875-Part-3 Wind Code - Dynamic Analysis |
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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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