[SEFI] Zero point accelaration and missing mass correction
281 views
Skip to first unread message
SUKANTA.ADHIKARI
May 5, 2008, 1:17:38 AM5/5/08
to gen...@sefindia.org
Dear Sefians,
Can someone explain how to corelate Zero point accelaration and missing mass correction and please explain zero point accelaration in elaborately.I know that the time period in case of zero point accelaration is zero but practically 0.03 sec.But I want to know the significance
Regards,
S.Adhikari
-------------------- m2f --------------------
Structural Engineers Forum of India
Read this topic online here:
http://sefindia.org/forum/viewtopic.php?p=7351#7351
-------------------- m2f --------------------
Can someone explain how to corelate Zero point accelaration and missing mass correction and please explain zero point accelaration in elaborately.I know that the time period in case of zero point accelaration is zero but practically 0.03 sec.But I want to know the significance
Regards,
S.Adhikari
-------------------- m2f --------------------
Structural Engineers Forum of India
Read this topic online here:
http://sefindia.org/forum/viewtopic.php?p=7351#7351
-------------------- m2f --------------------
scpatel_74
May 5, 2008, 3:17:41 AM5/5/08
to gen...@sefindia.org
Hi..
ZPA is called as 'Zero period accln.' Practically it is
accln. corresponding to 33Hz frequency (i.e. T= 0.03 s) on response spectrum curve.
ZPA is equal to the peak ground acceleration observed at a given site or as suggested by codes for a given region. When a structure is almost rigid, the response of the structure do not change to ground accln. and its response is equal to given input motion. So peak accln. of groud motion time history becomes the peak accln. anywhere in the rigid structure (i.e. ZPA=PGA for rigid structure).
ZPA has no direct relation with missing mass. Missing mass is the
summation of fraction of masses not excited for every DOF, upto modes considered in dynamic analysis.
Say if we have considered 10 modes for dynamic analysis. If cummulative mass participated is 85%, the remaining 15% of total mass is called as "Missing Mass'.Response of structure for above 10 mode is incomplete without response due to missing masses. Hence we need to give Missing mass correction. Internally software find out missing mass vector (missing mass for every DOF). Multiplying it with Response spectrum accln (Sa/g) for time period corresponding to last mode gives the force vector. The response of structure to this force vector is the contribution of missing mass. This is suitably added to response due to ten modes.
IS code requires that dynamic analysis should be carried out such that cummulative mass participation is atleast 90% of the total
mass.
For durther details, Book by A.K.Chopra or Clough & penzien is very good material.
I hope this clears the querry. Should need arise, i'll be happy to elaborate more...
Regards,
Sanjay Patel
E&D TICB, Mumbai
ZPA is called as 'Zero period accln.' Practically it is
accln. corresponding to 33Hz frequency (i.e. T= 0.03 s) on response spectrum curve.
ZPA is equal to the peak ground acceleration observed at a given site or as suggested by codes for a given region. When a structure is almost rigid, the response of the structure do not change to ground accln. and its response is equal to given input motion. So peak accln. of groud motion time history becomes the peak accln. anywhere in the rigid structure (i.e. ZPA=PGA for rigid structure).
ZPA has no direct relation with missing mass. Missing mass is the
summation of fraction of masses not excited for every DOF, upto modes considered in dynamic analysis.
Say if we have considered 10 modes for dynamic analysis. If cummulative mass participated is 85%, the remaining 15% of total mass is called as "Missing Mass'.Response of structure for above 10 mode is incomplete without response due to missing masses. Hence we need to give Missing mass correction. Internally software find out missing mass vector (missing mass for every DOF). Multiplying it with Response spectrum accln (Sa/g) for time period corresponding to last mode gives the force vector. The response of structure to this force vector is the contribution of missing mass. This is suitably added to response due to ten modes.
IS code requires that dynamic analysis should be carried out such that cummulative mass participation is atleast 90% of the total
mass.
For durther details, Book by A.K.Chopra or Clough & penzien is very good material.
I hope this clears the querry. Should need arise, i'll be happy to elaborate more...
Regards,
Sanjay Patel
E&D TICB, Mumbai
| Quote: |
SUKANTA.ADHIKARI
May 5, 2008, 6:52:20 AM5/5/08
to gen...@sefindia.org
Dear Sanjay Patel,
Thanks for your reply.I have raised this question because while doing response spectrum analysis in STAAD I came accross the same, the syntax is as follows,
STAAD SYNTAX
SPECTRUM CQC 1893 X 0.04 ACC SCALE 1 DAMP 0.05 LIN MIS ZPA 33
Can someone explain this.
Regards,
S.Adhikari
Thanks for your reply.I have raised this question because while doing response spectrum analysis in STAAD I came accross the same, the syntax is as follows,
STAAD SYNTAX
SPECTRUM CQC 1893 X 0.04 ACC SCALE 1 DAMP 0.05 LIN MIS ZPA 33
Can someone explain this.
Regards,
S.Adhikari
-------------------- m2f --------------------
Structural Engineers Forum of India
Read this topic online here:
nicee
May 6, 2008, 6:16:27 AM5/6/08
to gen...@sefindia.org
Dear Colleagues,
More than a year back, NICEE started its quarterly periodical
"Earthquake Engineering Practice" to disseminate current research and
state-of-the-art in earthquake engineering to professionals and
academics in developing countries. The periodical consists of articles
of wide interest already published in reputed journals. Contents of the
past issues may be viewed at http://www.nicee.org/EEPcontents.php .
In case you are not already a subscriber to the periodical, you are
welcome to subscribe free-of-charge by visiting the web site:
http://www.nicee.org/EEP.php .
You may also like to explore if your organization / library can
subscribe a paid copy.
The Periodical is free-of-charge for individuals and has a very nominal
charge to the organizations/libraries (Rs 1000/- per year in India, and
Rs 2000/- per year overseas). Further, it is not distributed in USA/ Canada.
The periodical has received tremendous response, with about 1,850
subscriptions already. We look forward to expanding the base of its
subscribers to further improve the effectiveness of the periodical
towards seismic risk reduction.
With best regards,
Sudhir K Jain
Coordinator, NICEE
More than a year back, NICEE started its quarterly periodical
"Earthquake Engineering Practice" to disseminate current research and
state-of-the-art in earthquake engineering to professionals and
academics in developing countries. The periodical consists of articles
of wide interest already published in reputed journals. Contents of the
past issues may be viewed at http://www.nicee.org/EEPcontents.php .
In case you are not already a subscriber to the periodical, you are
welcome to subscribe free-of-charge by visiting the web site:
http://www.nicee.org/EEP.php .
You may also like to explore if your organization / library can
subscribe a paid copy.
The Periodical is free-of-charge for individuals and has a very nominal
charge to the organizations/libraries (Rs 1000/- per year in India, and
Rs 2000/- per year overseas). Further, it is not distributed in USA/ Canada.
The periodical has received tremendous response, with about 1,850
subscriptions already. We look forward to expanding the base of its
subscribers to further improve the effectiveness of the periodical
towards seismic risk reduction.
With best regards,
Sudhir K Jain
Coordinator, NICEE
-------------------- m2f --------------------
Structural Engineers Forum of India
Read this topic online here:
SUKANTA.ADHIKARI
May 9, 2008, 1:08:47 AM5/9/08
to gen...@sefindia.org
Dear Mr Rahul Leslie,
I am not that much clear with your explanation on Zero point accelaration.Actually I was designing STG building for a small power plant in Zone V.I found that the masss participation in X direction is 90% in 13 th mode and the natural frequency of the structure at that mode is 2.77 hertz. Can you expalin clearly.
Regards,
S.Adhikari
I am not that much clear with your explanation on Zero point accelaration.Actually I was designing STG building for a small power plant in Zone V.I found that the masss participation in X direction is 90% in 13 th mode and the natural frequency of the structure at that mode is 2.77 hertz. Can you expalin clearly.
Regards,
S.Adhikari
-------------------- m2f --------------------
Structural Engineers Forum of India
Read this topic online here:
amritaban
May 10, 2008, 2:59:17 AM5/10/08
to gen...@sefindia.org
If your structure have brick infill panels then put structure type St=3. Then it will calculate as per 7.6.2 os IS1893-2002. If we consider structure type 1 or 2 then it will calculate by 7.6.1. You can try it.
Regards,
Amrita
Regards,
Amrita
| Quote: |
| ----- Original Message ----- From: rahul.leslie (fo...@sefindia.org) To: gen...@sefindia.org (gen...@sefindia.org) Sent: Friday, May 09, 2008 22:43 Subject: [SEFI] Re: Zero point accelaration and missing mass correction If you are getting 90% mass participation, you have met the codal requirements. You may however consider including more modes, provided that the freq. of the highest mode considered is less than 33 Hz. However, I've seen lots of opinion popping up in various discussion groups that response spectrum results you get from STAAD are incorrect (which is different from their view on seismic coefficient method of STAAD itself, which is popularly considered accurate). Still, if you happen to get results in such a way that the provision of Cl.7.8.2, IS:1892-2002 has to be applied, then this problem of inaccuracy discussed widely should vanish. But that is only in case your structure is a framed one with masonry infills. In that case, in place of 0.04, which is actually (ZI)/(2R), in the example |
SPECTRUM CQC 1893 X 0.04 ACC SCALE 1 DAMP 0.05 LIN MIS ZPA 33 |
|
put in its place, (VB_bar/VB)*(ZI)/(2R), where (VB_bar/VB) is as per Cl.7.8.2, IS:1892-2002. One can find VB_bar and VB by STAAD itself. Do two analysis with the same model. One with Seismic Coefficient method and the other with Response Spectrum method. Find sum of horizontal support reactions for all supports for each analysis, and you get VB_bar and VB. I would further suggest calculating time period by hand and entering it as a parameter in STAAD for Seismic Coefficient method, rather than allowing STAAD to calculate it by itself. There again, use formula from Cl.7.6.2, but as 0.09*h/root(d), since that ‘h’ is missing in print in IS:1893-2002 I was planning to bench mark the accuracy of Response Spectrum analysis in STAAD, but didn't find time and patience to do it. Had I done that, I could have given suggestions. |
-------------------- m2f --------------------
Structural Engineers Forum of India
Read this topic online here:
Reply all
Reply to author
Forward
0 new messages