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Empirical Expression for T of Tall Buildings and Minimum Sei
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Manoharbs_eq
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PostPosted: Mon Dec 03, 2012 4:23 am    Post subject: Reply with quote

If you are quite aware of etabs, please refer this model.



Rgds
Manohar
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Yogesh.Pisal
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PostPosted: Mon Dec 03, 2012 4:25 am    Post subject: Reply with quote

Dear Manohar,

Thanks for the reply. Good to know that ETABS and SAP use to perform Modal Analysis along with Compression only elements. Please ensure whether you have assigned compression only properties to the equivalent brace elements for brick infill.

Actually, my thought is based on number of past discussion on SEFI on this topic. Let me know if it is possible in STAADpro also. But, do remember we have to go with Compression only specifications for the brace elements.

Further, try to refer following discussions which are about assigning compression only springs for rafts.

http://www.sefindia.org/forum/viewtopic.php?t=6427
http://www.sefindia.org/forum/viewtopic.php?t=7674

Kindly note that compression only spring supports or compression only elements are having same effect on modal analysis.

Regarding creation of matrix in MATLAB - Can you explain how you have considered compression only specifications of brace elements.

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Yogesh Pisal
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Yogesh.Pisal
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PostPosted: Mon Dec 03, 2012 4:48 am    Post subject: Reply with quote

Dear Manohar,

Kindly check following links, which I have searched on google.

Reference:
"No loads are allowed in the modal analysis. All structural supports can be applied. Due to their nonlinear nature, compression only supports are not recommended in a modal analysis. Use of compression only supports may result in extraneous or missed natural frequencies."

Source:
http://www.kxcad.net/ansys/ansys/workbench/ds_modal_analysis_type.html

There are some more reference in this regards but not possible to post all the links. The posted above is from ANSYS documentation.

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Yogesh Pisal
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Manoharbs_eq
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PostPosted: Mon Dec 03, 2012 4:55 am    Post subject: Reply with quote

I agree, I checked it out but even Etabs take tension/ compression springs not only compression springs.


the Matlab equations we used were for only modal analysis, so we derived the effective stiffness.
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Manoharbs_eq
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PostPosted: Mon Dec 03, 2012 4:57 am    Post subject: Reply with quote

its true we cannot analyse if we have only compression element.
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Yogesh.Pisal
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PostPosted: Mon Dec 03, 2012 5:48 am    Post subject: Reply with quote

Dear Manohar,

Thanks for the nice and healthy discussion.

I too faced the same problem when I was doing the Raft for first time and got my doubt cleared from Respected Arun Kashikar sir via SEFI only.

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Yogesh Pisal
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Manoharbs_eq
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PostPosted: Mon Dec 03, 2012 5:51 am    Post subject: Reply with quote

Dear Yogesh,

But it doesnt solve the problem or even give an insight, how to consider compression only springs and perform a dynamic analysis. because we need to do a modal analysis.


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Manohar
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Manoharbs_eq
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PostPosted: Mon Dec 03, 2012 5:56 am    Post subject: Reply with quote

Line Springs - Line ObjectsForm:  
Assign Spring


Line springs can be assigned in any of the local axes directions of the line object. Line springs are linear; that is, they support both tension and compression. You cannot define tension-only or compression-only line springs.
ETABS distributes the springs associated with the line object to all of the nodes associated with the internal-to-ETABS (analysis model) representation of the line object. Note that internally ETABS may mesh (break up) a line object into several elements with associated points between each element.
If you are modeling a beam on elastic foundation with a line spring, you may want to mesh the line object yourself to ensure that internally in ETABS a sufficient number of springs are used in the analysis model. ETABS will automatically determine the required stiffness for each spring. This saves you a considerable amount of time when the points where the springs actually occur are not uniformly spaced.
To assign line springs to line objects

This is what Etabs has to say but for wall we cannot afford to consider tension at all. It should be only compression.

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Manohar
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Yogesh.Pisal
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PostPosted: Mon Dec 03, 2012 5:59 am    Post subject: Reply with quote

Dear Manohar,

You are right, this does not solve the problem. Rather it is created another question.

This was the reason behind my comment "We should have range of empirical formulas which will cover different types of buildings based on experiments". This will make our life simpler by avoiding complex modeling and practical difficulties in analysis while doing Response spectrum and time history analysis as per codal provisions.

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Yogesh Pisal
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Manoharbs_eq
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PostPosted: Mon Dec 03, 2012 6:08 am    Post subject: Reply with quote

Dear Yogesh,

In that case there must be numerous experiments to be conducted for different types of structures keeping few variables. But its really a tedious and long process.

But empirical time period now available in code itself is quite misleading  

Rgds
Manohar
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