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Analysis and Design of Two Structures in one STAAD
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MallikarjunG
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PostPosted: Mon Dec 21, 2015 3:40 am    Post subject: Reply with quote

Dear All,
As suggested in my previous post i suggest use ETABS  2013 and later version to handle such problem.
In these versions of ETABS software has the capabilities of analyzing multiple towers in one model and get results tower wise viz., Time period, drifts story EQ forces etc etc as if the two structures are analysed in two different model. Advantage is one can easily visualize the results in one model, mainly when pounding actions has to be visualized.
Also this option is very much useful when designing the foundations(especially Combined footings) for columns of two different structures/building as the node number/columns numbers are different and are in same model and this males its easy to /export to SAFE or any other software .


Last edited by MallikarjunG on Mon Dec 21, 2015 3:44 am; edited 1 time in total
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sudipnc
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PostPosted: Mon Dec 21, 2015 4:20 am    Post subject: Reply with quote

STAAD can handle multiple structures in one model. There would just be a warning in the output file regarding multiple structures - which is intended for the user in case this condition exists without the intent of the user. However, there are no problems with regard to analysis. If I can know your exact requirement I can guide you further on this. I do not think reading the frequencies would be difficult.
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MallikarjunG
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PostPosted: Mon Dec 21, 2015 5:57 am    Post subject: Reply with quote

Deleted post. Will repost

Last edited by MallikarjunG on Wed Dec 23, 2015 2:50 am; edited 2 times in total
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PREETI_SARKAR
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PostPosted: Mon Dec 21, 2015 12:42 pm    Post subject: Reply with quote

My understanding is the following. The stiffness matrix is based on the connectivity between various components of the structure. So, as there is no connectivity between 2 physically unconnected structures, there will be no load transfer from the members of one structure to those of the other. The loads applied on one structure will be resisted by the members of that structure only. So, any software based on the stiffness method is automatically capable of handling static analysis.

The stiffness matrix used in the frequency calculation is the same as the one from the static analysis. So, frequencies, modes and participations factors for the individual structures will be correct. Base shears will be correct too since those are based on the mode shapes and masses of each structure. Recall that for any given mode, only one structure will have a non-zero value for the mode shape. Member forces, joint displacements, plate stresses, etc. will all be right too for the same reason that base shear shears are right.

However, though the above terms will be correctly calculated, the softwares do not report them in a user-friendly form. If there are "n" structures, there should be "n" tables of each result - frequencies, participation factors, base shear, etc. Most programs I know provide only a single table.

But I think, some hurdles do come up when we do an earthquake analysis.

1) In IS1893, the calculation of base shear is as per section 6.4.2 of the 1893 code. The Sa/g depends on the fundamental period of the structure which is based on clause 7.6.1. So, if there is more than 1 structure in the model, the Sa/g used for the individual structures has to be based their respective fundamental frequency.

2) Equation 7.9.2 of the code describes the design eccentricities to be used in accidental torsion. one of the components of this equation is bi - floor plan dimension of the floor perpendicular to the direction of the floor. If there are many structures, each structure will have its own bi. So, the "bi" used for any structure has to be appropriate to that structure.

So, while most softwares are designed to handle multiple structures for static cases, their ability to handle the earthquake analysis issues mentioned above need to be clearly spelt out before one should use them for such situations.
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thirumalaichettiar
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PostPosted: Mon Dec 21, 2015 1:33 pm    Post subject: Reply with quote

Er.Preeti has explained well on this aspect. In order to have continuity it is necessary to link them together.then there will be one result and one table.
T.RangRajan.
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shefypattambi
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PostPosted: Mon Dec 21, 2015 6:14 pm    Post subject: Reply with quote

Dear Sefians,
     I agree with the views expressed by Er. Preethi Sarkar and also I had read from the ETABS 2013 Analysis manual that although ETABS can handle multiple towers in one file it could not distribute the auto lateral seismic force accurately between floors if multiple towers are there in one file. But in ETABS 2015 manual I could not find out the above warning regarding the multiple tower. However there is some useful information is available from CSI Etabs FAQ website from this link.:
https://wiki.csiamerica.com/display/etabs/ETABS+Modeling+FAQ

It is mentioned like this :
What should be considered when modeling multiple towers in the same file?Answer: A few notes which concern the modeling of multiple towers in a single file include:

     Diaphragms Different diaphragms should be defined and used for each tower.
     Loading For seismic loading ,do not use a Seismic  Auto Lateral Load type , use User  defined seismic loads  , perform a Response Spectrum or Time History analysis instead.
     Vertical systems In terms of height, unbraced length, response, and all other factors, design of columns and shear walls should proceed as normal.
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AmitD
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PostPosted: Tue Dec 22, 2015 9:56 am    Post subject: Reply with quote

I am a member of the technical support group for STAAD.Pro. My answer is in the context of STAAD.Pro only.

If the model is being analysed for seismic loads and, load generation per the IS1893 code is performed, then, one should not use models containing multiple structures. The reasons are explained quite well by Preeti Sarkar in her recent post.
We don't think there will be an error in any other situation if multiple structures are used.
On various seminars that we have conducted throughout India this year, we have made it a point to mention this.
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azaman
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PostPosted: Tue Sep 15, 2020 5:07 pm    Post subject: HELLO Reply with quote

Regards
ETABS  can not do a rational spec. analysis  for being two tower in one file because  if we have two tower  one is very stiff  and the other is very flexible   which  period is selected  for analysing ?
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