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Torsional mode as a first mode in Tall Building

 
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HMA7
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PostPosted: Sun Sep 16, 2018 7:57 am    Post subject: Torsional mode as a first mode in Tall Building Reply with quote

I just remodeled a building in ETABS that already build in Turkey, even so my model is approx. but i noticed that the first mode is torsional and the building tend to rotate for any excitement.




I wonder if should avoid that the first mode to be the torsion or we should do some extra provision if the first mode it torsion or we do need extra works.





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NEW Miodel 44 story torsion check.rar
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Building model (ETABS 16.2.1)

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AmarVenkat
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PostPosted: Sun Sep 16, 2018 4:40 pm    Post subject: Reply with quote

I have not gone through the model. Sometimes torsional mode may be due to peripheral columns  moving out of phase the  along the same axis - which  could be result of eigen value search algorithm.  Check the mass participation factor. What is the predominant mode with considerable mass participaation ? You  may verify with rigid diaphragm action.  And, torsional mode could be genuine first mode also.
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HMA7
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PostPosted: Sun Sep 16, 2018 8:17 pm    Post subject: Reply with quote

pavsefi wrote:
I have not gone through the model. Sometimes torsional mode may be due to peripheral columns  moving out of phase the  along the same axis - which  could be result of eigen value search algorithm.  Check the mass participation factor. What is the predominant mode with considerable mass participaation ? You  may verify with rigid diaphragm action.  And, torsional mode could be genuine first mode also.



let's say due to arch. plan we can't avoid to have torsion as a first mode, then what we should do? this my question.
You mentioned that it is ok to have torsion as a first mode, is there any code provision that support your suggest!
thanks
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sakumar79
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PostPosted: Mon Sep 17, 2018 1:01 am    Post subject: Reply with quote

Dear Sir,
     It is not okay to have torsion mode as 1st mode. Code gives recommendations on Torsionally irregular buildings in Table 5 of IS 1893 Part 1 2016 in which it prohibits torsional irregularity beyond a specified limit (even if the irregularity is within limits, recommendations include having torsional mode of oscillation under control). In this particular model you have shown, torsion can be easily controlled by splitting the building into two rectangles instead of L shape. Are you sure that no such separation joint is provided in the original building?

Yours sincerely
Arunkumar
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HMA7
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PostPosted: Mon Sep 17, 2018 6:21 am    Post subject: Reply with quote

sakumar79 wrote:
Dear Sir,
     It is not okay to have torsion mode as 1st mode. Code gives recommendations on Torsionally irregular buildings in Table 5 of IS 1893 Part 1 2016 in which it prohibits torsional irregularity beyond a specified limit (even if the irregularity is within limits, recommendations include having torsional mode of oscillation under control). In this particular model you have shown, torsion can be easily controlled by splitting the building into two rectangles instead of L shape. Are you sure that no such separation joint is provided in the original building?

Yours sincerely
Arunkumar




That is original plan and the building wan't split in to two rectangle,even so split building in two parts might solve some torsionality, but spliting the building by itself make new problem, because two new rectangle building should have a gap of 0.5-1.0 m even more to avoid impact loading to each other during seismic while there moving.

As you mentioned before if the torsional irregularity within in the limit it OK according to the code but  recommendation says to have torsional mode of osculation under control.after i checked your IS code i find this at table 5 as you mentioned.



"the natural period corresponding to the fundamental torsional mode of oscillation is more than those of the first two transnational modes of oscillation along each principal plan directions"

then the code mentioned that

" i)in the range 1.5 - 2.0 (dmax / dmin on same direction) (a) the building configuration shall be revised to ensure that the natural period of the fundamental torsional mode of oscillation shall be smaller than those of the first two transnational modes along each of the principal plan directions and then (b) there dimensional dynamic analysis method shall be adopted and
ii) more than 2.0 (dmax/dmin) the building configuration shall be revised "


in other word the torsional mode should be the third and later mode, am i right!

let say i have building that, the first two mode are transnational mode but no pure transnational they have some torsion too. and the the ratio of the (Dmax/Dmin) is less than 1.5, can I proceed the design? i can not rearrange any elements from the arch. plan.
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AmarVenkat
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PostPosted: Tue Sep 18, 2018 7:13 pm    Post subject: Reply with quote

Well, I noticed the plan of the building  & modal participation ratios from your posting ( did not notice earlier !!)

- ASCE 7 does not mention about time period in torsional mode.

- I dont know about the base vector considered by ETABS to evaluate RZ mass participation ratio .  First mode ( as reported here) seems to be due to  high mass moment of inertia associated with rigid diaphragm assumption . This mode may vanish, if semi-rigid diaphragm is assumed or if slabs are modelled as shell elements with realistic properties.  I would suggest to  ignore this mode : anyway its contribution to seismic base shear is nil.

- you may try Ritz vector approach in ETABS .

- Arch plan of this building is impressive. Apply  seismic spectral load in 45 degree axes , as principal direction of this structure seems to be so , as per mass participation in translation modes.

- Regarding codal provision, I have no  clue about the rationale behind avoiding  torsional mode time period. May be the intention is that one should not use this mode shape to arrive at base shears in the building. Also by pushing the torsional mode to third / fourth / n th mode, what are we going to achieve ?

- In the present case, you may go ahead ignoring first mode reported by ETABS : Storey shear distribution will not be governed by this mode.
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