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Time Period in design softwares
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Aatif
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PostPosted: Sat Nov 28, 2020 9:43 am    Post subject: Time Period in design softwares Reply with quote

Hello people.


i have this doubt regarding time period. should the time period be calculated as per IS 1893 formulas and input in the software or let software to calculate it and report it. please advice in what conditions we use either ones?

suggest with respect to both softwares please. ETABS & Staad Pro
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VPandya
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PostPosted: Sat Nov 28, 2020 9:51 pm    Post subject: STAAD has some VERIFICATION EXAMPLES IN DYNAMIC ANALYIS Reply with quote

Dear Er.  Aatif ,

STAAD  has some VERIFICATION EXAMPLES IN DYNAMIC ANALYIS . Proving that STAAD  Time Period calculations are correct.  That being the case you can easily rely on STAAAD for Structure Period  calculations if you want.

Make sure assumptions and limitation on the use  of IS 1893 formulas are close to your structure . In modeling actual structure in STAAD and then calculating period by STAAD is  a  more sure way I  think.  

Note :  Same comment applies to  ETABS  also .

Regards.

Vasudeo Pandya P.E.
Structural Engineer
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aditya
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PostPosted: Sun Nov 29, 2020 3:35 am    Post subject: Reply with quote

Dear Sefian,
Please find attached herewith the extract from ETABS Lateral Force Manual on time period calculation as applicable to IS 1893 Part 1: 2016 and its explanations for your reference.
with regards,
Aditya



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Time Period Calculation in ETABS as per IS 1893 Part 1.docx
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 Filename:  Time Period Calculation in ETABS as per IS 1893 Part 1.docx
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Aatif
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PostPosted: Sun Nov 29, 2020 1:34 pm    Post subject: Re: STAAD has some VERIFICATION EXAMPLES IN DYNAMIC ANALYIS Reply with quote

VPandya wrote:
Dear Er.  Aatif ,

STAAD  has some VERIFICATION EXAMPLES IN DYNAMIC ANALYIS . Proving that STAAD  Time Period calculations are correct.  That being the case you can easily rely on STAAAD for Structure Period  calculations if you want.

Make sure assumptions and limitation on the use  of IS 1893 formulas are close to your structure . In modeling actual structure in STAAD and then calculating period by STAAD is  a  more sure way I  think.  

Note :  Same comment applies to  ETABS  also .

Regards.

Vasudeo Pandya P.E.
Structural Engineer


Sir,
making assumptions from 1893 formulae close to our modelled strcuture, In Staad as we do not model infill walls and slabs, does that make a difference?

if yes, how can we make it closer to modelled structure?

because when i let staad to calculate time period, its giving same time period for X and Z direction even when the building is having different plan dimensions in X and Z for static analysis .
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Aatif
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PostPosted: Sun Nov 29, 2020 1:41 pm    Post subject: Reply with quote

aditya wrote:
Dear Sefian,
Please find attached herewith the extract from ETABS Lateral Force Manual on time period calculation as applicable to IS 1893 Part 1: 2016 and its explanations for your reference.
with regards,
Aditya


sir,

from your attached file, i could make out that for static analysis (seismic coefficient method) time period needs to be input manually calculating from formulaes given in IS 1893 and for dynamic analysis, option of program calculated can be chosen.

but as your note read even for dynamic analysis, for scaling purpose, static time period is needed. so even for dynamic analysis, option of user defined time period needs to be selected?
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va
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PostPosted: Sun Nov 29, 2020 2:30 pm    Post subject: Time Period in design softwares Reply with quote

It is always better to calculate the time period as per code provided empirical formula and provide the value in STAAD, so that , there is no confusion. Otherwise software will try to calculate the time period based on the data of height and base width as it understands.

Regards.
Hemant Vadalkar
Consulting Engineer Mumbai.




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aditya
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PostPosted: Sun Nov 29, 2020 3:12 pm    Post subject: Reply with quote

Dear Sefian,
As I stated earlier, clause 7.7.3 of IS 1893 Part 1: 2016 requires comparison of two values of base shears when we carry out dynamic analysis:
a) Base Shear obtained though Seismic Coefficient Method (SCM) by using the empirical time period, and
b) Base Shear obtained from dynamic analysis, which is done on the basis of modal analysis in case of response spectrum method (RSM).

In ETABS, three options for the input of time period are given as explained earlier. Since IS 1893 Part 1:2016 has given empirical formulae depending upon the structural system, either Option 1: Approximate (when structural system is bare RCC MRF frame or Steel MRF) or Option 3:User Defined may be used. Please note that even for bare frame RCC SMRF or steel SMRF, time period may be calculated using the empirical formulae and then directly input to the program through Option 3. In my opinion, Option 2: Program calculated is not generally used so as to comply with the codal requirement for the calculation of time period.
Thus to summarize:
i) Case I-When ONLY SCM is sufficient or adopted: Calculate time period using empirical formula and make input to the program using option 1 or option 3 as the case may be. Or just use option 3 for all structural systems by manually calculating the time periods from the codal formula.
ii) Case II-When RSM is carried out: SCM is also mandatorily to be done in this case to get the base shear from SCM, for which time period input is the same as given in case I. For RSM in ETABS, we define the response spectrum function and also define Response Spectrum Load Case. The Time periods for different modes are then auto calculated by ETABS and the modal combination base shear is also given by ETABS from which one has to decide on whether base shear scaling is required or not as per codal provision.

with regards,
Aditya
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Aatif
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PostPosted: Sun Nov 29, 2020 3:53 pm    Post subject: Reply with quote

aditya wrote:
Dear Sefian,
As I stated earlier, clause 7.7.3 of IS 1893 Part 1: 2016 requires comparison of two values of base shears when we carry out dynamic analysis:
a) Base Shear obtained though Seismic Coefficient Method (SCM) by using the empirical time period, and
b) Base Shear obtained from dynamic analysis, which is done on the basis of modal analysis in case of response spectrum method (RSM).

In ETABS, three options for the input of time period are given as explained earlier. Since IS 1893 Part 1:2016 has given empirical formulae depending upon the structural system, either Option 1: Approximate (when structural system is bare RCC MRF frame or Steel MRF) or Option 3:User Defined may be used. Please note that even for bare frame RCC SMRF or steel SMRF, time period may be calculated using the empirical formulae and then directly input to the program through Option 3. In my opinion, Option 2: Program calculated is not generally used so as to comply with the codal requirement for the calculation of time period.
Thus to summarize:
i) Case I-When ONLY SCM is sufficient or adopted: Calculate time period using empirical formula and make input to the program using option 1 or option 3 as the case may be. Or just use option 3 for all structural systems by manually calculating the time periods from the codal formula.
ii) Case II-When RSM is carried out: SCM is also mandatorily to be done in this case to get the base shear from SCM, for which time period input is the same as given in case I. For RSM in ETABS, we define the response spectrum function and also define Response Spectrum Load Case. The Time periods for different modes are then auto calculated by ETABS and the modal combination base shear is also given by ETABS from which one has to decide on whether base shear scaling is required or not as per codal provision.

with regards,
Aditya



got your point.
appreciate your patience and writing it out.

just curious, have you been using staad software, if i could get your views on that.. same applies to staad as well?
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aditya
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PostPosted: Mon Nov 30, 2020 8:24 am    Post subject: Reply with quote

Dear Sefian,
In Staad.Pro, one needs to define "Seismic Definition" for auto-seismic load generation. It can be done through graphical user interface screen or directly as command line parameter in an input file which is a text file.
For time period calculation, one needs to define the parameter named "ST" which means "Structure Type" whose value may be from 1 to 5 depending upon the type of structural system. Values of ST for different structural system are shown below:
Structure type of the seismic-resisting system. The program will calculate natural period as per Clause 7.6.2 of IS:1893(Part 1)-2016.

1 = RC moment-resisting frame building
2 = RC-Steel composite moment-resisting frame building
3 = Steel moment-resisting frame building
4 = Building with RC structural walls*
5 = All other buildings
Note: *ST 4 requires that wall data be entered in order to determine the natural period.
Another option would be to directly give the values of time periods as "PX" and "PZ" values in seconds from the empirical time period formulas given in the code. If PX and PZ are defined, program will not use the value of ST in the calculation of the time period.
Detailed syntax of STAAD input for seismic load definition is given in the attached file. You may also refer the inbuilt help in STAAD.
with regards,
Aditya



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Seismic Load Definition in Staad.Pro.docx
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 Filename:  Seismic Load Definition in Staad.Pro.docx
 Filesize:  43.22 KB
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Aatif
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Joined: 26 Jul 2016
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PostPosted: Tue Dec 01, 2020 9:17 am    Post subject: Reply with quote

aditya wrote:
Dear Sefian,
In Staad.Pro, one needs to define "Seismic Definition" for auto-seismic load generation. It can be done through graphical user interface screen or directly as command line parameter in an input file which is a text file.
For time period calculation, one needs to define the parameter named "ST" which means "Structure Type" whose value may be from 1 to 5 depending upon the type of structural system. Values of ST for different structural system are shown below:
Structure type of the seismic-resisting system. The program will calculate natural period as per Clause 7.6.2 of IS:1893(Part 1)-2016.

1 = RC moment-resisting frame building
2 = RC-Steel composite moment-resisting frame building
3 = Steel moment-resisting frame building
4 = Building with RC structural walls*
5 = All other buildings
Note: *ST 4 requires that wall data be entered in order to determine the natural period.
Another option would be to directly give the values of time periods as "PX" and "PZ" values in seconds from the empirical time period formulas given in the code. If PX and PZ are defined, program will not use the value of ST in the calculation of the time period.
Detailed syntax of STAAD input for seismic load definition is given in the attached file. You may also refer the inbuilt help in STAAD.
with regards,
Aditya



Okay.
Thank you for reply!
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