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[adutt]

Dear SEFI members,

In carrying out a dynamic analysis by response
spectrum analysis of 12 storeyed or higher buildings by STAAD it is
observed by me that the Ist mode shape time period of the building is
very large(2.3 seconds). The model does not consider brick infills .If I
compare this with fundamental time period of the empirical equation
Ta = 0.075条款中给出h.75 7.6的是1893 (part 1):2002 with
the STAAD model time period it is almost half.(That is IS equation
gives 1.3seconds).

The code (Clause 7.8.2 ) is asking us to enhance the forces by a ratio
of base shear obtained by the fundamental time period to that
obtained by the dynamic analysis.

My Ist question is that does STAAD give a larger time period in general
,the other users can give their observations

My 2nd question is do I follow the clause 7.8.2 even if there is a
very very large difference in the time periods.

With Regards

Amit Dutt

Posted via Email

[sdec.in]

Dear Amit
have u modelled the brick infills? The default formula used in STAAD is the
one without infills, however if u have infills and u have modelled them u
can change the time period formula by specifying ST3 as 3 while entering the
IS 1893 loads.By doing this u will be using the time period formula with
infills.pl check with this and see if ur problem gets solved
regards
sangeeta wij
----- Original Message -----
Message From
To:
Sent: Wednesday, November 16, 2005 11:20 AM
Subject: Time period of Building by STAAD

[SaurabhPurandare at co...]

Dear Amit,

It is usual to get such high period of vibration due to dynamic analysis.

As per IS 1893, the structure is required to be designed for a lower bound of EQ forces. The same can be done by enhancing the Base shear achieved by dynamic analysis proportionaly. This can be done only if the strucutral model is truely representative of actual strucutre.

The difference between static & dynamic analysis (as per above) is only in vertical (floor wise) distribution of eq forces. Normally in static analysis it will be parabolic. (dynamic analysis may be usefull in reducing total overturning moment on structure)

Now above advantage can only be taken if the model is representative of actaul strucutre. If any irregularity (soft storey due to infill masonary above stilt etc) in the structure is not considered in analysis, we can not use 7.8.2 as it is.

Regards,
Saurabh

-----Original Message-----
Message From adutt[AT]des... [mailto:adutt[AT]des...]
Sent: 16 November 2005 11:02
To: Saurabh Purandare
Subject: Time period of Building by STAAD

Dear SEFI members,

In carrying out a dynamic analysis by response
spectrum analysis of 12 storeyed or higher buildings by STAAD it is
observed by me that the Ist mode shape time period of the building is
very large(2.3 seconds). The model does not consider brick infills .If I
compare this with fundamental time period of the empirical equation
Ta = 0.075条款中给出h.75 7.6的是1893 (part 1):2002 with
the STAAD model time period it is almost half.(That is IS equation
gives 1.3seconds).

The code (Clause 7.8.2 ) is asking us to enhance the forces by a ratio
of base shear obtained by the fundamental time period to that
obtained by the dynamic analysis.

My Ist question is that does STAAD give a larger time period in general
,the other users can give their observations

My 2nd question is do I follow the clause 7.8.2 even if there is a
very very large difference in the time periods.

With Regards

Amit Dutt

Posted via Email

[vva at vsnl.com]

Dear Amit,
Calculation of time period is sensitive to mass and stiffness. Empirical
time period given in the code can not be compared with the dynamic analysis
of analytical model which will always differ as pointed out Sangeeta and
others.

It is always better to work out the time period for the appropriate type of
structure based on the empirical formula given in the code and use this as
an input to STAAD PX and PZ ( time period in x and z dir). This period given
by the user will be used by STAAD in calculating the base shear.

Hemant Vadalkar
Consulting Engineer
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118, Veena Beena Shopping Centre, Opp. Bandra Station(W), Mumbai 400 050
INDIA
Tel. / Fax : 91-22-2643 0668 / 91-22-2641 0857
Email : vva[AT]vsn...

Posted via Email