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Load combinations to be considered
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nshettyprasad
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PostPosted: Wed Aug 24, 2011 3:20 pm    Post subject: Load combinations to be considered Reply with quote

Dear SEFIans,

I have following queries,

1. IS1893:2002 clause 7.8 defines when to perform dynamic analysis. So when we perform dynamic analysis, is it necessary to design the structure for static load combinations also?

2. It is silent about the criteria to perform static analysis say if its just 1 floor or G+1 floor is it becomes mandatory to perform static analysis? No zone criteria?
    In that case none of the structures is to be designed for gravity case alone?

Thanks,
Prasad Shetty
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nshettyprasad
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PostPosted: Fri Aug 26, 2011 6:40 am    Post subject: Reply with quote

experts please thow some light on this........
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Abishek_Siingh
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PostPosted: Fri Aug 26, 2011 8:27 am    Post subject: Reply with quote

1. Static load combinations are unfactored loads. so they will NOT be used for design. They are used for checking the serviceability criteria only.

2. You can perform dynamic analysis for G+1 if you feel so but the problem is that results will not differ. The first mode will dominate the entire design. There are only zonal restrictions (EQ zones) specified in IS 1893 for dynamic analysis.

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Abishek Siingh

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prashant.ambulkar
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PostPosted: Sun Aug 28, 2011 7:02 am    Post subject: Reply with quote

Dear Prasad,

In clause 7.8, dynamic analysis method has been described for earthquake load calculation.

In clause 6.3, load combinations are given for design of structure. Please check those load combinations where gravity loads are also considered.
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santoshab
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PostPosted: Sun Aug 28, 2011 6:28 pm    Post subject: Reply with quote

Dear Friends,
1) The question seems to be different than to ones understanding. Its about the earthquake static analysis v/s dynamic analysis. The condition for serviceability is completely different as to what you are suggesting. The question raised here is debatable. When there is a criteria to perform dynamic analysis then how can we consider the load combinations  of static equilibrium? in our designs. It should be indeed 1.5(DL+LL), 1.2(DL+LL+RS) etc RS = Response spectrum cases.

2) Is there a need to perform static analysis or just gravity load i.e., DL and LL is sufficient to fulfill the design. There is no distinguishing note to perform static equilibrium analysis based on EQ zone or height of the building etc. Or Just all the buildings irrespective of zone or  according to height etc., be analysed for static equilibrium method. As IS 13920 which clearly mentions that this code applies to zone 3,4 and 5 clause 1.1.1. like that is there any mention in IS 1893 about the static analysis.

Seniors please advice.
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nshettyprasad
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PostPosted: Mon Aug 29, 2011 3:59 am    Post subject: Reply with quote

Dear Deshpande sir,

this exactly is my question. thanx for elaborating..........
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Abishek_Siingh
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PostPosted: Mon Aug 29, 2011 4:50 am    Post subject: Reply with quote

Mr. Deshpande,

I agree to what you are saying.
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kajal.chopra
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PostPosted: Mon Aug 29, 2011 7:10 pm    Post subject: Reply with quote

Dear Mr.Kumar Abhishek Singh,

I have been reading the book by Prof.Anil K Chopra. And, if you read the chapter 12, it is clearly proved through two examples, that the number of modes required to be included for computing response, depends upon the spatial distribution of forces.In the second example (of the 5 storey shear frame) by Chopra, it is clear that all 5 modes were needed for getting an accurate base shear computed whereaS 3 modes were needed to get an accurate roof displacement computed.

How can you say just first mode is important ??

Kajal
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Kedar
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PostPosted: Mon Aug 29, 2011 7:50 pm    Post subject: i st mode contains max excited mass Reply with quote

@sefindia                                                                                 hi Kajal                                                                                    the first mode generally covers more than 90% of excited mass. The physical interpretation can looked at as all mode shape ordinates are either +ve or -ve, meaning all massess at various locations are definitely displaced from original position during excitation. In othr modes some masses may remain staganent where modal displacement values change their sign. The stick bent on only one side as a cantilvr is good eg of i st mode shape with max. Excited mass. Hence first mode is so imp. Regards-Kedar
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kajal.chopra
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PostPosted: Mon Aug 29, 2011 9:18 pm    Post subject: Reply with quote

Dear Mr.Kedar,

I have attached 3 pages from the book by Prof Chopra of University of Berkeley, he has clearly mentioned and also proved later that in case of 5 storey shear frame, the contribution from second and third mode is significant for set of force sb.See the last four lines of page 3/3 which I have encircled in green.

Please comment.



Kajal



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