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IRC:6-2014 Partial Safety Factor (Seismic Combinations)

 
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bkballa
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PostPosted: Wed Dec 03, 2014 6:33 am    Post subject: IRC:6-2014 Partial Safety Factor (Seismic Combinations) Reply with quote

Dear Sirs/Madams,

I recently got a copy of IRC:6-2014. The major difference, it seems, is on the partial safety factor in seismic condition. The partial safety factor increased a lot in seismic case.
I was earlier trying to design abutments with IRC:6-2010 with partial safety factors for loads. The forces and moments using the partial safety factor was very less in seismic condition. That prompted me to continue using Working Stress Method as I thought seismic case should be governing in design of abutment stem (of reasonable height).
I again checked with partial safety factor provided in IRC:6-2014. To my surprise, it gives forces and moments a lot higher than expected. One of the reasons could be we are required to provide 1.5 partial factor in seismic force as well. In case of Zone V, Ah = 0.45 (upper limit w/o response reduction factor). If we multiply by partial safety factor, it comes out to be Ah=0.675. Even with response reduction factor of 3, Ah = 0.225.
Isn't that very conservative, even in high earthquake zone? Or am I missing something here?

Lastly, I would like to know why such huge difference was made in the partial safety factor for seismic combination in 2010 and 2014. Thanks.


Biswa Balla
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Dr. N. Subramanian
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PostPosted: Thu Dec 04, 2014 4:03 am    Post subject: Re: IRC:6-2014 Partial Safety Factor (Seismic Combinations) Reply with quote

Dear Er Biswa

I am not conversant with IRC codes. Universally a partial load factor of 1.2 only is used for EQ loads, because they occur rarely. I request my friend  Er Alok BHOWMICK to reply why it was considered necessary to increase this value from 1.2 to 1.5 in the 2014 issue of the IRC code.
I just read a paper by Dr Goswami and Prof. Murty. It says that the provisions in the 2000 issue of IRC codes are not sufficient for EQ resistant design and hence they were changed in the 2014 issue. Please read
http://www.iitk.ac.in/nicee/RP/2005_RCBridge_Piers_IRC.pdf


Best wishes,
NS

bkballa wrote:
Dear Sirs/Madams,

I recently got a copy of IRC:6-2014. The major difference, it seems, is on the partial safety factor in seismic condition. The partial safety factor increased a lot in seismic case.
I was earlier trying to design abutments with IRC:6-2010 with partial safety factors for loads. The forces and moments using the partial safety factor was very less in seismic condition. That prompted me to continue using Working Stress Method as I thought seismic case should be governing in design of abutment stem (of reasonable height).
I again checked with partial safety factor provided in IRC:6-2014. To my surprise, it gives forces and moments a lot higher than expected. One of the reasons could be we are required to provide 1.5 partial factor in seismic force as well. In case of Zone V, Ah = 0.45 (upper limit w/o response reduction factor). If we multiply by partial safety factor, it comes out to be Ah=0.675. Even with response reduction factor of 3, Ah = 0.225.
Isn't that very conservative, even in high earthquake zone? Or am I missing something here?

Lastly, I would like to know why such huge difference was made in the partial safety factor for seismic combination in 2010 and 2014. Thanks.


Biswa Balla
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bkballa
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Joined: 29 Nov 2010
Posts: 11

PostPosted: Wed Dec 10, 2014 5:23 am    Post subject: Re: IRC:6-2014 Partial Safety Factor (Seismic Combinations) Reply with quote

[quote="Dr. N. Subramanian"]Dear Er Biswa

I am not conversant with IRC codes. Universally a partial load factor of 1.2 only is used for EQ loads, because they occur rarely. I request my friend  Er Alok BHOWMICK to reply why it was considered necessary to increase this value from 1.2 to 1.5 in the 2014 issue of the IRC code.
I just read a paper by Dr Goswami and Prof. Murty. It says that the provisions in the 2000 issue of IRC codes are not sufficient for EQ resistant design and hence they were changed in the 2014 issue. Please read
http://www.iitk.ac.in/nicee/RP/2005_RCBridge_Piers_IRC.pdf


Best wishes,
NS



Dear Dr. N. Subramanian Sir,

Thanks for your reply. As you have pointed out the partial factor for EQ is increased from 1.2 to 1.5. (It is probably true that partial factor of 1.0 as given in IRC:6-2010 is inadequate). The question is is it really necessary to increase the partial factor by 25% than that provided in other codes (including IS codes)?
I really need sefians help in how to estimate the seismic coefficient using Sa/g method. The method I am using is giving me very high values of seismic coeff Ah in case of Zone V.

PS: IS it possible to use lower value of Ah in case of local road bridges (apart from using 1.0 in importance factor)

Thanks.
Biswa Balla
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Dr. N. Subramanian
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Joined: 21 Feb 2008
Posts: 5430
Location: Gaithersburg, MD, U.S.A.

PostPosted: Thu Dec 11, 2014 6:33 am    Post subject: Re: IRC:6-2014 Partial Safety Factor (Seismic Combinations) Reply with quote

Dear Er Biswa Balla,

It may be because Bridges are more imp. than buildings.

Regards,
NS

[quote="bkballa"]
Dr. N. Subramanian wrote:
Dear Er Biswa

I am not conversant with IRC codes. Universally a partial load factor of 1.2 only is used for EQ loads, because they occur rarely. I request my friend  Er Alok BHOWMICK to reply why it was considered necessary to increase this value from 1.2 to 1.5 in the 2014 issue of the IRC code.
I just read a paper by Dr Goswami and Prof. Murty. It says that the provisions in the 2000 issue of IRC codes are not sufficient for EQ resistant design and hence they were changed in the 2014 issue. Please read
http://www.iitk.ac.in/nicee/RP/2005_RCBridge_Piers_IRC.pdf


Best wishes,
NS



Dear Dr. N. Subramanian Sir,

Thanks for your reply. As you have pointed out the partial factor for EQ is increased from 1.2 to 1.5. (It is probably true that partial factor of 1.0 as given in IRC:6-2010 is inadequate). The question is is it really necessary to increase the partial factor by 25% than that provided in other codes (including IS codes)?
I really need sefians help in how to estimate the seismic coefficient using Sa/g method. The method I am using is giving me very high values of seismic coeff Ah in case of Zone V.

PS: IS it possible to use lower value of Ah in case of local road bridges (apart from using 1.0 in importance factor)

Thanks.
Biswa Balla
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msazarudhin
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Joined: 06 Apr 2015
Posts: 5

PostPosted: Tue Apr 07, 2015 1:01 pm    Post subject: Reply with quote

Dear Biswa,

The only difference between IRC:6-2010 and IRC:6-2014 is
a) For design of foundation, the seismic loads should be taken as 1.35 and 1.25 times the forces transmitted to it by concrete and steel substructure respectively (Cl. 219.8 of IRC:6-2014)
b) Same clause no of IRC:6-2010 states that the above value as 1.25 in general.
So as per IRC:6-2010 also, for Zone V, Ah = 0.675 (w/o response reduction factor) and  Ah = 0.225 (with response reduction factor of 3).
Now, what is the difference?


You mentioned "we are required to provide 1.5 partial factor in seismic force" which is subtle to me.

Greetings
Azar
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