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Bridge load combination, IRC:78 vs IRC:6

 
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saroj
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Joined: 24 Jan 2014
Posts: 6

PostPosted: Wed May 18, 2016 11:24 am    Post subject: Bridge load combination, IRC:78 vs IRC:6 Reply with quote

Dear friends

Here is a small confusion. IRC:6 gives a table of load combinations for bridges which indicates that if the loads are combined with seismic or wind case, the live load can be reduced to 20%. (Earlier it was 50%)

IRC:78 also has recommended load combinations for foundations which does not say anything about reduction, but mentions that the loads may be evaluated as per IRC:6.

Some people argue that no reduction in any case is allowed because the allowable stress is increased for the seismic or wind cases.

My opinion is that is is based on probability that while there is the critical seismic or wind condition there will not be the full design vehicle, but a portion of that.

Would be great if anyone could throw some light on this matter.

Thank you.
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rsn1980
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Joined: 03 Nov 2008
Posts: 30

PostPosted: Thu May 19, 2016 12:03 pm    Post subject: Reply with quote

Dear Mr.Saroj,

I presume that you are implying the increase in the net SBC of the soil while checking a footing for base pressure. It may be noticed that IRC specifies EQ loads only for verification of Structural Strength.. i.e in ULS only..

The serviceability limit state combinations doesn't specify EQ loads. So, there is no question of increase in the permissible stresses which comes under serviceability criteria.

It is prudent to reduce the live loads to 20% and check the base pressure against the actual SBC and not permit any increase.

Hope I am right.. Seniors may comment/ correct me if I am wrong.

Thanks
Subramanian
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saroj
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Posts: 6

PostPosted: Fri May 20, 2016 3:58 am    Post subject: Reply with quote

Thank you Mr. Subramaniam.


Actually I meant for structural analysis. Regarding the base pressure, IRC has clearly mentioned that the combination/increment does not apply.
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