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Response Reduction Factors for Bridges as per IRC : 6-2010

 
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er.adeb
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Joined: 19 Nov 2012
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PostPosted: Wed Jul 24, 2013 7:53 am    Post subject: Response Reduction Factors for Bridges as per IRC : 6-2010 Reply with quote

Dear SEFIans,
                      Please clarify me the Note - iii) under Cl. 219.5.5 of IRC :6-2010 regarding Response Reduction Factor. If I use Elastomeric bearing for any RCC simply supported or continuous span bridge shall I have take the reduction factor as 1.5 for the design of RCC substructure ? Or the clause is telling something else. Because some consultants are telling that this 1.5 factor will be applicable if elastomeric bearing is used in horizontal direction and if these bearings are used below superstructure to transmit the load from superstructure to substructure then this Note-iii) will not applicable.
                     But I have still doubt in my mind. So please give some expert opinion.


     I have also attached the page of that clause of IRC :6-2010 for your ready reference.


Thanks & regards
Amitabha Deb
Engineer, PWD



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IRC-6-2010 58.pdf
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d_bhawal
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PostPosted: Sat Mar 08, 2014 11:06 pm    Post subject: Reply with quote

Mr A. Deb
1. Assuming that horizontal bearings meant by you as impact-reduction measures of seismic loads there is no codal rule on how to design that particular elastomeric bearing; which may be designed following the IRC codes/relevant codes used for bearing-design. The design force for the stopper itself shall be based on the clause as mentioned in the code.

2. However for substructure or foundation design one has to consider the R value as 1.5 (as per code, unless effective dampeners are used) . The elastomeric-bearings which are generally used for transferring vertical loads from super-structures to substructures cannot effectively resist  the design earthquake loads  (unless these bearings are properly stiffened and manufactured/anchored to the substructure).Also one cannot simply use the lateral flexibility of elastomeric bearg. to increase the time period of the system and get lower seism. coefficients without proper analysis, which always becomes a matter of extensive study/research .  Finally one has to transfer the force to seismic stoppers ( otherwise the super-str.  would get dislodged or displaced ). Also the lateral displacements of elasto. beargs, under such magnitude of lateral loads will be very high and could almost behave like rollers. The superstructure will certainly pass the lateral load to stoppers (if used)  with very high impact (vertical pads may  reduce it ). Hence the substructure will have very little time to diffuse the load , by rotation etc at selected points of the substructure where ductility is introduced by some means of detailing (say for circular rcc piers one cannot assume R=3 /2.5  without proper justification)
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