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Seismic force calculation in Bridge

 
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Subramanian K
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PostPosted: Sat Oct 17, 2020 8:31 am    Post subject: Seismic force calculation in Bridge Reply with quote

Hello All,
I have the following criteria for a road bridge that I'm designing. Please guide me through the design procedure.
1. Seismic Zone = V
2. PSC Box Superstructure simply supported on RCC circular piers with one pier fixed and other free.
3. Foundation is of open type.
Now my question is how to arrive at the seismic forces using elastic response spectrum analysis.
As per table 18 of IRC: 6-2017, for prestressed structures a damping ratio of 2% shall be adopted for which the scale factor is 1.4. Where this value is to be used. In my opinion the spectral data has to be multiplied my this value. But the substructure system is RCC, so I propose this method. The forces in the superstructure alone will be multiplied with 1.4 after analysing the entire structure with 5% damping. But I dont know how this can be done. Please advise and provide guidance.
Regards
Subramanian
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s.bhushanraj
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PostPosted: Mon Oct 19, 2020 10:46 am    Post subject: Reply with quote

Hi Er.Subramanian,

The damping ratio corresponding to the "seismic force resisting element" is to be considered in the design. In your case, the PSC superstructure is simply supported and does not participate in the energy dissipation during earthquake. It is the RCC substructure which resists the earthquake. Hence, spectral acceleration corresponding to 5% damping has to be used.

However, in case you want to consider 2% damping, either you can directly input the spectral acceleration curve by scaling the values when defining response spectrum or apply a scale factor for the results after the analysis. Some software packages provide you the option of defining scale factor directly.

Best regards,
Bhushan Raj
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Subramanian K
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PostPosted: Mon Oct 19, 2020 11:37 am    Post subject: Reply with quote

Hello Er.
Bhushan Raj,

Thank you for your reply. Now I have a better idea. I have an another question. What damping ratio has to be considered if the structure is continuous. In this case do we design the superstructure for the seismic forces with R=1 forces?

Thanks in advance.

Regards

Subramanian K
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s.bhushanraj
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PostPosted: Mon Oct 19, 2020 12:08 pm    Post subject: Reply with quote

As per IRC SP 114, plastic hinge formation is not allowed in the superstructure. So you can either design the superstructure for R=1 or ensure that the superstructure is elastic when plastic hinge formation occurs in the substructure using capacity design approach.
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pvgraju
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PostPosted: Thu Oct 22, 2020 7:12 am    Post subject: BRIDGES Reply with quote

Table 4.1 of IRC SP 114 is specifing R=1 if plastic hinge is not formed and R=3 if plastic hinge is forming. So merely taking R=1 will not avoid plastic hinge formation. Whether plastic hinge is forming or not shall be checked by analysis and after that R factor shall be decided.
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pvgraju
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PostPosted: Thu Oct 22, 2020 7:46 am    Post subject: BRIDGES Reply with quote

This IRC SP 114 gives MCE for bridge life more that 100 years so R shall be 1 only. For bridge life less than 100 years DBE so R can be taken as 3
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Subramanian K
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PostPosted: Fri Oct 23, 2020 8:56 am    Post subject: Reply with quote

Thank you Er.
Bhushanraj & Er. PVGRaju for your valuable information.

But I have concerns with regards to the statement made by Er. PVGRaju. When we are considering R=1, we are computing the elastic capacity requirements of the element directly. So design for the same in turn will result in an elastic section which will not undergo any plastic deformations. That is the reason we do design for R=1 in abutment wall along transverse direction because it will not form Plastic Hinge. Applying the reverse of this logic will yield the statement, if we design for R=1 forces, the section will not form Plastic Hinges.


Also please clarify on the relation between the design life of the bridge and R factor. By your statement does it mean that, for design life of 100 years or more, all the components shall be designed for R=1 forces and only when the design life is less than 100 years we can opt for higher R factors.

In my opinion the design life of the bridge might affect the Importance factor and Zone factor and not the Response Reduction Factor. Please clarify.



Much Thanks again for your valuable information.



Regards

Subramanian K
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pvgraju
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PostPosted: Fri Oct 23, 2020 11:45 am    Post subject: BRIDGES Reply with quote

Ah = Z/2*I/R formula

Gives DBE

If we remove 2R from denomitaor it is MCE .So for MCE R=1. If your take R=3 plastic hinges will from upto certain distance from the end of the support. That is to be reinforced with closely spaced strriups to bring ductile behaviour. Though we take R=1 ductile detailing is to be done as per code and end regions are to be reinforced with closely spaced strriups.
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pvgraju
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PostPosted: Fri Oct 23, 2020 11:57 am    Post subject: BRIDGES Reply with quote

wall piers in transverse direction R=1 Thickness will be more. Chances of developing plastic hinges is less.

In longitudinal direction. R=3 thickness provided will be less. Chances of developing plastic hinge is more.
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