Advanced Seismic Analysis For Nuclear Power Plant Structures Using Inelastic Design Response Spectrum.

Inelastic Design spectra analysis is an approach to optimize the design of a non-safety related structure located next to safety related structure. TCE used this approach to qualify non-safety related structures in nuclear power plant projects to ensure that these structure do not collapse on nearby safety related structure under the occurrence of extreme earthquake like safe shut down earthquake. 

Nuclear Power plants (NPP), generally, have structures like safety related and non-safety related structures. Safety related structures are analysed and designed for site specific operating basis earthquake and Safe shut down earthquake responses. Non-safety related structures are generally designed considering Site specific operating basis earthquake or IS code of practice. The non-safety related structures however, may collapse under the occurrence of extreme earthquake like safe shut down earthquake. When a non-safety related structure is located next to safety related structure, it is required to ensure that the non-safety related structure does not collapse on safety related structure thus hampering the working of processes or damaging the safety related structure. TCE was required to demonstrate in a Nuclear power plant project that non-safety building under the site specific SSE intensity does not affect the safety of the adjacent safety related structure. Further, maximum displacement of the two adjacent buildings was also to be evaluated to arrive at the safe required gap between adjoining structures to prevent impounding effects on each other.

TCE adopted an approach of considering the benefit of higher energy absorption capacity of concrete structure beyond elastic region. In order to take the advantage of above property of concrete, an approach was adopted called “Inelastic Seismic Analysis method” considering suitable ductility factor. The analysis uses Response spectrum method applied to inelastic systems involving development of an Inelastic Design Response Spectrum as shown in figure-1


Figure-1 :-  IDR Spectra along with SSE spectra.


Analysis involved two stages considering gross moment of inertia of the RCC members along with inelastic design spectra and another with cracked moment of inertia of RCC members along with SSE spectra. Cracked moment of inertia of each RCC member is arrived considering M-φ curve for the initial designed section. Best fit curves are used to adopt reasonable accurate stiffness reduction factors. Safety of each RCC member was ensured with the forces generated due to SSE responses spectra. Displacement of non-safety building towards safety building was plotted under IDRS & SSE spectra analysis. Required safe gap between the two buildings was arrived.

This approach has been reviewed and approved by project safety committees. TCE carried out the design using in house developed software TATACADS.

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