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For seismic zone 2B and RCC, you may adopt Concrete 'Intermediate Moment Resisting Frame (IMRF)' for your building, with an R value of 5.5. This would require RCC member detailing as per section 21.3 of ACI 318-08. Alternately, you may also adopt 'Special Moment Resisting Frame (SMRF)' system for you structure. However, it would require a more stringent detailing, in accordance with ACI code Sections 21.5 onwards. Regards.

R factor is used to account for expected ductility of building in earthquake loading. There is no R in non-linear analysis. Lesser R means less post-yield ductility and hence more base shear and vice versa. If a building is ductile (more R) we design for less base shear because in reality building has considerable ductility even after yielding (energy dissipation in non linear range) so in simple words lets say the actual seismic force in reality is 100kn then in linear elastic code based design we design it for only 50kn (hypothetical) and for the remaining 50kn we assume based on structural framing system that building will resist by post-yield ductility. Most common lateral resisting systems are moment frame system, shear wall system, building frame system, flat slabs (not in higher seismic zones) etc. You first determine seismic design category and see limitations on type of lateral system you can "assume" to find R.

Wa-alaikum-assalam, Dear Mubeen, Selection of the structural system for a building would generally depend upon the seismic zone or category of the area where building is to be constructed and the preferred material of construction. For a B+G+5 building with Reinforced Concrete as preferred material of construction, my obvious choice would be a Concrete 'moment resisting system (MRF). Which MRF (IMRF, SMRF etc) would actually be used, will depend upon the selected 'Lateral Force Resisting (LFR) System' corresponding to the seismic design category (or seismic zone) of the area, and the code (UBC-97 in the present case) requirements / restrictions applicable for that area. Value of overstrength factor 'R', would correspond to the 'LFR System', adopted for the building. Types of various Building Systems, LFR Systems, their corresponding R values and restrictions on the use of these building / LFR systems are available in Table 16-N of UBC-97. BTW... Where is you building located? And, what is your preferred material of construction. Regards.