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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.

All, It is a must have and must read. Please go through the attachment. Thanks. Soil Structure Interaction For Building Structures.pdf