This chapter presents a comprehensive comparative study of seismic design code applications for reinforced concrete and steel building structures in the United States (ASCE 7-22), Canada (NBCC 2020), and China (GB50011/GB55002), using a unified 12-story reinforced concrete dual-system building to isolate the effects of code-specific requirements. With identical geometry, materials, and analytical properties applied to representative high-seismicity locations in Los Angeles, Vancouver, and Kunming, the study examines how each jurisdiction’s hazard model, spectrum shape, ductility framework, and analysis procedures influence base shear, story-force distribution, drift response, component-level demands, and nonstructural components acceleration. The results highlight fundamental differences among the three systems: ASCE’s risk-targeted collapse-prevention philosophy with advanced nonstructural components provisions; NBCC’s UHS-based reliability approach combined with strong capacity design and shear magnification; and China’s multi-level fortification strategy emphasizing robustness, drift limitation, and rare-earthquake stability. Through aligned discussions, comparative tables, and an integrated case study, this chapter provides a practical and analytically consistent reference for understanding how seismic design outcomes diverge across national codes despite identical building characteristics, offering valuable insight for global practice, cross-regional design, and future harmonization efforts.