A risk-based design method for preventing fire spread between buildings to achieve a safety level equivalent to the prescriptive standard

The 2019 amendment to Japan's Building Standard Law introduced performance-based provisions for fire spread prevention. However, existing verification methods rely on a relative comparison with a hypothetical prescriptive twin, which not only imposes a significant design overhead but also hinders the rational optimization of design parameters such as setback distances and opening sizes. To address these defects, this study proposes a probabilistic verification method that establishes an absolute safety benchmark derived from fire statistics. This approach ensures performance equivalence with the current prescriptive standard while streamlining the design process. A case study of a three-story office building demonstrates that the fire resistance of exterior openings and setback distance are dominant factors in performance, enabling a more flexible and optimized fire safety design. • A risk-based design method for preventing inter-building fire spread is proposed. • The method uses an event tree to probabilistically analyze building performance. • The method ensures a safety level equivalent to the existing prescriptive standard. • The method was applied in a case study of a three-story office building. • Exterior openings and setback distance were dominant factors for fire spread risk.