Directional building loads and safety distances for hydrogen cloud explosions: Applicability of shock-based methods to deflagration waves

Hydrogen explosion assessments commonly predict free-field incident overpressure, but structural consequence assessment requires directional loading on finite targets and consequence-based safety distances. This paper presents a methodology that uses large-scale hydrogen-air explosion data to translate free-field predictions into directional surface loads and safety distances. Reflected-wall measurements and simulations show that Rankine-Hugoniot reflection provides a conservative upper bound for peak reflected pressure. For long-rise-time deflagration waves, finite-target clearing substantially reduces the reflected peak, while steeper waveforms progressively converge towards classical reflection behaviour. In the tested configuration, convergence began at approximately 10-20 kPa incident overpressure, corresponding to imposed flame velocities of approximately 100-200 m/s; these values are configuration-specific, while the more general applicability criterion is governed by the relationship between clearing-wave arrival time and pressure rise time. Instrumented shed measurements also show that front, side, rear, and roof surfaces experience markedly different pressure and impulse histories because of reflection, diffraction, and wave wrap-around. The proposed framework links free-field blast predictions with directional structural loading, damage thresholds, human injury criteria, and combustion-mode-dependent safety distances.