Impact of Mezzanine Rack Shelf Spacing on Radiative Heat-Dominated Flame Spread Characteristics

The widespread use of mezzanine racks in modern logistics warehouses has significantly increased fire hazards owing to the dense storage of combustibles. However, systematic full-scale studies examining the influence of shelf spacing on radiative ignition between adjacent racks are lacking. In this study, we investigate the effect of shelf spacing on radiative flame spread using full-scale fire tests and cone calorimeter experiments. The decrease in radiative heat flux with an increase in the distance was consistent with the inverse square law. Adjacent shelf ignition was prevented when the spacing was at least 5 m. Cone calorimeter tests identified a critical radiant heat flux of approximately 8 kW/m2, and the ignition time decreased nonlinearly from 207.8 to 69.6 s as the radiant flux increased from 10 to 16 kW/m2. These findings were cross-validated with the full-scale results, which indicated that a minimum spacing of 5 m serves as a radiative flame-spread barrier under similar storage and ventilation conditions. This study provides practical guidance for the fire-safety design of mezzanine rack warehouses. The effects of storage geometry, surface reflectivity, ventilation, active protection systems, and varying storage densities may be considered in future work to ensure broader applicability.