Localized Fire Behaviour of Reinforced Concrete Columns with CN Panel Protection in Open Piloti Structures

This study experimentally investigates the thermal behavior and structural vulnerability of reinforced concrete (RC) columns in piloti-type buildings subjected to localized fire conditions and evaluates the fire resistance enhancement effect of CN panel strengthening. Full-scale RC columns and CN panel-strengthened RC columns were fabricated and exposed to a localized fire generated by 90 L of heptane using a 10 MW large-scale calorimeter. Temperature distributions within the columns were measured using thermocouples, and flame characteristics and damage patterns were analyzed. The RC column exhibited severe spalling over approximately 42.4% of the exposed surface area (1.2 m²), with a maximum concrete temperature of 828.7 ℃ and a reinforcing bar temperature of 199.0 ℃. In contrast, the CN-strengthened column showed no spalling, and significantly lower maximum temperatures of 207.7 ℃ in concrete and 74.9 ℃ in reinforcing steel were recorded. The maximum flame height exceeded 3.5 m, indicating that structural members in piloti spaces may be directly exposed to high-temperature flames. Comparison with the localized fire design model provided in Eurocode EN 1991-1-2 showed that the experimentally measured maximum flame height and duration were 1.15 and 1.58 times higher than predicted values, respectively, suggesting that conservative evaluation is required for real piloti conditions. The results demonstrate that localized fire can critically affect unprotected RC columns in piloti structures, while CN panel strengthening effectively mitigates spalling and thermal damage. The findings provide fundamental data for performance-based fire design of piloti buildings.