Energy distribution in a large-scale structural fire test: The convective heat loss

A key concept in structural fire engineering is the compartment fire, crucial for estimating thermal boundary condition in structural analysis. However, there are no complete quantifications on the energy distribution into load-bearing structural members for the steel-composite structures in large compartments in cases where the convective heat loss via the openings has been measured. This is due to the limitations in test scale, instrumentation resolution, and inability to estimate convective heat loss without calorimeter hoods for accurate HRR estimation. This study investigates the thermal energy distribution of a large compartment focusing on the convective heat loss through the opening, utilising the experimental data of a full-scale structural fire test conducted at the National Fire Research Laboratory (NFRL), National Institute of Standards and Technology (NIST), USA. In the experiment, gas burners operated for 131 minutes, providing a fire load density of approximately 1252 MJ/m². The peak gas temperature beneath the composite floor exceeded 1100 °C. In our study, the convective heat loss was quantified as 51.2 GJ utilising the measured gas velocities and thermocouple temperatures at the openings. In comparison, the 20 MW calorimetry hood estimated a total convective heat loss of 52.8 GJ. Those two estimates represent 63% and 65% of the total supplied energy of 81.3 GJ from the gas burners, respectively. Note that this apparent good agreement should be interpreted with caution, considering the uncertainties of this simplified bi-linear assumptions on the velocity profile along the opening height.