Revisiting the BS 8414 timber crib: An experimental investigation for enhancing facade fire safety

In the aftermath of the Grenfell Tower tragedy, the fire performance of external wall systems (EWS) and the timber crib fire source specified in BS 8414 have come under increasing scrutiny. This study presents a comprehensive experimental investigation into the burning characteristics and heat exposure effects of the BS 8414 timber crib. A two-scale approach was employed, combining bench-scale calorimetry (ISO 1716) and large-scale testing in both free-burn and chamber configurations, to evaluate the consistency, intensity, and thermal output of the fire source. Key parameters such as heat release rate (HRR), mass loss rate (MLR), temperature evolution, and incident heat flux on an inert façade were measured. The results demonstrate good repeatability and reveal that the actual heat output often exceeds the standard's prescribed values, exposing the façade to high-intensity thermal loads sufficient to ignite many cladding materials. The findings also indicate that the BS 8414 setup more closely resembles an external fire scenario rather than a post-flashover room fire, due to the chamber's geometry and fuel placement. This research provides valuable benchmark data that may inform regulatory bodies, standardization committees, and engineers, contributing to improved testing methodologies and the development of safer façade fire standards for high-rise buildings.