On the Effect of Exposed Timber on the Severity of Structural Fires in a Compartment and Required Firefighting Resources

Estimation of design fires and thermal exposure conditions is an important step in structural fire engineering procedures. Mass timber, as a combustible material, may contribute to the fire intensity, yet there lacks methods to estimate design fires in compartments with exposed timber. This paper summarizes available experimental data on the contribution of exposed timber to heat release rate, describes a simple analytical method to evaluate this contribution, and discusses the effects on time–temperature curves and required firefighting resources based on a case study. Results show that the total heat release rate in compartments increases with the surface of exposed timber. This total heat release rate can be conservatively estimated using empirical relationships for ventilation-controlled burning rate and charring depth. In estimating gas temperature–time curves, both external flaming and extended fire duration combustion models can be applied to obtain an envelope of fire severity inside the compartment and for external spread. The proposed assessment approach provides a method to evaluate realistic design fires in timber buildings and to estimate the water supply required to put out these fires. Accurate modeling of the contribution of timber to fire severity is important for the design of mass timber construction as well as for the safety of firefighters.