Application of Iterative Fire Severity Analysis to Fire Safety Design of Multi-storey Mass Timber Structures

The United Nation’s sustainable development goals to mitigate climate change and the New Zealand Government’s Climate Change Response (Zero Carbon) Amendment Act 2019 have encouraged the construction industry to utilize timber as a primary construction material. When exposed to fire, timber burns and forms an insulative char layer that provides protection to the unburnt load-bearing timber. Encapsulation can be provided to delay the start of charring. After flashover, exposed mass timber surfaces will burn and contribute additional fuel load. This paper illustrates the use of an iterative fire severity analysis based on a parametric compartment fire model to account for the contribution of non-encapsulated timber during post-flashover fires. A 5-storey, cross laminated timber residential building with sprinkler protection is used as a case study to investigate reasonable fire resistance ratings for design. Sensitivity analyses of fire resistance ratings show a linear relationship with fire load from 250–800 MJ/m2, and an exponential relationship with ventilation resulting from 19% to 64% of window breakage. For the post-flashover fire scenarios considered, the fire resistance rating ranges from 30–60 min subject to the design assumptions and inputs adopted.