Impact of Prescriptive Fire Design Provisions on Embodied Carbon for International Building Code Type IV Construction

Factors such as carbon storage potential, short construction schedules, prefabrication, and aesthetics are making mass timber a sought-after structural solution. For mass timber, fire design requirements can both drive and impact design decisions. These decisions must understand code limitations for exposed timber, as well as the options for fire protection. This article primarily discusses how the last motivator, aesthetics, affects fire protection design and the carbon storage potential of mass timber. To meet code requirements for different construction types, designers can use noncombustible protection installed directly to the timber, additional sacrificial timber thickness for char design to expose the timber aesthetically, or a combination of both. Although considerable information exists for these design options, there is little guidance on how these decisions affect sustainability goals for carbon storage. This paper presents a study of mass timber structural floor systems in the residential (R-2) occupancy with sprinklers using parametric design analysis. It found that assumed carbon storage for timber elements significantly affects embodied carbon (EC) of floor structures in their entirety and therefore requires further discussion and possible standardization in industry. For designs using half of the possible timber carbon storage material availability, the lowest-EC designs come from maximizing char design and minimizing noncombustible protection. Further, the timber concrete composite (TCC) and timber floor with girders (TG) systems offer the lowest-EC options.