Building Standard Law of Japan (BSL) requires buildings to maintain certain fire safety performance for preventing building-to-building fire spread in urban area. The level of requirement specified in BSL changes with site location, total floor area, and number of floors of the target building in a prescriptive manner. In this study, a performance-based method is proposed for verifying compatibility of fire spread prevention performance of buildings with that required by BSL. The criteria adopted for verification is the risk of fire spread: fire spread prevention performance of the target building is judged as compatible with the required level if the risk of the target building relative to that of the reference building designed under the prescriptive regulation is equal to or smaller than unity.
Squared value of radiant heat flux passing through the site boundary is used as the risk index that represents the magnitude of fire hazard. However, for the enhancement of lucidity of the risk index, the magnitude of fire hazard is transformed into the fire spread resistive time, which congruently represents the fire spread prevention performance of the target building, by using the theoretical relationship between the time required for an infinitely thick material to get ignited and the external radiant heat. With the transformed risk index, fire spread prevention performance of the target building is judged as compatible with the required level if the fire spread resistive time of the target building relative to that of the reference building designed under the prescriptive regulation is equal to or greater than unity.
In the proposed method, fire spread resistive time of a fire compartment comprising the target building is evaluated by the event-tree analysis in which the probability for a fire compartment to become a fire source to its adjacent is considered as the probability of successive occurrence of comprising events, i.e., ignition, flashover, and burn-through of compartment boundaries. Probability of each event is formulated as a function of the fire spread resistive time of component building members and equivalent fire duration time inside of the fire compartment. The overall fire spread resistive time of the target building is obtained by the exterior wall length-weighted harmonic average of those of individual fire compartments.
As a case study, the proposed method is used to evaluate fire spread prevention performance of a building, which heavily use wooden building members and furnishings in the internal space. The result showed that there are several combinations of design options such as the reinforcement of exterior materials, compartmentation, or installation of sprinkler systems that enables the building to maintain equivalent level of requirement in terms of the fire spread risk. Among the design options tested, compartmentation that fragments internal space into several smaller fire compartments was most effective in reducing the fire spread risk of the building. Installation of sprinkler system was also effective, but limited to the internal fire scenario among the two fire scenarios assumed.
