A risk-informed appraisal of prescriptive building separation regulations in Switzerland

Basler & Hofmann AG Zurich, Switzerland

Abstract

In Switzerland, prescriptive requirements apply to minimum separation distances between buildings in order to prevent building-to-building fire spread. These requirements are formulated by the regulatory body, for ease of use and enforcement, in a very simple manner: 10 m minimum separation distance between buildings, if both buildings have combustible exterior linings, 7.5 m if only one building has a combustible exterior lining, and 5 m if both buildings have non-combustible exterior linings. However, no quantitative guidance is given on how to reduce these distances, e.g. in cases where existing buildings are involved. In this paper, an overview on design methods for calculating separation distances in order to prevent fire spread between buildings is given. A model is described to calculate the probability of fire spread between buildings at several distances, taking into account relevant uncertainties. It appears that there is a 1 in 20 chance that a combustible exterior building lining can ignite if exposed to a burning building (room fire conditions) at a distance of 10 m, and a 1 out of 3 chance if at a distance of 5 m. The level of risk implicitly accepted by the Swiss fire codes is discussed, and a method is presented on how to derive separation distances imposing an equal level of risk for different building geometries and occupancies.