On the Danger of Threats Underestimating and the Resulting Unreliable Assessment of Building Fire Safety

: The paper gives an example showing that the utilization of prescriptive rules in some situations can lead to strong underestimation of the real risks and inadequate fire safety assessment of buildings. The issue seems to be very important as the prescriptive rules in many European countries are the only allowed and acceptable by the authority’s method of building fire safety assessment. The building presented in this paper is an exemplary bakery plant consisting of several premises of a different purpose and method of use, e.g.: technical facilities, production depot, distribution and storage spaces, long-term storage cool rooms, etc. The whole building that consists of single-story technological (production and storage) part and (located on two stories) office parts was approved as a singular fire zone with a total usable area of 6280 m 2 . The technological area includes production facilities, storage depots of raw materials, packages and finished products, as well as cold stores and a number of auxiliary function rooms. In the second (having two stories) part of the building some social rooms, administrative areas and offices are localized. The total height of the building (at the highest point) does not exceed 10.5 m. Due to the Polish regulations the parameters determining the fire-related requirements of individual structural elements of the building (especially in terms of their fire resistance) are the surface area, the average value of the fire-load density and the presence of the risk of possible explosion. The building was designed based on the assumption that the average fire-load density does not exceed the level of 1000 MJ/m 2 . The analysis and calculations carried out during the exploitation phase of the building confirmed the compatibility with the assumptions adopted, but the actual volume, estimated at the level of 974 MJ/m 2 proved to be very close to the limit value. Exceeding of the limit value of 1000 MJ/m 2 – due to provisions given in a state regulations - would automatically double the formal requirements for the resistance of the structural elements from R30 to R60. When assessing the real risk, especially in case of the large-surface-area buildings with varying ways of use of the premises, the average values of fire-load density may not properly reflect the real threat of fire. This is confirmed in the present facility, where in approximately 47% of the total area of the building the fire-load density doesn’t exceed 100 MJ/m 2 . Surfaces for which the fire load density exceeds 4000 MJ/m 2 (in extreme cases, it’s 5644 MJ/m 2 ) represent only about 11% of the total area. It is worth mentioning that the fire-load density exceeding 4000 MJ/m 2 due to the national regulations and codes of design would increase the criterion of structural resistance to R240. A completely separate issue is the fact that the oldest part of the building was completed in violation of some basic technical and construction requirements, so that the structure of this part of the building currently does not meet any criteria for fire resistance. This prompted the owner to implement some solutions that will not only lead the property to become fully consistent with the state regulations but also raise the level of security over the required standards, especially in the areas particularly vulnerable to fire. Presented case study shows that the adopted method of determining the requirements for fire resistance, especially based on the average value of fire-load density, in selected cases can lead to significant underestimations and result in incorrect assessment of a building fire safety.