The impact of horizontal projections on external fire spread - A numerical comparative study

Sammanfattning: The aim of the report was to investigate and compare the effect of horizontal projections along with different spandrel heights between the unprotected openings in the building facade. The effect was more specifically the effect on the risk level in regards to the external vertical fire spread. Furthermore, based upon the performance requirements of the Swedish building regulations (BBR), the aim was to compare the effects of the different protection methods. From the validation study it was shown that FDS version 6.2.0 is well suited as calculation tool for the problem area given the presented settings. The main conclusions drawn from the report was that the use of at least a 60 cm deep horizontal projection results in lesser consequences and lower risk levels at the facade above the projection compared with the scenarios built up by different spandrel heights. Also, the conclusion was that the use of at least a 60 cm deep horizontal projection results in lower risk levels above the projection compared with the risk level accepted in the prescriptive part of the BBR. The opposite was seen for the 20 cm and 30 cm deep horizontal projections. The use of 60, 80 and 100 cm deep horizontal projections resulted in 15-50 % reduction in the relative exposure of adiabatic surface temperature at the facade 1.2 m above the underlying opening. The main horizontal projections used in the report were 20 cm thick rectangular non-combustible balconies with open sides and no separation walls, extending 0.4 m on each side of the underlying openings. Based on the findings in this report, a spandrel distance of at least 1.2 m in the building exterior as stated by the BBR can be replaced by at least a 60 cm deep horizontal projection positioned at any height above the underlying opening. However, additional simulations on other setups are needed to determine the general use of this outcome. Furthermore, recommendations were given on the design of BBR for the problem area.