The results of this study indicate that the fire resistance of the load-bearing structures should not be reduced from R30 - R60 to R15, even if the wall surfaces have more than 1/3 open area fraction.

Relevant regulations in Norway, Sweden, Denmark and Finland have been surveyed. The following main rules apply to load-bearing systems in car parks in these Nordic countries:

• Car park with two floors : R 30 – R 60
• Car park with three and four floors : R 60
• Car park with more than four floors : R 60 – R 90 – R 120

In Norway and Sweden, subject to different prerequisites, the fire resistance may be reduced to R 15. In Denmark and Finland, however, the use of R 15 for the load-bearing system in car parks is not allowed. Sweden and Finland require the installation of an automatic sprinkler system if the fire resistance is reduced. Of the four Nordic countries, only Norway uses the open area fraction in wall surfaces as a basis for reducing the fire resistance.

A total of ten fire simulations with different wind conditions (direction and force) were carried out. Two generic car parks, one with less than 1/3 open area fraction and one with more than 1/3 open area fraction, were examined and a simplified structural analysis was conducted.

It is emphasized that the CFD simulations and structural analysis involve a number of uncertainties and limitations. The focus is therefore on a comparison of car parks with a different open area fraction.

The analysis showed that an increased open area fraction both entails a positive and a negative effect on the structure’s load-bearing ability in a fire, depending on whether wind conditions are favorable or not.

Regardless of wind conditions, the structural analysis showed that expanding the open area fraction from 21 % (i.e. less than 1/3) to 41 % (more than 1/3) has a smaller effect on the collapse time than reducing the fire resistance from R 30 to R 15. The difference is even more pronounced in a reduction from R 60 to R 15. By using R 60 none of the beams collapsed. The results of this study indicate therefore that the fire resistance for load-bearing structures should not be reduced even if the wall surfaces have more than 1/3 open area fraction.

In what way open wall surfaces impact a car park fire is highly dependent on the fire scenario and wind conditions. These two factors cannot be controlled. Dimensioning the fire resistance to the main load-bearing system in a car park based on the open area fraction of wall surfaces is therefore considered unreliable. Open wall surfaces contribute in some cases to improving visibility conditions in car parks, which may extend the available escape time. For this reason, open wall constructions are nevertheless considered advantageous.

The study is commissioned by the Norwegian Directorate for Civil Protection (DSB) and the Norwegian Building Authority (DiBK).
Read the full report in Norwegian with English summary (pdf):