Ventilation conditions are an essential factor in the development of car park fires. This study investigates if larger open wall areas can affect fires in naturally ventilated car parks such that a reduction of the fire resistance of the main load-bearing system is warranted. A set of ten fire simulations with different wind conditions (direction and force) were carried out. Two generic car parks were examined, one with a 21 % open area fraction and one with a 41 % open area fraction. A simplified structural analysis for all scenarios was, furthermore, conducted to investigate the effect of different open area fractions on the collapse time of individual steel beams. The results of this study indicate that the fire resistance of the main load-bearing structure should not be reduced from R30 or R60 to R15, even if the wall surfaces have a larger open area fraction.