Reliability-based evaluation of the inherent safety presumptions in common fire safety design

• A probabilistic model is developed for assessing the safety level of concrete slabs during fire.
• The safety level at the fire resistance time is compared for different slab configurations.
• The safety level of slabs during fire is highly sensitive to the variability of the concrete cover.
• Fire ignition frequencies are derived from reliability calculations.
• Reliability-based fire ignition frequencies are compared to empirical frequencies.

The structural safety of a simply supported one-way concrete slab exposed to the ISO 834 standard fire curve is investigated. The proposed methodology allows for an objective comparison of the structural performance of different concrete slab design alternatives and to derive indirectly a lower bound of the fire ignition frequencies that can be associated to the Eurocode guidelines. A comparison is made with empirical fire ignition frequencies found in literature and a detailed study is presented of the significance of the concrete cover and its uncertainty on the safety level and fire resistance time. Results indicate that the safety level during fire and the fire resistance time are both highly sensitive to the ratio of the variable to the permanent load and that the accepted design solutions given in EN 1992-1-2 are rather conservative.