Fire safety is of utmost relevance for steel industrial buildings. Steel, though highly versatile and performing, features an inherent vulnerability to fire due to its high thermal conductivity. In addition, steel structural members typically feature high section factors, condition which further exacerbates the structural vulnerability. To prevent structural failure, passive protection is usually adopted. Therefore, the structural members expected to be exposed to high temperatures are either (i) painted with intumescent coatings, (ii) covered with sprayed plastered or (iii) encased by means of plasterboards. Beyond the different response to high temperature due to their own nature (i.e. reactive vs. non-reactive), the fire-resistance of such materials is affected by several additional factors, relevant to both inherent material properties (e.g., microstructure, chemical composition, free and chemically bound water content), as well as external aspects (e.g., input flux, heating rate, installation methods, and eventual damage corrupting the integrity of the protective cover).

This article aims to provide insights on the effect of passive protection on the fire vulnerability of steel industrial buildings, since the path towards the application of modern Fire Safety Engineering principles to protected structures is still littered of open issues, also proposing a methodology of cost-benefit analysis, useful to guide both the designer and the owner of the structure in the design of the protection material. As a matter of fact, modern codes worldwide miss in providing detailed prescriptions about the application of the performance-based approach when passive protection is adopted. The authors, in this contribution, present some preliminary results of an extensive research activity, including experimental and numerical investigations. Relying on experimental evidence about the behavior of different protective materials exposed to non-standard heating regimes and the results recently published by authors related to fire fragility curves for steel and composite structures, fragility curves have been derived for a typological steel building (i.e. car park or vehicles storage), with respect to either bare and protected configurations. The above-mentioned fragility curves have been used to (i) quantify the beneficial effect of passive protection in terms of fire vulnerability reduction, and (ii) serve as basis for a cost-benefit analysis, to identify the best strategy to mitigate the vulnerability for a given set of fire scenarios. The proposed methodology combines (i) experimental investigation, (ii) numerical modelling, and (iii) fragility analysis. The experimental activity has allowed to detect thermal and chemo-physical properties of the protective materials of interest - in this study a water-based intumescent coating and a lightweight gypsum-based sprayed plaster - when exposed to non-standard heating regimes.

The experimental characterization has provided evidence to derive simplified correlations between the effective thermal conductivity of the protective layer and the temperature of the protected steel underneath, to exploit in the advanced thermo-mechanical analyses of the structure against 27 different natural fire scenarios. The latter have been defined by varying type and topology of the vehicles, contributing to the fire, within the car park. Each scenario has been related to two intensity measures, (i) heat release rate (HRR) peak and (ii) fire load (q_f). The advanced thermo-mechanical analyses have been performed, by means of the FE-based software SAFIR, on the bare steel structure, in order to set a reference, and on the same structure, protected with intumescent coating and sprayed plaster, alternatively. Then, for each analysis the demand-to-capacity ratio (DCR) has been calculated, where (a) the demand has been taken equals to the maximum displacement at the top of the column, normalized with respect to its height, and (b) the capacity has been set equals to either 1/100 and 1/250, relevant to the performance levels (PLs) of limited damage and full functionality, respectively. Finally, cloud analyses have allowed to derive the fragility curves for the above-mentioned PLs.

In this contribution, the authors propose a novel methodology to conduct cost-benefit analyses to identify the best fire design choice, conceived to fulfil the requirements in terms of fire resistance and economical sustainability. Relevant to the latter, it is worth mentioning that several aspects play a crucial role: (i) target performance levels, (ii) fire intensity, (iii) unit, installation and maintenance costs of the protective material. According to the mentioned aspects, the methodology shows that in some cases the use of passive protection materials is not the optimal solution, but redundant instead.