Computational and experimental substantiation of the fire resistance of unprotected steel mezzanine racks made of thin-walled profiles

Introduction. Mezzanine structures are subject to a regulatory requirement of 15 min for loss of load-bearing capacity under the standard temperature-time regime. At present, no approved calculation method is available for determining the actual fire-resistance limits of steel mezzanine rack structures or the limiting internal forces in load-bearing members needed to achieve the R15 rating under the applied loads, including loads from subsequent tiers.

Aim and Objectives. The aim of this study is to provide computational and experimental substantiation of the fire resistance of warehouse mezzanine racks made of thin-walled cold-formed steel profiles manufactured by PAO Severstal. To achieve this aim, the following objectives were addressed: calculation schemes and load cases were determined for two representative fragments of a mezzanine rack; the fire-resistance limits of the structural elements were calculated; fire tests were conducted to determine fire resistance according to the R criterion and to validate the calculations; and applicability limits for the obtained results were established.

Methods. Calculation schemes and load cases for two representative mezzanine rack fragments were determined, and the fire-resistance limits of structural elements were calculated using two methodological approaches: direct reduction of mechanical properties followed by recalculation of the load-bearing capacity in accordance with GOST R 56567–2015, and a method based on the provisions of TKP EN 1993-1-2–2009 for Class 4 cross-sections. Fire tests of two mezzanine-fragment specimens were conducted in accordance with GOST 30247.0 and GOST 30247.1 to determine fire resistance according to the R criterion and to validate the calculations.

Results and Discussion. It was established that unprotected thin-walled rack structures, when their cross-sections and section factors are selected correctly and effective section properties are accounted for with experimental validation, can provide an inherent fire-resistance limit of at least R8. This provides a basis for developing a design methodology for rack mezzanines with regulatory justification for use in facilities requiring a fire-resistance limit of R15.

Conclusions. The refined calculation dependencies and coefficients are not universal for all racking systems or all LSTC profiles. Their application is permissible only for structures similar to the tested specimens in terms of material, profile shape and thickness, perforation pattern, calculation scheme, joints, stress level, section factor, and fire exposure conditions.

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