PSI - Issue 44

Agnese Natali et al. / Procedia Structural Integrity 44 (2023) 2334–2341 Agnese Natali, Francesco Morelli / Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction Automated Rack Supported Warehouses (ARSWs) are storage solutions for palletized goods where steel racks are used also as the primary structural system of the building. To follow the fast-evolving market request, they acquired most of the structural features of traditional racks without being supported by a specific regulatory framework: structural types, profiles shapes and configurations (cold-formed thin walled elements), as well as technical solutions are kept from the traditional ones. The lack of dedicated design rules is especially felt in seismic applications, as highlighted by local damages and collapses happened after seismic events. Indeed, although this structural system is relatively new compared to the traditional steel structures (Natali et al., 2022), very little effort has been put into the development of a seismic design guideline for them (Haque and Alam, 2015). As a consequence, standards for buildings (like Eurocodes) may be used for their design, but they are hardly applicable to given the ARSWs’ structural peculiarities which make them different from standard steel buildings (Caprili et al., 2018). In case of difficulties in applying these prescriptions, current regulations for traditional racks are allowed to be adopted, like EN16681(2016), which in any case appear to lead to possible unsafe design (Natali et al., 2022a, 2022b). In this framework, possible solutions for dissipative seismic-resistant ARSWs are studied by the authors (Natali et al., 2022c), based on existing and under study solutions for similar structural types (Morelli et al., 2016, 2019, Braconi et al., 2015, Caprili et al., 2022). Indeed, a new design approach is proposed, named the “Over-resistant Connection Strategy” (OCS), which foresees concentration of dissipation in the braces constituting the racks arranged in the X tension-only structural scheme. The design procedure is based on the Eurocode 8 prescriptions for medium-Ductility Class DC2 X tension-only structural schemes (EN 1998-1:2004: Eurocode 8: Design of structures for earthquake resistance – Part 1: General rules, seismic actions and rules for buildings., 2004), with few adjustments to meet ARSWs’ structural needs. These modifications are properly calibrated on a Double-Depth case study structure (one of the most adopted structural typologies for ARSWs), whose configuration is showed in Fig. 2. As showed in this figure, a partial collapse mechanism is aimed to fully optimize and exploit the sources of the structure, given that the higher forces concentrated in the lower part. The core of the design method is the design of the dissipative element – the diagonal – and its’ connection to the upright, which must be over-resistant with respect to the brace. Indeed, according to Eurocode 8 capacity design rules for concentric bracings systems (2004), the connection of the dissipative element to the other components shall be designed by applying the (1): ≥ · ℎ · (1) where is the resistance of the connection; is the plastic resistance of the dissipative element, based on the nominal yield stress of the material; is the material randomness factor in the dissipative zones; and ℎ is the hardening factor in the dissipative zone. This condition is not easy to be fulfilled for ARSW structures: Fig. 1 shows the typical upright-to-diagonal connections, where diagonals are directly connected to uprights through a single bolt. Given the low thickness of the diagonals, the leading resistance of the connection is the bearing one , , which can be evaluated as in (2), according to EN 1993-1-3 (EN 1993-1-3:2006 Eurocode 3: Design of steel structures - Part 1-3: General rules - Supplementary rules for cold-formed members and sheeting, 2006): , = 2.5 · · · · · 2 (2) where depends on the geometry of the connection; depends on the thickness of the element; is the ultimate strength of the material of the element; is the diameter of the hole, and is the thickness of the element; 2 is the safety coefficient for checking steel connections. Disregarding the two coefficients and which can be easily maximized to the highest allowed value 1.00, to increase the bearing resistance of the connection it’s necessary to increase the thickness of the diagonal, use a stronger material, or using more than one bolt. The first two options also increase the diagonal plastic resistance , so increase the lower bound of resistance for connections,