Development of typological seismic fragility curves for chemical industrial installations

Legged steel storage tanks, widespread in the wine, food and process industries, are generally unanchored. They rest on their legs by gravity alone, and the frictional contact between the leg base plates and the floor governs their seismic response. Post-earthquake surveys in Chile, Emilia and New Zealand identify this configuration as one of the most vulnerable typologies of the process industry, with damage concentrated in the legs and at the base interface. Design codes provide no dedicated procedure for it. This dissertation develops seismic fragility functions and annual failure rates for unanchored legged steel tanks by nonlinear time-history analysis of equivalent mechanical (lumped-mass) models built in midas Gen NX. The liquid is the Housner–Malhotra two-mass analogue, the legs are propped cantilevers, and the base contact is a set of tension-free Coulomb friction elements. The modelling chain is first validated by reconstructing the five legged-tank archetypes of a published study. It is then applied to a real installation: the eleven legged vessels of a Veneto tank park, modelled from their fabrication drawings. Twelve configurations convert contents density and fill level into quantified sensitivities, and 2160 incremental dynamic analyses on fifteen recorded Italian three-component motions carry them to fragility. The park divides into two failure-mode domains: the slender head-mounted vessels uplift first (medians 0.08 to 0.14 g), while the bracket-mounted reactors plastify their legs before the first support opens (0.14 g for the 20 m^3 units); base sliding never governs. Convolution with the official site hazard gives annual rates and a vessel ranking in which the two 20 m^3 reactors exceed the 1.0e-4 per year ASCE 7 target for Risk Category III by an order of magnitude. Comparison with the archetype study, with 140 Italian wine tanks, with the New Zealand empirical fragilities and with anchored Chilean tanks extends the slenderness taxonomy into the uplift-governed regime and reproduces the damage hierarchy observed in the field.