"Irrigidimento" also means stiffening (also bracing), so I would go with stiffened plates.
Simply putting rigid doesn't work, as metal plates are rigid essentially by definition.

Here's a scientific study on these plates, referring to ships.

Studied here was how some particular stiffened plate structures in ships fail and thus ways to improve knowledge of the steps which can be implemented to avoid such failures. Incorporation of stiffeners alone does not lead to optimum design. The effects of varying stiffener-thicknesses, geometrical imperfections (due to constructional errors), material behaviour and modelling techniques can be equally important - the first two were considered. Many buckling and nonlinear analyses using the ANSYS program were performed.