plate forming side walls roof an floor in a tank according to the invention the plate comprises a sandwich structure 10 with two surface sheets 11 11 u2032 and a core material 12 between the sheets 11 11 u2032 there are grid stiffeners 13 running from one surface sheet 11 to the other surface sheet 11 u2032 the cross section is shown as a plane plate but may of course be arced to form a circular tank wall as shown in fig7 in fig9 is there shown one embodiment of the grid stiffeners 13 where the grid stiffeners 13 are plate elements with a width of the plate running from one surface sheet 11 to the other 11 u2032 the length of the plate element is running parallel to the surface sheet of the sandwich structure one may see from this figure that the outer surface sheet and the inner surface sheet will have a varying internal distance between them it can be seen from the width of the grid stiffeners 13 which at the corners of the structure have a larger width than for the rest of the walls it can be seen form the figure that the inner sheet will have rounded corners and the outer sheet will have right angled corners and therefore a varying distance between the surface sheets in the sandwich structure the grid stiffeners 13 may be plate elements or rods or other structures running from one surface sheet 11 to the other surface sheet 11 u2032 fig1 shows a detailed perspective view of a second embodiment of the grid stiffeners 13 arranged onto an outer sheet 11 of the sandwich structure the grid stiffeners.