1. Prestressed supporting structure for supporting an upright vessel comprising a generally horizontally disposed frame and spaced downwardly extending legs supporting the frame and rigidly connected at their upper ends to said frame, said legs being essentially straight when unstressed and when fully loaded, said legs being prestressed by beiNg bent by displacement of their bottom ends; and means prestressing said legs before loading of said supporting structure with a load including said vessel so that the bottom of each leg is displaced and held in a direction and by an amount essentially corresponding to the direction and the amount said bottom of said leg would move if said supporting structure was fully loaded with a load including said vessel and so that the leg is bent, said means comprising means supporting each of said legs from the bottom of the leg, before loading of said supporting structure, at a location that is off-center of the point at the bottom of said leg at which the force resulting from the load supported by the leg would be concentrated, said location being off-center in a direction extending opposite to the direction at which the bottom of the leg is displaced and off-center by a distance such that after the supporting structure is fully loaded there is no prestress remaining in any of the previously prestressed legs and no shear or bending moment at the bottom of any such leg.