A wires laid at a predetermined bend, a bend protection member, an atmosphere temperature and the initial shape and the final bent shape of the wires and the bend protection member are designated, and infinite element models are prepared for the wires and the bend protection member. A pair of caps can be connected to each other and sealed when in a pre-use state. The depth D1 of the point P1 from the tread face being in a range of from 0. The retaining structure can alleviate toppling or other loss of structural integrity of the container structures. The invention provides that the computer unit is provided to compare the parameter to at least one comparison parameter calculated by means of at least one model.