a structural metal sheet employed as an outer sheet for a railway vehicle , for example , is generally pretensioned in order to prevent buckling . hitherto , the pretensioning is effected by various methods after or during the assembly of a structure . according to the invention , the metal sheet itself is pretensioned by a plastic working for increasing the flexural rigidity thereof . thus , there is no fear of buckling , and the workmanship is improved as compared with the prior art .