Method for manufacturing an assembly capable of use in the production of a supporting structure, said assembly being constituted by parts (7,8) made of a fibrous material forming an elongate element that is at least partially hollow (10), with a longitudinal axis of symmetry (9), delimiting an inner area (11) and an outer area (12), characterized in that it consists of producing a localized densification of the fibrous material on the two end portions (13) of the elongate hollow element, in order to exert, simultaneously with or following this densification, on each of the two end portions of the elongate hollow element, outward tensile forces in opposite directions to each other along its longitudinal axis in order to create an axial prestressing force having a determined value, and to maintain the elongate hollow element in this prestressed condition during the installation of the assembly in the structure.