Abstract: A connecting device for tubular elements (1), for assembling the tubular elements into modular structures, includes a plurality of inserts (22), each insert having an expandable portion (2) and a head (3). Shells (9) are provided, each shell having a plurality of housings (10) having a shape approximately complementary to a shape of the head (3). The head may be fixed to a related housing (10). The insert (22) is locked into an end of the tubular element (1), when the expandable portion (2) is inserted into the end of the tube, and expanded. The tubular element (1) is connected to the shell (9), when the head (3) is in the locked condition, the tubular element (1) being engaged in a related housing (10) of the shell, and fixed to the shell by a fixing device.