Abstract: A rigid assembly of at least two intersecting constituent members which form between them at least one engagement canal made of two opposite holes in two brackets of one constituent member, each with an inwardly converging wall, and a registering transversal bore with divergent end portions in the other constituent member. A tightening device is accommodated within each engagement canal which comprises a first cylindrical member with a centrally bored frusto-conical head portion, a second cylindrical member slidably accommodated therein and having a screw-threaded central bore and a frusto-conical head portion, and a tightening screw connecting both cylindrical members, each of said frusto-conical head portions being of smaller size than the associated bracket hole and having a slant matching that of the side wall of the latter. When the tightening screw is tightened, the first and second cylindrical members of the tightening device are drawn together and the constituent members are fastened to each other.