FAVINI PRESTRESSING SYSTEM

1951
“Favini system” for prestressed structures
Invention patent no. 482296 of 18.12.1951

«…The equipment in question is composed of ferrules for the fastening of cables and a jack with a double effect, with an outer balancer crown, for simultaneous traction of a group of six lines. The standard ferrule is formed by a core of nodular cast iron, encircled in steel, bearing a suitably shaped central hole with a truncated conical form (upper base diam. 24 mm, lower base 19 mm) with three trapezoid-section hollow grooves. The groove has a width at the base that is less than the sum of the diameters of the two lines 5 mm in diameter, whereby the latter cannot rest against the inner surface of the groove if they are not powerfully pressed and deformed. A truncated conical blockage plug in mild steel ensures the anchoring of the rods against the walls of the groove by means of friction. Since the base width of the groove, as stated above, is less than the sum of the diameters of the lines, all the lines are either in direct contact with the steel plug or are pressed by the neighboring line lodged in the same groove. Therefore all six of the lines will necessarily have a generatrix in contact with the wall of the groove. The component force perpendicular to the surface of the groove itself ensures anchoring by friction. In the preliminary phase of the pull, also by virtue of a suitable line guide, the lines arrange themselves as in the detail…

…The jack for the tensioning of the lines has a double effect. The lines emerging from the ferrule are anchored in pairs, by forcing with special cotters, on a circular crown that rests on the central part of the jack itself thanks to a spherical joint. The crown will serve to balance possible lack of uniformity of the lines. The blocking of the lines on the ferrule happens by forcing the truncated-cone male into the corresponding central socket. In an initial blockage phase the male will run on the lines, then placing force on them to clasp them deeply inside the walls of the housings. The final position sees the six lines side by side and forced between the wedge of mild steel and the cast iron walls…

…In the light of what has been outlined above, we believe the main advantage of the ferrule described herein is that of ensuring excellent grip of the lines with a device whose working requires no particular finishing or grinding. The walls of the housings are rough cast and require no further mechanical workmanship. The two lines with a diameter of 5 mm forced into the hollow by the play of the different hardness of the cast iron walls and the mild steel cotter, anchor themselves by friction and by plastic deformation…

…From the whole of the results outlined above the following considerations can be derived: the efficacy of the blocking obtained with the examined device seems to be very satisfactory. It can be seen that the reduction of tension caused by settling of the blocking device is relatively moderate, which could be particularly advantageous for the tensioning of short cables, such as those used, for example, for the solidarizing of precast parts or the creation of prestressing on limited zones of hyperstatic structures; the grip in the time of blockage can be considered excellent; the tensions developed in the ferrule at the moment of blockage are moderate and guarantee a wide margin of safety. The steel rimming seems suitable to eliminate possible doubts that could arise due to the use of nodular cast iron for the central core; the fatigue tests have shown that at the moment of blocking the lines do not weaken dangerously, which would reduce the available margins of safety.
These considerations as a whole make it possible to conclude: the equipment in question, if used with the procedures described by its designer, is suited to ensure, in a satisfactory way, the tensioning and blocking of reinforcements, composed of groups of six steel cables with a high elastic limit, for use in structures of prestressed reinforced concrete.»

From: Cesare Castiglia, Un nuovo tipo di attrezzatura per strutture precompresse, in “Giornale del Genio Civile”, no. 1, January 1954, Roma.