Patent GB-709521-A (GB)
Title: Improvements in or relating to methods of stressing concrete
IPC: E04G21/12
Filed: 1951-04-26
Abstract: 709,521. Prestressed concrete beams &c. CHU, I. C. April 22, 1952 [April 26, 1951], No. 9733/51. Classes 20 (4) and 87 (1). [Also in Group XXIII Wire or cable reinforcement a for prestressed' concrete is tensioned by restraining the end portions against movement, determining unrestrained sections along the length by the use of elements b which engage the wire &c. at opposite points thereon to prevent twisting, and twisting these unrestricted sections. As shown, the ends of the wire &c. are embedded in anchorage blocks k while the remainder is located in recesses in the member and exposed to permit the twisting operation. Dowels b extending from the concrete member and free to turn about their axes restrain the wires at selected points. Twisting is effected by means of clips c, Fig. 11, each comprising two portions which grip the wire between them and are secured together by screws h, the clips and wire &c. being secured, after twisting, by pins o passing through holes in the clip into the concrete member. The clips may be turned by means of a two part slotted toothed wheel p assembled around the clip by means of screws and plates q, Fig. 12, and driven by a chain t from a second toothed wheel which is mounted on a spindle r and rotated by means of a handle u. The tension of chain t is adjusted by means of turnbuckles s in the frame v in which spindle r is mounted. In an alternative twisting apparatus, Fig. 13, the two part wheel assembled around the clip c has spaced peripheral holes or notches x into which a lever arm w is inserted. With an I-section beam the wires are grouped lengthwise preferably in groups of two, as in Fig. 4a. When the reinforcements are housed in the concrete members, as in Figs. 8a and 10a, openings m are left in the member through which the clips c are fitted and turned, the openings then being filled. When the member is solid as in Figs. 8 and 8a the reinforcements are diposed in tubes while the main portion of the member is cast with openings m, and the tubes then removed, and the anchoring members k cast. All sections may be tensioned or selected sections only as in Fig. 7a wherein sections g are twisted and sections f not twisted, and the clip c for any section may be arranged centrally, as in Fig. 4, or nearer to one pair of dowels to give a variable tension in the one section. The reinforcement may comprise looped wires to provide double runs for twisting ropewire. In addition to the twisting the wires may also be stressed by pulling, e.g. bolts and nuts or small jacks, as in Fig. 7a, wherein the dotted line represents the original position of the reinforcement and i the point of pulling. Figs. 14 and 15 illustrate continuous concrete beams constructed according to the invention, and a portal framed structure, a column, and a pile are also illustrated. Fig. 19 shows a paving block, the tensioned reinforcement being situated in recesses adjacent one or both faces. A table is given showing the percentage of elongation of the wire for particular d/e ratios where "d" is the diameter of the wire and "e" the pitch of the twist, and examples show the use of the table to calculate the number of half turns to be given to the clips to produce a required tension. The stressing of the reinforcement may be carried out before the concrete is cast, using temporary anchorages &c.