Patent GB-696097-A (GB)
Title: Improvements in or relating to pre-stressed reinforced concrete beams
IPC: E01D2/04
Filed: 1951-04-16
Abstract: 696,097. Pre-stressed reinforced concrete. LIPSKI, A. April 16, 1951 [April 24, 1950; March 1, 1951]. No. 8845/51. Class 20(iv) In a method of forming a reinforced concrete beam in which the.reinforcement is deflected in direction in which it will be deflected under load and concrete is made to adhere to the reinforcement before the cause of the deflection is removed, the reinforcement is a rigid metal girder which by itself resists bending and the concrete is applied only on a part of the deflected rigid beam which does not include the most compressed fibres. The concrete may be applied before, after, or at the same time as the bending. The beam may be bent mechanically by sup. porting it at its ends and applying a transverse force at its centre, e.g. by means of a screw jack bearing against a fixed support, the bending force being gradually reduced when the concrete 7 has set, and the uncovered part of the beam 2 then covered with concrete 11. It is stated to be advantageous to subject the reinforcement 2 to a tensile stress greater than that to which it is subjected in service, and which momentarily exceeds the original elastic limit of the beam. The flanges of the beam may be provided with integral or added projections 20 and with plates 21. to increase the adhesion with the concrete. Two beams may be constructed simultaneously as shown in Fig. 7, by connecting the reinforcements together at both ends by ties 23, a portion of the flanges 24 being cut away for this purpose moving the two centres apart by means of a jack 26, and holding them in their deflected position by struts 27. To equalize the deflections of the two girders by correcting the effect of the weight of the beams and struts, the centres of the beams may be raised by a jack 30 and held in that position by struts 31. When the applied concrete has hardened the struts 27, if they are adjustable, are gradually reduced in length, or the jack is again applied so that the struts 27 may be removed and the gap between hte beams 2 gradually reduced until a balanced state is assumed. In alternative arrangements the beams s are secured together at the ends and the centres moved towards each other, or secured together at the centres while the ends are moved towards or away from - each other. Fig. 12 shows a construction in. which part of the reinforcement 2 is removed after the concrete 7 has set, and the additional concrete 11 is bonded by holding bars 28. The deflection of the reinforcement may be brought about by a thermal treatment thus the reinforcement shown in Fig. 13 rests on supports at its ends, and the lower flange is heated by passing steam through tubes 50 in contact with the upper surface 51 of the flange, while the upper flange is kept cold, this difference in temperature resulting in the reinforcement becoming longer at its base than at its top part, so that it bends downwardly. The heating is applied until the concrete 7 has hardened. The flange 32 may be cooled by passing a refrigerant brine through tubes in contact with flange 32. The base of the reinforcement may comprise closely spaced tubes or rods 42 joined together and to the web 44 of the beam by transverse members 43. A heating fluid may be passed through the tubes, or if rods 46, Fig. 19 (not shown) are used they may be heated electrically. The deflection. of the beam may also be produced byapplying an expanding cement to that part which will be tensioned, the swelling of the concrete deflects the beam, and ordinary concrete may then be applied to the remainder of the reinforcement. Fig. 15 shows a beam of large dimension with two co-operating lattice reinforcements 2 which rest on two supports. The concrete 37 which forms a roadway is cast on the upper part and this load may be increased, e.g. by adding earth 38 and by the passage of vehicles 39, the combined load tensioning the lower part of the beams which are then covered with concrete 7. The invention may be applied to cantilevers, to continuous beams with numerous bays, :and to porticos &c. and the beams may be of lattice type &c.