Patent GB-698086-A (GB)
Title: Improvements in or relating to the post-tensioning method of pre-stressing structural members
IPC: B28B23/04
Filed: 1950-06-05
Abstract: 698,086. Pre-stressed concrete; pre-stressed metal beams. SNELGROVE, B. Sept. 5, 1951 [June 5, 1950], No. 13933/50. Classes 20(ii) and 20(iv) A concrete, metal or plastic member is prestressed by anchoring the reinforements at one end to the member, and at the other end to a second member, which may ultimately be joined to the first member, separating the members' to tension the reinforcements, observing, by measuring-gauges, the tendency of the member or members to move out of their original positions of axial alignment, and adjusting the lengths of individual reinforcements to correct this tendency. As shown, parts 51, 52 which are ultimately to form a single beam are engaged at their adjacent ends by shoes 53, 54 on jaws 39, 42 which are separable hydraulically by the ram 32 and are mounted on a crcsshead 6 vertically slidable in guides in uprights 7, the vertically moving assembly being connected to a counterweight by a cable passing over the. pulley 11. The jaw 39 is detachably secured to a block 25, screwed to the crosshead 6, by a T-section portion thereof engaging in a similarly shaped channel in, the block. The cylinder 31 is secured to the block 25 whilst the ram 32 is guided in blocks 27, 29 also secured to the cross-head 6. The jaw 42 is connected detachably with a member 41 by means of a T-sliding joint as in the case of the jaw 39, the member 41 being stepped in its bore to correspond with the stepped ram 32 over which it is threaded. The thrust of the ram is transmitted to the member 41 by a head 44 which is of vertical cylindrical form except that the face 45 is flatted. The member- 41 can thus rock within limits, on the head 44 to accommodate itself to the face of the beam part 52. The jaws 39, 42 each carry a shoe 53, 54 in one or two parts and shaped to be non-obstructive to the reinforcement bars 55. The shoe, or shoes, is flexibly supported for the corresponding jaw by one or more wires having enlarged heads which engage in recesses in the jaw and shoe. The pressure is applied to the shoe by a roller 62 which fits in a recess 63 in the shoe, bears upon a pressure plate 65 and is prevented from dropping out by a ledge 66. The interposed roller 62 allows the shoe to make limited up or down and tilting movements when the pressure. is applied. When the jaws are to be released from the beam parts 51, 52, fluid is admitted at 70, but as the jaw 39 is fixed relatively to the crosshead 6, this would not, in itself, release the jaw 39 from the part 51. Cam means 72, normally urged by a coil spring 76 to press the crosshead 6 to the left, is therefore provided, so that, by pressing the handle 77 downwards the crosshead is free to move to the right to the extent of the space 71 in the crosshead guide 7. The beam parts 51, 52 will, during the ten. sioning of the reinforcements, tend to make small angular movements in vertical and horizontal planes, and these movements are measured to determine the necessary adjustments to be made in individual rein. forcements. The beam parts 51, 52 are laid in line upon a flat table 80 and upon a number of supports 100 carried by ball bearings 101 in such manner that limited longitudinal and transverse movements of the supports relatively to the base 102 are permitted. The reinforcements 55, 56 are threaded through passages 121 which are joined by cross passages 122 to an opening 123 through which liquid grouting is finally injected. Cross-rods 126 are passed through the loops of the reinforcements and inner screwed ends of the reinforcements are connected by adjusters 125. The outer end of the part 51 is held by a resilient pad 85 forced down by a screw 84 in a bridge 81 having side clamps 83 with rollers 86 for engaging the part 51. The inner end of the part 51 is held against side movements by similar clamps 87 on a bridge 88 which carries a gauge 90, 98 for indicating up. ward deflections. The other part 52 is held down at its outer end by a resilient pac 107, but a gauge 110 indicates the amount of transverse swing of the beam end. The upward deflection of the inner end is indicated by a gauge 105, transverse move. ments being prevented by side clamps 103 The pressure in the ram cylinder is released to permit the adjusters 125.to be tightened or loosened, and is then reapplied. When the correct adjustment is obtained, a small excess pressure is applied to increase the gap between the parts 51, 52 and to permit a number of tiles 127, with shims if necessary, to be inserted in the gap. The tiles are held in position by guard plates 128 which are fitted over the ends of the parts 51, 52 and are clamped together by a bolt 129. The pressure is then released gradually, the gauges being watched to ensure that distortion of the beam is not caused by irregularities in the tiles, or if caused, is corrected by shimming. When the jaws are finally removed, and before or after the removal of the beam from the machine; the space between the tiles is filled with concrete and the grout is forced around the reinforcements. The ends are covered with concrete caps 131 if desired The reinforcement rods could be screwed at their outer ends to receive anchoring-nuts, instead of being looped, in which case the adjusters 125 may be dispensed with. The supports 100 may be replaced by a trolley movable longitudinally by rack and pinion and carrying on rollers or balls a transversely movable table to permit transverse movements of the part 52. In modifications, one or both beam parts is or are supported on pivoting means at the centres of gravity of the parts; or one beam part is held against all movement and the other is supported in a sling at its centre of gravity. Instead of making one beam of two parts, two separate beams may be pre-stressed and, if necessary, the ram may be disposed between the beam ends. One beam part only may be prestressed by connecting the reinforcements to an opposed member which is a permanent part of the machine.