Improvements in the design and manufacture of pre-stressed concrete structures subjected to bending moments in two opposite directions and to shear especially railway sleepers or the like

摘要

565,829. Pre-stressed concrete. PRESTRESSED CONCRETE CO., Ltd., and MAUTNER, K. W. Sept. 22, 1943, No. 15516. [Class 20 (iv)] Concrete structures, particularly railway sleepers, are provided with prestressed reinforcements which extend over the full length and are so arranged that, notwithstanding reversed bending moments at different sections, the tensile stress in the concrete is kept below its tensile strength. This is done by (1) forming the concrete with a different cross-section at places of bending moment having opposite sign, e.g. by a rebate in the middle portion of a sleeper, or by varying the width and side slope of the section, so that the relative positions of the centres of gravity of the concrete and reinforcement cross-sections are reversed ; (2) as in (1), but reducing instead of reversing the separation of the centres of gravity ; (3) as in (1), but keeping the separation of the centres of gravity constant, in conjunction with an increase of the pre-compression effect for sections remote from the ends sufficient to keep the tensile stresses within the required limit ; (4) by reversing the relative positions of the centres of gravity, or by reducing their separation while not reversing them, by removing the adhesive bond of some of the stretched reinforcement on a length which has the greater bending moment. Fig. 5 shows a half elevation of a railway sleeper with three layers r1, r2, r3 of stretched reinforcements. Fig. 5A shows 'a cross-section at AA, and Fig. 5B a cross-section at BB. The centre of gravity of the reinforcement is at S and of the concrete at the two sections at G respectively. In Fig. 5A, the precompression produces a higher compressive stress at the bottom than at the top: In Fig. 5B, the compressive stress at the top may be made equal to, or greater than that at the bottom by reducing the distance e1 between S and G to zero, or reversing it as at e2. When the point G still remains above S, but at a less distance than in the section AA, the compressive stress at the top is increased, but is smaller than at the bottom. If the adhesive bond between reinforcement and concrete is removed along a length m of the layer r3, the distance e, Fig. 5A, is increased, but not e1 in Fig. 5B. Specifications 338,864, 338,934, [both in Class 20 (iv)], 364,621, 393,576, 393,606, [all in Group V], 541,835, 554,693, 554,970 and 560,521, [Group XXX], are referred to.