Discussion: Reinforced concrete joints between prestressed concrete members

Abstract

The paper by Mr Reeves and Professor Morice provides further support for the use of reinforced concrete joints between prestressed concrete members. In their discussion of joint design, and specifically of the behaviour of this type of joint at ultimate load, the authors suggest that, at transfer of prestress, the stresses in the lower fibres of the span members at sections adjacent to the supports should be low compressive or tensile stresses. Tests of joints of the type shown in Figure lc of the paper, recently carried out in the Structural Development Laboratory of the Portland Cement Association (1), have shown that this requirement is not always necessary.Pairs of precast prestressed I beams were joined together by deformed reinforcing bars placed longitudinally across the joint in a deck slab cast on top of the girders, as shown in Figure I. The space between the ends of the girders was filled by concrete forming part of a transverse diaphragm placed at the joint. The specimens were supported below the joint and loaded by equal point loads at each end. In these tests, the amount of normal reinforcing bars in the deck slab, expressed as a percentage of (depth of girder x width of bottom flange), was 0·83, 1·66 or 2·49 /~. The initial precompression of the bottom flange of the pr,ecast girders was 0, 2,100 or 3,100 Ibjin 2 • The ultimate strength of each connexion measured in the tests was compared with the calculated ultimate strength, any influence of precompression of the bottom flange of the precast member being neglected, and also with the calculated ultimate strength taking this precompression into account. From a study of these measured and calculated ultimate strengths it was concluded that, for the practical range of continuity reinforcement of from 0·5 to 1·5 %, the influence of precompression of the bottom flange may be neglected in the calculation of the ultimate strength of this type of joint. These tests and the conclusions drawn from them are discussed fully in PCA Development Department Bulletin D34(1).Similar tests have been carried out on connexions between prestressed double-T members (2), in which deformed bars were placed in the topping concrete ·Pages 13-20 of Magazine No. 37. 160 covering the top flange of the member. Equally favourable results were obtained in these tests.The behaviour of this type of connexion has also been studied in tests of continuous composite beams(1-4), and has been found to be most satisfactory. One of these tests is seen in progress in Figure II. The specimen consists of two 33-ft long precast prestressed I beams joined by deformed bars placed in a 39 x 3 in. thick cast-in-place concrete deck slab. The beam is a half-scale model of one beam and its portion of deck slab taken from a two-span continuous highway bridge. The beam was designed for the envelope of maximum moments caused by the standard H20-S16 loading of the American Association of State Highway Officials. The test loading simulated the equivalent H20-S16 lane loading, arranged so as to cause maximu...