Kazinczy

Kazinczy (1914) tested two steel beams, each about 6 m long, which were embedded at their ends in substantial abutments; the loading, which consisted of increasing numbers of courses of bricks, was uniformly distributed. The steel beams were in fact encased in concrete, but Kazinczy easily dissects out the conclusions that apply to the steel alone. If the ends of the beam in fig. 9.1 (a) were perfectly fixed, then conventional elastic theory gives the bending-moment diagram sketched in fig. 9.1(b); the beam must be designed for a maximum bending moment of value wl2/12. The explicit question asked by Kazinczy, to which the experiments were designed to provide the answer, was whether the end embedment may be taken to be complete and, if not, what degree of fixity may be assumed.

The concrete provided an effective tell-tale to monitor the progress of the experiments. As the loading was increased, cracks in the casing first appeared at the ends of the beams, indicating yield at those points. However, the beams could carry further load, and it was not until a substantially greater weight had been added that deflexions became very large. Upon unloading, each beam was found to have permanent kinking deformation, at the two ends and at the centre. Kazinczy called these kinks ‘hinges’, and he states that a fixed-ended beam cannot collapse (undergo increasing deflexions) until three hinges have formed. Two (end) hinges merely transform the fixed-ended into an effectively pin-ended beam; the third central hinge is necessary for collapse. Moreover, says Kazinczy, the degree of end clamping is irrelevant, provided the embedment is strong enough to allow the hinges to develop.