Determination of the bending stiffness for a spiral strand

Owing to their peculiar construction, axially preloaded spiral strands undergo plane-section bending only for sufficiently small maximum lateral deflection/span ratios. Beyond a certain limit of this ratio, plane sections do not remain plane and, depending on the level of axial tension and imposed radius of curvature, interlayer slippage takes place, starting from the outer layer and spreading towards the centre of the strand. On the theoretical side, apart from one publication in the mid-1980s and some fairly recent publications by the first author and his associates, all other theoreticians have previously assumed either plane-section bending or, in many cases, have totally ignored the ever-present interwire friction, assuming that the individual helical wires act as simple helical springs. Previously reported experimental techniques, aimed at measuring the effective bending stiffness of helically wound cables, also largely suffer from major shortcomings. In the present paper, a novel experimental method is reported for obtaining reliable measurements of the effective bending stiffness of axially loaded spiral strands. It is very simple and relatively inexpensive to use in practice, for even very large (greater than, say, 100mm outside diameter) spiral strands. Effective bending stiffness measurements, based on this approach, are reported here for a 164mm outside diameter spiral strand (with axial loads of up to 3 MN), with these supported by a detailed theoretical analysis, which reinforces the generally sound nature of the proposed method.