The prediction of axial damping in spiral strands

Abstract

This paper addresses the problem of predicting the axial hysteresis in multi-layered helical strands from first principles.Using previously reported orthotropic sheet theory for helical strands with wire compliances derived from results in available contact stress theories, normal forces on the interwire/interlayer contact patches have been estimated. Using such data, the full slip histories on the contact patches from the micro-slips on the periphery at low loads, to the onset of gross slip at higher loads and beyond are predicted. It is shown that cable overall axial hysteresis is dominated by the frictional energy dissipation over the line-contact patches within individual layers. Strand overall hysteresis is predicted by two rather different theoretical methods which provide an analytical double check. In the first method, the overall load-deflection curve, as derived by a previously reported orthotropic sheet model, is used to follow the loading and subsequent unloading response of an axially pre-loaded strand with its ends fixed against rotation. The calculated enclosed hysteresis loop area gives an estimate of strand damping which should closely match alternative predictions based on the summation of energy dissipation over the individual contact patches throughout the strand. The match between these two alternative predictions and available experimental data on some fully bedded-in (old) and substantial 39 mm spiral strand is very encouraging.The practical limitations of the proposed analytical models are critically addressed in the light of recent experimental findings.

Free bending characteristics of sheathed spiral strands under cyclic loading

“…The present model, which is based on well-established models in the field of contact between elastic bodies, does not predict damping for very small amplitudes of interwire movement accurately. In this context, experimental observations suggest internal material damping as the predominant source of energy dissipation (5) (11) (28). Finally, Figs 5(a) and 5(b) present the way sheathed spiral strands lose their bending stiffness, with reductions in the imposed radius of curvature, p, covering a wide range of water depths.…”

“…The assumed coefficient of friction is p = 0.12, which has been found to be a realistic value for galvanized steel wires (5) (11). As discussed elsewhere (28), in the context of axial damping of axially preloaded spiral strands, hysteresis predictions based on orthotropic sheet theory are best applicable to fully bedded-in and old specimens whose pattern of interwire contact has, with passage of time under service conditions, fully stabilized. Experimental data on new specimens reported elsewhere (9) suggest that new specimens need (depending on their construction details) a rather long period of bedding-in for their pattern of interwire contact to stabilize : during this period, hysteretic behaviour changes in a very complex way.…”

Prediction of axial hysteresis in locked coil ropes

“…For a given Sl, corresponding to the rope axial strain under the action of the full value of the applied axial load range, the value of S6T.i in the layer with lay angle ai may be obtained from equations (1) to (8). Alternatively, following the theoretical parametric studies reported fully by Raoof (10, Il), s6T, may be estimated from the following:…”