Loss of prestress in spiral strand guy ropes in masts

Abstract

ABSTRACTIt's known since long that guy ropes in masts lose prestress over time. In many cases this leads to increased utilization of the mast. The problem is usually solved by regularly retightening the guys. The cause of the loss of prestress is not entirely clear, but guys in masts exposed to frequent and large vibrations, seem to be the most affected.Lately at manufacture, the guys undergo a number of large load cycles, thereby enabling the threads to get relocated and tightened in the spiral strand guy rope, in order to reduce the loss of prestress. Certificates describe the load cycles, strains and calculated Young's Modulus during this process. Apparently Young's Modulus doesn't only vary with the load range, but it also differs when loading and unloading. All together this means, when unloading the guy during a large enough load cycle, it will not completely regain its previous length, but instead gain a small remaining elongation.By being exposed to large vibrations in masts, guys can be elongated, something that in this paper is referred to as an explanation for the loss of prestress.This paper presents an analytical model, describing the loss of prestress over time in a guy, exposed to large vibrations in a mast. Prestress measurements from mast inspections also show consistency with the model.Based on three prestress measurements of a guy, carried out with a minimum of a few years interval, the model offers an efficient strategy for choosing when to retighten, or estimating if the expected loss of prestress is small enough not to require adjustment. In addition, the model offers guidelines for choosing guy dimension and initial prestress, in order to actively reduce the loss of prestress and extend the time between retightenings.