Ice and wind loads on guyed masts

Sammanfattning: Atmospheric icing is a large problem for many structures located in regions with cold climate. Structures as guyed masts and overhead transmission lines and towers are particularly exposed. A 323 meter high guyed TV and radio mast in Arvidsjaur in Northern Sweden was equipped with an extensive data collection system in the winter 1988-89. The winter before another mast, similar but weaker than the mast in Arvidsjaur, had collapsed due to an overload of heavy ice. The primary purpose of the measurements was to study the effects of ice load and also the effects of combined ice and wind load. It was of particular interest to find out how large ice loads that could be expected, since the ice loads used in the design of masts was solely based on visual experiences of how much ice that could be observed in masts. The older masts, built during the fifties and sixties, were not designed for ice load at all since this type of load was deemed as negligible in comparison to wind. A detailed analysis of the measured ice and wind loadss is presented. Suitable distributions for the evaluated loads are proposed and design values, i.e. characteristic values, for the reference mast in Arvidsjaur are given. In addition to the characteristic loads, appropriate load reduction factors are determined for combined load cases. Those load reduction factors are obtained by a probabilistic analysis of the combined load effect of ice and wind load. The combined load effect is however a strongly nonlinear function and the analysis has therefore to be based on simplifications. A reliability analysis must also include statistical parameters and uncertainties connected with the resistance of the components. Much of this statistical information is found in the background documents of the Eurocode 3, and it will be briefly presented herein. Today, the design of guyed masts is based on the semi-probabilistic partial coefficient method. The partial coefficient used for the variable ice and wind load is very much influenced by the strongly nonlinear load effect function for the combined load. Appropriate partial coefficients for masts that experience the same environmental loads as the reference mast in Arvidsjaur are proposed herein. The usage of these partial coefficients is intended to correspond to an annual target probability of failure of 0,0001.