Ramboll has prepared detailed fatigue analyses of two typical 300 m guyed masts in Sweden. The purpose of the investigation was to establish the lifetime of guyed masts now that they are around 50 years old.
The client Teracom is the main TV and Radio Broadcast provider in Sweden and Denmark and operates more than 700 guyed masts and self-supporting towers.
Quite some of Teracoms tall guyed masts have now been in operation for 50 years, and they shall still be in operation for many more years. Guyed masts are normally not designed for fatigue, but as they are sensitive to the dynamic wind load Teracom would like to have an indication if their old masts remaining life time to fatigue.
The two selected masts were analysed for fatigue vibrations due to vortex shedding, buffeting and galloping.
The response from vortex shedding was found using the theory of negative aerodynamic damping taking in to account the different turbulent scenarios, the Weibull distribution of the wind and the different wind scenarios from different wind directions.
The fatigue from buffeting was analysed using the stochastic theory for buffeting.
Finally was the fatigue from galloping found using the modal analysis.
In the analyses the life time of the critical hot spots in the masts were found.
One of the main parameters in the analyses is the structural damping and no proper values are available in the literature, so it was decided to measure the structural damping of one of the masts.
In order to perform the analyses the computer software Mainmast was used. Mainmast is developed by Ramboll and is able to prepare stochastic, modal and patch wind analysis of self-supporting towers and guyed masts.
An important parameter for the analysis is the actual wind climate on the sites since this information is used to find the life time of the structures.
It was found that the most critical phenomena for the fatigue analysis is vortex shedding, and with the measured damping it could be concluded that the masts have not reached their fatigue lifetime
Apart from the fatigue analysis the foundations was measured for air voids around the bolts using a Impact-Echo (IE). The system is a Non-Destructive Test system (NDT). The advantage of the system is that it gives strong reflections from air voids/pockets until certain depths and depending of the size of the void.