Guyed masts are a specialized type of structure commonly used in the broadcasting industry to support equipment at substantial heights.The dynamic analysis of these structures under seismic loading is a much understudied field that requires investigation.
The complex nature of their analysis arises from the nonlinear force-deflection relationship of the cable supports as well as P-D effects in the mast. These lead to the structure exhibiting significant nonlinear characteristics even under working load conditions. Full nonlinear analysis of guyed masts is rarely performed as it is complex and time consuming. Masts are usually designed by equivalent static methods for wind and ice loading only, with the seismic load case often assumed to be less onerous. The validity of this assumption is investigated as part of this research.
In this project four existing guyed masts in the UK with heights ranging from 99m to 312m are accurately analysed under various seismic loading conditions using SAP2000 structural analysis software. The research aims to gain an understanding into the distribution and magnitudes of forces developed during typical seismic and design wind events, establish indicators and trends that may aid in guyed mast design, and identify the circumstances in which seismic loading may be the governing load case. Investigations into the ‘travelling wave’ effect, the significance of vertical motion, and the suitability of a response spectrum analysis are also undertaken. Applicable sections of Eurocode 8 are followed wherever possible.
It is shown that when subjected to substantial seismic events with peak ground accelerations in the region of 4m/s2, significant forces can develop in masts that are comparable to those produced during a wind assessment using the Patch Load method. The distribution of forces can be appreciably different from a wind loading analysis and any regions with irregular or inconsistent distribution of wind response forces can be vulnerable to seismic loading, particularly when mean hourly design wind speeds are less than 22m/s. It is also shown that enough common or predictable seismic behaviour exists between masts to suggest that the development of a simplified seismic analysis method for guyed masts is feasible.
Source: University of Oxford
Author: Matthew Grey