ABSTRACT: Self-supporting antenna towers in the Philippines
The study then first establishes guidelines for the selection of a GEF formulation. From among seven (7) formulations found in literature, the gust response factor (GRF) formulation for trussed towers in ASCE’s “Guidelines for Electrical Transmission Line Structural Loading
Data from measurements of thirty-four (34) actual trussed towers from other countries is analyzed, and estimation formulas for the natural frequency and structural damping ratio are suggested. An estimation formula for the aerodynamic damping ratio is also suggested. The estimates are generally on the conservative side, considering the variance in the available data. Using these estimates for antenna towers in the Philippines, those with typical heights from around 30 to 60 meters are estimated to have very low damping, and that the GEF and correspondingly the wind loads in the flexible design are significantly larger than the rigid-structure value in a rigid design, even for natural frequencies greater than 1 Hz. A flexible design is thus recommended for all towers in the Philippines, regardless of natural frequency.
A case study is finally performed on a 60-meter antenna tower, which arrives at similar conclusions in a simple, general one-degree-of-freedom analysis of trussed structures: that if all other parameters in the wind load calculation are correct, antenna towers designed using the NSCP-01 rigid-structure GEF are under-designed according to the requirements of the NSCP-01, although failure in terms of yielding of steel members is not expected. The study then recommends that supplementary material to the NSCP-01 outlining a flexible design procedure for trussed towers, be provided.
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