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This paper focuses mainly on the transverse loading of tangent type wood transmission structures due to ice and wind loads and the numerical results illustrate the differences between the two methods.
The most critical task in the design of any structure is to determine the loads that the structure must withstand. In the case of transmission line pole structures, currently there are two available methods commonly utilized to calculate the environmental loads: wind and ice. The first method is suggested by the National Electrical Safety Code (NESC). This is an ultimate stress method where all factors of safety are included in the loads. The second option, recommended by the American Society of Civil Engineers (ASCE), calculates the forces that must be resisted by the structure and may be used in an ultimate strength method, where wood is the pole construction material. This later technique may also be used in a load and resistance factor design (LRFD) with other common materials. This paper compares the advantages and limitations of the two methods. Numerical examples will be provided showing how the design may differ depending upon which method is employed.
There are two available options that may be used to calculate the design loads for transmissions structures. The minimum design requirements are provided by the National Electrical Safety Code. The American Society of Civil Engineers suggests an alternative method. Even though, in the 2002 edition of the NESC, efforts have been made to conform the two loading methods, differences still exist. The two methods result in differing design criteria for choosing structures.
The NESC has traditionally been an ultimate stress design method where all factors of safety are included in the loading conditions by applying applicable overload factors. Three cases for transverse loading are considered.
The NESC defines three general loading areas in the United States: heavy, medium, and light. Figure 1 defines these loading areas. For each of these loading areas general wind and ice loads are also defined as described in Table 1. Wind load is calculated including ice on the conductor but not on the structure.
|Structural Loading Calculations Of Wood Transmission Structures|