Reliability analysis of pole-type transmission structures

Abstract

The unguyed single-pole transmission structure is widely used in the U.S. to support transmission lines of up to 345 kV capacity. Most poles are naturally grown wood, but some poles of prestressed concrete and some of steel are used, especially in high decay or high load situations. The resistance of each structure is a random variable which can be significantly influenced by time and location. Material strength and stiffness properties, especially of wood, vary from structure to structure, even for nominally identical poles. The life expectancy of poles varies with geographical location with poles in hot, humid climates having lower life times than those in hot, dry areas. The individual load causing phenomena of dead weight, wind velocity, and accretion of ice and snow on the structure are also random variables. Additionally, wind velocity, terrain conditions, temperature and structure shape interact to affect ice and snow accretion. Design of a structure to provide a known level of reliability requires that a reliability-based design methodology be established. To accomplish this the variable nature of the resistances and load causing phenomena must first be defined. Next, a correct method of analysis which properly considers all important response characteristics such as the highly nonlinear behavior of pole structures must be developed. The function of the analysis is to convert loads to load effects directly comparable to resistance. Finally, a procedure which allows the designer to size structures so they will provide the desired level of reliability must be created. A major part of producing such a practical design procedure is developing a method for performing the reliability analysis of transmission structures. As part of an eight-year effort to develop reliability-based design methods for transmission structures, several methods for performing reliability analyses of single-pole transmission structures were established. Implemented in program POLDAR (POLe Design, Analysis and Reliability), these procedures enable the user to perform a reliability assessment of unguyed planar single-pole structures of any material and configuration for the major loads normally considered in design. It is shown that the load and resistance factor design (LRFD) method developed based on reliability analysis results obtained with POLDAR produces structures having reliability levels very close to the target levels assumed by the designer. Thus a reliability-based design method is now available for use in the design of new structures and the assessment and upgrading of existing structures. Since over 130,000,000 wood poles are now in place, and several million additional poles are put into service each year at an in-place cost of $1000 to $2000 each, it is expected that improved design procedures will have a significant financial impact.