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Project: T153700 #33/104
 

The report presents a systematic methodology for determining the reliability of site-specific overhead line transmission structures considering the time dependent deterioration of decay for wood pole structures and metal loss (thickness loss) due to atmospheric corrosion for steel structures. In an earlier study entitled “Reliability Based Calibration Model for Latticed Transmission Towers: A Guide for Utility Engineers,” a step by step methodology was provided to determine the probability of failure of a lattice steel tower considering load and strength interference as lognormal distributions. An Excel-based program was developed to determine tower reliability when no deterioration has taken place.

This study is an extension of earlier work where the load distribution is extreme value type I and strength is a lognormal distribution. Calibration of the model is validated by comparing the results with the published work, and results show agreement between the computed probability of failure of a structural component based on the present formulation.

Two specific time dependent deterioration models are developed where pole decay due to fungal attacks at ground line level and metal loss of steel structures due to corrosion are considered. The reduction in capacity is directly related to the pole’s bending capacity (for wood pole and steel tubular pole structures) and a steel tower leg member’s capacity for axial compression. The time dependent capacity model at discrete ages (during the service life of the asset) is later used to determine the expected mean and coefficient of variation of the respective deteriorated capacity following FOSM (first order second moment). The input from these models is later used in a fragility model where the probability of failure of the respective structure is determined by convolving (by integrating) the fragility distribution function of the strength with load density function. The fragility model is applied to three types of structures: wood pole H-frames, steel lattice towers and tubular poles.

A sensitivity study is carried out where several key parameters are considered and their impacts on the probability of failure of these three structure types are assessed and presented. Finally, a simple line reliability model is presented considering the line as a “series” system where it is shown how the correlation of strength of several similar elements (structures) can influence probability of failure when the line is treated as a system. A simple cost risk model is also developed to show how a decision to replace the structure or not can be made by balancing the cost of replacement (depreciated capital value at a specific age of asset’s life) against the present value of the expected cost of future losses of the structure.

Keywords:

Structural Reliability, FOSM, FORM, Aging Issues, Deterioration, Transmission Lines, Line Maintenance, Wind and Ice Loads, System Reliability, Pole Decay, Corrosion of Lattice Tower, Tubular Pole Structures, Decision model, Asset Refurbishment, Replacement