Reliability of poles in NESC grade C construction

The reliabilities of steel and wood poles designed according to the 1997 National Electric Safety Code provisions for Grade C construction were evaluated. It was found that the reliability of steel poles may be significantly higher than that of wood poles. Reasons for the discrepancy relate to the historical development of the deterministic design approach used in the National Electric Safety Code specifications. For correction of this discrepancy, fundamental principles of structural reliability were used to develop modified load factors and strength factors applicable to all materials under Grade C construction. A load factor of 1.75 is proposed for all materials, and a table of strength factors is proposed. The strength factors are a function both of the coefficient of variation of the pole strength and the lower exclusion limit corresponding to the nominal design strength value. As an interim measure, it is proposed that the modified load and strength factors for Grade C construction be allowed as an alternative approach in the National Electric Safety Code. Ultimately a complete rewriting of the National and-resistance-factor-design approach will be necessary to guarantee consistent safety levels across all materials and structure types.