NESC Rule 250B and Reliability Based Design

Abstract

The National Electric Safety Code (NESC) specifies two load cases for structural design which include radial ice thicknesses in combination with wind pressures/wind speeds. These two load cases are NESC Rule 250B and NESC Rule 250D. Load case NESC Rule 250B “Regional District Loads” is a Deterministic load case which originated in 1908. After several catastrophic failures in Iowa in the 1990s from ice and wind loading, engineers agreed there was a need for a new load case which more accurately accounted for combined ice and wind loads. Engineers analyzed years of meteorological data and created the load case known as NESC Rule 250D, “Extreme Ice with Concurrent Wind Loading.” NESC Rule 250D is a probabilistic load case with a 50-year Mean Recurrence Interval. The Extreme Ice and Concurrent Wind Map, NESC Rule 250D, was first published in the 2002 edition of the NESC, and the NESC specified a Load Factor = 1.0 to be applied to the NESC Rule 250D loads. The NESC Rule 250B with the NESC required load factors (Vertical = 1.5, Trans Wind = 2.5, Trans Wire Tension = 1.65) often results as the controlling load case. This paper will discuss the structural analysis of a 90-degree self-supporting steel deadend from a 345 kV line. This analysis will compare the base reactions for this structure for the 38 unique/overlapping load regions that exist between NESC Rule 250B and NESC Rule 250D within the continental United States. The ratio of the base reactions, (NESC Rule 250B with load factors)/(NESC Rule 250D with load factor = 1.0) will be calculated and mapped. This structural analysis will show that certain regions of the country are designed with probabilistically “high” design loads while many other regions are designed with probabilistically “low” design loads. This can lead to inconsistent levels of reliability between loading regions. Recommendations will be made on how to design structures to have more consistent reliability levels across the continental United States.