Probability and reliability, the 1990 NESC

The change proposals for the load and strength sections of the 1990 National Electric Safety Code (NESC) are discussed. In the transition from prior code editions, loads and load factors approach independence from structure materials. Working Group 5.1 of the SC5 subcommittee of the NESC proposed an additional method to load wood structures and crossarms with load factors similar to those for other materials. The dual methods permit use of a logical rationale for analysis and design of composite structures and components under equivalent loads. Direct comparisons of the performances of structures or components with composite materials and economic evaluation of structures of unlike materials can be made on an equal loading basis. The performance under the required load will allow review of the true safety inherent in overload factors. Potentially, the separation of the overload capacity factor into component factors of safety, load change, materials, and location will be possible. The probability of load, the reliability of materials, and safety expectations can then be taken into account