Effects of Surface Decay on Remaining Strength of Transmission-Line Wood Cross-arms

This paper develops a model that correlates the amount of surface deterioration to the remaining strength of rectangular Douglas-fir cross-arms used in a 115-kV transmission line. One challenge that utility companies face today is to develop effective strategies to replace cross-arms that are potentially detrimental to the transmission-line system. In service cross-arms are continuously exposed to the environment and experience wide ranging temperatures and moisture conditions. Furthermore, rainwater entrapment on surface and bolt holes causes rotting and decay of wood, resulting in gradual loss of strength. The evaluation of the remaining strength of in service cross-arms is complicated further by a large variability associated with the wood microstructure, the decay process, defects, and environmental conditions. To understand the effects of surface decay on the strength of wood cross-arms, an image-processing technique is employed to accurately quantify the amount of deterioration and examine the effects of decay location. Full-scale bending tests are used to assess the remaining strength. Statistical models have shown that surface decay, depending on its location, has a direct linear relationship with the strength loss for wood cross-arms in bending.