Consideration of Sustained Loads and Creep Effects in Specifying and Designing Fiber Reinforced Polymer (FRP) Utility Poles

Fiber reinforced polymer (FRP) utility poles are used to support overhead utility lines for the electric power industry. Manufacturers commonly determine ultimate strength of their product using full-scale physical testing of poles under short-term loading and then base the design strength on the 5% lower exclusion limit, as outlined in the National Electric Safety Code (NESC). This is adequate for characterizing the strength of FRP poles under short term load conditions, but does not adequately address cases of long-term, or sustained loading. FRP materials are affected by creep, which is a time-dependent deformation under sustained (long-term) loads. For most FRP materials, design for sustained loads is limited to a fraction of the short-term strength. The NESC, however, does not provide guidance on the effects of creep on FRP structures, and the NESC strength factors do not explicitly account for creep. Utility providers and their engineers must therefore interpret manufacturer data to understand if creep has been considered in the manufacturers’ published design strengths. This paper reviews the existing industry standards and guidelines currently applicable to FRP utility pole design in the United States, discusses approaches used by manufacturers to obtain design strengths for FRP poles, and recommends code clarifications to assist manufacturers, designers, and specifiers in understanding how sustained loads and creep effects are incorporated into FRP pole design.