STRENGTH OF SINGLE-POLE UTILITY STRUCTURES : Chapter 4

Single-pole utility structures are generally loaded as cantilever beam-columns. Their load capacity varies with geometry, structural mate­ rial, manufacturing process, and support conditions. These parameters, often characterized with varying degrees of uncertainty, are used with mechanics-based as well as empirical-based models (that also introduce some uncertainty) to obtain load capacity or resistance estimates. For these reasons, the design procedure described in Chapter 2 incorporates resistance factors (φ factors in Eq. 2-1a) to account for the uncertainties inherent in the estimates of pole capacity. Pole resistance is a random variable that may be characterized using a probability density function (PDF). For the typical range of pole resistance and load coefficients of variation (COVs), a relatively consistent reliability can be achieved across material types by setting nominal resistance values that represent a 5% to 10% lower exclusion limit (LEL) equal to the design load effect corresponding to a predetermined return period (RP) (IEC 2002; Peyrot and Dagher 1984). For example, setting the 5th percentile pole strength, R5%LEL' equal to the 50-year RP load effect Q50, R5%LEL = Q50, yields resulting reliabilities that are relatively insensitive to the respective PDFs and COVs of the load and strength parameters. In this manual, nominal resistance (Rn) is defined as the strength that will be exceeded by 95% of poles in the target population (see Fig. 1-1 in Chapter 1). In statistical terms, this value is often referred to as the 5th per­ centile (5% of the population lies below it) or the lower 5% LEL. Because it is not practical to require a precise evaluation of the 5% LEL, statistical methods are commonly used to obtain an estimate that has an associated level of confidence. Confidence refers to the probability that a randomly selected sample will have an LEL greater than or equal to the target value. The lower confidence bound on a LEL is called a lower tolerance limit (LTL). In this manual, we refer primarily to a 5% LTL having a 50% or 75% confidence.