Construction in any industry is often guided by empirical results, but calculated designs are always preferred over experiential values. In building transmission and distribution lines within the power industry, the preference is no different. However, providing better designs is challenging when most U.S. states only require the lines to meet the minimum safety requirements of the National Electric Safety Code (NESC).

The NESC was never intended to serve as a design code. Nevertheless, it has become the de facto standard by which many lines have been designed and constructed. One problem with the NESC is that the specified design load and strength factors do not have a strong theoretical foundation. Instead, these factors are primarily based on “successful” experience. As such, some utilities find the NESC requirements to be too conservative, while others feel they are not conservative enough and have seen fit to develop loading criteria to supplement the NESC requirements.

A desire to achieve more consistent structural reliabilities across materials was the impetus for the development of American Society of Civil Engineers (ASCE) Manual 74, “Guidelines for Transmission Line Structural Loading” (ASCE Committee on Electrical Transmission Structures, 2001) and the IEC 60826, “Design Criteria of Overhead Transmission Lines” (International Electrotechnical Commission, 2002). While ASCE Manual 74 offers consistent methods to calculate loads, there is still a need to provide a design methodology for both transmission and distribution poles that yields consistent structural reliabilities across all material types: wood, steel, concrete, fiberglass, as well as any other materials that might be used. To accomplish this goal, for the past two years an ASCE Structural Engineering Institute (SEI) committee has worked to develop a Manual of Engineering Practice entitled “Reliability-Based Design of Utility Pole Structures.” The committee included pole producers, utility representatives, industry consultants and university faculty.

Consistency in Definition of Nominal Strength

One of the primary issues in developing this methodology was to provide consistency in establishing a pole's “nominal strength.” The nominal strength R_n of poles is calculated using a variety of approaches, depending on the pole material type. This includes, for example, ASCE Manual 72 for steel poles (ASCE 1990a), ANSI O5.1 for wood poles (ANSI 2002), PCI guide for prestressed concrete poles (PCI, 1999) and, most recently, ASCE Manual 104, “Recommended Practice for Fiber-Reinforced Polymer Products for Overhead Utility Line Structures.” Additionally, manufacturers of concrete and FRP poles typically use in-house models for predicting the nominal strength of their poles.

These strength guides, standards and in-house methods have evolved independently, so there is little consistency in the definition of nominal strength across pole materials. For some pole materials and design methods, the nominal strength value is close to a mean strength, whereas for other pole materials, the nominal strength represents a more conservative value such as a 5% or 10% lower exclusion limit (LEL). As a result, there is currently a wide range of possibilities as to what the nominal strength of a pole represents. This results in inconsistent and unknown reliability levels for different pole materials.

Figure 1 helps to illustrate this problem. Any strength property of a pole, such as the bending strength of a Class 1 wood pole as determined by cantilever testing, is a random variable. For example, testing 100 identical poles in the same manner will result in 100 different values of the strength. The many sources of uncertainty in observed pole strength inherently includes variabilities in material properties, geometry, manufacturing and testing methods. The strength of a particular pole type is thus best characterized by a probability density function (PDF) with a mean value (m) and a coefficient of variation (COV). The COV (the standard deviation divided by the mean) varies between pole materials and pole types. For example, the COV is typically higher for wood poles than for steel poles. The higher coefficient of variation causes the PDF for wood poles to be wider and flatter than for steel poles. The procedure used to calculate the nominal or characteristic strength of the pole (whether ASCE Manual 72 or ANSI O5.1) yields a nominal strength R_n that falls somewhere on the horizontal axis of the PDF in Fig. 1.

Figure 1 shows three possible positions of the nominal strength R_n = R₁, R₅ and R₅₀, representing a 1%, 5%, and 50% LEL, respectively. For example, a 5% LEL nominal strength is a value that is not achieved by 5% of the poles. It is also referred to as a fifth percentile strength. For a normal probability density function, the 5^th percentile is 1.645 standard deviations below the mean:

R₅ = m - 1.645 (m × COV) (1-1)

Similarly, a 50% LEL or 50^th percentile nominal strength is a value that is not achieved by 50% of the poles. For a normal density function, R₅₀ is also the mean strength. It follows that R₅₀ > R₅ > R₁ and the distance between these values grows with increasing coefficients of variation. Clearly, unless strength design methods for different pole materials yield consistent nominal value definitions (e.g. all materials at the 5% LEL), it is difficult to achieve consistent reliability among material types.

Reliability-Based Design

Failure of a component or a structure occurs when the load (Q) exceeds the resistance (R). If the PDF of the strength and the load are known, then failure of a component may be estimated using the area where the PDFs overlap (Fig. 2). Minimizing the overlap between the two curves is commonly achieved by moving the curves apart (such as by applying safety factors or partial safety factors to the mean or nominal design values). The distance between the means of the two curves, measured in number of standard deviations of R and Q, is referred to as the reliability index (ß). Larger reliability indices mean more distance between the two curves and a smaller probability of failure. Components with equivalent reliability indices ß have relatively equivalent probabilities of failure.

In reliability-based design (RBD), load and strength factors (also called partial safety factors) are selected so that components have relatively equivalent reliability indices. Target reliability indices are typically selected by reliability calibration with respect to conventional or historic designs.

Relation to NESC and Other ASCE Guides

The rules of the NESC contain basic provisions that are considered necessary for the safety of employees and the public under specified deterministic load conditions. As previously noted, the NESC is not intended to be used as a total design specification. The RBD Manual provides a design methodology that would be used in conjunction with any NESC safety requirements.

The RBD Manual refers to ASCE Manual 74's procedures for computing design loads and load factors that are independent of the materials used and types of supporting structures. It is consistent with ASCE Manual 74's approach in that the loading agenda should reflect uncertainties in the loads and the accepted risk that these loads will be exceeded. The RBD Manual provides reliability-based loads and strength factors that may be used along with the ASCE Manual 74 load conditions.

Benefits of RBD

The methodology presented in the ASCE/SEI Manual of Practice is simple and straightforward. RBD of pole structures will be as simple to accomplish as traditional deterministic designs. In addition, because of the way the methodology was calibrated, pole designs, on average, will be equivalent to historical NESC designs for both grades B and C construction. Departures from traditional designs will only occur where those designs were either unsafe or too conservative. While the current RBD Manual focuses on single-pole tangent structures, future editions will have an expanded scope to cover RBD for all utility structures and components.

In summary, using the methodology presented in the new ASCE/SEI Manual of Engineering Practice offers many real benefits:

• Achieves relatively uniform structural reliability across all pole materials and different locations in the United States, thereby allowing utilities to compare the cost of equivalent lines constructed using different materials.

• Provides a means for quantifiably adjusting reliability whenever needed or justified. An essential line can be made quantifiably more reliable than a less important line.

• Defines minimum reliability levels for both NESC grades B and C construction based on reliability analyses of existing NESC pole designs. This enables designers to produce designs to either NESC grades B or C construction with more consistent results.

• Opens the door for innovation by enabling the introduction of new materials into pole design.

• Encourages manufacturers to continually improve their products by providing design incentives for more reliable poles and better databases for strength. A manufacturer that develops better statistical data on pole strength is allowed to adjust the strength factors accordingly.

• Provides uniform procedures for defining the nominal strength of transmission and distribution structures to be used in conjunction with the available strength guides (such as ASCE Manual 72 and ANSI O5.1-2002). It suggests that all future editions of the strength guides provide nominal or characteristic strength values at or near the 5% LEL.

• Complements ASCE Manual 74. The RBD Manual does not provide new methods for calculating loads and load combinations. It refers to ASCE Manual 74's procedures for computing design loads and load factors that are independent of the materials used in the supporting structures. It is consistent with ASCE Manual 74's approach that a loading agenda should reflect uncertainties in the loads and the accepted risk that these loads may be exceeded.

• Brings pole structural design in line with well-established RBD codes such as the American Association of State Highway and Transportation Officials LRFD Bridge Design Specifications (AASHTO, 2002), the American Institute of Steel Construction Load and Resistance Factor Design Specification for Structural Steel Buildings (AISC, 1999) and the National Design Specification for Wood Construction (AF&PA, 1996).

Dr. Habib J. Dagher is director of the Advanced Engineered Wood Composites Center at the University of Maine, and a professor of civil/structural engineering. He has more than 20 years of experience in reliability-based design (RBD) of utility structures and has published extensively on the subject. He currently chairs the ASCE/Structural Engineering Institute Committee on the Structural Reliability-Based Design of Utility Poles. Dagher is a registered professional engineer.
hd@umit.maine.edu

Ron Randle is vice president of engineering at EDM International Inc. (Fort Collins, Colorado, U.S.). Since joining EDM in 1997, Randle has been involved in the design, development, evaluation and testing of wood, steel, concrete and fiberglass pole structures and components. Presently, he is chairman of the American Society of Civil Engineers' (ASCE) standard's committee responsible for drafting, balloting and publishing the first ASCE standard for the Design of Steel Transmission Pole Structures. He also serves as secretary on the ASCE task committee on Fiber-Reinforced Composite Structures for Overhead Lines. He is an active member on the ASCE standards committee for the Design of Latticed Steel Transmission Towers, and the ASCE task committees for Structural Reliability-Based Design of Utility Poles and for Design of Guyed Electrical Transmission Structures. Randle is a registered professional engineer.
Rrandle@edmlink.com