Loading Analysis of Transmission and Distribution Structures based on NESC 2017

Posted by

National Electrical Safety Code (NESC) Part 2 aimed for the practical safeguarding of persons during installation, operation, or maintenance of power and communications lines and their associated equipment.

Loading Analysis of new and existing structures is vital in achieving safety and reliability. This series of articles will present the NESC rules relating to loading of structure with step-by-step example calculation.

3 Classifications of Load

The forces acting on a power line structures can be classified according to their direction: vertical load, transverse load, and longitudinal load.

Vertical Load

Vertical Load is defined as a force acting vertically due to gravity.

On the other hand, uplift loads, which occur due to uneven terrain and cold temperatures, are acting against gravity.

The vertical load on the structure includes its own weight plus the weight of the insulators, hardware, wires and cables (iced or non-iced) together with the effect of any difference in elevation of supports.

Transverse Load

Transverse Load is defined as force or pressure acting perpendicular to the direction of the line. However, for angled structures and angled dead-ends, it is parallel to the bi-sector of the line angle.

Transverse load on the structure includes the following:

>Wind load on the structures

This load shall be computed by applying, at right angles to the direction of the line, the appropriate wind pressure determined by NESC weather loadings below.

This load shall be calculated using the projected surface of the structures and equipment without ice. Appropriate force coefficients (shape factors) should be used.

>Transverse load from conductors

The calculated transverse load from conductors shall be based on the wind span, the average of the two spans adjacent to the structure concerned.

>At Angles Structure

The loads on angled structures shall be the vector sum of the transverse wind load and the wire tension load. In calculating these loads, a wind direction shall be assumed that will give the maximum resultant load.

Note that the wire tension to be used is due the ice and/or wind plus the “k” constants in Table 251-1. The sag-tension spreadsheet on the previous posts can be used to easily calculate this.

Longitudinal Load

Longitudinal Load is defined as force or pressure acting parallel to the direction of the line. For angle structures and angled deadends, it is perpendicular to the bisector of the line angle.

Longitudinal Load may be due to the following;

>Single DeadEnd Structure

This is simply the total wire tension on one side of the structure.

>Double Dead-End Structure

This is due to the unbalance pull created by difference in tensions.

>Unequal spans and unequal vertical loads

>Stringing Loads

Design Factors to Consider

A. NESC Grade of Construction

The grade of constructions generally determines different margins of safety. Higher grades of construction translate to a higher level of structural reliability and safety to withstand the environmental conditions.

The three NESC-defined construction grades are:

  • GRADE B – This grade of construction provides the highest margin of safety and is required when the pole supports spans that cross limited access highways, railroads, and waterways.
  • GRADE C – This grade of construction is most common and provides a basic margin of safety. It is often utilized for the typical power and joint-use distribution pole.
  • GRADE N – This is the lowest grade of construction and is most often for emergency and temporary construction.

B. Strength Factor and Load Factor

The fundamental design philosophy behind current transmission structure design is that all structures shall be designed and detailed in such a way so as to sustain imposed factored design loads without excessive deformations and stresses.

The Load Factor (LF) accounts for the uncertainty of the given load and/or simplifying assumptions made in the analysis. This factor increases the applied load on the structure based on the required construction grade. LF values is based on Table 253-1.

Strength factor (SF) decreases the efficient strength of the structure.The Strength Factor accounts for the variability of the resistance property. SF values is based on Table 261-1.

C. NESC Weather Loadings (Rule 250)

NESC specifies three weather loading requirements in which the one that has the greatest effect shall rule the structure design.

The structural capacity provided by meeting the loading and strength requirement of NESC Rules provides sufficient capacity to resist earthquake ground motions.

Note that Rule 250B and Rule 250D does not include height adjustment factor for wind speed unlike Rule 250C.

Another important note here to the reader is that NESC 2017 does not require the Rule 250C and 250D if no portion of a structure or its supported facilities exceeds 18 m (60 ft) above ground or water level.

1. Combined Ice and Wind District Loading (NESC Rule 250B)

The data available from the United States Weather Bureau and from wire-using companies relating to the frequency, severity, and effect of ice and wind storms in various parts of the country provided the basis for dividing US into four loading districts. They are shown on the map below as heavy, medium, light and warm island loading.

For a given transmission line, only one of the four zones is applicable unless the line crosses more than one zone.

Warm island loading applies to islands located from latitude 25 degrees south through 25 degrees North.

2. Extreme Wind Loading (NESC Rule 250C)

The structure and its supported facilities shall be designed to withstand the extreme wind load associated with the Basic Wind Speed. It shall be applied without ice. The Basic Wind Speed is found in the figures (maps) below.

The following formula shall be used to calculate the wind load.

\begin{align*} Wind_{Pressure} &= Q * V^2 * k_z * GRF * I * C_F\\ Wind_{Load} &= Wind Pressure * A \end{align*}
Q -Numerical CoefficientV -VelocityWind PressureA-Area
0.00256mph (miles / hr)psfft^2
0.613m/s (meters / second)Pam^2
47.3 x 10^-6kph (kilometers / hr)kPam^2
  • Q – Velocity-pressure numerical coefficient reflects the mass density of air for the standard atmosphere
  • kz – Velocity pressure exposure coefficient
  • V – Basic Wind Speed, 3 s gust wind speed in m/s at 10 m above ground
  • GRF – Gust Response Factor
  • I – Importance Factor, generally taken as 1.0 for utility structures.
  • CF – Force Coefficient (Shape Factor)
  • A – Projected wind area

The wind pressure parameters (kz, V and GRF) are based on open terrain with scattered obstructions (Exposure Category C in ASCE 7-10). This exposure is the basis of the NESC extreme wind criteria.

Topographic features such as ridges, hills, and escarpments may increase the wind load on site-specific structures. A Topographic Factor, kzt, from ASCE 7-10, may be used to account for these special cases.

Notes on kz and GRF:

Velocity pressure exposure coefficient, kz:

  1. kz for structure is based on 0.67 of the total height h of the structure above ground line.
  2. kz for wire is based on the height h of the wire at the structure
  3. kz for a specific height on a structure or component is based on the height h to the center-of-pressure of the wind area being considered.

The following formulas will determine the value of velocity pressure exposure coefficient, kz. Alternately, Table 250-2 can be used.

\begin{align} k_{z-Structure} &= 2.01 * {(0.67 * \frac{h}{275})}^{\frac{2}{9.5}} &h \leq 275m\\ k_{z-Structure} &= 1.85 &h \geq 275m\\ k_{z-wire} &= 2.01 * { (\frac{h}{275})}^{\frac{2}{9.5}} &h \leq 275m\\ k_{z-wire} &= 2.01 &h \geq 275m\\ \\ Minimum\;k_z=0.85 \end{align}

Gust Response Factor, GRF:

  1. GRF for structure is based using the total structure height h. When calculating a wind load at a specific height on a structure, the GRF should be determined using the total structure height h.
  2. GRF for wire is based on height of the wire at the structure and the span length L.
  3. GRF for components, such as antennas, transformers, etc., shall be the structure gust response factor determined in #1.

The following formulas will determine the value Gust Response Factor, GRF. Alternately, Table 250-3 can be used.

\begin{align} GRF_{Structure} &=\frac{1+(2.7E_s{B_s}^{0.5})}{{k_v}^2} \\ E_s &=0.346{[\frac{10}{0.67h}]}^{1/7}\\ B_s &=\frac{1}{1+0.56(\frac{0.67h}{67})}\\ \\ \\ GRF_{Wire} &=\frac{[1+(2.7E_w{B_w}^{0.5})}{{k_v}^2} \\ E_w &=0.346{[\frac{10}{h}]}^{1/7}\\ B_s &=\frac{1}{1+0.8(\frac{L}{67})}\\ \end{align}
  • Es – Structure Exposure Factor
  • Bs – Dimensionless response term corresponding to quasi-static background wind loads on the structure
  • Ew – Wire Exposure Factor
  • Bw – Dimensionless response term corresponding to quasi-static background wind loads on the wire
  • kv – 1.43
  • h = Structure or wire height, in meters
  • L = Design wind span, in meters

3. Extreme Ice with concurrent Wind Loading (NESC Rule 250D)

This refers to the situation where ice build-up on a transmission wire is accompanied by small wind.

The structure and its supported facilities shall be designed to withstand loads associated with Uniform Ice Thickness and Concurrent Wind Speed as specified in the figures (map) below.

The wind pressure for the concurrent wind speed shall be as indicated in Table 250-4.

Ice is assumed to weigh 913 kg/m^3 (57 lb/ft^3).

  1. For Grade B, the radial thickness of ice shall be multiplied by a factor of 1.0
  2. For Grade C, the radial thickness of ice shall be multiplied by a factor of 0.8

Steps in Conducting Loading Analysis

  1. Evaluate the applicable Grade of Construction
  2. Apply NESC Rule 250B by selecting the appropriate loading district.
    • Calculate the transverse, vertical and longitudinal load.
    • If angled structure, calculate the transverse component of wire tension using the sag-tension principles.
    • Apply Load Factors
  3. Apply NESC Rule 250C (Extreme Wind)
    • Select Basic Wind Speed in the appropriate maps
    • Calculate the transverse, vertical and longitudinal load.
    • If angled structure, calculate the transverse component of wire tension using the sag-tension principles.
    • Apply Load Factors
  4. Apply NESC Rule 250D ( Extreme Ice with Concurrent Wind)
    • Select ice thickness and wind speed in the appropriate maps.
    • Convert the wind speed into wind pressure using Table 250-4.
    • Calculate the transverse, vertical and longitudinal load.
    • If angled structure, calculate the transverse component of wire tension using the sag-tension principles.
    • Apply Load Factors

What’s Next?

Applying the steps above would yield to these three load cases.

Load Case 1 – Either Light, Medium or Heavy Loading District

Load Case 2 – Extreme Wind

Load Case 3 – Extreme Ice with concurrent Wind

These load cases will be used in selecting the appropriate wood poles to be installed which will be presented later. Also, these will be used in designing new steel pole.

On the succeeding articles, sample calculations will be presented to illustrate the principles above. Also, a spreadsheet in aid of manual calculation will be presented.

Example 1: Pole Loading Analysis of Tangent Structure based on NESC 2017

References:

  1. National Electrical Safety Code 2017 Edition
  2. Design of Electrical Transmission Lines.2017- by Kalaga,S. and Yenumula,P.
  3. Pole Loading 101-Osmose Utility Services, Inc.
  4. Guidelines for Electrical Transmission Line Structural Loading 2010-ASCE
  5. Design of Steel Transmission Poles 2012 – ASCE 48-11
  6. CIGRE Brochure 324: Sag-Tension Calculation Methods for Overhead Lines