1 Structural Loading Calculations Of Wood Transmission Structures Keith Malmedal Member IEEE Sen, Ph. D, , Senior Member IEEE. Senior Engineer/Project Manager Professor of Engineering NEI Electric Power Engineering Colorado School of Mines Arvada, Colorado 80001 Golden, Colorado 80401. Abstract: The most critical task in the design of II. NESC METHOD. any structure is to determine the loads that the structure must withstand. In the case of The NESC has traditionally been an ultimate stress Transmission line pole structures, currently there design method where all factors of safety are included are two available methods commonly utilized to in the Loading conditions by applying applicable calculate the Environmental loads: wind and ice.
2 Overload factors. Three cases for transverse Loading The first method is suggested by the National are considered. Electrical Safety Code (NESC). This is an ultimate stress method where all factors of safety are 1. General Loading due to wind on wire and pole included in the loads. The second option, with ice. recommended by the American Society of Civil Engineers (ASCE), calculates the forces that must 2. Extreme wind on all structures without be resisted by the structure and may be used in an conductors or ice. This provision is new in the ultimate strength method, where wood is the pole 2002 NESC. construction material. This later technique may also be used in a load and resistance factor design 3.
3 Extreme wind on conductor and pole without (LRFD) with other common materials. This paper ice if the structure exceeds 60 ft in height. compares the advantages and limitations of the two methods. Numerical examples will be provided Case 1: showing how the design may differ depending upon which method is employed. The NESC defines three general Loading areas in the United States: heavy, medium, and light. Figure 1. defines these Loading areas. For each of these Loading I. INTRODUCTION areas general wind and ice loads are also defined as described in Table 1. Wind load is calculated There are two available options that may be used to including ice on the conductor but not on the structure.
4 Calculate the design loads for transmissions structures. The minimum design requirements are provided by the National Electrical Safety Code. The American Society of Civil Engineers suggests an alternative method. Even though, in the 2002 edition of the NESC, efforts have been made to conform the two Loading methods, differences still exist. The two methods result in differing design criteria for choosing structures. This paper focuses mainly on the transverse Loading of tangent type wood Transmission structures due to ice and wind loads and the numerical results illustrate the differences between the two methods. Figure 1: Loading Map .
5 1. Table 1: Loading Per District  Table 3: Gust Response Factor GRF. Heavy Medium Light Hgt. Structure Wire GRF, Span Length (ft). Radial Thickness 0. of ice (inch) (ft) GRF <250 250- 500- 750- Horiz. Wind 4 4 9 500 750 1000. Pressure (lb/ft2) < 33 Temp. 0 F 15 F 30 F 35-50 50-80 Cases 2 and 3: 80-115 115-165 Load cases 2 and 3 require the extreme wind pressure 165-250 to be calculated. The method for making this calculation is also new in the 2002 NESC. The following equation is utilized to calculate the force due to extreme wind. Loading in pounds =. (Vmi/h ) 2 k z G RF I C d A (1). Where: Vmi/h = Basic Wind Speed at 33 ft above Ground kz = Velocity Pressure Coefficient GRF = Gust Response Factor I = Importance factor ( for utility structures).
6 Cd = Shape Factor for circle or ellipse A = Projected wind area in ft2. The basic wind speed Vmi/h is taken from Figures 2 or 3. The thickness of ice is taken as 0 for extreme wind Loading . The velocity pressure coefficient (kz) is dependent upon conductor height or pole height and is found by using Table 2. Table 2: Velocity Pressure Coefficient (kz) . Figure 2: Basic Wind Speed . Height (ft) Structure Wire < 33 35-50 For final Loading calculations, two different rules are described in the NESC. Both rules require multiplying 50-80 the loads by an overload factor and multiplying the 80-115 ultimate pole strength by a strength factor.
7 115-165 165-250 For transverse wind Loading and wood construction the overload factors and strength factors to be used for the The gust response factor (GRF) is a function of height first rule are shown in Table 4. and span length. It may be found from Table 3 for span lengths of 250-1000 ft. Table 5 shows the overload and strength factors if the second allowed rule is applied. 2. The overload and strength factors may be combined into a single overload-multiplying factor that will used to multiply the load. Since the strength factors in rule 2 are all , the multiplying factor for rule 2 is the same as the overload factors in Table 5.
8 However, the overload and strength factors from rule 1 may be combined into the single set multipliers shown in Table 6. Table 6. Rule 1 Overload Multipliers Construction Grade B C. Wind Extreme Wind The overload multipliers thus produced are comparable to the rule 2 multipliers. III. ASCE METHOD. The ASCE calculation technique is applied to an ultimate stress method of design. It also lends itself to Figure 3: Basic Wind Speed  a load and resistance factor design. But for comparison purposes the ultimate stress application is Table 4 only examined. For transverse Loading due to wind Rule 1 Overload and Strength Factors and ice, two Loading calculations must be examined.
9 (Transverse Loads). Construction 1. Calculated design wind on wire and structure with no ice. Grade B C. 2. 40% of calculated design wind on structure Wind and wire with ice. Extreme Wind Strength Factor (wind) The following equation is suggested for calculation of Strength Factor (extreme wind) force due to wind Loading . Table 5 F = (Z v V) 2 GC f A (2). Rule 2 Overload and Strength Factors (Transverse Loads). Construction Where: Grade F = Force in lbs B C. Zv = Terrain Factor Wind (at crossings) V = Fastest mile wind speed (from map) in mph (elsewhere) G = Gust Response Extreme Wind Cf = Force Coefficients ( is recommended ).
10 Strength Factor (wind) A = Area exposed normal to the wind direction in ft2. Strength Factor (extreme wind) 3. The fastest mile wind speed may be obtained from the There are two gust response factors, one for the map in Figure 4. conductor and one for the structure. For exposure C. the gust response factor (Gw) for conductors is shown in Figure 5. Figure 4: Fastest Mile Wind Speed . The terrain factor Zv is dependent upon the type of Figure 5:Conductor Gust Response Factor Gw . terrain, which is divided into three exposure types. Exposure B is urban, suburban, or wooded areas, The gust response factor (Gt) for structures is shown in exposure C is flat open country, and exposure D is Figure 6.