APPENDIX C

DESIGN EXAMPLE

Example 1: For the TUC-1 pole structure and loading conditions given below, determine the "standard class concrete pole":

General information:

Line voltage: 138 kV

Design by: ACME Engineers

Structure type: TUC-1 Concrete Pole Structures

Geometry of the structure and location of loads:

 

Distance from Pole Top, Ft.
OHGW 0.25
COND-1 7.50
COND-2 17.50
COND-3  27.50
At Gd. Line-assumed 70.00
Pole-End  80.00

 

Overall pole length is 80 feet. The above dimensions assume a 10.0 foot embedment depth for the concrete pole (using standard rule for wood poles of 10 percent pole length plus 2 feet). Assume top of the pole has a 12 inch diameter, and the groundline diameter is 30 inches.

Overload Factors (OLFs) used in this example:

For NESC Light, Medium, or Heavy Loading District Loads

 

Vertical 1.50
Transv. Wind 2.50
Longitudinal Loads 1.65
For Extreme Wind Loads 1.10
Wind on Pole 2.50
Transv. Line Angle 1.65

Conductor and OHGW Data:
OHGW:  3/8"HSS
R.B.S = 10,800 lbs.
138 kV Conductor:  Drake (795 26/7 ACSR)
R.B.S = 31,500 lbs.
Vertical Span 900 ft.
Horizontal Span 750 ft.
Line Angle 0 degrees

Load Cases:

Load Case A: NESC Medium District Loads
With an unbalanced longitudinal
Load of 700 lbs. at each conductor

Load Case B: 80 Mph Extreme Wind Load
(1.1 OCF applied)

 

Loading Information (summary):

NESC Medium Loading Data
   

Transverse

Vertical
 

Cond. Tension (kips)

lb./ft.

lb./ft.

Drake – 138 kV

7.91

.536

1.516

OHGW – 3/8 HSS

2.56

.287

.463

Extreme Wind Loading Data (16 psf)
   

Transverse

Vertical
 

Cond. Tension (kips)

lb./ft.

lb./ft.

Drake – 138 kV

6.54

.9091

1.0940

OHGW – 3/8 HSS

1.23

.4800

.2730

Calculate forces and moments at the groundline:
 

Load Due to Wind on Wire

Load Due to Line Angle (kips)

Total Transv. Load W/OCF (kips)

Moment Arm Feet

Ultimate Moments Ft. kips
@Groundline

OHGW

.22

0

0.54

69.75

37.5

COND-1

.40

0

1.01

62.5

62.8

COND-2

.40

0

1.01

52.5

52.8

COND-3

.40

0

1.01

42.5

42.7

Groundline

0.0

 

Totals for Wire Loads

3.57

  

195.8
       

 

Wind on the Pole

1.23

  

36.8
 

Moments due to unbalanced vertical Wire Load

8.2
 

Moment due to deflection for weight of pole and for wires (p-delta moment)(Approximated)

29.1
 

Moments due to rise of insulators

    

negligible
 

Total Transverse Shear and Moments at Groundline

4.78

  

269.2

For the unbalance longitudinal load, the shear is 2.1 kips and the longitudinal moment is 121 ft.-kips (.7k @ each cond.)

 

TOTAL RESULTANT GROUNDLINE MOMENT = 296 ft. kips

Similar calculations are performed for the extreme wind load.

TOTAL GROUND LINE MOMENT FOR THE EXTREME WIND LOAD = 324.1 ft. kips

Determine which "standardized" concrete pole design to use:

Distance 2’ from top to groundline = 70’ – 2.0’ = 68'

Load 2’ from the top to cause a 324 ft.-kip moment at groundline: = 324 ft-kips/68’ = 4770 lbs.

Based on the above calculated tip load, use a C-04.7 pole (The strength is within one percent of the required strength)

Perform a quick check to verify the assumed embedment depth using Bulletin 1724E-205, "Design Guide: Embedment Depths for Concrete and Steel Poles".

 

Example 2: An existing 115 kV single pole line is composed of Douglas Fir wood poles. In several locations, concrete poles are to replace wood pecker damaged wood poles. The existing damaged poles are 80 ft class 1 wood poles with the TUS pole top assembly. Determine which standard size pole should be used to replace the wood pole. Extreme wind design load is 16 psf (80 mph). The line is located in the heavy loading district. The conductor is 795 Drake and the overhead groundwire is 3/8" HSS.

NESC heavy district loads with an overload factor of 4 controlled the design of the original wood pole line. A quick comparison of the unit loads for the extreme wind and the NESC heavy district load with overload factors for concrete, indicates that the NESC heavy district and not the extreme wind load will control the design of the concrete pole. Because extreme wind does not control the design, the engineer may use Table A-1.

Table A-1 indicates that a C-02.8 may be used. However, in order to account for the additional moment due to deflection from the dead weight of the heavy concrete pole and the loaded weight of the conductors, the engineer should select a pole, one class greater in strength from this table. A standard class C-03.4 should be used.

 

Example 3: Same as example 2, except the line must sustain 100 mph extreme wind loads.

NESC heavy district loads with an overload factor of 4 controlled the design of the original wood pole line. A quick comparison of the unit loads for the extreme wind and the NESC heavy district load with overload factors for concrete, indicates that the 100 mph extreme wind load will control the design of the concrete pole (an overload factor of 1.1 is used for extreme wind loads). Because the extreme wind load with an overload factor of 1.1 is greater that the NESC district load with an overload factor of 4.0, extreme wind will control the design. Table A-2 should be used to select the appropriate pole.

In order to account for the additional moment due to deflection from the dead weight of the heavy concrete pole and the loaded weight of the conductors, the engineer should select one pole class up from the table. A standard class C-04.0 should be selected.

 

In example 2, calculations for the actual line conditions may actually show that a C-02.8 may be used to replace the 80 class 1 wood pole. Likewise, the engineer might find that a C-03.4 may be adequate for example 3 if calculations are performed using actual line conditions.