Example:
- Calculate Pole foundation size and Wind pressure on Pole for following Details.
- Tubular Street Light Pole (430V) height is 11 Meter which is in made with three different size of Tubular Pipe.
- First Part is 2.7 meter height with 140mm diameter,
- Second part of Pole is 2.7 meter height with 146 mm diameter and
- Third part of Pole is 5.6 meter height with 194 mm diameter.
- Weight of Pole is 241 kg and there is no any other Flood Light Fixtures Load on Pole.
- Total Safety Factor is 2.
- Wind zone category is 3.
- The Pole is installed in open terrain with well scattered obstructions having height generally between 1.5 m to 10 m.
- Foundation of pole is 700mm length, 700mm width and 1.95 meter depth. The Average weight of foundation concrete is 2500 Kg/M3.
Calculation:
Wind Pressure according to Location:
- Wind Zone is 3 so Wind Speed as per following Table.
| Basic Wind Speed-Vb (As per IS 802-Part1) | |
| Wind Zone | Basic Wind Speed, vb m/s |
| 1 | 33 |
| 2 | 39 |
| 3 | 44 |
| 4 | 47 |
| 5 | 50 |
| 6 | 55 |
- Wind Speed (vb) = 44Mile/Second.
- Co-efficient Factor (K0)=1.37
- K0 is a factor to convert 3 seconds peak gust speed into average speed of wind during 10 minutes period at a level of 10 meters above ground. K0 may be taken as 1.375.
- The Pole is used for 430Vand wind zone is 3 so Risk Co-efficient (K1) as per following Table
| Table 2 Risk Coefficient K1 for Different Reliability Levels and Wind Zones (As per IS 802-Part1) | ||||||
| Reliability Level | Wind Zone-1 | Wind Zone-2 | Wind Zone-3 | Wind Zone-4 | Wind Zone-5 | Wind Zone-6 |
| 1 (Up to 400KV) | 1 | 1 | 1 | 1 | 1 | 1 |
| 2 (Above 400KV) | 1.08 | 1.1 | 1.11 | 1.12 | 1.13 | 1.14 |
| 3 (River Crossing) | 1.17 | 1.22 | 1.25 | 1.27 | 1.28 | 1.3 |
- Risk Co-efficient (K1) =1
- Terrain category (K2) for Open terrain with well scattered obstructions having height generally between 1.5 m to 10 m is 1 as per following Table
- Terrain category (K2)=1
| Terrain Roughness Coefficient, K2 (As per IS 802-Part1) | |||
| Terrain Category | Category 1 | Category 2 | Category 3 |
| Exposed open terrain with no obstruction and in which the average height of any object surrounding the structure is less than 1.5 m. | Open terrain with well scattered obstructions having height generally between 1.5 m to 10 m. | Terrain with numerous closely spaced obstructions. |
|
| Coefficient, K2 | 1.08 | 1 | 0.85 |
- Reference Wind Speed (Vr)= Vb / K0.
- Reference Wind Speed (Vr)= 44 / 1.37 =32 Mile/Second.
- Design wind Speed (vd)= Vr X K1 X K2.
- Design wind Speed (vd)= 32 X 1 X 1 =32 Mile/Second.
- Design Wind Pressure (Pd)=0.6 x vd2
- Design Wind Pressure (Pd)=0.6 x (32)2 =614.4 N/m2
- Design Wind Pressure (Pd)=614.4/10 =61.4 Kg/m2
Foundation Detail:
- Total Weight =Pole Weight +Foundation Weight.
- Total Weight = 241 +(0.7×0.7×1.95×2500) =2620.75 Kg
- Stabilizing Moment = Total Weight X (Foundation Length/2)
- Stabilizing Moment = 2620.75 X (0.7/2) = 920.41 Kg/Meter.
Pole Detail:
- First Part of Pole (h1) = 2.7 meter
- Diameter of First Part (d1) =140mm
- Second Part of Pole (h2) = 2.7 meter
- Diameter of Second Part (d2) =146mm
- Third Part of Pole (h3) = 5.6 meter
- Diameter of Third Part (d3) =194mm .
Wind Pressure on Pole:
- Overturning Moment due to the wind on 1st Part of the pole=pdxh1xd1x(h1/2+h2+h3)x0.6
- Overturning Moment due to the wind on 1st Part of the pole=61.4×2.7x(140/1000)x(2.7/2+2.7+5.61)x0.6
- Overturning Moment due to the wind on 1st Part of the pole=134.47 Kg/meter—I
- Overturning Moment due to the wind on 2nd Part of the pole=pdxh2xd2x(h2/2+h3)x0.6
- Overturning Moment due to the wind on 2nd Part of the pole=61.4×2.7x(146/1000)x(2.7/2+5.61)x0.6
- Overturning Moment due to the wind on 2nd Part of the pole=112.76 Kg/meter.—-II
- Overturning Moment due to the wind on 3rd Part of the pole=pdxh3xd3x(h3/2)x0.6
- Overturning Moment due to the wind on 3rd Part of the pole=61.4×5.6x(194/1000)x(5.6/2)x0.6
- Overturning Moment due to the wind on 3rd Part of the pole=112.14 Kg/meter.—III
- Total Overturning Moment on Pole due to Wind=134.47+112.76+112.14=359.36 Kg/meter.
Safety Factor:
- Calculated Safety Factor= Stabilizing Moment / Total Overturning Moment on Pole.
- Calculated Safety Factor=920.41/ 359.36 =2.56.
- For safe Design Calculated Safety Factor > Safety Factor
- Here Calculated Safety Factor (2.56) > Safety Factor (2) hence
- Design is OK
- B : If Calculated Safety Factor < Safety Factor then Change Foundation Size (Length, width or depth)
View more at https://electricalnotes.wordpress.com/2019/04/14/calculate-size-of-pole-foundation-wind-pressure-on-pole/.
Published by Department of EEE, ADBU: tinyurl.com/eee-adbu
