Calculate Size of Pole Foundation & Wind Pressure on Pole

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.

1

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)

 

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

One Response to Calculate Size of Pole Foundation & Wind Pressure on Pole

  1. N.Mathur says:

    Sir, What about wind load on conductors

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