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.

Calculation:

 Wind Pressure according to Location:

• Wind Zone is 3 so Wind Speed as per following Table.

• 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

• 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

• 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)