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[dubeyratnesh]

hello sir i want to design a pedestal with footing on soil having sbc 8 t/m2.pedestal is supporting 9m high light pole in an area having basic wind speed 50 m/s.
what approach we can follow to design this pedestal and footing

[vikram.jeet]

Pedestal & footing  for high mast

Wind load is predominant and substructure is designed for moment due to wind forces on high mast body.

Generally square pedestal and square shape footing for ease of reinf placement vis a vis circular one.

Footing size as also foundation depth shall be sufficient enough to provide soil weight over footing . Footing shall be checked for overturning  . Sliding may not be a problem but to be checked .

Base pressures needs to be checked considering some upliftment of footing due to wind.
e = M/W
M is the moment at base due to wind forces
W is the wt of footing + soil wt over it + wt of steel mast
The contact length X = 3*(B/2 - e)
B is the dimension of footing in direction of moment
Contact Length percentage = (X/B)*100

Contact %  can be 67% to 80% depending on FOS against OT

Max base pressure = 2*W /( X*L)
L is the dim of footing in other direction

Some designers also consider passive soil resistance relief with some FOS provided by rcc pedestal and footing

[dubeyratnesh]

SIR ON WHAT BASIC DEPTH OF PEDESTAL WILL BE CALCULATED OVER FOOTING.

[vikram.jeet]

Pedestal shall also be rcc in square crossection .

Pedestal is designed as vertical cantilever  from footing .

Depth of foundation for 9.0m mast may be minimum 2.5 m . But all depend on soil weight needed over footing . If foundation size is restricted , then higher depth of foundation is needed to achieve soil weight.

[dubeyratnesh]

Thank you sir
From where we can start ? On what basis we can select hight of pedestal.? Taking factor of safety about 1.5 for overturning the hight of pedestal is coming more than 4 meter which is not feasible.a 9 meter hight pole on cyclone effected area gives more wind moment? Weight of pedestal resisting will be more? How to decide pedestal hight.further footing design?

[vikram.jeet]

You are thinking of block foundation for light pole . I think in this case Pl think of rcc spread footing with soil over it  with rcc pedestal. The concrete block foundations are feasible for low ht light poles , but not for high mast which require spread footing with rcc pedestal.

[dubeyratnesh]

Sir can you provide any design example of high mast from any refrence book or previous design?

[vikram.jeet]

I don't think that any ready standard design / details available , but you can search NHAI or Highways / Expressways for any ready material.

But it is a simple case of designing an isolated spread footing for Uniaxial moment. Footing need to be square as wind can blow in any direction .

[vikram.jeet]

Pl see in google search details  for standard street light pole upto 35 feet high Search P-660.pdf on Google.

[vikram.jeet]

Please see in google search for light pole design calcs , are available in net