15 Jul 01 5:18
I would agree with Ron that your pole will need a substantial footing. However, I don't entirely agree that 102 km/hr is remarkably low. It all depends on how that speed is to be used.
For example, if I were to be building your pole in one of the southern State capitals in Australia (outside the areas subjected to occasional cyclones), and using permissible stress design, then the Australian Standard would require a design speed of 28.5 m/sec (102.6 km/hr - pretty close correlation to your speed) for the bottom 20 metres (65 ft or so), increasing slightly for the top 15 ft of the circular sign.
The above assumes that the signs are not intended to rotate, are both oriented to face the N-S wind direction, and thus not exposed to our strongest wind direction, and that the surrounding landscape consisted of tall multi-storey buildings for at least 250 metres in all directions.
The design speed for limit state ('ultimate strength') design would be 21% higher (124 km/hr), and the pressures 46.5% higher. Put the same structure in Darwin, and I would be looking at a working stress design speed of 173 km/hr, and ultimate design speed of 230 km/hr.
My quick analysis (at 28.5 m/sec) gives me a maximum working flexural stress in the bottom of the 3 ft diameter section of 17,100 psi. which seems reasonable, and allows some margin for the axial stress due to selfweight.
One point that you might care to consider - if it were my responsibility, I would want to make sure that my foundation design would remain intact under the full plastic moment in the pole base plus some margin. That should avoid any legal difficulties if the pole designer had got it wrong. With your circular pole base, the plastic moment would be about 2.5 times the calculated working moment.
In concept, the piled footing that you describe might well be a good answer, provided that the 8 piles are not equally distributed around the base. One of my preliminary designs would use a rectangular pilecap (long side perpendicular to the plane of the signs) with piles distributed along each short side. (You do not need the same moment capacity in the other direction, of course).
Another possibility, could be a single 'caisson' about 5 ft diameter. I have no knowledge of construction practices in Pakistan, and recognize that could be an inappropriate idea, but not so long ago I was shown such foundations being hand-dug by small family teams in Hong Kong.
A simple rectangular block of concrete could be made to work. My rough figures would suggest that you would need about 20ft*10ft*10ft, giving a working bearing pressure (maximum in a triangular distribution) of about 2750 lbs/sq.ft. Since it is more than 30 years since I last concerned myself with dimensions in feet, treat this result with a good dose of suspicion.
Good luck with all this.