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80 Ft High Sign Board Pole , Foundation design help required.Helpful Member!(2) 

mahesar (Civil/Environmental) (OP)
11 Jul 01 1:02
There is 80 ft high sign board tower which is conisiting  1/2" thick & varying 2.5 (Average) Dia MS steel pipe with total weight of about 4.2 Tons.While the bearing capacity of soil is upto 0.9 tons/sft , one have designed 8 Nos 40 ft deep & 2 ft dia Reinforced concrete pile foundations for the said tower.Is there any alternative design to confirm the value engineering.However a wind pressure is not more than 102 KM/hr and there is dry clay soil in main city area where many multi story buildings are already existing.
Your comments will be highly appreciated.
Regards

Ron (Structural)
11 Jul 01 6:49
Your question is a bit different than the one posted in "Foundation" forum, but check that answer as well....

You did not give the sign area, so computing the bending stresses on the pipe is not possible with the info you have provided.  The base moment of the pipe supports will control the design of the foundation, since you have little vertical or horizontal load.  I'm not sure where you are located, but 102 km/hr is a very low wind speed.  That would not be allowed anywhere in the US.  The probability of a wind event to exceed this speed is quite high and unacceptable under most codes.

The fact that you are inside an urban area might create some wind effects that even the code values do not accomodate.

I suspect your foundation will have to be quite substantial.  The sizes you have noted would not be out of the expected range, however without knowing the loads, it would be impractical to guess that you have enough foundation resistance.
mahesar (Civil/Environmental) (OP)
11 Jul 01 12:50
I really appreciate & thank you so much to post your word in reply to my question .I would like to further clarify as per your desired data.

- The Sign pole is to be installed on filling station consisting logo sign , similar to that of installed in USA at Gas Stations On Motorway Or Highways.

- The sign pole already designed consisting three segments, 3ft dia MS pipe 20ft high at lower portion, 2.5 ft dia at center 18ft high and 2ft dia on upper portion 22ft high. Where is circular logo sign of 20ft dia at top with lights (Illumination).
-While at the center of above 22ft portion consisting 4ft x 33ft long sign board (Rectangular). This is all what , I have received a design.

-The location where the sign pole to be installed is ISLAMABAD (PAKISTAN). My actual concern is alternate design to Pile foundation for economical construction & time saving.

I shall appreciate your further advice.
Regards
Helpful Member!  austim (Structural)
15 Jul 01 5:18
Hi, Mahesar,

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.

     


Helpful Member!  Trussdoc (Structural)
9 Aug 01 13:48
There are two very useful formulas in the American "Uniform Building Code (1997)" Chapter 18 for specifically this type of caisson installation. Base moment is resisted by the lateral soil capacity surrounding the caisson, and designs for both a restrained (such as a surrounding slab) and unrestrained can be done.
mikeh (Structural)
23 Aug 01 0:10
The design of the foundation for signs can be unexpectedly complex. Check out the Florida Department of Transportation website. They have an abundance of information that can guide you in your design. (Unfortunately I do not have the link)

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