Pile Foundations in sign board tower of 80ft high 1

[mahesar]

There is 80 ft high sign board tower which is conisiting varying 1/2" thick & 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 minimal.

Regards
Mahesar

 

[Ron]

Even though the wind pressure might be minimal, it usually must be designed for a basic wind speed of at least 90 mph. Considering this and the typical "sail area" of a sign plus the fact that your sign is relatively tall, the overturning moment at the base of the sign supports can be quite high. This leads to either massive shallow footings or deep foundations. The bearing capacity of the soil is not the controlling factor in this case, though it does help to size the shallow foundation if you choose to go that route. 0.9 tsf is not a great bearing capacity, but adequate for most conditions.

 

[KAM_imported]

Drilled shafts are typically used by the DOT in Florida for the support of signposts as they are very efficient in carrying lateral loads. A comprehensive design guide for drilled shafts can be obtained from the ADSC at

http://www.adsc-iafd.com.

 

[ishvaaag]

0.9 TSF is so modest capacity that to use piles is good. In any case try a single footing big enough (do it square, not round, will have better stability) to take the forces.

Considering the piling drive machines transport, using more concrete and steel can make sense. Also, ascertain what a reasonably modulus of subgrade reaction will provide you with one good assessment of the rotation of the footing interface (use compression-only Winkler's springs). I would start using 4 (kgf/cm^2)/cm for a bad soil, that yours is. Further widening the footing will diminish rotation.

For poles giant lateral deformations nowhere else (in buildings seen) use to be allowed. So if the displacement at the tip under worst wind load is about H/150 a lots would applud, where in buildings rarely we would accept but H/400 or stiffer.

 

[ishvaaag]

You can also use a better approximation for the modulus of subgrade reaction from geotechnical data such SPT or plate load test.

 

[Polecat]

Mahesar:

I agree with Ron --- you are going to have to deal with a rather large groundline reaction here. Even with basic wind velocities of 75 or 80 mph, after you get thru considering the gust factor, height factor and shape factor of the sign and pole shaft, you will have some formidable forces to consider coming into the foundation.

Also, Ishvaaag is correct in noting that you can allow huge lateral deflections and still not be detrimental to the pole (as long as you consider the P-Delta or secondary moments induced).

In my work in Florida (mostly sandy soil) with PCS towers and monopoles, as well as with high voltage transmission structures, I am faced with this problem every week. There are a couple of good programs available to help you in determining the proper burial depth to use. Vertical bearing is generally small enough to be handled with skin friction. It is the lateral resistance required (usually peaking at about 1/3 of the depth) that is hardest to cope with and generally determines the burial depth.

http://www.spiraleng.com

If you wish to visit my website, we can discuss these or other issues further.