Pile Head Deflection For Shallow Pier Foundations

[dreber]

Hello Eng-Tips,

I'm trying to design a flagpole foundation for deflection without doing soils testing. The concrete foundation is 1ft diameter, 3 to 6 ft deep, so I feel safe assuming it is rigid (i.e. Shallow Foundation).

The code says you can double the allowable lateral pressure if a half inch of deflection is permissible at the soil head, and I was wondering if anyone had experience with converting these values into an equivalent soil-spring value and then using that soil spring to determine expected worst-case deflection at varying depths.

I realize this is a bit on the "creative" side, but I would appreciate hearing from anyone who has been down this particular rabbit hole before.

 

[BigH]

Surely you must know something about the soils at the site? Any nearby information? How tall is the flagpole? I did a quick search on flagpole foundation and picked up some information readily. Most images show that there is a sleeve and then the pole goes down the sleeve.

As an example:

http://www.colonialflag.com/external-35x5x-125-residential-flag-poles/

(I have no interest whatsoever with this company or its website)

 

[dreber]

The foundation supports a small array of photovoltaic panels bolted to 1.5" pipe.

I actually don't know anything about the site.

 

[dreber]

I think I have it figured.

Soil info here:

http://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx

If I'm permitting 1/2 deflection at the pier top (by doubling allowable pressures), then there is a 1/2" deflection at the pile top.

If I'm not permitting that deflection (by not doubling the allowable pressures), I'll assume that there is no deflection at the pier top (which is good enough for my needs in this application).

 

[oldestguy]

Depending on location, in some very cold areas that type of foundation gets jacked out of the ground under successive annual frost effects.

 

[dreber]

Thanks for bringing that up, Oldestguy! I once saw a utility scale PV projecct, with hundreds of piles getting jacked out of the ground!! WHOOPS! "Frost heave"

Also, if someone wants to figure out how to install a ground screw literally coated with Teflon (or equivalent), the PV industry in Canada will probably thank you (read give you money).