Pole Footing Uplift

[ivanga7]

Hi, I am designing a pole footing where the dead load is not sufficient to resist the uplift created by wind. Is there a provision in the code that gives soil friction values to resist the uplift? There is no geotech involved in this project.

 

[sticksandtriangles]

I am curious how your pole is achieving uplift.

Typically if only a flag is attached the pole will only want to overturn, not lift out of the ground.

 

[msquared48]

Flag? Flag? What flag?

Mike McCann, PE, SE (WA)

 

[jayrod12]

I'm expecting he's talking about pole barn type foundation. I've provided collars on the bottom of posts to increase the soil cone holding it down. Take a look at ASAE EP486 I think.

 

[ivanga7]

I guess I didn't really specify. This is not for a flag. There will be an umbrella attached to the pole (11'x11' umbrella to be specific).

 

[Lomarandil]

Typically, soil friction used to resist uplift is limited to 1/2 or sometimes 2/3 of the allowable value used to resist downward loads. Granted, that presumes you have some geotech information for the downward resistance -- or maybe you can estimate one out of NAVFAC (be conservative!).

Is this a one-off design? Add more concrete and be done with it. That's the cheapest option.



----
The name is a long story -- just call me Lo.

 

[oldestguy]

Hey, don't forget about bouncy effect of high ground water that sometimes is present when strong storms are present. There I'd use an inverted truncated cone with side angles at 20 degrees and soil unit weight of 110 damp and 60 #/cf submerged to be safe, but as ultimate. Use some safety factor also.

 

[spieng89]

Was there a geotchnical investigation/report conducted at this site? The geotechnical engineer could provide an equation or value for uplift. I'm not sure you'll find skin friction values in the code references. Maybe a scholastic textbook probably

 

[ivanga7]

No geotechnical report. I'm probably just going to increase the depth of the footing given that there are only two of them on this project.

 

[MrHershey]

That would tend to be my recommendation. You can probably dig in deep and justify some side friction induced by any active pressures on the side of the pole foundation. But it probably won't help you much and there's not much sense turning two umbrellas into an academic pursuit. I'd just throw concrete at it and be done.

 

[oldestguy]

Isn't thee some wind load hat will cause the umbrella to collapse upward? That would be my design load, not max wind pressure.

 

[MotorCity]

If the water table is an issue, skin friction won't work (or at least it would be negligible). Just add enough concrete to resist wind uplift and buoyancy.