Uplift in combined footing supported by geopiers

[tbone73]

I'm designing a combined footing supported on geopiers where one column has uplift and the other does not. I've not encountered this condition before, so I'm looking for some input. The geopiers will be designed to resist the uplift, so I have to size the footing for bearing and then design it for the moments and shears. So what is an acceptable approach to analyzing it? I was thinking that I could assume that the uplift column will have zero load as the geopier takes the uplift and design the footing accordingly. Does this seem reasonable? Thanks in advance for any input!

 

[KootK]

So what is an acceptable approach to analyzing it? I was thinking that I could assume that the uplift column will have zero load as the geopier takes the uplift and design the footing accordingly. Does this seem reasonable? Thanks in advance for any input!

Sounds reasonable to me. For the design case with uplift, pretend the uplifting column isn't even there. Potentially, if your resulting analysis showed that part of the footing was uplifting from the soil even without the uplift column, you could use the geopiers to pin it down to some degree.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BadgerPE]

Out of curiosity, how does on attach to a geopier to resist uplift?

 

[KootK]

I had the same question. They're a little vague about it on their website.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BadgerPE]

Koot,

I contacted our Geopier consultant to get more information. Will follow up when I hear something.

OP,

Sorry for the hijack.

 

[SteelPE]

I have always sized the combined footing to support the loads w/o assistance of the Geopier. The problem you describe (one column has uplift the other does not) is problem I associate with a bracing system and usually the uplift problem can be solved by placing the columns on the same footing. I don't know if this is your problem or not.

I seem to remember asking a similar question when I used Geopiers before. I believe they can embed a plate in the ground somewhere in the pier and then use rods to attach the plate the the footing and gain uplift resistance. However, I have never utilized this option. I even seem to remember them using grouted piers (don't know how this wouldn't be a "Frankie" pile) under locations that may have high compression loads

 

[tbone73]

It is a braced frame and where possible I do place both columns on the same footing. Under lateral load I'm getting a substantial net uplift and there is no realistic way (the footing size is not practical) to get a footing to work without the assistance of a tension element. This is where the geopier comes in. In my understanding they embed large anchors into the footing enough to develop the tension capacity and extend them down to the geopier base. This is supposed to develop a large cone of soil to act to resist the uplift. So assuming they load test on site and get acceptable SFs, I was wondering how does one design this footing. Thanks again all.