Analysis of an irregular shaped mass pier/footing

[dm3415]

I have a project with some existing concrete piers supporting existing pipe columns that support equipment. For non-engineering reasons, the supports are being relocated a few feet away from the center of the pier. The loads are relatively small (1 to 3 kips) vertically and laterally and the existing pier is fairly substantial with quite a bit of factor of safety, as I designed it.

I plan on epoxy doweling into the existing pier and adding a "blob" mass of concrete to the existing pier that tapers on it's way down, but doesn't go to the bottom of the existing pier. This would resemble a wedge on the side of a cylindrical pier. The new post will have a base plate and anchor bolts. There will be moment and shear as well as gravity loading on this new eccentrically loaded pier mass

My question is this thing acts as both a footing and a pier, I could determine the Sx and also model as some sort of pier for lateral loading, but I'm not sure how to combine the two methods.

Is there any techniques or literature (I've already reviewed NAVFAC 7.02) someone could point me towards for an efficient analysis of this hybrid animal?

 

[dm3415]

I should add to my previous question for clarification, I am just asking for advice on the soil side of things, like bearing and lateral loading.I have the concrete covered.
TIA!

 

[bootlegend]

Can you provide a sketch? I'm having a hard time picturing this. If you develop the dowels to ensure that the two foundations act as a unit then the bearing pressure is still (P/A) + (M/S). Lateral load at the top of the "pier" multiplied by the distance from top of pier to bottom of footer will add to any moment due to eccentric gravity loads. Is this what you are asking?

I got 350 heads on a 305 engine; I get ten miles to the gallon, I ain't got no good intentions.

 

[dm3415]

P/A + M/S doesn't account for the support a pier provides, that's the problem.

There's a flagpole, or pile type analysis that takes lateral loads laterally along the side of the pier, and a footing analysis that takes loads at the bottom of the footing, but I think it's overly conservative to just use one or the other in this instance. See attached sketch

Thanks

 

[bootlegend]

Okay, I thought the pier was above grade.

Question number one is why did you need a pile for such light loads to begin with?

It is hard to tell without dimensions on the sketch, but can you get a small spread footer next to the existing pile that will do the job? You could tie into the pile and use it as extra weight to counteract the overturning due to lateral load if needed.


I got 350 heads on a 305 engine; I get ten miles to the gallon, I ain't got no good intentions.

 

[dm3415]

Worst soil. Ever. The pier is about 7 feet deep, and it is literally the only component on this project that hasn't previously failed, so I'm totally comfortable with the previous design.

What you are suggesting sounds like what I have drawn basically. The angle of the taper on the new footing addition can vary of course based on the moment couple with the epoxy dowels. But it sounds like you are suggesting treating it like a pad footing in terms of analysis and neglecting the pile-load resisting characteristics?

 

[bootlegend]

Yes, that's what I am thinking. I wouldn't taper it I don't think. Just use the weight of that pile/pier to keep the spread footer from overturning. So you wouldn't need to transfer a moment through the dowels, just the shear. If the lateral load is to the right in the picture, the weight of the pile multiplied by the eccentricity from new column location will need to be greater than the overturning moment due to lateral load. If the lateral load is to the left in your sketch, the bearing capacity of the pile can be used to counteract the overturning moment. If the overturning moment is out of plane in your sketch you'll need to make sure the resultant is still in the middle third of the new spread footer, which it probably will be when you count on the weight of the pier. You'll have to check, but the loads seem light enough that this might work.

I got 350 heads on a 305 engine; I get ten miles to the gallon, I ain't got no good intentions.