Micropile Bearing Plate Design

[jreit]

I wanted to get some suggestions on the best way to design a micropile bearing plate other than using FEM.
I was planning on using Roark's formulas for a flat plate with all edges simply supported and a load applied over a small radius as per Case 1b.
Then, I would only analyze a certain part of the plate which I would assume takes the entire load from that particular micropile.
For example, if I have a 24 x 24 plate and assume the plate loaded area is only 8x8 and analyze that as per Roark would that be a reasonable approach?

 

[jayrod12]

I don't understand why you wouldn't just design it as if it were a column baseplate, just upside down.

 

[bones206]

With 4 reinforcing rods attached to a common bearing plate, I think it would be overly conservative to assume a small portion at one corner transmitting the entire force into the concrete. I think a yield-line approach of the entire plate would be a more realistic approach. Similar to how baseplates are designed, as jayrod said.

Alternatively, the Case 1b approach, but with 1/4 of the load. That should give similar results to a yield line pattern with localized yielding centered over the rod.

 

[TLHS]

Is there moment on the group or just an axial load? If it's just axial, I'd likely just apply the full load as a distributed load on the plate, check what thickness if would need if it were a one way plate with the full width engaged, and then see how thick the plate is and if that makes sense.

If the plate size isn't crazy, then I'd roll with that. This is generally going to be a pretty conservative scenario for this kind of system and, realistically, your bearing is going to be much more concentrated around your pile locations and not evenly spread.

If there's moment, or it you get a crazy plate thickness, then I'd likely do 'upside down baseplate' or I'd cluster the load near the anchors and ignore the center of the plate.

Also note that the cone breakout of the full plate may govern over concrete bearing in this sort of arrangement.

edit: this is also very much a question of how many plates you're installing. If it's just a couple, go with something thick, prove that it works using something conservative and rough, and keep moving. You're not going to save enough steel to justify getting complicated. If you're going to install dozens of the things, then start spending more time on it.

 

[jreit]

Thanks for the suggestions everyone.
The demands include shear, axial and moment.
I was able to use a simplified baseplate approach for uplift as laid out in a Structure mag article and designed the plate using the resultant tension/compression in the piles.
I did check for cone breakout also.

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