Unequally loaded pile caps

[Berniedog]

I need some insight. I am using a 6 pile pile cap as per CRSI, 1996 edition. The problem is I have a column load in the middle of the cap but I also have grade beams bearing into the ends of the pile cap. The average load per pile is 66.7k but the cap is not equally loaded.

Can I still use the 6 pile pile cap? Will the center 2 piles settle slightly allowing the load to go to the other piles?

See attached sketch. Also, should the grade beam bear on top of the pile cap?

 

[SteelPE]

It's not really clear to me how your system is not equally loaded.

However, an unequal loading will equate to a moment at the centroid of the footing. Figure the moment from the unbalanced loads at the footing centroid like you would any other type of footing and then design your pile cap (figuring the reaction in each pile due to the additional moment and then moment and shears in your cap).

The following is a quick reference from Teng.

The load in each pile can be developed from elastic theory (assuming the pile cap is rigid, pile heads are pinned and piles are elastic columns whose deformations and stress distributions are planar).

Qm = Q/n +/- MyX/(sum X^2) +/- MxY/(sum Y^2)

Qm = axial load in any given pile
Q = total axial load on centroid of piled group
n = number of piles
Mx,My = moment with respect to the X and Y axes
x,y = distance of pile from Y and X axes

 

[Berniedog]

Thanks SteelPE. Can you look at my loading diagram? You can download it. See above.

 

[SteelPE]

I have looked at your drawing and it still is not clear where the eccentricity is. I would assume it would be due to the fact that the grade beams are of different length. Otherwise if the grade beams are of the same length, would they produce equal an opposite moments on the pile cap? If you want to figure some eccentricity in the system (due to uneven loading) then you can use the above equation to get the loads in each pile and design your cap.

 

[Lion06]

It looks pretty concentrically loaded to me. As SteelPE notes, unless the grade beam reactions are different, there is no net eccentricity. I doubt that the two center piles would settle without bringing the other four along with them. Why would they. The pile cap is so thick and short that it acts as a rigid body, it's not acting like a beam spanning from the outer two piles to the center two piles and then to the other two outer piles.

The pile caps I've done with grade beams have had the grade beams frame into the side of the pile cap (with the same top of concrete elevation - though I don't think there's any necessity for that to be true)

 

[Berniedog]

Thank you StructuralEIT and SteelPE

 

[ToadJones]

Bernie-
Can I ask where you are located?

 

[Berniedog]

Located in Akron, Ohio.

 

[ToadJones]

I wondered....I saw the bottom line on your sketch.
That's a wonderful hardware store.

 

[Berniedog]

Yes. The Owner is very good to the community including my child's school. Where are you located?

 

[ToadJones]

Due east about 40 miles.
I worked for quite a few years building homes.
That place was the mecca for tool junkies like myself.
Good quality people there.