Large Pile Cap Load Eccentricity

[gmannix1000]

Hi All,

Looking for some advice, have a large pile cap load offset (on the edge of a 5 pile cap)and was thinking about adding a beam back to the pile cap behind this. Once the stirrups go through the pile cap, am I correct in saying this pile cap will still work as intended i.e. concentrically loaded? Or is there something else to consider?

Thanks in advance.

 

[MIStructE_IRE]

Hmmm... I’m not entirely convinced that it will since the beam is considerably less stiff than the pile cap/pile group.

I think you would need to analyse as a group. However I suspect that the majority of load will go to the two piles nearest the column.

Another option could be to keep the beam slightly above the pile cap and provide a little reinforced pedestal/stub which ensures the beam’s reaction is directly applied to the centre of the pile cap.

 

[SlideRuleEra]

Agree with MIStructE IRE about two piles nearest the new column receiving the majority of the added load.
The suggested spacer will solve that problem... if the single pile (including pile cap) at the other end of the beam will take uplift.

http://www.SlideRuleEra.net

 

[gmannix1000]

Thanks for the input. I also agree that most of the load will shed into the two piles closest to the point load; however, as these piles settles 2mm say, wouldn't the load shed into the centre pile also and then as that settles, load will go into the back two piles accordingly? Provided the 'beam' can cantilever the load back to the centre of the pile? I'd find it hard to think just those two piles will be doing all the work in the stiff pile cap?

 

[hokie66]

I think you are on the right track. Hope so, because I have done it many times.

 

[gmannix1000]

I'm not convinced, I don't believe there is anyway of backing up with calculations that the load will be carried back to the center of the pilecap by the beam (less stiff) instead of just strutting into the piles as the pile cap itself is much stiffer...or be able to guarantee if the two front piles will be over-utilized and if so, by how much.

Not without testing anyway...

 

[BridgeSmith]

As you alluded to gmannix, much depends on the settlement of the piles. If the front 2 piles settle only slightly, then they will continue to carry almost all the load. If they settle more, then they will shed load to the other piles, primarily the ones in the cap. Under that scenario, the center pile would see downward axial load, but the back 2 may see uplift. The strut will do almost nothing until the rotation of the pile cap becomes fairly large, unless the beam is much larger than you have it shown.

I would start by calculating the forces on each pile using a pile group analysis assuming the pile cap to be rigid and without the strut beam. Can the piles handle the bearing loads and any uplift loads? Is the rotation of the footing acceptable? If not, (assuming driving more piles at the front is not an option) I suggest driving more piles near the back and extending the pile cap to encompass them, then check the pile group loads and cap rotation again.