Columns directly on piles without pile cap

[bratty]

There are a few questions to raise
1. I have seen foundation drawings showing the pile caps are unrestrained ie there is no tie beams to the stumps. In fact it is practice in Down Under. But in Asian countries, it is a norm to have ground beams tie to the stumps. What wil be the consequences
2. Similarly with columns or piers directly on piles and there is no restrain to the pile head and will eccentric load cause a P-Delta effect?
3. I have seen a wind turbine pier nicely located at the centre of a round foundation. But the weight of the turbine is offset and hence there is a constant eccentric moment on the foundation. Is it a normal practice to have a nice looking symettric foundation causing a constant trapezoidal bearing pressure or should the foundation be offset as to have a uniform bearing pressure?

 

[phamENG]

bratty, I wouldn't do a column directly to a pile with no cap. It leaves you with minimal (if any) construction tolerance, and the connection details would be a nightmare. (That being said, I have used long piles cantilevered out of the ground as columns for an elevated structure, but I don't think that's what you mean here).

Whatever you do, you have to ensure there is stability in your pile cap. If you have at least three piles, you're good without tie beams. If you have two piles, you need a tie beam perpendicular to the axis passing through the two piles. If you one pile, you need two tie beams at right angles to one another. The angles could be skewed, but you'll have to resolve the forces accordingly to ensure stability. If you're in a seismic area, tie beams are good regardless of the stability question as it restrains the pile caps under cyclic ground motion - in the US, they are required in high seismic areas.

As for your last question, it's all about the bearing pressure. In that sort of structure, you'll never have uniform bearing pressure. You may design it be uniform for the dead load, but the wind can come from any direction (and if the head can rotate with the wind, the dead load will never be uniform, either). The goal is usually to keep the peak bearing pressure for all load combinations below the allowable while finding the right mix of material savings and simplified detailing (to reduce labor costs). That balance varies based on where you are in the world and how the economics of construction play out there.

 

[BridgeSmith]

I have to disagree with phamENG on the first point. Assuming we're talking about cast-in-place concrete shaft foundations (which we, and AASHTO, would call drilled shafts), we do single shafts directly to columns on a regular basis. It requires careful consideration of the unbraced length of the combination and analysis to determine the moments and fixity of the shaft (the max moment will be larger and occur at a location below the column to shaft interface). We use either Allpile or Lpile for this analysis.

We provide construction tolerance by sizing the drilled shaft 6" larger than the column, but keeping the same reinforcing cage diameter through both (4.5" cover to spirals in the DS and 1.5" in the column).


Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

BridgeSmith - I'll agree with you there. I'm used to driven piles, and that's where my thought process went immediately. I can certainly see what you're talking about working, though I think it would be strange to see in a building structure.

 

[BridgeSmith]

...I think it would be strange to see in a building structure.

Agreed. It doesn't seem it would usually be difficult to provide some kind of restraint in a building foundation without adding much extra structure. I didn't really know what kind of structures the OP was considering, though.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bratty]

BridgeSmith
If the shaft or pier is extended into the piles, the restraint will be at the top of the shaft. There is no restraint at the ground level. Further how do we have proper continuity ie connection. Sometimes the shaft rebars are more than pile as the area could be smaller. Further at what depth is the point of fixity?

phamENG
I agree there will be never uniform bearing pressure due to wind. But if under dead load is under uniform bearing pressure then under wind load, the resulting bearing pressure will be less. Imagine, with the off centre turbine, uneven bearing pressure exist and since in the wind turbine is usually facing the direction of wind, the uneven bearing pressure will be much greater. Further, if the foundation is eccentric with uniform bearing pressure under dead load, then whatever lateral loads in any direction, then resulting uneven pressure will be the same in direction as compare to a concentric foundation which has higher bearing pressure in the direction of the turbine and less in other directions.

I have seen many foundations designed in Australia by top consultants in this way; nice and symmetric about the superstructure.

Is there other reasons?

 

[BridgeSmith]

The restraint for the shaft and column combination is from the soil or rock surrounding the shaft. The structural design of the shaft is the same as for a column. The longitudinal reinforcement in the shaft is the same or larger than in the column and typically the bars are extended a lap length from the shaft into the column and lap with the column bars. The only difference in reinforcing is that the spiral in the shaft is at a 6" pitch to allow for good consolidation of the concrete. Only the top 10' of the shaft can be vibrated, so the clear spacing of the bars must be wide enough to allow the concrete to flow under gravity (AASHTO LRFD specifies 5 times the max aggregate size; the older Standard Spec. said 3 times).

There is some debate over where fixity is assumed. Most assume it at zero deflection or max moment. Per the recent editions of AASHTO, the unbraced length can be assumed to end 5' below where it's embedded in soil, or at bedrock contact.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

Simplicity is probably a big reason. Surveyor locates the center of the foundation, mark off a giant circle, and start digging. It helps to avoid field conflicts and mistakes. If the concrete contractor accidentally pours his eccentric foundation 45 degrees off of the prevailing wind and turbine direction, no harm done as long as you have a symmetrical bolt pattern. There are some wind fields near me and I've noticed several of the turbines sit at various angles. I'm not an expert in wind farm design, but I would guess it's to take advantage of subtle variations in the prevailing winds of the region and ensure a more uniform power output. In that case, having a single, directionally independent foundation design would be of some value to the construction manager.

This is mostly conjecture at this point.

 

[BridgeSmith]

For wind turbines, like any other design for wind, the maximum wind loading is assumed to be possible from any direction, and in worst combination with the turbine direction. The wind turbines I've seen can also face any direction, so the design loading is the same in all directions.

Rod Smith, P.E., The artist formerly known as HotRod10