Lateral deflection of a pile with pilecap 3

• Jun 15, 2022

• #2

EZBuilding

Structural

Aug 26, 2014

399

US

I think you have a free head condition with your single pile under a pile cap. There is nothing to provide frame action which would introduce the fixed head behavior.

I generally rely on the project's geotechnical engineer for the lateral analysis of the piles on my projects, but generally I have seen them use the bottom of the pile cap as the top of the pile for the purposes of L-Pile analysis. I think this would make the semi submerged portion of your pile cap not relevant in the lateral behavior of your pile. If you are considering your pile cap in your lateral analysis, I would think you could resolve this by considering just the submerged portion of the pile cap.

 

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• Jun 15, 2022

• #3

HTURKAK

Structural

Jul 22, 2017

3,366

- It is true that equation 6.21 is for deflection at head of fixed-headed pile..

- Fixed head may develop at a pile within a pile group with rigid pile cap but the pile would be deep and flexible .

Refer to Figure 6.30 ( Coefficients for calculating deflection of pile carrying both moment and lateral load )

Pls notice that Mt/(HT)= −0.93 for fixed-headed case and Mt/(HT)= 0 for free -headed case. You will see in-between cases where ,Mt/(HT) is (-0.1,-0.2,-0.3...-0.7..) as usual ..

 

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• Jun 15, 2022

• Thread starter
• #4

mte12

Structural

Mar 1, 2022

146

AU

Thanks for advising.
I thought perhaps the ground would act as a fulcrum providing the fixed head, although I didn't really believe this due to assumption of an air gap from underside of pilecap to ground.
But it's not readily apparent in reading the text, the definition of free or fixed head.

EZBuilding, I agree that for a single pilecap, the pilecap being semi submerged, wouldn't make much of a difference to the deflection. But for a continuous pilecap, I thought it may be more significant. It's more of a question of what's normally accepted practice. Then again, the top portion seems to be ignored by some, in for example stability calculations.

If the pilecap is continuous along a single row of piles, I presume this is still a "free head" scenario. And for two rows of piles, this tends toward "fixed head".

HTURKAK, Figure 6.30 will be helpful actually.

Also, generally speaking, if a pile was modelled with springs (proportional to modulus of subgrade reaction), in your experience, would the deflection be similar, a bit less/more or significantly less/more?

 

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• Jun 16, 2022

1
• #5

HTURKAK

Structural

Jul 22, 2017

3,366

Then modell the pilecap ( together with the piles ) with FEM etc and see how fixity develops..

The left picture, free head at short dir. loading and fixed ( or probably partial fixity develops ) at long direction ..

 

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• Jun 16, 2022

1
• #6

le99

Civil/Environmental

Jan 23, 2022

380

US

This is my understanding on "fixed head" vs "free head" piles. I think Lpile can provide information on the deflection and slope of the shaft.

 

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• Jun 16, 2022

1
• #7

EZBuilding

Structural

Aug 26, 2014

399

US

mte - I think the distinction for the load and deformation comes based on the modeling assumption. If the assumption is that the lateral load occurs at the top of the pile/bottom of the pile cap, then the soil above this load is not necessarily contributing to resisting it's load. It's also usually a beneficial assumption as the soil will tend to be stiffer as you get down from the top could of feet.

I align fixed head as a consideration that the pile and the pile cap are able to form "frame action" and transfer moments from one element to the other. In this condition, your modeling assumption would be similar to a fixed column and beam moment frame. The free head assumption would be similar to a cantilever column fixed at the base and free to rotate at the top. That capabilities for the fixed head condition to present itself would usually come about by having a second pile in the direction of the lateral loading which can become the second part of that frame action. In reality there may need to be some considerations for the relative stiffness of the piles and the pile caps in determining how much fixity you are getting - but I don't think that is typically considered in the geotechnical modeling of the fixed-head parameters.

 

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• Jun 16, 2022

• Thread starter
• #8

mte12

Structural

Mar 1, 2022

146

AU

Thanks to all for the advice.

 

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• Jul 26, 2022

• Thread starter
• #9

mte12

Structural

Mar 1, 2022

146

AU

Hi
I was surprised to read this:
[URL unfurl="true"]http://www.findapile.com/laterally-loaded-piles/pile-groups[/url]

Which is a contradiction to what's expected, as well as the formula below (refer to attachment in initial post for more detail).
Am I missing something?
I also read somewhere that this is a topic of debate.

Would a "fixed head" definition then be for a single pile with a pilecap at the top?

 

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