L-Pile and Fixed vs Free Head Conditions

[garrettk]

If you have a large diameter drilled shaft foundation (7 to 8 feet) which is properly embedded in the overlying concrete cap, you have a 'fixed head condition'. This I am comfortable with.

The structural engineer has provided us with axial, shear and moment forces for this pier. By having a 'fixed head condition', can you still have a moment force on the top of the pier? There is a debate in our office right now as to whether you can or cannot have this force. Everyone agrees you can have axial and shear, the question is on moment.

In L-Pile there are 6 options for Data-Boundary Conditions and Loading. The user's manual is not the most friendly to use and I need to know which of the options models the "fixed head condition".

I think the Shear & Slope (option 2) boundary condtion is the fixed head. If this is the case, how do you convert a moment (kip-ft) into radians (provided you can have a moment on a fixed head)?

Thanks for you help.

 

[GeoPaveTraffic]

I agree completely about the manual for LPile.

It has been several years since I used the software, so I don't remember the boundry condition options very well. However, if the drilled shaft is fixed by the cap, then the rotation at the top is zero. I know that LPile will allow you to enter the moment. If you would like, post the boundry condition options and I should be able to remember.

Alas, I don't have access to LPile or the manual at this time.

 

[Panars]

Specify the shear and slope to model a fixed-head condition. The value of the shear will be your applied lateral load and the slope should be zero. This is described on page 3-17 of the LPile Plus 5.0 user's manual.

In reality, no shaft will be absolutely fixed. Even with a perfectly rigid connection at the pile cap, you will still get some rotation or flexing of the pile cap. So the theoretically correct boundary condition should be shear and rotational stiffness for a "fixed-head" shaft. However, calculating what that rotational stiffness should be is a very complicated process and not usually worth the effort.

How has the structural engineer calculated the moment at the top of the shaft? If your moment calculation from LPile using a fixed-head condition is vastly different than his moment, I would investigate further into his calculation method. There may be some miscommunication about the geometry.

 

[dmoler]

There will still be bending (rotation) of the pier below the interface of the pile cap/pile to the point of fixity.

While the embedded portion of the pile in the pile cap is not free to rotate, the portion below the interface will rotate, the degree depending on the py characteristics of the soil, pier stiffness, and applied lateral load. Check out the ASTM Lateral Load Test testing method for illustrations of what a 'fixed head' condition is defined as for lateral testing, and this becomes clearer, since the portion of the pile above the groundline is braced to prevent rotation, while that which is below will rotate (bend). In contrast, a free head does not have the extra bracing, and subsequently more deflection for lateral loading.

In any event, there will always be a moment when a lateral load is applied. I have not used Lpile in a while, but will check on the model that may be appropiate.

 

[garrettk]

It appears that the proper boundary condition for a fixed head scenario is Boundary Condition #2 'Shear and Slope'. This allows for the input of a shear force, an axial load, and a 'slope' value. The slope value, for a fixed head condition, is zero. I understand the slope value to be the measurement, in radians, of the tilting of the top of the pier (this is information I don't have but should be zero).

My question at this point is what to do with the moment force? It seems that moment forces are basically ignored and not included in the calculation for this boundary condition.

We are using L-Pile version 4.0 for those wondering.

 

[TTK]

It would be helpful to have more information on the type of pier that you are evaluating. Be very careful about just assuming the shaft will be "Fixed-Head" based on embedment in a pile cap. If you have only a single line of shafts, for instance, they could effectively behave as a "Free-Head" condition.

One of the primary purposes of a pile cap is to re-distribute these column moments to the foundation elements (piles or shafts) as axial forces. But this will depend on the pile layout and the relative stiffnesses of the pile cap to the individual foundation elements.

Based on the size of the shafts and the fact the structural engineer gave you moments at the top of the shaft, I would suspect that you have only a single line of shafts at each pier and these should be treated as "Free-Head" for your analysis. The L-Pile input for "Free-Head" should then be Boundary Condition #1 -Shear & Moment (with axial force included as well).

 

[garrettk]

TTK,

I believe the shafts are to be arranged on a 2x3 grid (6 shafts per cap). The request for fixed head analysis came directly from the Structural Engineer.

Since posting, I have spoken with two people who have more experience with L-Pile than myself and the boundary condition they each recommended is #2 - Shear, Slope, and Axial.

For a fixed head condition, the Slope is set to zero, and the moment force is basically not included in the analysis. Neither person could adequately explain why the moment is ignored, except that it is the way L-Pile works. It has something to do with the moment being "absorbed" by the pile cap. I'm not a structural guy (and neither are the two people I spoke with) so if you have any comments I'm all ears.

 

[TTK]

garrettk,

Looks like you're working on a very big pier!

If the shafts are in a grid AND the structural enginer is confident the cap is stiff enough to cause fixed-head moments then I agree with your L-Pile modeling approach.

If this is the case, the structural engineer should not be giving you shaft moments since the cap should be converting these moments to axial loads. I would ask the structural whether the axial loads they gave you include the moment effects (if not, they should). The moments increase the axial load on some shafts while simultaneously decrease the axial loads on other shafts...this could effect your analysis. The GROUP software would handle this structure much better but L-Pile can be used with some proper "book keeping"

Good Luck!

 

[garrettk]

Thanks for your help TTk - I appreciate it!

 

[Robertofg]

When you have a fixed connection between the pile and pile cap, a moment develops at the top of the pile due to the restrain that the cap imposses on the rotation of the top of the pile. If you have a free ended pile (pinned connection), the pile will rotate freely at its top about the axis perpendicular to the application of the lateral load and consequently, the slope will be different to zero and the moment will be zero. It helps if you think of it as a beam, the difference between fixed end and pinned connection at the pile cap is the same as having a beam fixed at one end and pinned at the other or a simply supported beam (pinned at both ends), when you restrict the rotation at the top of the pile a moment force appears to balance the moment at the fix connection in order to maintain static equilibrium. Now the thing is, the magnitude of the moment at the top of the pile depends on the deflected shape of the pile which in turns depend on the lateral soil resistance, so in theory, the structural engineer should calculate the moment after you complete your lateral analysis. The way structural engineers calculate the moment at the top of the pile is with a column analogy, modeling the pile as a column fixed at the pile cap and fixed at the pile point of fixity, that is why structural engineers always request that you provide them with the location of the point of fixity. I have not worked with LPile so I am not familiar with its output, but other simmilar programs (such as Florida Pier now known as FB-Pier) will give you the moment force throughout the pile. This moment is not necessarily equal to that calculated by the structural because is estimated using a different approach

 

[jxzohio]

I use LPILE Plus 4.0 quite often. It is correct to select boundary condition #2 'Shear and Slope' for fixed-head condition (slope =0). You can not input moment in the LPILE program for fixed-head conditions. I thought about this before when I first used this software. I agree with TTK that there should be NO applied moment if you assume fixed-head condition. The reason is that any moment applied to the concrete cap will be converted to compressive and uplift axial loads of piles in a pile group. That explains that one row of piles although capped in a concrete cap is not having "fixed-head" condition in the perpendicular direction.

 

[geotechnist]

I have also found LPILE not the easiest to use. My firm has recently purchased a program called PYGMY, which was written by Dr. Doug Stewart and is marketed and supported by GSS (

http://www.geotechsolutions.net).

Both the manual and the prgram are very user-friendly.

Jake

 

[dmoler]

I got the new LPILE Version 5 Plus, since this thread started, and I find it really easy to use.

 

[cyclinggeo]

What are your thoughts regarding large diameter shafts and using L-Pile? My understanding is that L-pile is more geared for "smaller" diameter shafts (slender elements). Any thoughts on the strain-wedge method of analysis?

sdz