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Raft pile - a bug? URGENT

Question:

Hello,

plese see the model.

The pile cap for pressing in and pulling out was calculated. The results, however, are highly unbelievable.

For the pressing analysis, the GTS NX shows an upward displacement.
And for the puuling out analysis - the forces in the virtual beams are also compressive. WHY?

The defined load and its signs are rather correct.

Why the displacements look like this?

Regards,
Aleksandra 

Answer:

Hello Aleksandra,

In GTS NX, the client ran a non-linear static analysis with the self-weight analysis option enabled. Hence software will automatically consider all the mesh sets (including concrete) and run a linear analysis. Hence in the self-weight analysis, we could find huge compressive forces in piles as the unit weight of concrete is very high.

But the client was interested in comparing the results with Plaxis where he ran construction stage analysis (Note: definition of phases is nothing but construction stage analysis in plaxis). So you need to run construction stage analysis in GTS NX as well. So I ran the construction stage analysis and you can find results similar to plaxis. 

The construction stages are as follows,
Stage 1: Activate only Soil mesh sets
Stage 2: Activate Foundation mesh sets (Since you are activating foundation mesh sets in the second stage, you won't be finding huge compressive forces in foundation mesh sets because linear Insitu analysis (K0 condition) is made to run only for Stage-1)
Stage 3: Activate Load

And in the main analysis control, activate insitu analysis as shown in the following figure,



Insitu Stresses,


Now you can find tension due to the pull load in the virtual beam. 


Please find the attached model.

Regards,
Harsha
Geotechnical Engineer

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As requested, results of the Plaxis insit analysis.



As you can see, the stresses change from 0 to about -330kPa (compression)

The next stage with the foundation slightly increases these values to about -350kPa.

For comparison, please note in the screenshot that you have sent - that the maximum compressive stresses exceed -2000kPa and moreover, on the ground surface we have tensile stresses of + 45kPa ?????

Regards,
Aleksandra 


Hello Aleksandra,

Can you please confirm with the client what are the Total Stresses in the Vertical direction (S-ZZ Total) in the insitu analysis in Plaxis?



Are the SZZ values in GTS NX similar to plaxis in the case of Insitu analysis?

Also, as the client said, it is generally recommended to go for construction stage analysis in geotechnical scenarios irrespective of any particular software. 
In the current model, for the case of construction stage analysis, we can proceed for following stages,
1. Insitu analysis
2. Pile foundation
3. Load

Regards,
Harsha
Geotechnical Engineer

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Dear Harsa, please see the client's comment below. Do you think he is right?

This example was calculated not only in Plaxis, but also in other programs, eg RFEM or DC Pfahle, and despite taking into account the self weight, tensile forces were generated.

I think we're getting closer to clarifying the topic.

It seems that in Midas the sequence of calculations in NL analysis must always be carried out with the use of Construction Stage Analysis not by a single NL analysis case with in-situ analysis - although such a possibility exists. 
It is probably an important tip for the developer to add this info in the manual.

Reagrds,
Aleksandra 


You are most welcome :)


Hello Harsha,
thank you for detailed explanation! Very helpful.

Regards,
Aleksandra 


Hello Aleksandra,

The behavior of compression or tension in the virtual beams is related to the solid piles to which virtual beams are applied. 
So hence the stresses in the solid piles play an important role in the virtual beam results. Tension/compression in the virtual beams is directly related to the Szz stresses in the Solid piles. 

For example, if you see the model (virtual beam_test.gts)  that I had attached in my previous replies, you can find the following, 



Similarly, you can find tension in the Pull load case,


The above-stated virtual beam test model was made to run a linear static analysis with only external loads. No self-weight is applied and no insitu analysis was made to run. 

Now coming to your model,
An insitu analysis with Self Weight was made to run in both push and pull load cases.


The stresses because of self-weight (which are basically compressive) are then taken into non-linear static analysis where push or pull loads were applied. The stresses because of the pull load are much smaller than that of the insitu stresses. Hence the pile solids are still in compression and the same is reflected in the virtual beam. 


So I ran a linear static analysis by removing the self-weight load and disabling the self-weight analysis. You can find the results of tension in the virtual beam as shown in the following picture which is what you were expecting,



So in your model, basically the applied pull load was not sufficient to create tensile stresses in the piles as the insitu self-weight stresses are high. And if you try to apply a more pull load, then the analysis may result in divergence.

Regards,
Harsha
Geotechnical Engineer

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Hello Harsha,

please see teh naswer from client

Thank you for your comprehensive answer, but I have one attention.

I understand and accept the answer that in Midas we will achieve the correct assessment of settlement only when we additionally cut off the effective negative pressure (because of the algorithms used). This procedure is not needed, for example, in Plaxis, but it seems logical in this context.

However, the answer to the virtual beam is probably unacceptable from an engineering point of view.

If you really means that compression forces are generated for the extraction loads in virtual beams (where the pull load was applied)? Of course, from a physical point of view, this is not possible, except when the heavy piles are based on the rock and the pulling forces are negligible.

How do you think?

Regards,
Aleksandra 


Hello Aleksandra,

The heaving (upward displacement) in the analysis even after applying the compression load is due to the negative effective pressure. This negative effective pressure developed when we run the In-situ analysis which is linear in nature. Hence it is always recommended to enable 'cut off negative effective pressure' in the analysis control.

 


After enabling this option, you could able to find the following results,




And regarding the virtual beam,
In GTS NX for beams, the negative sign in the axial force indicates compression, and a positive sign indicates tension. 

When the model is applied with compressive load, the virtual beam is showing compressive behaviour as shown in the following figure,



In the case of the analysis set where the pull (upward load) was applied, the piles are still showing compressive behaviour. It is due to the interaction with the soil (i.e, Shear stiffness modulus of interface elements affects this behavior). 

There is no problem with the virtual beam elements. You can check the same in the attached model. 

Regards,
Harsha
Geotechnical Engineer

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Hello,
please see the model.


Hello Aleksandra,

Please attach the model file. 

Regards,
Harsha
Geotechnical Engineer

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Creation date: 11/1/2021 6:33 PM (Harsha)      Updated: 11/1/2021 6:33 PM ()
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