Elevated water tank foundation calculation under sismique loads

[REDO10]

Hi All, @Stephane.kapetanovic @Simau 

I have to study the foundation of the elevated water tank, and I’m struggling with how to properly model the hydrodynamic case. (I apologize for repeating a question I have already asked  here, but so far I haven’t mastered how to model the impulsive mass required to perform seismic calculations.)

According to my hand calculations, and as shown in the image below, the impulsive mass is equal to 4694.35 kN, which should be applied at an elevation of 40.95 m over half of the tank circumference.

This mass should be introduced as a dynamic mass for seismic analysis, and I am therefore struggling with how to apply this mass at that specific level?

To address this, I added a new load case named “masse_impulsive”, in which I can assign this mass. However, I am still unsure about the correct way to apply this mass at the required elevation!

As shown in the image below, I decided to subdivide the conical tank pqnel vertically into two parts at that elevation. This allows nodes to be generated at that level after meshing (36 nodes are created). I then selected 19 nodes representing the required half-circumference and applied the mass to them.

My questions are:

• Am I using the correct approach to model and assign masses?
• Should the load case masse_impulsive be included in the seismic combinations, or would this lead to a duplication of mass?
• How can I obtain the overturning moment required to check foundation stability and to calculate the reinforcement?

Please check my attached file below.

Any help or guidance would be appreciated.

Thanks.

[Simau]

Hi @REDO10 

First, I haven't read the numerous threads related to your message.

• Am I using the correct approach to model and assign masses?

What you have done is correct
To take into account the impulsive mass, I assign it to a node located on the vertical axis of the reservoir at a "hi" height. This master node will be linked to the secondary nodes located on the surface of the cone.
I got approximatively same reactions.

• Should the load case masse_impulsive be included in the seismic combinations, or would this lead to a duplication of mass?

As you choose to use modal analysis, impulsive mass must be included, maybe you also need to include convective mass at it’s "hc" height, but i am not sur if it's correct using modal analysis

 
• How can I obtain the overturning moment required to check foundation stability and to calculate the reinforcement?

You can find it in reactions table

[REDO10]

Thanks @Simau for your reply

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@Simau  a écrit :

Hi @REDO10  and 

What you have done is correct
To take into account the impulsive mass, I assign it to a node located on the vertical axis of the reservoir at a "hi" height. This master node will be linked to the secondary nodes located on the surface of the cone.
I got approximatively same reactions.

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I have a few questions regarding your approach:

• Did you use a rigid link to model the impulsive mass? If so, is this rigid link defined as a diaphragm, or is it another type of constraint?

• Could you please show how you modeled the impulsive mass, or possibly share your model file?

I am not fully confident that my approach produces the same results as yours. As you can see in the image below, the reactions sum of values are not zero, unlike in your case.

• Have you previously carried out a seismic analysis including hydrodynamic loads?

• Could you please help interpret the magnitude of my results and suggest the necessary corrections to my model?

Thanks.

[Simau]

Hi @REDO10 

I used rigid link "Membrane" in my model, master node is node 5827 and secondary nodes are finites elements of panels 104 105.

In your model, sum of values are not null because you introduced impulsive mass only in half of the circle. You'll get null value after correction.

Have you previously carried out a seismic analysis including hydrodynamic loads?

Only steel cylindric tanks laying on the ground, not concrete surelevated ones.

Could you please help interpret the magnitude of my results and suggest the necessary corrections to my model?

Sorry, but I don't have sufficient expertise in this area.

Models attached

[Simau]

Models attached

[REDO10]

@Simau @ 

first of all I forgot to notice that I'm using RSA 2021 so I can't open your attached file...could you please reattach the file?

As you can see in the image below, I tried to reproduce your model using membrane as regid link, is this correct?

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@Simau  a écrit :

Hi @REDO10 

I used rigid link "Membrane" in my model, master node is node 5827 and secondary nodes are finites elements of panels 104 105.

In your model, sum of values are not null because you introduced impulsive mass only in half of the circle. You'll get null value after correction.

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As it's described in the extract document indicated in the image below, the author states that the impulsive mass "mi" should be applied over half of the tank circumference at the elevation of this mass "hi", wheras in your model you applied the total of impulsive mass which is equal to 4964.35 KN over the entire circumference using membrane as regid link, Would this produce the same reaction results?

Thanks.

[Stephane.kapetanovic]

How to insert a part of a structure defined in another file to the current project.

[Simau]

Hi @REDO10 

Sorry but Robot 2021 is no longer installed here

Some documentation states that the impulsive mass is rigidly attached to the reservoir and that the liquid is incompressible. Therefore, my interpretation is that the mass is attached to the entire surface of the reservoir, not just half of it. Again, I'm not an expert to definitively rule on this.

Furthermore, I re-ran your initial model and found that the sum of the values ​​is zero.

Could you re-ran your model check this and share it again?

As you can see in the image below, I tried to reproduce your model using membrane as regid link, is this correct?

Please check if node 9599 is at level "hi"

Secondary nodes are  those belonging to panels 104 105,  not only those at level 45.39

[REDO10]

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@Stephane.kapetanovic  a écrit :

How to insert a part of a structure defined in another file to the current project.

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Tthanks @Stephane.kapetanovic for sharing this link!...now I know how to insert an RTD file created in a newer version and I can open @Simau's file

[REDO10]

Hi @Simau 

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@Simau  a écrit :

Furthermore, I re-ran your initial model and found that the sum of the values ​​is zero.

Could you re-ran your model check this and share it again?

Please check if node 9599 is at level "hi"

Secondary nodes are  those belonging to panels 104 105,  not only those at level 45.39

---

I was finally able to open your model and see how you applied the membrane rigid link. As shown in the image below, I grouped all the results, including those from your model (which I ran without changing anything). However, the results differ from yours; moreover, the sum of the values is not zero in all cases. What could be causing this discrepancy?

Thanks.

[Simau]

Hi @REDO10 

I agree with you.
Reactions are very sightly different, but i cant't explain the discripencies in "sum of values"

[REDO10]

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Hi @Simau 

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@Simau  a écrit :

Hi @REDO10 

I agree with you.
Reactions are very sightly different, but i cant't explain the discripencies in "sum of values"

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Getting to the heart of the matter: as you can see in the table below, I grouped the sums of reactions for the maximum combinations in order to design the raft foundation, where:

• for vertical force FZ:  maximum is for case N°6 (ELU) and N°7 (ELS).
• for Horizontal force FX and bending moment MY: maximum is for case N°13 (sismic)

To determine the radial and tangential moments acting at the base of the raft, I used Timoshenko’s theory for circular plates, as described in the extract document below, where the bending moments resulting from the two load cases discussed must be summed:

• case n°1: represents the soil pressure acting in the +Z direction.
• case n°2: represents a linear vertical load coming from the shaft and acting in the −Z direction.

In my case, and for the application shown in the image below, I defined "a" as the intermediate radius of the shaft and "b" as the raft radius. Thus, I have:

• a = 3.60 m
• b = 9.00 m (after calculating the raft diameter by checking the overturning moment condition)

The total radial moment diagram (Case 1 + Case 2) should match the diagram shown in the image above. However, when I run the attached file below, I obtain an incorrect diagram, as shown below, and I dont know where I was wrong ?

Edit:

Please check the rtd file + hand calculations

Thanks.

[Simau]

Hi @REDO10 

The way you modeled the raft is not correct. The raft is not supported on its circumference; it is a slab on elastic supports.

You may design your raft using Robot and without hand calculation
You can transfer the reactions from your main model (source file) to the raft (target file) using this API.

https://forums.autodesk.com/t5/robot-structural-analysis-forum/only-reaction-for-new-model-api/m-p/3...

[REDO10]

Hi @Simau 

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@Simau  a écrit :

The way you modeled the raft is not correct. The raft is not supported on its circumference; it is a slab on elastic supports.

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1. First, I’d like to point out that I did not choose to use elastic supports, as illustrated in your image, because this parameter does not appear in the geotechnical report and I do not have sufficient data to compute it. Only the allowable soil pressure is provided in the report, with a value of 0.18 MPa.
2. I tried using the API you attached to import reactions from the "source" file to the "target" file, as shown in the image below, but I could not clearly understand how to use it correctly. I therefore have the following questions:

• From what I understand, the API imports reactions only from bar elements that have support nodes at their ends. How can these reactions be imported when dealing with panels, as in my case?
• It seems that only single load cases (without load combinations) are imported via the API. In this case, do the load combinations need to be redefined?
• If load combinations must be redefined, how should this be done in order to regenerate seismic combinations, which in my case should include the impulsive_mass load case as a dynamic mass for seismic analysis?

Please refer to the attached folder below, which contains the source file “Chateau_d'eau_1000 m3” and the target file “radier_chateau_eau”, for more details.

Thanks.

[Simau]

Hi @REDO10 

First, I’d like to point out that I did not choose to use elastic supports, as illustrated in your image, because this parameter does not appear in the geotechnical report and I do not have sufficient data to compute it. Only the allowable soil pressure is provided in the report, with a value of 0.18 MPa.

You may do some tests with kz = 25000 50000 100000 kN/m3 and check soil stress. You must ask the geotechnical engineer as this info is necessary for this kind of foundation.

From what I understand, the API imports reactions only from bar elements that have support nodes at their ends. How can these reactions be imported when dealing with panels, as in my case?

No, the API imports also reactions from panels with supports (in your case panel 4)

It seems that only single load cases (without load combinations) are imported via the API. In this case, do the load combinations need to be redefined?

Correct, you have to redefine combinations

If load combinations must be redefined, how should this be done in order to regenerate seismic combinations, which in my case should include the impulsive_mass load case as a dynamic mass for seismic analysis?

The API will importe EX and EZ as they are simple cases

In your model, no reactions were imported.

[REDO10]

Hi @Simau 

I tried to import the reaction forces by applying your suggestions, as shown in the image below, but without success. Perhaps I did not fully understand the procedure. Could you please show me how to do this, possibly with a video?

Thanks.

[Simau]

Hi @REDO10 

First step

- Run the API, reactions will be imported to the target file.

- Open target file

- Modify nature of load cases if necessary.

- Add the raft and mesh it, delete the panel 4, delete the supports.

- Generate again combinations you need

- Run the target file and check your raft.

[REDO10]

Hi @Simau,

I followed your steps to import the reactions but wasn’t successful, as you can see in the image below.

I’m a bit confused about how to achieve this...maybe it’s due to fixed linear supports defined in the "source" file that should be replaced with fixed node supports? ...Sorry to ask again, but could you post a video showing how to import these reactions, or possibly share your “target” file?

Thanks.

[Simau]

Hi @REDO10 

I followed your steps to import the reactions but wasn’t successful, as you can see in the image below.

I agree the steps aren't so obvious, but you just have to follow them and repeat as many times as necessary.

...maybe it’s due to fixed linear supports defined in the "source" file

No, that has nothing to do with it, but it is not necessary to use fixed supports, pinned supports are sufficient.

that should be replaced with fixed node supports?

No

Target file with pinned support in the source file attached

[REDO10]

Hi @Simau 

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@Simau  a écrit :

Target file with pinned support in the source file attached

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Are you sure you attached the correct file? As you can see in the image below, it looks like it’s the same original file I previously posted!?

Thanks.