Buckling of RC slab

[mathias_peron]

Hi everyone,

I am trying to verify a funny shaped / ground supported slab under compression and would like to seek advice.

My problem is as follows:

A secant pile wall will be used to allow the installation of a new manifold in X city.

The secant pile wall will be propped at multiple level just like this :

My task is to make sure the propping system will work.

No problem to check the steel props. But the secant pile wall designer assumed that the secant pile wall will also be propped by a 200mm thick RC slab at the very bottom of the excavation.

I need to check that the slab can take the compression and will not buckle.

My model is the exact shape of the slab using 'parameters of foundation elasticity' for support (uplift Z) with a load all around its perimeter. See below:

I have performed a buckling analysis and read the critical coefficients for the first 10 modes.

First of all, I'd like to know if there is a better way to check this slab. As the buckling analysis do not take into account any imperfections, this model don't represent the reality. 

Have a good day,

Mathias.

[Artur.Kosakowski]

Hi @mathias_peron

First of all, I'd like to know if there is a better way to check this slab. As the buckling analysis do not take into account any imperfections, this model don't represent the reality. 

After freezing the mesh(es) you can move the panel nodes to include the influence of imperfection in the buckling analysis.

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

[mathias_peron]

Hi Artur and thank you for your answer.

I already thought about that but it seems to be a very time consuming thing to do. Also, it is difficult to deform the mesh 'smoothly' by hand.

But I still have one question.

The critical coefficient of my first mode is less than 1.00, all the others are above 50.00. But I think my first mode is a 'fake' one.

When I check the deformation of the first mode, it looks like this :

The mesh seem to have moved downward and on the side. I think it is due to the absence of supports in X and Y (which exist in the reality).

The second mode looks way more like a buckling shape, see here :

I'd like to know what you think about this, and if I can ignore the first mode.

Please find the model attached,

Have a good day,

Mathias.

[mathias_peron]

Also, I'd like to know how to take into account the friction slab/soil when undertaking the buckling analysis.

When I set a planar support like this, the analysis do not converge :

[Artur.Kosakowski]

Hi @mathias_peron

I already thought about that but it seems to be a very time consuming thing to do. Also, it is difficult to deform the mesh 'smoothly' by hand.

See: https://forums.autodesk.com/t5/robot-structural-analysis-forum/api-deformed-geometry-update/m-p/4905...

message by @Rafal.Gaweda

The critical coefficient of my first mode is less than 1.00, all the others are above 50.00. But I think my first mode is a 'fake' one.

Your first critical factor is actually 1.19 (above 1) however I'd rather have all boundary conditions in place before running the buckling analysis.

[mathias_peron]

Thank you Artur,

I have put correct boundary conditions to model the piles and tried the macro.

The problem is that I have an error message 'Linear load has not been applied to a panel.'. I can't find a way to correct it.

Could you take a look at it please?

Please find model attached.

Thank you in advance.

[Artur.Kosakowski]

Hi @mathias_peron

It seems that you moved the edge nodes 'slightly away' from the positions of the edges.

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

[mathias_peron]

Thank you Artur!

I'll go with what I have then.

Last question before to accept the solution: Do you think I am using a good approach for the verification of this slab? Linear buckling + initial imperfection?

[Artur.Kosakowski · Accepted solution]

Hi @mathias_peron

You may want to try to run nonlinear buckling and compare the results. 

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.