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Slab with edge moment

Peter Everett

I am analysing a ground bearing concrete slab which has a moment along the edge.  It is the base of a retaining structure.

I have put the slab into STAAD as plates on a plate mat foundation (subgrade modulus 250 kN/m2/m).  The edge moment is applied at the nodes as a point moment:

The slab analyses fine.  When I do a plot of the base pressure however, I get this:

I was expecting the pressure to vary along the length of the slab (parabolic or similar).  I'm also not sure why the pressure changes sense halfway along - the plates are all orientated the same way.

The slab deflection is similar to the pressure plot.  The slab bending moment doesn't look right at all.

I've attached my STD file.  The load is Load 3 - Wall moments.

Can anyone help figure out what's going wrong?

 

Tank slab.std
  • 0 Offline

    Hi,

    I think that you wanted to apply bending moments about local X axis of the plates. Please note that nodal loads are applied in global coordinate system of the structure. So if you want to apply those moments in local X of the plates, then you should to change nodal moments MX into MZ. Now you should get the expected results.

    Regards,

    Modestas



    Answer Verified By: Peter Everett 

  • 0

    Hi petere,

    Have a look at your input load. I think you might have defined your wall moment rotating around the wrong axis.

    There are other problems with your model that I think you should look into:

    1. A soil-structure interaction modelling such as your requires that you use compression spring only, as soil does not provide you with tension support, which means this model is a non-linear model.
    2. Since this is a non-linear model, iterations will be required. Iteration limit should be set to something higher to ensure that. Look into SET ITERLIM command.
    3. Also, non-linearity also means that you cannot linearly add up your analysis results for different load cases. I suggest that you either lump all your input the same load case, or use Reference Loads feature.
    4. At 50 mm, your base slab seems to be too thin. Typically, structural stiffness contribution is higher than soil stiffness contribution, so it is important to correctly model the structure.
    5. The subgrade modulus of 250 kPa / m that you have defined seems to be a very low value.

    Hope my suggestions help. Refer to Technical Reference Manual for more information about the command / feature that I have mentioned.

    Regards,

    Hean

  • 0 Offline

    The moments that you applied are trying to twist the slab. I think you intended to apply nodal MZ moments as Modestas suggested in his post. So please modify the Load case 3 as shown next.

    LOAD 3 LOADTYPE Dead  TITLE WALL MOMENTS (SLS)

    JOINT LOAD

    *1 4 5 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 -

    *54 56 MX 4.423

    *2 3 1571 TO 1596 MX -4.423

    *

    1 4 5 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 -

    54 56 MZ -4.423

    2 3 1571 TO 1596 MZ 4.423

    Moreover hean brought up some excellent points which you need to look into. The mat is too flexible and the subgrade modulus seems approx 100 times too small.



  • 0 in reply to Sye

    It's always the obvious answer!

    Changing the direction of the moment gives a more sensible pressure distribution.

    It was a hypothetical slab to verify a hand calculation rather than an actual slab, which is why it was so thin and the modulus was so low.

    I'll have a go with the non linear analysis as well when I get a chance.  Many thanks for pointing it out.

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