Raft foundation with ground beams

[Anonymous]

Hello everyone,

it's the first time I'm modelling the raft foundation in the RSA and I have got some problems:

1) the mesh is regular exc. one of the panels (in red circle) - how can I solve it?

2) do I have to chose rigid or flexible foundation type? How can I take into account the water table (which occurs between the 2nd and 3rd layer of soil) during calculation of Kz coefficent?

3) what about the uplift (none or UZ+)?

4) estimated foundation load is equal to 47.13 kN/m2 and I suppose that the extreme soil reaction should have +/- the same value (while it is equal to 72.32 kN/m2) - what do you think about it?

5) I don't have certainty that the Mxx map is good (outside ground beams should caused higher values of the moments?)

6) how can I calculate the reinformcement in the raft and beams - using function 'Provided reinformcement of RC elements' for beams (what about the supports?) and function 'Slab - required renforcement' for the raft or something else?

Figures presenting my questions are attached below. I attached a file with the model, as well.

Thanks a lot in advance.

Regards,

[mustafahesenow · Accepted solution]

Hi @Anonymous

1st I had checked your model and I found out that some panels overlapped check attached pictures 

1) the mesh is regular exc. one of the panels (in red circle) - how can I solve it?

this is happened because this slab is not connected properly to beam 23 

Yo need to delete the mesh for panels 41 and 49 then fix the panel 49 then re mesh it see attached picture after fixing this issue

[Anonymous]

Hi @mustafahesenow and thank You for your reply.

I've remeshed the model and now it's regular. Could You give me a help with other questions?

2) do I have to chose rigid or flexible foundation type? How can I take into account the water table (which occurs between the 2nd and 3rd layer of soil) during calculation of Kz coefficent? I've chosen flexible type, but have no idea how to take into account the water table during the Kz coefficent calculation

3) what about the uplift (none or UZ+)? I've chosen UZ+ uplift (it hasn't  affected on the results)

4) estimated foundation load is equal to 47.13 kN/m2 and I suppose that the extreme soil reaction should have +/- the same value (while it is equal to 72.32 kN/m2) - what do you think about it?

5) I don't have certainty that the Mxx map is good (outside ground beams should caused higher values of the moments?)

6) how can I calculate the reinformcement in the raft and beams - using function 'Provided reinformcement of RC elements' for beams (what about the supports?) and function 'Slab - required renforcement' for the raft or something else?

[mustafahesenow · Accepted solution]

Hi @Anonymous

2) do I have to chose rigid or flexible foundation type? How can I take into account the water table (which occurs between the 2nd and 3rd layer of soil) during calculation of Kz coefficient?

Usually for Raft we consider it as flexible foundation because of the deferential settlement between its points especially if you have concentrated loads coming from columns. 

I think you need to input new layer  with properties of Saturated density not the normal density .

3) what about the uplift (none or UZ+)?

This option will allow you to assign the kz in one direction whether uplift +z means the the reaction will work only for -z direction (this option should be active for the raft especially when you have water uplift) to get accurate result when you activate this option the analysis will be nonlinear analysis.

[mustafahesenow · Accepted solution]

Hi @Anonymous

4) estimated foundation load is equal to 47.13 kN/m2 and I suppose that the extreme soil reaction should have +/- the same value (while it is equal to 72.32 kN/m2) - what do you think about it?

When you have linear or concentrated  load on edges is for sure the result of soil reaction will be more than the expected because the stiffness of the edge is smaller than inside area and you will get also higher displacement there.

[mustafahesenow · Accepted solution]

Hi @Anonymous

5) I don't have certainty that the Mxx map is good (outside ground beams should caused higher values of the moments?)

I looked at it and it looks good and normal (at the ground beam connections you will encounter negative moment and at raft mid span you will encounter top moment which is same the result shown in the map ).

[mustafahesenow · Accepted solution]

Hi @Anonymous

6) how can I calculate the reinformcement in the raft and beams - using function 'Provided reinformcement of RC elements' for beams (what about the supports?) and function 'Slab - required renforcement' for the raft or something else?

You will do the design using function of provided reinforcement it  will work and you will obtain good results .

For the beams The support will be the perpendicular beams 

For raft use provided reinforcement for slab module RSA will call calculate it accurately.

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