Beam Force Difference in ETABS and SAFE

[Enhineyero]

Hi all:

I dont know where i should post this, so I apologize if I posted this in the wrong thread. so here is my question:

Why is there a huge difference in beam shear & moments between ETABS and SAFE? I did a model a 1 storey model on etabs and directly exported that floor on safe. but the beam force difference was quite huge ( may be 2x). I modeled my slab as a membrane in etabs.

I did my research. I have read that I need to change the bending stiffness to almost zero for the slab for the result of safe & etabs to tally ( but I dont know why would the stiffness of the slab needs to be modified). I did this and the result became closer but the forces still have some significant difference, about 30%.

can anyone help me with this.

thanks in advance

 

[slickdeals]

In ETABS, the slab and beam are taking a share of the load based on the stiffness. If you do a section cut through the beam and slab and compare the results, you will get similar results between SAFE and ETABS.

SAFE on the other hand, computes the combination of moments carried by the slab and beam and reports it.

When you use a membrane element or shell element with negligible stiffness, it does not carry any load and all the loads are carried by the beam. Hence, the results in ETABS (w/membrane) match the results from SAFE.

 

[bookowski]

In addition to Slickdeals comments - SAFE will have a much tighter mesh (unless you really meshed a lot in etabs) and will also be accounting for cracking. If you are using V12 of safe have a look in the advanced modeling options, you need to tell SAFE if you want to consider the offset of the beam centerline from the slab - this also has a big impact. Be careful if you tell it to apply a rigid diaphragm though, in that case you can not also use the offset or it will become artificially stiff.

In general I would not trust etabs for slab and beam forces in concrete unless you spend a lot of time calibrating the model.

 

[Enhineyero]

Thanks for your replies. generally the result for beam forces on etabs is relatively accurate for framed structures. the only time i see too much variation if the structure is a flat slab (with perimeter beams & some internal beams). i have always modeled my slabs in etabs as membrane, but when i have a flat slab ( with some cantilever) and assigned it as a membrane the deformed shape is not correct, because the beams appear to be rigid (very small deflection. I also made a similar model on safe & the deformation is correct, unlike with etabs.
so i decided play with my etabs model and assign my flat slab as a shell (with zero stiffness in M12) then suddenly the deformation became correct? can anyone give light on this?

FYI i only considered gravity loads.

 

[slickdeals]

M12 is the twisting moment in the slab. Usually it should not be ignored.

Usually when you assign a membrane option, you should not mesh it such that you have an "unsupported" node (ie) all nodes should be supported. In case, you have a unsupported node, the deflection will be extremely high at that node because a "membrane" element has no out-of-plane stiffness.

If you don't mind could you post your model so we can comment on it better?

 

[Enhineyero]

Hi slick thanks for your advice on this matter. I think M12 is the torsion on the element right? for my model my slab is not subjected to any torsional forces.

see attached two models.

http://files.engineering.com/getfil...d7c485fdf72&file=correct_slab_deformation.$ET

http://files.engineering.com/getfil...bc49ffb35a5&file=correct_slab_deformation.EDB

http://files.engineering.com/getfil...96f2cff2f&file=incorrect_slab_deformation.$ET

http://files.engineering.com/getfil...9e5920ed6&file=incorrect_slab_deformation.EDB

when i assign my slab as a shell the output shows the deformation that i expected, while if i assign it as membrane the deformation is a bit incorrect (the beams become very stiff). I usually assign my slabs as membrane (for framed systems), now this structure is generally a flat slab and i don't know why, when i assign my slab as a membrane the deformation is a bit incorrect.


Thank you for your help.