Beam Force Difference in ETABS and SAFE
Beam Force Difference in ETABS and SAFE
(OP)
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
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






RE: Beam Force Difference in ETABS and SAFE
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.
RE: Beam Force Difference in ETABS and SAFE
In general I would not trust etabs for slab and beam forces in concrete unless you spend a lot of time calibrating the model.
RE: Beam Force Difference in ETABS and SAFE
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.
RE: Beam Force Difference in ETABS and SAFE
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?
RE: Beam Force Difference in ETABS and SAFE
see attached two models.
http://files.engineering.com/getfile.aspx?folder=0...
http://files.engineering.com/getfile.aspx?folder=6...
http://files.engineering.com/getfile.aspx?folder=2...
http://files.engineering.com/getfile.aspx?folder=e...
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.