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Equivalent frame Re: A structural engineering journal

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My experience has been that the equivalent frame method gives very close
results to that of a FEM method used by a program such as SAFE. In fact
from equilibrium, the full load must be carried separately in each
direction (cut a panel along the points of zero shear so that it is
centered over a column). As stated by someone earlier, the FEM just gives
you a distribution of the forces along the slab width.

I don't think that analyzing the slab twice in two different directions
corresponds to designing for twice the load. If that was the case, nobody
would design using the equivalent frame method. I think it just means
doing two checks to make sure it works, but not adding the results of the
checks together.

> The equivalent frame method is "very conservative" because it analyzes the 
> panels as one-way slabs in each direction, with each carrying the total 
> load.  Therefore, the shear forces in the panel at the column lines in 
> direction 1 have to equal the total load on the panel.  Similarly, the shear 
> forces in the panel at the column lines in direction 2 also have to equal the 
> total load on the panel.  Therefore, the total resisting shear on a panel is 
> equal to twice the load on the panel, a gross violation of statics.
> There is nothing in the equivalent frame method that I have seen that 
> considers consistent deformation, one of the requirements of a two-way slab 
> or statically indeterminate system.
> I cannot tell you why your FE program gives you the same results as the 
> equivalent frame method.  If the shear forces at all 4 panel edges total 
> twice the total load on the panel, I would suspect that the FE program was 
> written as an equivalent frame program and marketed as an FE program.
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona