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Re: A structural engineering journal

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Gil Brock wrote:

>>Roger,

In what way is the equivalent frame method "very conservative". A finite 
element analysis results in the same total moments in a panel in each 
direction as an equivalent frame analysis for a rectangular grid of 
columns. This also shows that the total load needs to be carried in both 
directions. The difference from a FE analysis is that it gives an elastic 
distribution of those moments across the width of the panel and will also 
show the distribution of moments for irregularly placed columns. The total 
design moments in a panel, however, are the same from the 2 approaches for 
the rectangular grid of columns.<<

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