question about fiber section

[yuejun]

Ex9c.build.RCSection.RectUnconfinedSymm2D.tcl

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set numBarsSec 4;	
set nfY 16;	
set nfZ 4;
section fiberSec $SecTag   {;
patch quadr $IDconcU $nfZ $nfY -$coverY $coverZ -$coverY -$coverZ $coverY -$coverZ $coverY $coverZ; 
layer straight $IDreinf $numBarsSec $barAreaSec  $coreY $coreZ  $coreY -$coreZ;
layer straight $IDreinf $numBarsSec $barAreaSec -$coreY $coreZ -$coreY -$coreZ;	
};

In this code, 4 reinforcements are placed on top and bottom of the column section, respectively. And the column section is divided into 64 patches. My question is why it defines so many patches.
There is notation following the code

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set nfY 16;

saying:

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# number of fibers for concrete in y-direction

what's the meaning of this comment? What is the physical subject this command describing? It is straightforward that LAYER defines the locations, areas, and arrangements of the reinforcements of the section. But it is so confusing to me what the PATCH command does.
Thanks a lot.

[silvia]

please review the command language manual.

[yuejun]

please review the command language manual.

I could not find help from reading the manual. So I post it here for help. Thanks.

[silvia]

look at some workshop material, it might help.

http://opensees.berkeley.edu/workshop/OpenSeesDays.html