questions about the fibre discretization of section

[davidjingyu]

Dear Dr.silvia:

I have some questions about the fibre discretization of section.

1. Should the reinforcing bar fibre and the concrete fibre have the same place to satisfy the same stain?:
section fiberSec $Col_Tag {
patch circ $IDconc1 $nfCoreT $nfCoreR 0 0 0 $rc 0 360;
Patch circ $IDconc2 $nfCoverT $nfCoverR 0 0 $rc $ro 0 360;
set theta [expr 360.0/$numBarsSec];
layer circ $IDreinf $numBarsSec $barAreaSec 0 0 $rs $theta 360; # Define the reinforcing layer
}
Can $rs doesn't equal to $rc and $ro?
And also, can $nfCoreT doesn't equal to $numBarsSec?

2. When using nonlinearbeamcolumn element, does the strain of different material fibres in cross section always satisfy the straight-section hypothesis.

Best！

david

[silvia]

they overlap, yes, but no, the number of concrete fibers doesn't have to equal the number of bars.
yes, the assumption is that plane section remain plane!

[davidjingyu]

Thank you very much for your reply.
Do you mean that
1. $rs has to equal $rc or $ro? Otherwise, straight-section hypothesis of strain would not be satisfied.

2. $nfCoreT can don't have to equal $numBarsSec?

/////////////////////
(cover and the core have the same material IDconc1,
$IDreinf placed at the borderline of cover and core)
section fiberSec $Col_SecTag1 {
patch circ $IDconc1 $nfCoreT $nfCoreR 0 0 $ri $rc 0 360; patch circ $IDconc1 $nfCoverT $nfCoverR 0 0 $rc $ro 0 360;
set theta [expr 360.0/$numBarsSec];
layer circ $IDreinf $numBarsSec $barAreaSec 0 0 $rc $theta 360; # Define the reinforcing layer
}
as i find that the from the results, the strain of $IDconc1 at -$ro & $ro (epsc_, epsc) and the strain of $IDreinf at -$rc & $rc (epss_, epss) don't satisfy straight-section hypothesis.
And also the curvature from results equal (epss-epss_)/(2*rc), do not equal (epsc-epsc_)/(2*ro).
So i am puzzle about it.
Does it caused by the centroid of fibre?

[silvia]

please work through it.
remember, the strain is sampled at the center of the fiber in a patch.