Double sections

[MarzieDastigerdi]

I need to model double back to back UNP sections with back-to-back distance of 25.4 mm. I used fiberSec to do so .To test the accuracy of the section which was built, I modeled a frame with 1 bay 1 story. But there's difference between fundamenal period in elastic model and this model. Also when I change the back-to-back distance, T changes again. logically they should be the same . Is there any special point for modeling double back to back UNP sections that i haven't considered?
When i model the same frame , with W section , T is the same.
Here's the code i used to generate my sections:
proc twoUNPsection { secID matID d tw bf tf nfd nftw nfbf nftf s} {
set dw [expr $d - 2 * $tf]
set bn [expr $bf - $tw]
set y1 [expr ($s/2) + $bf]
#set y2 [expr $y1 - $tw]
set y2 [expr $y1 - $bn]
set y3 [expr $s/2]
set z1 [expr $d/2]
set z2 [expr $z1 - $tf]
section fiberSec $secID {
# nfIJ nfJK yI zI yJ zJ yK zK yL zL
#patch quadr $matID $nftw $nfd -$y1 -$z2 -$y2 -$z2 -$y2 $z2 -$y1 $z2
patch quadr $matID $nftw $nfd -$y2 -$z2 -$y3 -$z2 -$y3 $z2 -$y2 $z2
patch quadr $matID $nfbf $nftf -$y1 -$z1 -$y3 -$z1 -$y3 -$z2 -$y1 -$z2
patch quadr $matID $nfbf $nftf $y3 -$z1 $y1 -$z1 $y1 -$z2 $y3 -$z2
#patch quadr $matID $nftw $nfd $y2 -$z2 $y1 -$z2 $y1 $z2 $y2 $z2
patch quadr $matID $nftw $nfd $y3 -$z2 $y2 -$z2 $y2 $z2 $y3 $z2
patch quadr $matID $nfbf $nftf $y3 $z2 $y1 $z2 $y1 $z1 $y3 $z1
patch quadr $matID $nfbf $nftf -$y1 $z2 -$y3 $z2 -$y3 $z1 -$y1 $z1
}
}

[fmk]

there is no special requirement other than you get the fiber section definition correct .. if you are getting different answers it means the command is wrong .. you might start out with inputting just 1 of these UNP sections and compare with result for just that section .. another idea would be to create a routine in Matlab that would both generate OpenSees input using the fiber command and also allow you to see the location of the fibers.

[MarzieDastigerdi]

thx . I used Matlab, and it helped. The section should have been rotated 90 degrees.

[Mohammadf]

Hi Dear Marzieh
I suggest you to read my book titled "The Most Complete Applied Reference of OpenSEES" in Persian.
Best