Eigen Analysis

[ach0121]

I am currently breaking down my model components and making sure they are correct. I have modeled a simple column with a fixed base in both opensees and SAP2000 and get completely different results. Is there something wrong with my model? My SAP calculations gives me a period of 0.38276 sec and my Opensees seems a little long at 1.1777 sec

# clear clache
wipe all

# basic 2D model with 3 degrees of freedom
model basic -ndm 2 -ndf 3;

# make files to store data
file mkdir Data;
file mkdir Model;
source Wsection.tcl; # wsection

node 101 0 0
node 301 0 312;

fix 101 1 1 1;


mass 301 0.34083 1e-9 1e-9;

uniaxialMaterial Steel01 1 55 29000 .03;
puts "material defined"

geomTransf PDelta 1;
puts "pdelta defined"

#Element Properties
#W14x257
set d 16.38
set bf 15.995
set tf 1.89
set tw 1.175
set nfdw 16
set nftw 16
set nfbf 16
set nftf 16
Wsection 1 1 $d $bf $tf $tw $nfdw $nftw $nfbf $nftf

puts "wsection"

#Defining exterior column Iyy
element forceBeamColumn 1 101 301 5 1 1


puts "elements"

set numModes 1

# record eigenvectors
#----------------------
for { set k 1 } { $k <= $numModes } { incr k } {
recorder Node -file [format "modes/mode%i.out" $k] -nodeRange 1 6 -dof 1 2 3 "eigen $k"
}

# perform eigen analysis
#-----------------------------
set lambda [eigen $numModes];

# calculate frequencies and periods of the structure
#---------------------------------------------------
set omega {}
set f {}
set T {}
set pi 3.141593

foreach lam $lambda {
lappend omega [expr sqrt($lam)]
lappend f [expr sqrt($lam)/(2*$pi)]
lappend T [expr (2*$pi)/sqrt($lam)]
}

puts "periods are $T"

# write the output file cosisting of periods
#--------------------------------------------
set period "modes/Periods.txt"
set Periods [open $period "w"]
foreach t $T {
puts $Periods " $t"
}
close $Periods


# create display for mode shapes
#---------------------------------
# $windowTitle $xLoc $yLoc $xPixels $yPixels
recorder display "Mode Shape 1" 10 10 500 500 -wipe
prp 312 312 1; # projection reference point (prp); defines the center of projection (viewer eye)
vup 0 1 0; # view-up vector (vup)
vpn 0 0 1; # view-plane normal (vpn)
viewWindow -200 200 -200 200; # coordiantes of the window relative to prp
display -1 5 20; # the 1st arg. is the tag for display mode (ex. -1 is for the first mode shape)
# the 2nd arg. is magnification factor for nodes, the 3rd arg. is magnif. factor of deformed shape


Thanks in advance

[esi_opensees]

most probably the mass source in two programs are different. please apply sigma (pAL/2) to your nodal mass in opensees.

[ach0121]

I'm sorry is sigma a command in opensees? I am not quite sure what you mean. Thanks for your help!