unexpected deformation of BRB in the rocking frame

[cxc08]

dear all,
I attempt to simulate a rocking frame with bucking restrained brace test which was conducted on E-Defense before.
The specimen is illustrated in figure 4 in this paper: http://nees.illinois.edu/hosted/Control ... 202011.pdf

I use three "forceBeamColumn" elements to define BRB.
The end two elements are assigned with BRB elastic zones properties, and the middle element is assigned with BRB core elastoplastic properties.
The connection between BRB and the steel frame is defined through "equalDOF" to simulate pin connection.

During the time-history analysis, the shape of BRB developed into "Z" shape. The elastic part and elastoplastic part of BRB deformed into different directions.
I can't find the reason yet.
I built a simplified 2d model with static analysis as following, but the "Z" shape deformation still occurs.
(To see the deformation, you need DisplayPlane.tcl and DisplayModel3D.tcl.)

##########################################################################################################
#units: N,mm,Mpa

wipe
model basic -ndm 2 -ndf 3

# Node --------------------------------
node 1 0 0
node 2 0 3e3
node 3 6e3 0
node 4 6e3 3e3
node 5 3e3 3e3
node 6 3e3 0
node 7 3e3 0
node 8 3e3 1e3
node 9 3e3 2e3
node 10 3e3 3e3
node 12 0 0
node 14 6e3 0

# Boundary condition --------------------
fix 11 1 1 1
fix 12 1 1 1
fix 13 1 1 1
fix 14 1 1 1
fix 7 1 1 1
fix 1 1 0 0
fix 3 1 0 0
# pin connection between BRB and frame
equalDOF 7 6 1 2
equalDOF 10 5 1 2

# Materials -------------------------------------------------------------------------
# "SM490":
uniaxialMaterial Steel02 2 325 205000 0.001 18.5 0.925 0.15
# "LYP-225" BRC fiber section
uniaxialMaterial Steel02 3 213 205000 0.001 20 0.925 0.15 0.07 1.0 0.0 1.0
# "lift up foot"
# uniaxialMaterial ElasticPPGap $matTag $E $Fy $gap <$eta> <damage>
uniaxialMaterial ElasticPPGap 10 1e6 1e15 0.0 0.0

# Sections ---------------------------------------------------------------------------
# BRB plastic part section "PL-16x138" LYP-225 fiber section
section Fiber 3 {
patch quad 3 10 2 -69 -8 69 -8 69 8 -69 8
}
# BRB elastic part section "+160x16" SM490
section Fiber 5 {
patch quad 2 10 1 -80 -8 80 -8 80 8 -80 8
patch quad 2 1 10 -8 8 8 8 8 80 -8 80
patch quad 2 10 1 -8 -80 8 -80 8 -8 -8 -8
}
# Column section "H-250x250x9x16" SM490
section Fiber 1 {
patch quad 2 10 1 -125.0 -125.0 125.0 -125.0 125.0 -109.0 -125.0 -109.0
patch quad 2 1 10 -4.5 -109.0 4.5 -109.0 4.5 109.0 -4.5 109.0
patch quad 2 10 1 -125.0 109.0 125.0 109.0 125.0 125.0 -125.0 125.0
}
# Beam section "H-250x250x12x25" SM490
section Fiber 2 {
patch quad 2 10 1 -125.0 -125.0 125.0 -125.0 125.0 -100.0 -125.0 -100.0
patch quad 2 1 10 -125.0 100.0 125.0 100.0 125.0 125.0 -125.0 125.0
patch quad 2 10 1 -6.0 -100.0 6.0 -100.0 6.0 100.0 -6.0 100.0
}

# Geometric transformation -------------
geomTransf Corotational 1

# Elements ------------------------------
# Beams and columns
element forceBeamColumn 1 1 2 5 1 1
element forceBeamColumn 2 3 4 5 1 1
element forceBeamColumn 3 2 10 5 2 1
element forceBeamColumn 4 10 4 5 2 1
element forceBeamColumn 5 1 10 5 2 1
element forceBeamColumn 6 3 10 5 2 1
# BRB elastic parts
element forceBeamColumn 7 6 8 5 5 1
element forceBeamColumn 8 9 5 5 5 1
# BRB plastic part
element forceBeamColumn 9 8 9 5 3 1;
# upliftable base
element zeroLength 11 1 12 -mat 10 -dir 2;
element zeroLength 13 3 14 -mat 10 -dir 2;

# recorder ------------------------------------------------------
recorder Node -file D1.out -time -node 1 -dof 2 disp
recorder Node -file D3.out -time -node 3 -dof 2 disp
recorder Node -file R1.out -time -node 12 -dof 2 reaction
recorder Node -file R3.out -time -node 14 -dof 2 reaction

# display --------------------------------------------------------
source DisplayPlane.tcl
source DisplayModel3D.tcl;
set xPixels 500;
set yPixels 300;
set xLoc1 0;
set yLoc1 0;
set ViewScale 30;
DisplayModel3D DeformedShape $ViewScale $xLoc1 $yLoc1 $xPixels $yPixels

# Gravity load ----------------------------
pattern Plain 1 Linear {
load 2 0 0 -100000
load 4 0 0 -100000
}
constraints Transformation
numberer Plain
system BandSPD
test NormDispIncr 1.0e-8 60 2
algorithm Newton
integrator LoadControl 0.1
analysis Static
analyze 10;
puts "Model Built"
loadConst -time 0.0;

# Reversed cyclic load ----------------------
pattern Plain 2 Linear {
load 10 1000 0 0
}
wipeAnalysis
constraints Transformation
numberer Plain
system BandGeneral
test NormDispIncr 1.0e-8 60
algorithm Newton
analysis Static
foreach disp {50 100 150 200 250 300} {
integrator DisplacementControl 10 1 1e-1
analyze [expr 1*$disp]
integrator DisplacementControl 10 1 -1e-1
analyze [expr 1*$disp]
integrator DisplacementControl 10 1 -1e-1
analyze [expr 1*$disp]
integrator DisplacementControl 10 1 1e-1
analyze [expr 1*$disp]
}
##########################################################################################################
Can anyone help me to find where the problem is?
Thank you for your kind help!

[cxc08]

You can download DisplayPlane.tcl and DisplayModel3D.tcl from this page:
http://opensees.berkeley.edu/wiki/index ... _W-Section

[cxc08]

Some possible reasons I have checked:
1. Display mistake
I checked the data. It also showed same deformation.
2. Geometric transformation
Both "Corotational" and "Linear" gave the same "Z" shape deformation
3. Element type
no influence
4. Constraint type
no influence
5. Dynamic or static analysis
no influence

I only found one way to fix the problem: assigning the same section for the elastoplastic and elastic parts of BRB.
But I can't figure out why..

[cxc08]

I also checked the element force of the three elements for modeling BRB. The bending moments are quite small.

[brag006]

Not quite sure what your problem is. It seems to be working fine. Why is this Z shape wrong?

[cxc08]

There are two reasons:
1. I don't think there is force which can cause such deformation.
2. In the full rocking frame model the "Z" shape deformation became very large and caused the analysis fail.