New problem in modeling the buckling of a very slender brace

[zishendemi]

Hi everyone,

I'm trying to model a very slender brace (It's a rectangular section (2"*0.5") and the length is 100". kr/L>1000). At first, I try to use tension only material to model it. But later I find it can't capture the buckling behavior very well.

Then I use the method in which Uriz models Concentrically Brace Frame. (http://peer.berkeley.edu/publications/p ... ZMahin.pdf)

To model the brace, I create a fixed supported beam (horizontal DOF at right node is not restrained) and give it an initial imperfection (5"). I subdivide the brace into 2 elements and then assign a fiber section to them. The material is Steel02 and the element type is displacement_based BeamColumn.

I apply cyclic axial load (displacement control) to the right node of the brace and plot the force-displacement relationship. The model is very good at the first 2 cycles. But after 2 cycles, the brace won't buckle in compression! Instead, it yields! Something's not right.

Anyone has any idea? My code is below. Below is also a link to the plot of force-axial displacement relationship of the right node and the time history of the lateral displacement of the mid-point.
https://dl.dropbox.com/u/67665569/brace ... roblem.zip

Thanks a lot!

## Units kips-inches
model BasicBuilder -ndm 2 -ndf 3
## Nodes
node 1 0.0 0.0
node 4 50.0 0.5
node 7 100.0 0.0

#Boundary Conditions
fix 1 1 1 1
fix 7 0 1 1
## Materials

uniaxialMaterial Steel02 1 36.0 29000.0 0.003 20 \
0.925 0.15 0.0005 0.01 0.0005 0.01

# brace section
section fiberSec 2 {
patch quadr 1 6 6 -0.125 1.0 -0.125 -1.0 0.125 -1.0 0.125 1.0
}
## Transformation
geomTransf Corotational 1

## Define Model

element dispBeamColumn 1 1 4 3 2 1 ;
element dispBeamColumn 2 4 7 3 2 1 ;

## Apply the nodal Load
pattern Plain 1 Linear { load 7 1.0 0.0 0.0 }

## Recorder
recorder Node -file DFree.out -time -node 7 -dof 1 disp;
recorder Node -file DMiddle.out -time -node 4 -dof 1 2 disp;

## Static Analysis parameters
test EnergyIncr 1.0e-8 300 0
algorithm KrylovNewton
system UmfPack
numberer RCM
constraints Plain

set peaks [ list 0.25 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 ]
for {set i 1 } { $i <= 9 } {incr i } {
set dU [expr -1.0*pow((-1.0),$i)*[lindex $peaks [expr $i-1] ]/50.0 ]
integrator DisplacementControl 7 1 $dU 10 $dU $dU
analysis Static
analyze 50
}

[zishendemi]

Hi, everyone!

Can anyone help me about this? I have some new problems now.

Thanks a lot!

[vesna]

I'm sorry, but you will have to figure it out yourself.