collapse load

[im_pce]

Hi all

I want to determine the ultimate load in nonlinear static analysis, without considering the displacement control because I don't know what is the displacement corresponds to ultimate load. As you know, during the nonlinear analysis iteration, the determinant of the overall stiffness matrix is checked to determine whether at any load increment it becomes negative. This is an indication of a loss of stability of the structure and the load factor which causes this is identified as the ultimate load factor.
Would you please tell me how can I determine ultimate load in opensees?

Here is my script. Thanks in advance.

# Units: Kg,cm,sec
wipe
model BasicBuilder -ndm 3 -ndf 3
# nodal coordinates:
node 1 300 0 0
node 2 150 259.8076 0
node 3 -150 259.8076 0
node 4 -300 0 0
node 5 -150 -259.8076 0
node 6 150 -259.8076 0
node 7 0 0 23.6
#--------------------------
mass 1 353.25 353.25 353.25
mass 2 353.25 353.25 353.25
mass 3 353.25 353.25 353.25
mass 4 353.25 353.25 353.25
mass 5 353.25 353.25 353.25
mass 6 353.25 353.25 353.25
mass 7 706.5 706.5 706.5
#--------------------------
fix 1 1 1 1
fix 2 1 1 1
fix 3 1 1 1
fix 4 1 1 1
fix 5 1 1 1
fix 6 1 1 1
#--------------------------------------------
uniaxialMaterial Elastic 1 2.1E6
#------------------------------------------------
# tag node1 node2 A matID
element truss 1 1 2 10 1
element truss 2 2 3 10 1
element truss 3 3 4 10 1
element truss 4 4 5 10 1
element truss 5 5 6 10 1
element truss 6 6 1 10 1
element truss 7 1 7 10 1
element truss 8 2 7 10 1
element truss 9 3 7 10 1
element truss 10 4 7 10 1
element truss 11 5 7 10 1
element truss 12 6 7 10 1
#
pattern Plain 1 Linear {
load 7 0 0 -1000
}
# --------------------------------------------- #
constraints Transformation
numberer RCM
system BandGeneral
test NormDispIncr 1.0e-6 30
algorithm Newton
integrator LoadControl 0.1
analysis Static
analyze 10

[silvia]

if you have strength degradation you definitely have to go with displacement control. just go to a very large displacement and monitor the response.

[im_pce]

Dear Silvia

what is large displacement for any kind of structures? it is different for any one. can opensees check stiffness matrix whether at any load increment it becomes negative? if so, how?

Thanks

[silvia]

you need to adjust for each case.
you need to check the unloading in the post-processing, i believe.

[susan]

Hi Silvia & im_pce

Would you please tell me how can i check the unloading in the post-processing? How much is large diplacement? Does opensees give error message when structure collapse?

[silvia]

nope, it doesn't -- unless you go wayy far away.
look at the overall response, and you decide.

opensees lets you make all these decisions.