no degradation in pushover curve!

[aminamini]

Hi everybody!

I've modeled a 2-bay 10-story steel moment frame using beamwithhinges elements. Fiber sections have been used to model hinges in two ends of elements. steel01 material has been used too. pushover analysis has been done for this stucture. analysis stops when the top displacement of structure reaches 8.07 m. moreover, there is no degradation in pushover curve! why? what should I do to observe degradation in pushover curve?

can anyone help me?
Thank you very much.

[nskok]

If I am not mistaken, the the post-yield tangent for the Steel01 material does have a limit associated to it. So if you are doing a pushover test, the stress in the steel will just keep increasing in accordance to this tangent line.

If you want to see degradation, I believe you will have to perform some type of cyclic analysis to generate hysteresis in the material.

-Nick

[aminamini]

Dear Nick
thank you for your reply;

But when I model a column using same element and material and push it, I can see degradation in pushover curve.
Moreover, I want to perform a monotonic pushover analysis not a cyclic analysis.

Here is my code for column:


wipe
wipeAnalysis

model BasicBuilder -ndm 2 -ndf 3;
file mkdir data1;

node 1 0 0;
node 2 0 5.

fix 1 1 1 1;

mass 2 200. 1.e-9 1.e-9 ;

geomTransf PDelta 1

uniaxialMaterial Steel01 1 2.4e8 2.e11 0.01

section fiberSec 1 {
patch quad 1 2 3 0.0348 0.023 0.0348 -0.023 0.04 -0.023 0.04 0.023
patch quad 1 2 8 -0.0348 0.0019 -0.0348 -0.0019 0.0348 -0.0019 0.0348 0.0019
patch quad 1 2 5 -0.04 0.023 -0.04 -0.023 -0.0348 -0.023 -0.0348 0.023
}


section Elastic 10 2.e11 7.4288e-4 77.701e-8
element forceBeamColumn 1 1 2 1 "HingeRadau 1 .4 1 .4 10"

eigen 1 ;
set T1model [expr 2*3.1416/(pow([eigen 1],0.5))]
puts "T1model=[expr $T1model]sec"

recorder Node -file Data1/DFree.out -time -node 2 -dof 1 2 3 disp;
recorder Node -file Data1/DBase.out -time -node 1 -dof 1 2 3 disp;
recorder Node -file Data1/RBase.out -time -node 1 -dof 1 2 3 reaction;
recorder Drift -file Data1/Drift.out -time -iNode 1 -jNode 2 -dof 1 -perpDirn 2 ;
recorder Element -file Data1/FCol.out -time -ele 1 globalForce;

pattern Plain 1 Linear {
load 2 0. -2000. 0.;
}

constraints Plain;
numberer Plain;
system BandGeneral;
test NormDispIncr 1.0e-8 6 ;
algorithm Newton;
integrator LoadControl 0.1;
analysis Static
analyze 10;
loadConst -time 0.0;

#wipeAnalysis

pattern Plain 2 Linear {
load 2 2000. 0.0 0.0;
}

constraints Plain;
numberer Plain;
system BandGeneral;
test NormDispIncr 1.0e-8 60 ;
algorithm Newton ;
integrator DisplacementControl 2 1 0.001;
analysis Static
analyze 1000;

puts "Done!"

[nskok]

What are you comparing with your pushover plot, roof displacement vs base shear? What do you mean by degradation in the curve? Does your curve for the frame follow a linear path?

[aminamini]

yes, roof displacement vs base shear.
strength degradation in curve means: roof disp increases without any increase in lateral loads (base shear).
no, my pushover curve isn't linear.

but now, when I change hardening ratio of steel material to 0.5% (instead of 2%), I can see strength degradation in pushover curve.

[andreascj]

Did you try considering P-Delta effect?

[aminamini]

yes, I've considered p-delta effect in analysis.