I am trying to run moment-curvature analysis for a beam section. Its a rectangular section with width = 12 in, Depth = 3 in, 3 bars of area 0.03 sq.in each. Depth of reinforcement from top of section is 2.5 inches. f'c =5 ksi and fy of steel is 60 ksi.
I defined a fiber section and then perform the moment curvature analysis (with zero axial load) as suggested in the example script (not exactly the same script but it does the same thing).
When doing this, I am facing three problems:
1. The moment capacity given by OpenSEES is about 5 times the one I get by hand calculations [using the formula Mn=As*fy*(d-a/2)]. OpenSEES gives me a value of 65 kip-in while hand calculation gives 13.2 kip-in.
2. I have defined two recorders to record the Stress Strain in steel but the files generated are empty !! I used the same syntax as given in the manual.
3. Since the section only has reinforcement on one side (bottom of the section), the moment capacities in +ve and -ve direction should be different, but OpenSEES gives me the same moment capacities in both +ve and -ve direction !
I am attaching the code, please let me know if am making some silly mistake.
wipe;
wipeAnalysis;
model basic -ndm 3 -ndf 6;
# Define Two Nodes with same X,Y & Z coordinates
node 1 0. 0. 0.;
node 2 0. 0. 0.;
# Define Fixicity
fix 1 1 1 1 1 1 1;
fix 2 1 1 1 1 0 1;
# Area of Steel
set As 0.03;
#
set mat_tag_conc 1;
set mat_tag_steel 2;
# Define the Concrete and Steel Properties
uniaxialMaterial Concrete01 $mat_tag_conc -5. -0.002 -1.0 -0.01;
uniaxialMaterial Steel01 $mat_tag_steel 60.0 29000.0 0.0;
# Define the Section - Local Origin assumed at the center of the section
## Section 1 - Positive Reinforcements Only
section Fiber 1 {
patch quad $mat_tag_conc 100 30 -6.0 -1.5 6.0 -1.5 6.0 1.5 -6.0 1.5;
layer straight $mat_tag_steel 3 $As -4.0 -1.0 4.0 -1.0;
}
# Define the ZeroLength Section Element
element zeroLengthSection 1 1 2 1;
# Define Recorders
recorder Node -file Curvature.dat -node 2 -dof 5 disp;
recorder Node -file NodeRcn.dat -node 1 -dof 5 reaction;
recorder Element -file SS1.dat -ele 1 section 1 fiber 4.0 -1.0 stressStrain;
recorder Element -file SS2.dat -ele 1 section 1 fiber -4.0 -1.0 stressStrain;
#
#
pattern Plain 1 Linear {
load 2 0. 0. 0. 0. 1. 0.;
}
integrator DisplacementControl 2 5 .0001; #USE -0.0001 for bending in other direction
system SparseGeneral;
test NormUnbalance 1.0e-20 10 2;
numberer Plain;
constraints Plain;
algorithm ModifiedNewton;
analysis Static;
analyze 200
One more question, when I record the reaction at node 1, which is fixed, I also get significant axial loads even though I only apply displacement controlled rotation !!
Thanks.
