Quad element - Stresses are identical at all Gauss points

[ckay]

Hello,

I have problems when plotting my stresses. The results are identical at all 4 Gauss integration points.

I'm trying to identify the location of my vertical stresses in relation to depth.
I currently only analysing a 2x2 matrix (4 elements).
The output shows identical values for sigma11 and sigma22 for all 4 Gauss points (initialStateAnalysis).
The stresses plotted are equivalent for the middle of the element.
I used quad elements with pressureDependentMultiYield material during InitialStateAnalysis.

Do you know why the stresses at all Gauss points within an element are all identical and why the results reported correspond to the stress at the mid height of the element?

Thanks

[ffirouzei]

hello
i make a model off 22 story building (3D model) but there is an error when i run the analysis that i cant handle . please help me
the error is:
force beam column 3d::set section pointers -max number of sections exceeded lobboto integration -- max integration points is 10

[ffirouzei]

ffirouzei wrote:
> hello
> i make a model off 22 story building (3D model) but there is an error when
> i run the analysis that i cant handle . please help me
> the error is:
> force beam column 3d::set section pointers -max number of sections exceeded
> lobboto integration -- max integration points is 10
>

[fmk]

to 1) are you sure the bounary conditions are such that the element is not in constant stress state. If not, swicth in an Elastic material to see what the stresses are.

to 2) we do not deal with that many integration points AS IT MAKES NO SENSE TO DO SO. if you need finer discretizations use multiple displacement beam columns.

[ckay]

Hello,

Thank you for your reply. I have checked my boundary conditions and changed to an elastic material. Nevertheless, my results are still the same at all four Gauss points and the results represent the stresses in the middle of the element.
Please see below a simplified model with only one element. I also realised, that the updateMaterialStage does not work for this particular example. Do you know if there is anything wrong with my example?
I'm thinking of using SSPquad element instead, but I'm not sure about its use for dynamic problems. Do you know if the SSPquad element is suitable for dynamic analyses?

Thank you for your help in advance.

wipe;
model basic -ndm 2 -ndf 2;
node 1 0.0 0.0 ;
node 2 1.0 0.0 ;
node 3 1.0 1.0;
node 4 0.0 1.0 ;

fix 1 1 1;
fix 2 1 1;
fix 3 1 0;
fix 4 1 0;

nDMaterial ElasticIsotropic 5 25000.0 0.35 2.0

set xWgt1 0.00
set yWgt1 [expr -9.81*$rho]

nDMaterial InitialStateAnalysisWrapper 1 5 2

element quad 1 1 2 3 4 1.0 "PlaneStrain" 1 0.00 0.0 $xWgt1 $yWgt1;

# create the pre-gravity recorders
set step 0.1
file mkdir ./Results_quad
recorder Element -xml Results_quad/Gstress1.xml -time -dT $step -ele 1 stress

constraints Transformation
numberer RCM;
system ProfileSPD;
#system SparseGeneral
test NormDispIncr 1.e-12 25 1;
algorithm Newton;
integrator LoadControl 1;
analysis Static;

InitialStateAnalysis on
analyze 4;
InitialStateAnalysis off
wipe ;

[fmk]

yes .. the element simply takes the weight and determines element forces that are applied at the nodes .. in your case resulting in a constant stress state
you could simply add a load to get out of this state
pattern Plain 1 "Linear" {
load 3 0.0 1.0
}
or add an element
node 5 2.0 0.0;
node 6 2.0 1.0 ;
fix 5 1 1;
fix 6 1 0;
element quad 2 2 5 6 4 1.0 "PlaneStrain" 1 0.00 0.0 0. 0.;

SSPQuad is suitable ..(it only uses 1 gauss pt at middle of element)