I am using the four node quadrilateral element to model a 40m deep soil column under earthquake excitations in OpenSees. However, the dimension of the stress output file is different from that of the strain output file. Therefore, I have trouble finding out which stress output is corresponding to which strain output. Can anyone please tell me how many stresses does the 'stress' recorder actually recorders per element? Or how to plot the shear stress vs strain curve in general?
Hello,
In general, you should plot the stress-strain at nodes. For elements, you need to know the integration points in the element. When you know the position of the integration points, you should be able to plot.
"In general, you should plot the stress-strain at nodes."
I totally and emphatically disagree. Finite element programs calculate stresses and strains at integration points; extrapolated values at nodes may be off by a factor of 2, for example in plastic materials.
This is two different questions.
Your comment is regarding the accuracy of the extrapolation from integration points to nodes.
You will find the answer for the first question if you look for instance at the Command Manual in the User Documentation of OpenSees and then the NDMaterial Command Lists:
Let's propose that you have assigned "PressureDependMultiYield" as a material model for the 2D soil column. the arrangement of the stress and strain components are as following:
σxx, σyy, σzz, σxy, ηr
and
εxx, εyy, γxy
These components will be recorded for one Material point or Gauss point once you call the Element Recorder Command for each soil element.
Now if you have soil elements type "SSPquad Element" then you have for each element one single integration point which records the response of your material.
http://opensees.berkeley.edu/wiki/index.php/PressureDependMultiYield_Material
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