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OpenSees AMI
Rectangular Patches
Original Post - 05 Jun 2022 - Michael H. Scott
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Defining rectangular patches is one of the more tedious aspects of
building a fiber section in OpenSees. Using the quad patch command,
you have to define the four corner points (I, J, K, and L) in
counter-clockwise order from I to L around the patch. If you go
clockwise, the fiber areas will be negative, which is probably not what
you want.
Some time in the mid 2000s, I added a rect (rectangular) patch command
that requires only two corner points, I and J. This command is
documented–it’s no secret–but,
confusion remains about the orientation
of the corner points in order to ensure the fiber areas are positive.
Like the immutable laws of reduced dimensionality in Flatland, counter-clockwise means nothing when there’s only two points.
Instead, the requirement here is that the I and J coordinates satisfy (yJ-yI)/(zJ-zI) > 0, i.e., the slope of the line from I to J is positive in the section y-z plane. If the slope of this line is negative, the generated fibers will have negative area.
Because rectangular patches are so common and easy to mess up, pull request #851 makes a simple correction so that both rectangular patch scenarios shown above will give positive fiber areas.
If you must create patches with negative fiber areas, you can still define a quadrilateral patch with corner points in clockwise order. I can see defining negative fiber areas for layers, e.g., to avoid double counting concrete and reinforcing steel bars in RC sections, but I’m not aware of a practical application where you would intentionally define patches with negative fiber areas.
For example rect patch code, see other posts, e.g.,
here and
here.