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Replacing Solid with Beam -- Results Don't Make Sense

Question asked by John Willett on Dec 5, 2017
Latest reply on Jan 11, 2018 by John Willett

My question arises out of attempting to replace the glass bar, highlighted in the screen shot below, with a beam.  The reason for this replacement, and for the whole modeling exercise, will become apparent (I hope) later, assuming you have the patience to read that far.

Make Glass Bar into Beam.png

First I must describe the assembly:  The plates that support it against the restraints shown, and also the end pieces bonded onto the glass bar, are defined with very low density so that the COM of the assembly is essentially the COM of the glass bar.  The plates have an elastic modulus for steel -- much larger than that of the glass -- so that they can flex some, but the ends are defined as very high elastic modulus so that they flex very little.  The plates are bonded to the ends only at their top faces, with gaps along their sides for no contact, as shown in this magnified image of the right end:

Contacts Right End.png

When I simulate this assembly with all solids, all the results agree with simple algebraic calculations on the simplified "free-body diagram," below, where the plates have been eliminated and replaced by the main forces and torques they exert on the bar ends.  (The ends have also been eliminated from this diagram for simplicity.):

Free-Body Diagram.png

Specifically, F2y = -F1y, F1x = W, and F2y = (T1z - W*delta)/L, which is small compared to W.


So here's the conundrum:  When I replace the highlighted solid bar with a beam of the same material and cross section, I get a much larger result for F1y, although F2y remains small (not quite the same value as before).  The bottom ends of both plates, however, show equal y-directed reaction forces that are both small and equal to the result with the solid bar.  Am I doing something obviously wrong here?


Background:  I'm out of time and will try to explain the motivation for this in my next post... -- John Willett


Update:  I have now found an answer to the question below, as briefly explained in my third post.  The question above, however, still remains open.  Something seems wrong with my changing this part from solid to beam in the simulation study. -- J.W.