Concern about RBE3

[Victor]

I'm trying to implement my own RBE3 according to the Mystran manual and I've noticed something that looks like a familiar type of wrong behavior. I'm not really sure if it is wrong or not or if Mystran also behaves this way. I haven't used Mystran before but I'd like see what other people think of it before setting up Mystran and learning how to define a model.

In my implementation, when I apply a moment load about the x axis, it causes the correct moment about x but also another moment about the y axis as shown in the attached picture. Note that the independent nodes are on an inclined face, not in a plane normal to the applied moment.

If you've used RBE3 before, would you expect this beam to bend as well as twist?

I'm suspicious of this because CalculiX has a similar issue with its moment loads on beams which are internally converted to MPCs similar to an RBE3.

[Victor]

I've made a simpler example and solved with Mystran. It's a single CHEXA element supported on one face (grids 1,2,3,4) by SPCs and with a moment about Z applied to the opposite face (grids 5,6,7,8) via RBE3 from a separate grid 9.


Input file:

Code:

SOL STATICS
CEND

SPC = 1
LOAD = 1
SPCFORCES = ALL

BEGIN BULK
GRID    1       0       0.      -1.     0.0
GRID    2       0       1.      0.      0.0
GRID    3       0       0.      1.      0.0
GRID    4       0       -1.     0.      0.0
GRID    5       0       0.      -1.     1.2
GRID    6       0       1.      0.      1.0
GRID    7       0       0.      1.      0.8
GRID    8       0       -1.     0.      1.0
GRID    9       0       0.      0.      1.0

C Element Connectivity: CHEXA
CHEXA   1       1       1       2       3       4       5       6       +CH1
+CH1    7       8

PSOLID  1       1       0       2       FULL
MAT1    1       200.E9          0.
SPC     1       1       1       0.
SPC     1       1       2       0.
SPC     1       1       3       0.
SPC     1       2       1       0.
SPC     1       2       2       0.
SPC     1       2       3       0.
SPC     1       3       1       0.
SPC     1       3       2       0.
SPC     1       3       3       0.
SPC     1       4       1       0.
SPC     1       4       2       0.
SPC     1       4       3       0.

MOMENT  1       9       0       100000. 0.      0.      1.

RBE3    2               9       123456  1.0     123     5       6       +1
+1      7       8

ENDDATA

Here's the solution for reaction forces on the SPC nodes (T1-T3) and my hand calculated moments about the 3 axes due to those forces (Moment X, Moment Y, Moment Z):

Code:

           GRID     COORD      T1            T2            T3       Moment X      Moment Y      Moment Z
                     SYS
              1        0 -2.431020E+04  9.081890E+02  5.563799E+02 -5.563799E+02               -2.431020E+04
              2        0  5.401138E+02 -2.440502E+04  4.443620E+03  0.0          -4.443620E+03 -2.440502E+04
              3        0  2.581279E+04 -1.975163E+03  5.563799E+02  5.563799E+02               -2.581279E+04
              4        0 -2.042702E+03  2.547199E+04 -5.556380E+03  0.0          -5.556380E+03 -2.547199E+04
            Sum                                                     0.0          -1.000000E+04 -1.000000E+05

The total reaction moment about Z (-1.000000E+05) is the opposite of the applied moment which looks correct. However, there's also a reaction moment about Y (-1.000000E+04) which I think should be zero. It doesn't balance with the applied moment.

Any thoughts?

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