# OS-E: 4000 Rotating Bar

This is an introductory example of the optimization of a multibody dynamics system. The object of the rotating bar problem is to minimize the maximum stress of the bar.

## Model Files

Refer to Access the Model Files to download the required model file(s).

The model file used in this example includes:

rotating_bar_design.fem

## Model Description

The structure consists of 5 bar elements. The driving motion has a velocity of sin(2t), which is applied to the left end of the structure. Sections of the bar elements are solid circles. Design variables are the radii of the sections. The mass of the structure should be less than 10kg.

A portion of the input file:
DESGLB   = 4
MINMAX   = 14
STRESS   = ALL
SUBCASE    1
MBSIM  =   1
MOTION =   1
SPC    =   10
:
:
$DESVAR 1 RAD1 10.00 0.05 100.0 DESVAR 2 RAD2 10.00 0.05 100.0 DESVAR 3 RAD3 10.00 0.05 100.0 DESVAR 4 RAD4 10.00 0.05 100.0 DESVAR 5 RAD5 10.00 0.05 100.0$
DVPREL1 10      PBARL   1       DIM1            0.0
+   1   1.0
DVPREL1 11      PBARL   2       DIM1            0.0
+   2   1.0
DVPREL1 12      PBARL   3       DIM1            0.0
+   3   1.0
DVPREL1 13      PBARL   4       DIM1            0.0
+   4   1.0
DVPREL1 14      PBARL   5       DIM1            0.0
+   5   1.0
$DRESP1 33 STRESS STRESS PBARL SNMAX 1 + 2 3 4 5 DRESP1 100 MASS MASS$
DOBJREF 14      33      1       -1.0    1.0
\$
DCONSTR 4       100             2.5

:
:

SPC1    10      123456  1

:
:

ENDDATA

The input file is just like one for an ordinary min-max problem. The maximum normal stress of the flexible body of the subcase 1 is to be minimized by using DOBJREF and MINMAX.

## Results

Notable points include:
• Because the stress in multibody dynamics systems is a time variant quantity, the minimization of stress in multibody dynamics analysis subcases should be a min-max problem.
• The SPC1 card fixes only 6 DOF of node 1 of the flexible body in order to remove 6 rigid body motions. If you fix more than 6 DOF of the flexible body, the additional fixed DOF become constraints of the flexible body.
• Tress output request is placed above the first subcase. If you place output request inside subcase, your output request will be ignored.