This example involves the optimization of a box defined entirely by equations (there is no finite element model in the solution).

The optimization problem is defined as:

The volume and surface are defined as equations using DRESP2 and DEQATN:

$

$ VOLUME

$

DEQATN  1       VOL(W,L,H)=W*L*H

$

$ SURFACE

$

DEQATN  2       AREA(W,L,H)=2.0*(W*H+L*H+W*L)

$

DRESP2  1       VOLUME  1

       DESVAR  1       2       3

DRESP2  2       SURFACE 2

       DESVAR  1       2       3

$

DESVAR  1       W       1.1     0.1     10.0

DESVAR  2       L       0.9     0.1     10.0

DESVAR  3       H       2.0     0.1     10.0

$

 

Then, in the optimization problem, the objective and constraint are global responses (for example, DESOBJ and DESGLB are used outside of a SUBCASE).

To trick OptiStruct into solving this problem, a dummy finite element model must be provided. Here, a single shell element with some load is used.

As expected, the solution yields a cube with even sides of about 0.707, a surface of 3.0, and a volume of 3.53.

The input file can be found in <install_directory>/demos/hwsolvers/optistruct/box.fem.