Size Optimization

OptiStruct has the capability of performing size optimization simultaneously with the other types of optimization.

In size optimization, the properties of structural elements such as shell thickness, beam cross-sectional properties, spring stiffness, and mass are modified to solve the optimization problem.

Defining size variables in OptiStruct is similar to other size optimization codes. Each size variable is defined using a DESVAR bulk data entry. If a discrete design variable is desired, a DDVAL bulk data entry needs to be referenced for the design variable values. The DESVAR cards are related to size properties in the model using a DVPREL1 or DVPREL2 bulk data entry. Each DVPREL bulk data entry must reference at least one DESVAR bulk data entry to be active during the optimization. HyperWorks includes a pre-processor called HyperMesh that can be used to set up any number of size variables for the properties.

The following responses (see Responses for a description) are currently available as the objective or as constraint functions:

Mass	Volume	Center of Gravity
Moment of Inertia	Static Compliance	Static Displacement
Natural Frequency	Buckling Factor	Static Stress, Strain, Forces
Static Composite Stress, Strain, Failure Index	Frequency Response Displacement, Velocity, Acceleration	Frequency Response Stress, Strain, Forces
Weighted Compliance	Weighted Frequency	Combined Compliance Index
Function	Temperature

In finite elements, the behavior of structural elements (as opposed to continuum elements), such as shells, beams, rods, springs, and concentrated masses are defined by input parameters, such as shell thickness, cross-sectional properties, and stiffness. Those parameters are modified in a size optimization. Some structural elements have several parameters depending on each other; like beams in which the area, moments of inertia, and torsional constants depend on the geometry of the cross-section.

The property itself is not the design variable in size optimization, but the property is defined as a function of design variables. The simplest definition, as defined by the design-variable-to-property relationship DVPREL1, is a linear combination of design variables defined on a DESVAR statement such that:

Where, p is the property to be optimized, and Ci are linear factors associated to the design variable di.

Using the equation utility DEQATN, more complicated functional dependencies using even trigonometric functions can be established. Such design-variable-to-property relations are then defined using the DVPREL2 statement.

For a simple gage optimization of a shell structure, the design-variable-to-property relationship turns into:

Where, the gage thickness, T is identical to the design variable.

If a discrete design variable is desired, a DDVAL bulk data entry needs to be referenced on the DESVAR bulk data entry for the design variable values.