DTRUSS

Bulk Data Entry Activates Truss Layout Optimization and defines the corresponding parameters for design optimization, including truss cross-sectional area limits, stress, symmetry, and buckling constraints.

Format

(1)	(2)	(3)	(4)	(5)	(6)	(7)
DTRUSS	`ID`	`AINIT`	`AMAX`	`SCOMP`	`SEXTEN`	`VRELAX`
	`SYMMETRY`	`AXIS`	`LOC`
	`METHOD`	`TYPE`
	`BUCKLING`	`ALPHA`

Example

(1)	(2)	(3)	(4)	(5)	(6)	(7)
DTRUSS	10	0.2	1.3	150.0	150.0	0.02
	`SYMMETRY`	X	45.0
	`BUCKLING`	10.0

Definition

Field	Contents	SI Unit Example
`ID`	Identification number No default (Integer > 0)
`AINIT`	Initial cross-section area of truss members No default (Real > 0.0)
`AMAX`	Maximum cross-sectional area of truss members No default (Real > 0.0)
`SCOMP`	Compression stress limit on truss members No default (Real > 0.0)
`SEXTEN`	Extension stress limit on truss members No default (Real > 0.0)
`VRELAX`	Volume relaxation parameter, which is the ratio of how much volume can be added to simplify the structure. 8 Default = 0.02 (Real > 0.0)
`SYMMETRY`	Flag to activate symmetry constraints in truss layout optimization; the corresponding parameters are to follow.
`AXIS`	Indicates that the plane of symmetry is perpendicular to the defined axis. No default (X, Y, Z)
`LOC`	The plane of symmetry is perpendicular to the defined axis on the `AXIS` field and located at the corresponding location defined by `LOC` on the specified axis. No default (Real)
`METHOD`	Flag to control the method type for truss layout optimization and indicate that the corresponding parameters are to follow.
`TYPE`	Controls the method type for truss layout optimization. 0 Local stress constraints are used for the entire solution (single phase approach). 1 Global stress constraints are used for the entire solution (single phase approach). 2 (Default) Global stress constraints are used first, followed by local stress constraints being used; the switch occurs based on certain conditions (two phase approach). 11
`BUCKLING`	Buckling continuation line to activate the consideration of Euler Buckling constraints during truss layout optimization. 12
`ALPHA`	Diameter-thickness ratio to adjust the stress bound for truss layout optimization.

Comments

The DTRUSS Bulk Data Entry activates truss layout optimization. It is currently only supported for Linear Static Analysis.
Truss layout optimization is useful for applications where structures include trusses such as in the architectural industry for building design.
If truss layout optimization is activated (DTRUSS is present), then all shell and solid elements in the model are automatically converted into non-tapered CROD elements to generate the ground structure. From the ground structure of CROD elements, during the initial adaptive member handling process, some CROD's can be added into the structure.
1. Some nearby grids can be connected with CROD elements
2. Some far-away grids can be connected with CROD elements
After the initial adaptive member handling process, a filtering process is performed in which some CROD elements with small cross-section are removed. This simplifies the Truss structure.
Subsequently, a Geometry optimization process is carried out, where:
1. The location of grids can change
2. The length of each CROD can change
3. The cross-section CROD elements can change
Subsequently, some additional operations may also be performed.
1. If some CROD elements cross over each other, new grids are added to divide the intersecting rods.
2. If some grids are very close to each other, they are merged to form a single grid.
The filtering process, geometry optimization, and grid operations are carried out iteratively until a steady state is reached.
The final optimized truss design is saved in the <filename>_opt.fem file in the working directory. This file can be opened in a Text Editor or imported into HyperWorks and the corresponding final design can be visualized. Additionally, if a more simplified structure could be generated during the optimization, this is saved in an optionally generated <filename>_opt_simp.fem file. This simplification is carried out by sacrificing some amount of volume, which is controlled by the parameter VRELAX.
Note: Sometimes the simplification process cannot find a simpler structure. In such cases, a <filename>_opt_simp.fem file is not generated.
Since local stress constraints are applied to each rod, the number of constraints can be very high even for small models. Therefore, the recommended number of CROD elements during truss layout optimization is lower than 2000. Hence, it is recommended to use coarse meshes for truss layout optimization.
During Geometry optimization, OptiStruct internally creates an optimization setup to minimize volume subject to local stress constraints during geometry optimization. User-defined responses via DRESP1, DRESP2, and DRESP3 and corresponding user-defined constraints via DCONSTR or objective via DESOBJ are currently not supported in conjunction with truss layout optimization.
If the TYPE field is set to 2 (Default), the two-phase approach is used. Phase 1 is driven by global stress constraints and Phase 2 is driven by local stress constraints. The switch to local stress constraints occurs:
1. After the filtering and validation stage, if the number of rod elements is less than 500, OR,
2. After joint merging stage, if the number of rod elements is less than 500, OR,
3. Before the simplification stage.
Euler buckling constraints are only considered at the final stages of truss layout optimization (at the sizing optimization step). At this step, the stress bound of the CROD truss elements are updated based on the defined ALPHA value. The general formulation of truss layout optimization is updated to consider Euler buckling constraints as:
Minimize volume, subject to (1)
$- σ_{p c} \leq σ_{i} \leq σ_{p e}$
(2)
${\bar{σ}}_{i} = \min \{σ_{i b}, σ_{p c}\}$
(3)
$σ_{i b} = \frac{α E A_{i}}{8 L_{i}^{2}}$
Where,

$σ_{i}$

Stress in the truss element,

$σ_{i b}$

Updated stress bound based on the buckling constraint.

$σ_{p e}$

Permissible extension stress limit.

$σ_{p c}$

Permissible compression stress limit.

$α$

Diameter-thickness ratio given by the ALPHA field.

$E$

Young's modulus.

$A_{i}$

Cross-sectional area of rod element $i$ .

$L_{i}^{}$

Length of rod element $L_{i}^{}$ .