# Section : Fabricated and line segment sections
# Definition
Line segment sections are defined in terms of a series of lines of specified thickness. The section properties are calculated assuming the section is a thin-walled section, making no attempt to tidy up the section at segment junctions except to remove any overlaps to prevent double-counting. Where a section may not be considered as thin-walled, a perimeter section should be used.
The section is described by a series of thickness, line to and move to instructions. An initial thickness is specified; this is applied to all segments until another thickness is specified. After the initial thickness the start position in specified by a move to instruction. Thereafter, line to and move to instructions are given to complete the definition of the section.
The centroid is calculated for the section and the section is assumed to lie centred at its centroid, – not at the datum coordinates.
The shear factors,
# Line segment sections
There are two types of geometric section: perimeter and line segment. Both are described in terms of 2D vertices. A line segment section is specified by a number of lines of specified thickness.
The syntax for geometric sections gives instructions for constructing the shape of the section. The section must start with a flag to give the type of geometric section. This followed by a series of instructions defining the shape of the section, separated by spaces. The instructions that are used are:
| Type | Type code |
|---|---|
| Perimeter | P |
| Line segment | L |
and
| Instruction | Values | Code |
|---|---|---|
| Move to position | (y, z) | M(y|z) |
| Line to position | (y, z) | L(y|z) |
| Set thickness | t | T(t) |
The (x,y) coordinates referred to here are in section definition axes. These translate to (−y,z) coordinates in element axes. (See Section profile for more information on this.) This translation happens automatically requiring no action by the user.
Thickness is only required for line segment section.
In general a geometric section is defined as
GEO <Type(unit)> <Instruction> <Instruction>…
The available units are m, cm, mm, ft and in, defaulting to mm.
So for example a line segment section definition is of the form:
GEO L T(10) M(5|5) L(195|5) L(195|395) L(5|395) L(5|5)