Section geometry

Define concrete cross-section outlines using parametric templates or custom polygonal drawing, including voids for hollow sections.

Overview

Every ACS analysis starts with the section geometry — the concrete outline that defines the cross-section shape. The geometry determines gross section properties and provides the boundary within which reinforcement is placed.

ACS offers two approaches to defining geometry: parametric templates for standard shapes, and manual vertex editing for custom sections.

Parametric templates

Templates generate section geometry from a small set of dimensions. Select a template type from the dropdown above the canvas, enter the required dimensions, and click Apply.

Available templates

Template	Parameters	Description
Rectangular	$b$ , $d$	Solid rectangular section
T-beam	$b_w$ , $d$ , $b_f$ , $h_f$	Web with top flange
L-beam	$b_w$ , $d$ , $b_f$ , $h_f$	Web with asymmetric flange
Circular	$d$	Solid circular section (approximated as polygon)
Hollow circular	$d_{outer}$ , $d_{inner}$	Ring section with central void
Box girder	$d$ , $b_{top}$ , $b_{bot}$ , $t_w$ , $t_{f,top}$ , $t_{f,bot}$	Closed box section
I-beam	$d$ , $b_{top}$ , $b_{bot}$ , $t_w$ , $t_{f,top}$ , $t_{f,bot}$	Double-flanged section
Trapezoidal	$b_{top}$ , $b_{bot}$ , $d$	Section with sloped sides

Template parameters

All dimensions are in millimetres. Common parameters:

Parameter	Symbol	Description
Width	$b$	Overall section width
Depth	$d$	Overall section depth
Web width	$b_w$	Width of the web (for flanged sections)
Flange width	$b_f$	Width of the flange overhang
Flange thickness	$h_f$	Depth of the flange
Web thickness	$t_w$	Thickness of the web (for box and I-sections)

You can apply a template and then manually adjust vertices afterwards. The template metadata is preserved, so you can re-apply the template with different dimensions at any time.

Custom geometry

For non-standard shapes, draw the outline directly on the canvas:

Vertices define the section boundary as a closed polygon
Drag any vertex to reposition it (snaps to the active grid)
The outline must be a simple polygon (no self-intersections)

Grid snapping

Configure the snap grid size from the canvas toolbar: 1, 5, 10, 25, or 50 mm. Vertex positions snap to the nearest grid point when dragged, ensuring clean geometry with round coordinates.

Voids

Hollow sections (such as box girders or sections with ducts) include voids — interior polygons that are subtracted from the gross section area.

When using a template that includes voids (e.g., hollow circular, box girder), the voids are generated automatically. For custom sections, voids can be defined as additional interior polygons.

Voids affect:

Gross section properties — area, centroid, and second moments of area exclude the void regions
Stress distribution — the concrete stress block skips void regions during integration
Fire analysis — void edges can be designated as fire-exposed surfaces (e.g., for a duct exposed to fire)

Coordinate system

ACS uses a Cartesian coordinate system with:

X-axis: horizontal (width direction)
Y-axis: vertical (depth direction, positive upward)
Origin: typically at the bottom-left corner of the section (templates centre the section at the centroid)

All coordinates, cover distances, and bar positions are measured in millimetres.

Getting started — your first section
Reinforcement placement — placing bars within the defined geometry
Materials — concrete and steel properties