Read and produce engineering drawings of civil structures in third-angle orthogonal projection in accordance with AS1100, including sectional views, dimensioning, line types and symbols

What this dot point is asking

NESA wants you to read and produce engineering drawings of civil structures using third-angle orthogonal projection, following Australian Standard AS1100. You must know the standard line types, dimensioning conventions, sectional views, the third-angle projection symbol, and how to arrange views on a drawing sheet.

The answer

Third-angle orthogonal projection

Orthogonal projection shows an object using multiple two-dimensional views taken perpendicular to each principal face. AS1100 specifies third-angle projection as the Australian default.

In third-angle projection, the views are arranged as if the object sits behind a glass box and you look in from each face. The view appears in the direction you looked from:

• Front view at the centre of the sheet.
• Top view above the front view.
• Right side view to the right of the front view.

The third-angle projection symbol (a truncated cone shown as a circle and a trapezium, trapezium on the right with the smaller end toward the circle) is placed in the title block to declare the convention.

Line types under AS1100

Each line type carries information:

Dimensioning

AS1100 requires:

• Dimension lines are continuous thin with arrowheads touching projection lines.
• Projection lines extend from the feature with a small gap (about 1 mm) at the feature.
• Text is placed above the dimension line, oriented to read from the bottom of the sheet or from the right.
• Use millimetres as the unit (no unit symbol on each dimension; declared once in the title block).
• The smallest dimension is closest to the view; chain dimensions accumulate outward.
• Do not duplicate dimensions: each feature is dimensioned once.

Sectional views

To show internal features, AS1100 uses a cutting plane line (chain thick with arrows) on one view, with the resulting section shown on the adjacent view. The cut faces are hatched with thin continuous lines at 45 degrees. Different materials use different hatching patterns (concrete is hatched as triangular aggregate, steel as evenly spaced lines).

Civil structures application

A civil engineering drawing of a reinforced concrete beam typically shows:

• A plan of the beam location on the floor.
• An elevation of the beam with overall dimensions and clear spans.
• A cross-section through the beam showing rebar layout, cover, stirrups and dimensions.
• A schedule of reinforcement (bar marks, diameters, lengths, shapes).

Sydney Harbour Bridge fabrication drawings of the 1920s used the same orthogonal projection conventions in pencil and ink on linen, with hand-lettered dimensions in imperial units. The geometry was identical to a modern AS1100 drawing in metric.