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Engineering graphics

The use of the graphical representation of items by engineers is almost universal. In fact it is a form of communication that is often not hindered by differences in spoken language. Graphical representation extends from quickly done freehand sketches to machine produced orthogonal views of complex items done by computers.

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NOTE - Source of above image unknown.

Drawings are used to convey impressions of completed items, to produce accurate proportioned views of items, and their component parts, and to represent the detail of those parts in whole or in section so they can be manufactured or assembled without difficulty.

Drawing techniques are also used to solve problems relating to the loads and stresses set up in mechanical components. This is referred to as graphical mechanics. In days without computers loads and induced stresses were calculated using slide rules, or drawn as force/stress polygons. These days loads, stresses and deflections caused by the loads can be calculated by computers during the drawing phase of a project, and not done separately.

In the days that drawings were prepared for the Sydney Harbour Bridge there were, of course, no digital computers as we have today. There were machines used for computations, and many mathematical functions were carried out using logarithmic tables or engineer's slide rule. Typically engineering drawings of the 1920s and 1930s were completed using drawing boards and hand drawing implements (drafting machines). All drawing was done by hand on cartridge paper or tracing linen with pen or pencil and copies were mainly produced using either the 'blueprint' or dyeline process.

The photograph shows a typical drawing office of the period. You can see the inclined drawing boards, with drafting machine attached, and the use of set squares and computational tables.

Pictorial Drawings

Sydney Harbour Bridge and Circular Quay Railway Station artist unknown, from Bradfield's doctoral thesis, 1924 Bradfield Collection, Rare Books and Special Collections Library, University of Sydney</cite>
Sydney Harbour Bridge and Circular Quay Railway Station artist unknown, from Bradfield's doctoral thesis, 1924 Bradfield Collection, Rare Books and Special Collections Library, University of Sydney

Pictorial drawings and sketches were used to give the engineers and the public some idea of the final structure. Like the perspective sketch below which was to give people an idea of how the cantilever bridge, first proposed by Dr. J. Bradfield, would 'fit' into the landscape of the growing city.


Comparative perspective sketches:

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Sydney Harbour Bridge compared in size with St. Paul's Cathedral, London
Source: Dorman Long (1932) Sydney Harbour Bridge. Dorman Long & Co. Ltd., p15

Comparative perspective sketches were also used to convey the idea of scale to people, such as the comparison of the bridge with St. Paul's Cathedral London. Not that this would have been very useful for Australians who had not been to London!

However, a drawing was produced of a section of City from Town Hall to North Sydney Grammar School by the government printer in 1903 showing the relative proportion of existing structures to the then proposed cantilever bridge (Mackaness, 2006, p92 and 93).


Rendered Pictorial Drawings:

Tender A2 - Proposed arch bridge across Sydney Harbour
This tender, Plan No. 10, is also for a two-hinged arch bridge of 1,650 feet span, but with alternative masonry abutment towers faced with pre-cast concrete blocks above plinth level in lieu of granite facing. The length of the main bridge and approach spans is 3,770 feet. Owing to the design of the abutment towers, which are much longer than those provided in the official design, four steel approach spans of 193 feet 9 inches are required on the southern side, and on the northern side four spans of 166 feet 6 inches centre to centre of bearings. Tendered cost, £4,233,105 4s. 7d.
The bridge is attractive in appearance, but the abutment towers are too massive.
Tender A2 - Proposed arch bridge across Sydney Harbour. Cyril A Farey, 1923 from the Tender of Dorman, Long & Co Ltd, Middlesbrough, England, January 1924
pencil, ink and watercolour on paper, 51 x 96.4 cm State Records NSW

As well as comparative perspectives done in black they were often coloured, or rendered. These types of drawings were used to convey to people the concept of size, shape and context with colour being used to provide a more realistic impression than the black and white sketch.


Orthogonal (orthographic) Drawings - Assembly Drawings

Longitudinal section and front elevation of Sydney Harbour Bridge pylons.
The granite faced towers and pylons, simple and elegant, are the architectural features of the bridge and harmonise with the lines of the arch. These pylons, the tops of which are 285 feet above mean sea level, give an expression of solidity to the abutment towers which resist the enormous thrusts from the arch, and by their weight steepen the resultant arch thrust, and so minimise the size of the skewback foundations. The towers have a definite function to perform in the erection scheme of the arch as well as to add to the architectural adornment of the structure which would otherwise be purely utilitarian.
Longitudinal section and front elevation of Sydney Harbour Bridge pylons.
Harold Own (attrib), undated hand-drawn in pencil and ink on paper, 70 x 80 cm (sight) Courtesy the Boaden family

These types of drawings are designed to show, in correct proportion, the interrelationship between each of the elements of the structure. Clearly, orthogonal assembly drawings were made of the complete bridge. One of these was at least 5m long. Another good example was printed in the Dorma, Long & Co. Ltd., publication on the bridge (Dorman, Long & Co. Ltd. 1932, p17-18) this drawing covers both pages of the book and is not suitable to be included here. However, a good example of an assembly drawing from the Sydney Harbour Bridge is given below. It shows the complete assembly for the pylons from the front and from the side. In this case the side view is also sectioned to show internal detail.


Orthogonal (orthographic) Drawings - Detail Drawings

Elevation of Link Plate
Elevation of Link Plate
Source: Dorman Long (1932) Sydney Harbour Brige. Dorman Long & Co. Ltd., p39

Typical of the hundreds of detail drawings done for components of the bridge is the following elevation (front view) for one of the link joints used to temporarily hold the cables used to support each half arch during construction.


Sectioned Orthogonal Drawings - whole cross section:

Cross Section of Deck and Hangers
Cross Section of Deck and Hangers
Source: Dorman Long (1932) Sydney Harbour Brige. Dorman Long & Co. Ltd., p35
Cross Section of the Beam
Cross Section of the Beam
Source: Dorman Long (1932) Sydney Harbour Bridge. Dorman, Long & Co. Ltd.

These drawings were used to show the detail across a complete section of the bridge such as the section across the roadway shown below. Or the section across the pylons shown above.

Or sectional drawings were used to show detail of local elements such as this riveted composite beam with its internal diaphragm.

Drawing Standards:

It is important to realise that every engineering and architectural drawing drawn for the bridge contractors was made to a strict set of drawing standards. Separate standards used to be developed by individual countries, such as the United Kingdom, or America in days gone by. However, today the standards used across the world for engineering and architectural drawing tend to be unified under the International Standards Organisation (ISO). And ISO standards tend to be adopted by most countries. In Australia one of the relevant standards is referred to as AS1100.

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