Engineering drawing is a language in which the ideas of designers, engineers or draughtsman are expressed in a manner that is clearly understood by the technician or person concerned with the job. A detailed drawing or a sketch is made of a particular job for completion, manufacture or construction. It is the best possible way to prepare a concise record of the numerous architecture, and engineering works in a manner that can be readily understood and translated into reality by people whose duties commence when the designer and draughtmen have finished their work. Engineering drawing is normally the starting point in a long chain of events which finally results in the production, manufacture or construction f a job required by some section of the community.
Classification of Civil Engineering Drawings
Civil engineering drawings are classified as:
· Tender Drawings
· Contract Drawings
· Working Drawings
· Completion Drawings
Tender Drawings:
It is necessary for an engineer to decide the extent of preliminary design to be carried out at the briefing/report stage. Once formalities are over and the final selection has been cleared, an engineer must prepare a set of engineering drawings called Tender Drawings. Tender drawings together with the other tender documents, bill of quantities, specification etc, describe the project scheme to the contractor so that he can price the construction work accordingly. Tender drawing are the first evidence of the project scheme regarding type and quality of the work involved and hence these are prepared by engineers with clarity and understanding. Tender drawings are prepared to various scales using convention and concepts of engineering drawing for production, manufacture and construction etc.
Contract Drawings
The engineer can carry on with detailed design only after the completion of tender drawings. If tendering has been straight forward and without any alternative proposals, the contract drawings will be the same as tender drawings. If alternate proposals have been accepted, new or additional drawings will to be prepared by the owner’s engineers according to the accepted tender proposals and alternatives. Contract drawings are printed on good quality paper and are provided with cloth backing to withstand handling and long storage.
Working Drawings
The working drawings fill the gaps in constructional details not reflected in tender drawings. As the name itself indicates, working drawings are used for working, manufacturing , constructing or building purpose and must therefore represent the engineers final decisions and design details. However, for large projects with only outline drawings at the tender stage, the drawing office will have to prepare detailed working drawings. In case working drawings lack finer design/construction details, notes and instructions are incorporated on the drawings to facilitate construction and avoid delays.
Completion Drawings
Complection drawings are also called Record or As-built drwwings. There are bound to be certain variations, additions or alterations due to unforeseen site conditions and advancement in technology. Howsoever small the variations, additions or alterations might be, these are recorded on a set of drawings called Completion , Record or As-built drawings. Record drawings should be prepared simultaneously as the work proceeds.
Interpretation of Engineering Drawings
An engineer reads and interprets many more drawings then he produces himself. The various architectural and engineering drawings are prepared according to codal provision so that these many be read clearly without any fear of misinterpretation and confusion. For proper interpretation of engineering drawings, the engineer must have knowledge of the the following:
· Scales
· Symbols
· Projections
· Line work
SCALES
The scales adopted for civil engineering drawings depends upon the degree of accuracy required. Drawings for civil engineering range from location maps with ‘Representative Fraction’ of 1:100,000 to plans and elevations of 1:50. It has been estimated that the scaled distances may be read only to an accuracy of 0.5mm. This represents an accuracy of 50m in drawings prepared to a scale of 1:100,000 and 25 mm in drawings to a scale of 1:50.
A range of suitable scales for various purpose are listed in Table. Scales, other than those recommended in Table should not be used unless there are good reasons for doing so. It is not usual for dimensions to be scaled from a civil engineering drawing as errors will occur due to paper instability caused by shrinkage or expansion. All dimensions should be clearly indicated on the construction/civil engineering drawings.
TABLE: SCALES FOR DIFFERENT PURPOSES
Scale Ration (RF
|
) Purposes
|
|---|---|
1: 100,000 / 1:100,000
|
Location maps
|
1: 50,000 / 1:25,000 / 1:10,000 / 1:5,000
|
Town Surveys
|
1:2,500 / 1:1,250
|
Site maps
|
1: 500 / 1: 200
|
Site plans
|
1: 200 / 1: 100
|
General arrangement
|
1: 50 / 1: 20
|
Plans, elevations and sections
|
1: 5 / 1: 2 / 1:1
|
Enlarged details
|
Symbols
A standard range of symbols used in drawings saves a lot of time in understanding and avoids confusion in interpretations. Careful attention should be given to the size of these symbols in relation to the scales of drawings. Wherever possible, symbols should be drawn to a suitable scale. Sometimes, the symbols may have to be enlarged slightly for clarity .
Projections
Engineers, architects and artists show the three dimensional creations clearly and accurately by the two-dimensional medium of drawings. Three dimensional pictures find their use for qualitative description only. They are inadequate for a complete definition of shape and for dimensional interpretation. An engineer expert in projections can quickly build up a useful image of the scheme from a set of drawings for construction.
In civil engineering, the following types of projection are used:
First Angle Projection : In first angle projection, each view is placed insuch a way that it represents the side of the object far away from it in the adjacent view. In the first angle projection, images are projected forward. Characteristics of first-angle projection ae:
· The plan is directly below the front view,
· The side views stand on both sides of front view and on one ground line.
Third Angle Projection:
The third angle projection, each view is so placed that it represents the side of object near to it in the adjacent view. This method has the advantage that features of adjacent view are in juxtaposition. As a result, it becomes easier to associate these features at the time to dimensioning or reading the drawing.
Generally, engineering shapes have some symmetry and it is usually found that three views namely the plan, the front view and one of the side view are sufficient to describe the shape completely. Although two views would describe any object if all hidden details are shown by dotted lines, the result would be confusing. Third angle projection is generally recommended as the standard method of projection.
Characteristics of Third angle projection
· The plan (top view) is above front view.
· The side views stand on either side of front view, between a pair of horizontal lines.
· Each view is so placed that it represents the side of object near to it shown in the adjacent view.
Combination of First and Third Angle Projection
In relation to the elevation, end views are so placed that they are in third angle projection and plan views in first angle projection. For building drawings, this method is widely used and combines the advantage of bothe first and third angle projections.
Various types of lines used in engineering drawings are shown
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