How to read civil engineering drawings?

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How to Read Engineering Drawings?


A standard range of conventions and symbols are used to understand and read the engineering drawings. The engineering drawings, at first glance, look incomprehensible, they need to be comprehended by means of specific types of symbols and codes. Engineering drawings are prepared on the basis of technical standards and professional codes, in order to be read by anyone without any difficulty and misinterpretation. Following elements are required to comprehend for reading an engineering drawing. These elements and codes act as alphabets of the engineering drawing.  

?    Scales
?    Projections
?    Symbols
?    Lines


01. Interpretation of Scales in Civil Engineering:


The scales adopted for drawings in civil engineering are dependent upon the degree of accuracy and detail required. The ratio between the dimensions in the drawing and the dimension in actuality is indicated by a representative fraction. For instance, a location map may have a scale of 1:300,000, meaning that the dimensions of the actual object or space, in reality, is 100,000 times larger than that of the drawing.
In the case of distances, the scaled distances 0.5 millimeters in the drawing will be equal to 50 meters in actuality. The ratio between the scaled distance on drawing and the actual distance is 1:100,000 millimeters. 

Civil Engineering Scales and Their Typical Purposes:
Representative Fraction        Typical Purpose
?    1:100,000                            Location maps
?    1:50,000                              Town survey
?    1:25,000                              Town surveys
?    1:10,000                              Town surveys
?    1:5,000                                Town surveys
?    1:2,500                                Site maps
?    1:1,250                                Site maps
?    1:500                                   Site plans
?    1:200                                   Site Plans
?    1:200                                   General arrangement
?    1:100                                   General arrangement
?    1:50                                     Plans, elevations, and sections
?    1:20                                     Plans, elevations, and section
?    1:5                                       Enlarged details
?    1:2                                       Enlarged details
?    1:1                                       Enlarged details

02. Interpretation of Symbols in Civil Engineering Drawings:


Engineering drawings use abbreviations and symbols to communicate and detail the characteristics of an engineering drawing. The following list includes some symbols to make it comprehensible for the readers of this article.


 

03. Interpretation of Abbreviations in Civil Engineering Drawings:


Abbreviations are the shortened form of the terms and words. Writing the complete word and sentence each time takes time. Just like another subject, in civil engineering and its drawing (order to save time and space) abbreviations. Some of the abbreviations frequently used in civil engineering drawing as given below as an example, so that it may completely be comprehended.  
The common abbreviation used in the subject of civil engineering are as follows:

A.A.S.H.T.O:     (American Association of State Highway Transport)
A.B:                  (Anchor Bolt Or Asbestos Board)
AC:                   (Asphalt Concrete)
D.L:                  (Dead load)
ELCB:              (Earth Leak Circuit Breaker)
MRC:               (Material Receipt Challan)
TB:                  (Tie Beam)
TBM:               (Tunnel Boring Machine)
W.S.D:            (Working stress design)
WL:                 (Working Level)
WO:                (Work Order)


04. Interpretation of Lines in Civil Engineering Drawings:


Many basic elements are used in a single drawing, and this work is carried out by different types of lines. Various lines of different styles and designs represent different physical objects in a drawing.  including visible, hidden, center, cutting plane, section, and phantom. Each style of these lines can further be divided into different types in a drawing. Most important and most commonly used lines are as under:

01: Continuous Thick Line

A straight thick line is used, when we have to represent the outline and edges of the main drawing, done with a pencil softer than HB.
__________________________________

02: Continuous Thin Line:

A long continous thin represents dimensions, extensions, leader line, and projection, etc. A sharp and harder pencil is used for drawing such a line. A 2H pencil is very suitable for drawing thin lines. 
_____________________________________________________________

03: Continuous Thin Free Hand Line:

A thin non-straight hand line is used to display a rough and irregular boundary. 
~~~~~~~~~~~~~~~~~~~~~~~~~
04: Continuous Thin Zigzag Line:

A continous thin zigzag line is used to show long break.
_______/l______/l______/l______/l______/l______/l______/l


05: Dashed Line:

A dashed line is used to show the backside/ hidden edges of the main object.
_  _  _  _  _  _  _  _  _  _  _  _  _  _  _  _  _  _  _  _  _  _  _  _  _  _  
06: Chain Thin Line Long-Dotted (Dashed Thin Lines with Dots):

A chain thin line with dots along is used to represent the centerline for circles and arcs.
_____  _   _____  _   _____  _   _____  _   _____  _   _____  _   _____ 
07: Chain Thin with Thick Ends:

The location of a cutting plane is displayed by means of this type of line.
____ _ _____ _  _____ _ _____ _ _____ _ _____  _ ____ 

08: Long Thin Dashed and Double Short Dashed Lines:
It is located in front of cutting planes, outlines of adjacent parts, censorial Lines, and to state center of gravity.
_____ _ _ _____ _ _ _____ _ _ _____ _ _ _____ _ _ _____ _ _ _____ _ _ _____ 


05. Interpretation of Projections in Civil Engineering:


Projections are used to represent the three-dimensional view of an object in two dimensions. On plain paper, a three-dimensional picture of an object is needed to be drawn. Different techniques are used to perform this job. Drawing projections should be as per relevant standards like British Standards in order to prevent misunderstanding and avoiding errors while interpreting them. 
Orthographic projection:
It is a type of 'parallel' projections in which the four orthogonal views of an object are represented. In the UK, the orthographic projection is known as ''First Angle Projection''. 
Axonometric projection:
The second method of drawing a projection is Axonometric projection, which has become popular during the 20th century and started being used as a formal representation technique. Till the late 20th century, it had remained very popular, but by the time of the introduction of CAD, it became an outdated technique. Axonometric projection creates a true plan set at 45º, which retains the original orthogonal geometry of the plan. It is particularly suitable for representing interior designs, such as kitchen layouts. Planning drawings can also be effectively represented as axonometric projections, showing the relationships between buildings and topography.
Isometric projection:
The isometric remained as the standard view until the mid of 20th century. Unlike the axonometric projection, the isometric plan view is slightly distorted, using a plan-grid at 30º from the horizontal in both directions. It can be used to show the nature of the design and explain construction details more clearly than an orthographic projection. It is sometimes used during concept design to help the client grasp the mass of the proposal.
Oblique projection:
When primary information is drawn in elevation, the interpretation can be enhanced by an oblique projection. This is a simple method of producing two-dimensional images of three-dimensional objects. The differentiating characteristic of oblique projection is that the drawn objects are not in perspective, and so do not correspond to any actual obtainable view.
Parallel projection:
'Parallel projections have lines of projection that are parallel both in reality and in the projection plane.' Ref Drawing for Understanding, Creating Interpretive Drawings of Historic Buildings, published by Historic England in 2016.
 


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