Classifications of Civil Engineering Drawings and Interpreting Engineering Drawings
Engineering drawings are how engineers and draftsmen communicate the designs for a building project to a contractor, builder, or technician. In civil engineering, detailed drawings are produced for each stage of a building project, from tendering the contract to post-completion. Drafting is the best possible way to prepare a concise record of a building project. Engineering drawing is normally the starting point in a long chain of events which finally results in the production, manufacture, or construction of a building project.
This article will discuss:
- The the types of drawings that go with each stage of a building project.
- A brief discussion of the elements of civil engineering drawing: scale, projection, and lines.
Classification of Civil Engineering Drawings
There are four basic types of civil engineering drawings.
- Tender Drawings
- Contract Drawings
- Working Drawings
- Completion Drawings
The function of each drawing corresponds to its stage in the building project, from tender to completion. The differences between each drawing may be negligent or significant depending on the specific project. In small-scale and simple projects, for example, there is often little difference between the tender drawing and the completion drawing. If drastic changes are made mid-project, the working and completion drawings may be noticeably different from the tender and contract drawings.
1. Tender Drawings
A tender drawing is prepared early in the building process, and is used by contractors for developing bids. Tender drawings, together with the other tender documents (bill of quantities, specifications, etc.), describe the project scheme to the contractor so that he or she can price the construction work accordingly. Tender drawings are prepared by engineers with clarity of understanding in mind.
2. Contract Drawings
The engineer can carry on with a more detailed design only after the completion of tender drawings and the bidding process. After evaluating the contractor's bid, the engineer compares the contractor's assessment of the project to the tender drawings to make the necessary modifications based on the proposed constructional methods and budget. These drawings and the engineer's report are included in the documents prepared for the legally binding contract between the contractor and the project's commissioner.
For some projects, if tendering has been straightforward and without any alternate proposals, the contract drawings can be the same as tender drawings. If alternate proposals have been accepted, new or additional drawings will need to be prepared according to the accepted tender proposals and alternatives.
Contract drawings are printed on good quality paper and given a cloth backing, and are meant to withstand long-term storage.
3. Working Drawings
Working drawings are generally more detailed than tender and contract drawings. These plans inform the actual work and manufacture of a building or other project, and represent the engineer's final decisions regarding various details.
For simple projects, the working drawings may not significantly differ from tender or contract drawings. However, for very large projects or projects with minimalist tender drawings, working drawings need to be much more detailed than previous drawings. They are often supplemented by finer design and construction details in the form of notes and additional written instructions.
4. Completion Drawings
Completion drawings are the final set of drawings made in a building project, and their purpose is to record the project as it was built.
During any given project, ad hoc changes are likely to have been made that diverge from the working drawings. These variations, additions, and alterations may be due to unforeseen site conditions or budgetary concerns. Even the smallest variations are recorded in completion drawings, also known as record or as-built drawings.
How to Read Engineering Drawings
Engineering drawing uses a standard range of conventions and symbols which may seem incomprehensible at first glance. Engineering drawings are prepared according to professional codes and technical standards so that they may be read by anyone without misinterpretation or confusion. Reading engineering drawings requires knowledge of the following elements:
How to Interpret Scales in Engineering Drawings
The scales adopted for civil engineering drawings depends upon the degree of accuracy and detail required.
The representative fraction indicates the ratio of the dimensions in the drawing to the dimensions in reality. For example, a location map may have a scale of 1:100,000, meaning that the dimensions of the actual object or space in reality is 100,000 times larger than that of the drawing.
It has been estimated that the scaled distances may be read only to an accuracy of 0.5 millimeters in the drawing. This indicates an accuracy of 50 meters in drawings prepared to a scale of 1:100,000 and 25 millimeters in drawings with a scale of 1:50.
The typical range of scales used for specific purposes is in the table below.
Civil Engineering Scales and Their Typical Purposes
Plans, elevations, and sections
Plans, elevations, and sections
Orthographic Projections in Civil Engineering Drawings
Orthographic projections help engineers and architects draw three-dimensional creations clearly and accurately using the two-dimensional medium of drawing. Three-dimensional drawings are included as a qualitative description of what the object should look like when completed, but are inadequate in conveying accurate shape or dimension for the purpose of building. Three-dimensional objects are therefore broken down into two-dimensional drawings that convey their elements with accuracy.
Two types of projections are used in engineering drawings. The information these different projections communicate is the same, but their layout is different. They are:
- First-Angle Projection, standard in Europe and Asia (excluding Japan).
- Third-Angle Projection, used in the United States, Japan, Canada, and Australia.
First Angle Projection
An easy way to visualize how projections work is to imagine a three-dimensional object suspended in a glass cube. Each side of the object—front, sides, top, and bottom—is projected onto each plane of the glass cube. The glass cube is then taken apart in a flat, two-dimensional formation. First- and third-angle projection are differentiated by the direction from which the sides are projected onto the plane. When flattened, the information in first- and third-angle projection is the same, but the projections are rearranged differently. In each, the front view is in the center of the projection configuration.
First-angle projection assumes a point of view in which the object is in between the observer and the plan of projection. Each side of the object is projected onto the far plane of the imaginary glass box (see image below), on the opposite side of the object from the viewer. Thus, when the imaginary glass cube is flattened, the sides appear to be flipped around: the right side is drawn on the left of the front view, the top is drawn underneath the front view, and so forth.
How to Visualize a First Angle Projection Drawing
The illustrations above show two different ways to visualize a first angle projection. The illustration on top shows the arrangement of the different sides of a three-dimensional object when the imaginary glass box is dismantled. The lower image shows the direction in which the object projects onto the drawing plane.
Third Angle Projection
A third-angle projection uses the same object-in-a-glass-cube visualization, but assumes a point of view in which the object is on the other side of the projection plane from the viewer. Each side of the object is projected onto the near-side plane of the glass cube, on the same side as the viewer (see image below), rather than the far side like in first-angle projection.
When the cube is dismantled and flattened with the front view in the center, the sides appear on their same orientation from the middle. The top side view is above the front view, the left side view is to the left of the front view, and so forth.
Generally, engineering shapes have some symmetry, and the object can be described sufficiently using three views—the front, one side, and the top—rather than all six angles.
How to Visualize a Third Angle Projection Drawing
The illustrations above show two different ways to visualize a third angle projection. The illustration on top shows the arrangement of the different sides of a three-dimensional object when the imaginary glass box is dismantled. The lower image shows the direction in which the object projects onto the drawing plane.
Videos: First-Angle vs. Third-Angle Projection
Lines Used in Engineering Drawings
In a civil engineering drawing, different types of lines convey different things.
Drawing Lines are the standard lines in engineering drawing, simply representing an edge and designating nothing particular about it.
Construction lines are preliminary lines used for laying out a drawing, not a standard line type and will not be included in a final drawing.
Center Lines designate the axis of symmetry of a given circular object.
Hidden Lines are short-dashed lines that represent edges that are not visible from a given angle.
Phantom Lines designate imaginary features such as the positions of moving parts, like a door swinging open.
Break Lines show imaginary breaks in objects which are too long or big to be represented in their entirety in a drawing. The dimension of the object will be included to show how big it is supposed to be.
Dimension Lines are imaginary lines (as in, they don't represent a tangible or real edge) that show the length, width, or height of a designated edge. These lines are bracketed off by arrows and with the measurement number placed in a break in the middle of the line.
Leaders are thin imaginary lines with a arrows on only one end. The leader connects a dimension-labeling line to the edge it's describing.
Cutting Planes are used in drawing cross-sections of an object, defining the sections of a whole object that will be represented as more detailed drawings of its parts. For example, if a designer wanted to include a drawing of the inside of an object, he or she would draw the entire object with a cutting plane to show where the drawing of the whole object would be "cut" to reveal the inside.