Drawings are the most important means of communication for an engineer. It is said that a single sketch is worth a thousand words. As a supervisor, you will not be required to prepare drawings at sip. Nevertheless it will be very useful to learn basics of drawing preparation. So, let us understand the problems that may arise in preparation and interpretation of drawings. Basically, we are required to construct structures which are three-dimensional (structures having length, breadth and height). However, the drawings contain only two-dimensional views.
For example, in a plan you can find only length and breadth. From the front elevation view you can get length and height but not the breadth. This procedure is followed for convenience of drawing. For simple objects one could draw isometric views (three-dimensional views). But, even for a simple structure, isomerric views become difficult to handle, and too complicated to dimension and interpret. The problem, therefore, basically is to visualize three-dimensional objects from their two-dimensional views such as plans, elevations and sections.
Organization of Drawings
Scale
Engineering drawings should always be drawn to a scale. Drawings not drawn to scale give a distorted visual impression of the structure. However, all the views in any drawing may not always be drawn to the same scale. For example, a particular element is frequently drawn to a particular scale in elevation and to a larger scale in section. If the scale used in the section is the same as in the elevation, details required for construction cannot be incorporated. The scale should be so chosen that, Ten necessary, dimensions may be cross-checked by scaling the view. From this point of view, scales such as 1:100, 150, 1:20, etc. are convenient and should be used. Scales such as 1 :30, 1:75 etc., are normally, best avoided.
Size
Drawings should be prepared on standard size sheets. The Bureau of Indian Standard stipulates the following size:
A0 841 mm x 1189 mm
A1 594 mm x 841 mm
A2 420 mm x 594 mm
A3 297 mm x 420 mm
Drawings for a particular job should preferably be prepared using the same size for the entire job. Drawings of different sizes make filing and handling them at site, rather difficult.
Type
Broadly,drawings may be grouped into the following categories:
- Architectural drawings,
- Structural drawings, and
- Service drawings.
work, doors and windows, etc.
Structural Drawings: These drawings provide all the information regarding structural (load bearing) aspect of construction. Structural drawings may be grouped as follows :
General Arrangement Drawings (GA Drawings): These drawings provide necessary information for defining the structural framing. These drawings are also sometimes called 'Framing Plans'. concrete construction, these plans provide all the information regarding the sizes and spans. For structural steel drawings they show the complete arrangement of the structural system with member sizes.
Detailed Drawing: These drawings provide construction details of various members described in the GA drawings. For RCC construction, reinforcement details are shown in these drawings. In case of structural steel drawing, enlarged views showing the connection details are shown in these drawings.
Fabrication Drawing: These are required only for steel construction. These drawings show the actual fabrication details as cut lengths, gusset plate sizes, etc. and are accompanied by a material list. These are usually prepared by the fabricator. These drawings are required to be approved by the Consulting Engineer.
Bar Bending Schedules: These are required for reinforced concrete construction and contain the fabrication details of reinforcing bars such as cut lengths bending dimensions, radii of bends, etc., accompanied by a material list. These are usually prepared by the Site Engineer and handed over to the Fitter. For major jobs, they may be supplied by the Consulting Engineer.
Service Drawings: Service drawings provide necessary information about plumbing for water supply and drawing of wiring for electrification etc.
Notes
The information given in the drawing is usually supplemented by "Notes" which appear at the right hand bottom corner of the drawing. Notes include a list of reference drawings, grades of materials to be used and some other typical guidelines useful for construction.
Revisions
The drawings may need revision during the course of construction. Usually an alphabet is added at the end of the drawing number. Dwg. no.XY-200 becomes XY-200A on first revision, XY-200B on second revision, etc. Sometimes the revision is indicated by adding R1, R2 etc. at the end of the number. For example, we may have XY-2WR1, XY-200-R2; etc. The nature of revisions carried out should be clearly slept out on the drawing. When a new revision arrives at site, it is a good practice to stamp the earlier drawing 'SUPERSEDED'. The date on which the drawing is so stamped should also be entered. This will avoid future usage of that drawing through oversight. However, all the drawings including those superseded should be carefully preserved. These will prove very useful records for resolving any differences/dispute that may arise in future.
Scaling of Missing Dimension
If any dimensions are missing on a drawing, it is best to request for a clarification from the Engineer who has prepared the drawing. Scaling of dimensions is usually discouraged, since this may lead to serious mistakes, especially if the drawing has not been prepared to scale. The shrinkage of paper on which drawing is prepared may also lead to erroneous dimensions.
Conflicting Dimensions
If the same dimension is repeated elsewhere with a different value or if the dimensions are inconsistent, it is best to bring this to the notice of engineer who has prepared the drawing. While revising drawings, mistakes are frequently committed when one view is revised without revising the other associated views, which get affected by the revision. For instance, beam depth is altered in elevation but may remain unaltered in section. If a particular dimension appears in different drawings, during revision the person concerned may revise the dimension in one drawing and forget to do so in another. In all such cases, it is best to check with the concerned person issuing the drawings.
Coordination
While preparing drawings the Architects, Structural Engineers and Service Engineers interact and sort out any interference. Even so, it is good practice to study the drawing and recheck that there is no conflict. For instance, the service drawing shows a pipe which cuts into a beam but the drawing does not show any opening in that beam. Either the beam has to be redesigned or the pipe relocated.
Thanks for sharing Fabrication Drawings preparation
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