The CSWD are based on good CAD practice
This section contains a brief description of what is considered to be good CAD practice in the production of drawings, together with some definitions that arise from this, which are referred to later in the document. The CSWD have been developed to support these principles.
CAD is not just an electronic drawing board
The benefits of CAD will be limited if the CAD system is used simply as an electronic drawing board. This simple approach to using CAD sees drawings as single entities, each one unrelated to another and closely mimics traditional drawing office practice. Instead of using a sheet of drawing film, which gets more and more battered as time goes by, the drawing is held as a computer file. From time to time, clean paper copies are made using a plotter. The crucial thing in such a system is that each drawing corresponds to a separate computer file.
CAD is a tool for co-ordination
CAD can be much more than that. If used correctly it can be a powerful tool for co-ordinating a project and overcoming two fundamental problems that occur in both manual drafting and simple CAD systems used as a manual replacement; namely:
How can CAD be used to solve these problems?
An unlimited drawing size
The traditional drawing is limited by the size of the film and the size of the drawing board. CAD files are not so limited. They can represent drawings that are far too big to plot in one piece. A large building or site may therefore be drawn complete in one file and only split up into more useable areas when plotted. In this way drafting and design work is not hindered by sheet boundaries.
There is a slight difficulty in doing this. With the simple approach to CAD, the drawing frame, title and revision notes can be carried in the file and plotted with everything else. This is not possible if the plot is a proportion of a larger drawing.
The best solution is for the CAD system to provide features for plot assembly. A plot is made by selecting areas from any number of drawings, combining and positioning them (perhaps scaling and rotating as well), and plotting as a whole. The master drawings are not modified by this process, and the system remembers the composition and layout of the plot, so reissue is no trouble.
A Co-ordination Model
Once CAD files are used in this way then the concept of a 'drawing' in the traditional sense becomes less important. The computer file is now representing a large part of the building; perhaps an entire floor plan. It is beginning to be used as a co-ordination model of the project.
For the successful co-ordination of project data, it is essential that the data remains unique. Unique data will be maintained by referencing the model files and never copying their data.
Drawings are views of Model Files
Drawings are then produced in one of two ways:
| (i) | The simplest and most common method is to build up the project data in a series of planar 2D models, typically relating to plans, sections and elevations, by discipline, and to generate the project drawings by referencing these model files. |
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| (ii) | The approach being adopted by the latest CAD software is to build up the project data in a series of 3D models. Drawings are then generated from views of the 3D model. |
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The CAD model principle involves the structuring of the project CAD data into a series of model files and drawing files which are then combined to form the project drawings.
Model Files
Model files are used to store all of the common project data either as 2D or 3D information. The majority of co-ordination work is carried out by combining the model files, through referencing, and establishing clashes etc. It is common practice to split model files up into discipline, categories and zones with the access status of the files being controlled.
The model files are then shared by all disciplines working on the project to co-ordinate and progress their part of the design in parallel with the overall design.
Drawing Files
Drawing files are merely windows on the project model, which record the information necessary to create a specific drawing. Drawing files will contain very little data and little of the production work is carried out in drawing files. Typically they will store annotation e.g. drawing number, title, revision, notes, dimensions and any information which is unique to that particular drawing and is unlikely to be used elsewhere.
The information presented in the drawing file is constructed by referencing the project model files. The degree of information and the appearance of that information which is displayed in the project model files can be controlled for that particular drawing.
These good drawing practice principles are used as the basis for the recommendations made in BS 1192-5:1998 Construction Drawing Practice ?Guide For The Structuring And Exchange Of CAD Data. The standard is thorough and well thought out and has been used as the basis for many of the recommendations made in the CSWD Study.