Using Construction Drawings & Overcoming Idiosyncrasies

Technology offers many more choices of how to generate, update, store, retrieve, and reproduce construction drawings than existed even ten years ago. Each type of technology has its idiosyncrasies to learn. For a project to run smoothly, you should remember several tricks of the trade relating to these idiosyncrasies when using drawings.


Other aspects of using drawings are almost timeless: proper field measurement techniques, taking field notes, and making modifications and updates. They are timeless because no matter what technology is used, there are opportunities for human error that translate into contractual difficulties.

There are many ways to generate or reproduce drawings, which in some ways have added to the complexities of duplication. For example, although sophisticated copiers that shrink or enlarge an image at the touch of a button facilitate reproduction, they can also create pitfalls.

Print Sizes

Print sizes are somewhat standardized in the United States. Standard sizes are start at typical copier paper measurements of 8-1/2 x 11 inches, designated as drawing size A. The following list shows the standard widths and heights in inches:
  • A: 8-1/2 w x 11 h
  • B: 11 w x 17 h
  • C: 17 w x 22 h
  • D: 22 w x 34 h
  • E: 24 w x 44 h
  • F: 28 w x 40 h
These letter designations are often found in the title block of the print set. They also appear in selection menus for CAD software programs.

Blueprints and Ozalids

Blueprinting is a process in which a special paper coated with light-sensitive chemicals is placed in a glass frame. In the traditional method, an original drawing was placed over the coated paper and both pieces were held together by the glass frame. This assembly was exposed to a measured amount of light. The chemical-coated paper was then cleaned and allowed to dry. The result was a coated piece of paper dark blue in color except where the lines from the transposed original tracing blocked the light.

More recent technology reversed this process in what is now known as white-printing or ozalid printing. In this process, chemically treated paper and the original drawing are exposed to high-intensity fluorescent lamps. This light desensitizes the exposed area (the area not covered by lines or shading on the original drawing) and has no effect on the protected area. After exposure to light, the print is passed through ammonia vapors. These vapors react with the sensitized areas (those that remained covered during the light-exposure process) and create the blue line commonly seen on prints today. Through this process, design concepts and ideas are transferred to paper.

Large-Scale Photocopiers

The technology of conventional fused-toner copying used for office copiers has greatly advanced in recent years. Toner-on-paper copies of even E-size drawings, to accurate scale, are now possible. Such technology permits regenerating and restoring a document that formerly would have had to be redrawn from scratch; it is possible to generate an accurately scaled drawing on any medium (paper, plastic film, et cetera) without regenerating the drawing because, unlike ozalid printing, light is projected on, rather than through, the original drawing.

CAD and CAFM Drawings

Computers have revolutionized the development, use, and application of construction documents, especially drawings. CAD and CAFM systems have been at the forefront of these changes. The ability to generate changes to drawings very quickly allows high responsiveness to changing project needs, but it also increases potential for the participants on a job to have different, incompatible versions of the same drawing. Thus, tight project control of document versions becomes more important than ever.

CAD and CAFM systems create scaled drawings based on mathematical coordinates for each line and figure. The programs are designed to store these coordinates and generate a drawing at any scale. This capability has changed the way many drawings are developed and produced.

Problems of Scale, Shrinkage, Distortion, and Scaling

All this technology has added to a long-standing pitfall in the generation of drawings: the distortion of scale. Even in ozalid and other dated technology, a drawing done exactly to scale often stretches when run through a high-temperature reproduction machine. The resulting print may be longer than its original (which returns to normal size when it cools off). 
  • No matter what the source of a drawing, scale off a long dimension string and check if the dimension stated on the plan matches what has been scaled out. If it does not, check another dimension string. It is common for a print to scale out slightly longer than the original it is copied from (for the reasons discussed previously). It is also common for the original to stretch more lengthwise than crosswise.
  • Architects often calculate correct dimensions but do not draw them exactly to scale, especially when making last-minute minor modifications on non-computer-generated drawings. Sometimes discrepancies of a few feet may result. You’ll then find yourself trying to install eight 5×5 workstations in a 38-foot long space. Conscientious revisers may add the notation NTS (not to scale), but don’t count on it.
  • If scaled from a plan, a sharp pencil and careful measurements are necessary: on a 1/8 in. scale, a 1/32 in. wide line is 3 in. wide when constructed.
  • Many marketing firms prepare visually elegant but grossly mis-scaled floor plan drawings for inclusion in marketing brochures distributed to potential tenants. These drawings are notoriously inaccurate. If a drawing is needed for such a space, require a print of the architectural floor plan from the construction set, and check a dimension string as described above.
Desktop copiers that reduce or enlarge images are another trap. Any person working in an office can unwittingly distort the scale of a drawing, especially a small sketch, by adjusting the image size button to reduce and copy a slightly oversize drawing to fit on the copy paper. The original may say the scale is, for example, 1/8 in. = 1 ft 0 in., but the copied image is not at that scale. The following tips can help you avoid common pitfalls when scaling dimensions on a drawing.

This article is adapted from BOMI International's Building Design and Maintenance, part of the SMA® designation program. More information regarding this course is available by calling 1.800.235.2664. Visit BOMI International’s website, www.bomi.org.

Comments

Popular posts from this blog

Everyday Protocols to Reduce Environmental Impact

Evaporator: Types and Design Applications

Designing a Pest Management Program