By Hugh Martin
This article original appeared in Precast Today’s Q2 2023 edition.
A picture is worth 1,000 words.
In precast concrete engineering, it easily could require 1,000 words to describe a single object in detail
And that is assuming the reader and the author both speak the same technical language.
Instead, engineers use singular markings to illustrate universal ideas, describing them in mere instants, regardless of commonality between author and reader. This applies to items that we can either see or visualize.
The power of a visual representation to replace a multitude of words and phrases is common in precast concrete shop drawings and necessary to efficiently communicate important details.
Detailed Plans for Detail-Oriented People
When manufacturing full-scale products for sale in the marketplace, particularly products that weigh several tons and require heavy machinery for handling, shipping and installation, it is vitally important to have a detailed and accurate plan for fabrication.
In the hustling world of precast concrete manufacturing, producers are hard-pressed to shorten the drawing-creation step as much as possible, sometimes even eliminating it altogether.
Yet for products that require so many resources to produce, detailed plans are a crucial step in the process. Properly prepared shop drawings with thoughtfully selected views, drawn to a practical size or uniform scale with crucial information such as dimensions, tolerances and weights take time to deliver but save days in the overall job cycle.
Here is a good thought experiment.
Draw a circle on a sheet of paper, then ask several people to name the object the circle represents.
Some will see a ball. Others, a ring. Still others, a coin or a disk – even the end of a circular column or solid bar. Without another view showing the third dimension, it is impossible to know.
For maximum usefulness, a shop drawing must depict a fabricated product in at least three dimensions: length, width and depth. And at a minimum, shop drawings should include at least two views:
- A plan view either from top down or the primary view along its long axis.
- A profile view, or elevation view, that captures the third dimension, usually from the side looking horizontally.
If the drawn object’s makeup cannot be accurately determined from only two views, add additional views – usually side views or end views – to provide clarity and eliminate ambiguity.
The more profile views there are, the easier it becomes to visualize the final product, but that also requires more space on the drawing page.
Make Good Use of Space
It can be a struggle for a designer or drafter to balance the amount of information needing to be conveyed with the amount of space available on a page. This especially is true for software systems that automatically generate their own shop drawings with only a plan view and a single profile view.
This limitation can, in turn, make reading those shop drawings challenging, such that further training is necessary to become more familiar with the format of a particular program’s output drawing.
It often is tempting to take whatever drawing is available, usually an isometric catalog drawing, and repurpose it as a shop drawing. However, even with a judicious use of sections cuts, at least two walls are partially hidden from view with some of the product viewed from the outside and some viewed from the inside. This causes confusion.
Arguably the best format for precast shop drawings is a variation of the standard multiview projection format used in machine drawings.
Profile views in precast shop drawings usually show each side of the product in the vertical orientation with each side being depicted on the drawing in the same orientation in which it is being manufactured on the plant floor. Whereas, in a true multi-view projection, profile views are orthogonal to the plan view, that is, aligned with and adjacent to the plan view so some profile views are actually shown sideways or upside-down on the page.
Orthographic projections are a way of representing three-dimensional objects by several two-dimensional views. This method works well for certain types of manufacturing, but in a precast concrete plant environment, turning a drawing multiple different directions in order to orient the drawing with the product can be tedious and can lead to mistakes.
The Butterfly Effect
Some manufacturers use another variation of an orthographic projection, colloquially referred to as a “butterfly layout drawing.” In a butterfly layout, each wall is shown as having been folded flat yet still attached to the horizontal slab, either the top slab in the case of a top section or the bottom slab in the case of a base section. This type of layout drawing is relatively easy to automate with software, making it a favored format for some software programs.
One of the main problems with this drawing format is that all the openings and cast-in elements are shown from the inside-looking-out perspective, where in actuality, forms or molds are prepared from the outside-looking-in perspective.
Sometimes, workers using this type of shop drawing can be seen holding it up to the light, looking through the back of the page so that the objects’ orientation on the page matches the same objects’ orientation on the formwork. It is easy to see how this can lead to mistakes and increased setup time unless each user is trained to recognize and interpret how these drawings match the actual formwork.
Regardless of the layout type or orientation, shop drawings contain all the details necessary to produce a final product.
They include such things as dimensions, locations of openings, types and locations of embedded items or hardware, fabrication tolerances, handling weight, connection details, materials needed, design details such as the design concrete strength and what type and quantity of reinforcement is needed.
They also contain any special instructions needed for fabrication, such as special coatings or hardware that are needed for the product to function properly. Like most fabrication drawings, they should follow a universally accepted set of rules regarding line thicknesses, line types, text size and style, hatching, shading, abbreviations, common symbols and phrases.
Make sure to learn what these standard rules and conventions are and what they mean.
Different Systems for U.S. and Abroad
Outside the United States, both engineering and shop drawings follow the International System of Units. Within the United States, a reader also must be able to recognize two other U.S. customary scales.
Most U.S. engineering and construction drawings received from project owners are drawn to the Engineer Scale in tenths of a foot to match surveyors’ instruments. Shop drawings, however, should be drawn and dimensioned using the Architect’s Scale to match the common U.S. customary tape measure.
Some shop drawings show dimensions in feet and inches while others depict only inches.
For example: 2′-8″ versus 32″ (2 feet, 8 inches vs. 32 inches).
Both are correct. The advantage to showing only inches is that it matches the numbers on the tape measure. The disadvantage is that an overall sense of scale can be lost. Most people are not able to tell how many feet are in 168 inches without stopping to do the math but have no problem visualizing a length of 14 feet.
Some shop drawings do not match any scale at all. Such drawings should have NTS or Not To Scale somewhere on the page, usually in the title block with the other project information. Another issue in this age of photo-realistic scanners and printers, the scale listed on the drawing may not be accurate because of multiple cycles of scanning and duplication over multiple revisions.
Use Common Sense But Don’t Assume
Manufacturing tolerances, vital to ensuring the finished product conforms to specifications, should be included in every drawing. If dimensional tolerances are not shown on the shop drawing, make sure to check the plant-specific quality control manual for all applicable tolerances to the dimensions shown. Typically, dimensional tolerances used for formwork setup should be about half as much as the tolerances for the finished product.
Since precast concrete products are designed to be handled and moved by heavy equipment, individual section weights should be listed prominently on the shop drawing. Make sure all lifting inserts cast into the product and equipment used for handling are properly sized. Even though others likely will have reviewed the shop drawing before it was approved for use in fabrication, it is best not to assume the product weights have properly been checked.
In fact, it is best not to assume anything when it comes to safety.
For instance, if a product weight is listed as 12,000 pounds, but it required four cubic yards of concrete to pour, the weight listed on the drawing clearly is not correct since one cubic yard of concrete weighs about 4,050 pounds. Knowing the product weight is important not only for safe handling but also for shipping to make sure trucks are not inadvertently overloaded.
Drawing scale, tolerances and material weights are especially important when it comes to the size and spacing of reinforcing steel, particularly rebar. The spacing between bars is the on-center (OC) dimension, whereas longitudinal distances between bar bends are measured to the outside surface of the bar.
Pay particular attention to the spacing tolerances between bars to ensure the correct total number of bars are tied to the cage, and make sure handling equipment is properly sized to lift and move the fully assembled cage from the fabrication area to the production area.
On the profile view of reinforcing drawings, readers may sometimes count the dots on a reinforcing section and notice that the number does not match up with the same quantity of bars shown in the plan view. In another example, a reader may notice an object in one view but notice the same object missing in an adjacent hidden view.
When there are conflicts between different views on a drawing, always first try to get the correct information from the drawing’s creator. In general, though, object lines should take precedence over hidden lines or dots in adjacent views.
Communication Helps Avoid Confusion
These are just a few practical tips for reading shop drawings. The importance of training for a specific manufacturing operation cannot be overstressed. Every manufacturing environment is different, and every manufacturer has its own preferred drawing format.
Just ask any of the consulting companies that provide drafting services for our industry. Owners and managers need to ensure the time necessary to train personnel on the proper use and interpretation of shop drawings.
For a reader, experience is the best teacher. Building familiarity with a company’s documentation goes a long way toward making the reading of a particular style of shop drawings easier over time. If there is ever any question about what is being depicted on the page, never assume.
While stopping to ask questions may slow down the manufacturing process, it is much more important that the product be made right the first time.
The goal in any communication is to make all expectations as clear as possible, whether through words or though pictures, especially in business. Shop drawings are an effective and universally understood way of communicating those expectations to others.
Hugh Martin, P.E., is the director of technical resources at NPCA.