What are rules of thumb, and when should they – and shouldn’t they – be used?

Have you ever been around engineers who seem to solve complex issues quickly, sometimes without even a pen and paper? Know the type who can look at a design and tell you whether it’ll work, or that you should be looking more closely at your assumptions? They’ve accumulated enough experience and knowledge to understand intuitively what should and shouldn’t work. They’ve amassed an internal rule book and can use those rules to break down complex problems to solve them quickly.

There’s a risk less experienced engineers don’t always have the fundamental ability to see errors. This can be for a range of reasons, including:

  • a lack of “hands-on” experience
  • not understanding the basics before using computer models
  • not understanding what to look for when checking their work
  • business pressures resulting in less investment in staff training
  • tips, tricks and rules of thumb not being passed down to younger engineers.

How it all began

Engineering has been around in one form or another since humans began to use tools and understand the power of the lever and uses of the wheel. Before modern mathematics and understanding of material properties, people designed and built using the question “does it look right?” As designs either succeeded or failed, we learned from the mistakes and general rules of thumb emerged.

As geometry became more widely understood, engineers and architects used it to proportion structures following the rules of thumb they were devising. By using these principles, spectacular engineering feats were built, from aqueducts and bridges to castles and cathedrals.

The great Scottish engineer Thomas Telford, who lived from 1757 until 1834, used several general rules and concepts to design a wide variety of structures. In his lifetime, he designed roads, bridges and ports and was the first president of the Institute of Civil Engineers.

He didn’t use calculation as we would recognise it. He relied on experience, rules of thumb and building scale models to check his designs. Many of his structures, such as the Menai Bridge, still stand today.

The advantages of rules of thumb

Using rules of thumb enables you to work heuristically, meaning while the answer won’t be perfect, it’ll be in the right ballpark. It means you’re less likely to be out by an order of magnitude and lets you quickly understand whether or not a computer model is accurate. That knowledge can be constructive whether you’re looking at your work or checking others'.

These concepts exist in every engineering discipline, from traditional civil engineering to modern computing and robotics.

An example I used every day was when designing timber pole retaining walls. I know the pole should go as deep as it is high, the diameter should be one-tenth of the height, the pole spacing should be about 1m apart. If the wall is more than about 2m high, the whaling planks require doubling up. If the design doesn’t work like that, then I want to know why.

However, these rules only work within specific parameters; there can’t be too much backslope, and it changes with surcharge, a slope at the toe or a vehicle impact barrier at the top. It takes a certain amount of experience to become aware of where these parameters lie. Casually designing without understanding them leads to engineering failure, so you must still check your work carefully.

The disadvantages of rules of thumb

Rules of thumb are only valid within strict parameters. When the designer exceeds these, the system fails. There are multiple examples of these failures throughout history, from the failures of the Aqua Claudia, to the Leaning Tower of Pisa and the various failures of Beauvais Cathedral. You or your checking engineer must have sufficient experience to understand where you can use rules of thumb, and the parameters. Where you make assumptions, you must state these so they stand out to any checking engineer.

Tips and tricks project

Engineering New Zealand is working with various technical groups and engineers from multiple disciplines to compile tips, tricks and rules of thumb. We’re collating some of the wisdom of more experienced engineers, looking at common mistakes they see, information on how these can be avoided, and information on how engineers can quickly estimate the sizes of components.

If you have any tips, tricks, or rules of thumb, please contact Martin Pratchett at martin.pratchett@engineeringnz.org

Martin Pratchett MEngNZ is Engineering New Zealand’s Engineering Practice Leader.