It must be 35 years since I first read Structures, or Why Things Don’t Fall Down, and a companion volume on The New Science Of Strong Materials, or Why You Don’t Fall Through The Floor. They were recommended by my Structural Mechanics professor, who acknowledged that they were more interesting than his lectures. This is no accident: in the introductory chapter Gordon observes: “I am afraid that a really formidable number of unreadable, incomprehensible books have been written about elasticity, and generations of students have endured agonies of boredom in lectures about materials and structures. In my opinion the mystique and mumbo-jumbo is overdone and often beside the point.” Happily, he does not merely share the student’ pain, but provides a very powerful preventative treatment with two very entertaining books that share deep knowledge of materials and structures.
Last Friday I was involved in a troubleshooting discussion about three-dimensionally woven structural materials. Yarns have been breaking in a specific region of the structure, and we can’t quite figure out why. Without isolating a root cause, it is tempting to replace the existing yarns with something stronger, and charge ahead. At least as far back as the Gospels, we can read of the perils of mismatched fibers (Mark 2:21 No one sews a patch of unshrunk cloth on an old garment. If he does, the new piece will pull away from the old, and a worse tear will result.) But I recalled that J.E. Gordon goes beyond stating what will happen and tells us why it happens. In Chapter 5 he points out “One can also cause stress concentrations by adding material, if this induces a sudden local increase of stiffness.” Beyond this practical explanation, he then emphasizes real-world consequences of this approach to repair: “The inspectors employed by insurance companies and government departments who insist on pressure vessels being ‘strengthened’ by the addition of extra gussets and webs are sometimes responsible for the very accidents they have tried to prevent.”
Although Gordon’s discussion of stiffness discontinuities didn’t appear fully relevant to our situation, I photographed the page and shared it with a couple of colleagues. It was enough to cause us to pause and brainstorm on root causes for our current difficulty, and the potential for introducing unintended consequences from repair. Our prioritized set of actions changed as a result, to focus first on reducing load rather than increasing strength. Soon we’ll learn whether the problem is fixed before we resort to changing the materials being used. Regardless of the outcome of this particular effort, clearly this book carries the power to influence decision-making of experienced engineers.
Having consulted Structures for a specific concern, I browsed further. Towards the end of the book, in a section devoted to accidents, he shares two specific examples of failure through strength. In one case, failure propagates from gussets that introduce stress concentrations. The other example emphasizes the danger of test-induced damage: tests intended to demonstrate strength can cause weakness. A vessel failed in service, at low pressure, due to propagations of damage that was initiated by high pressure
“proof” loading. This situation is highly relevant for spacecraft, where the structural test article may well be the flight vehicle. We must be vigilant against overtesting, and very sure that critical damage will be detectable in post-test inspection. This attention to test details may be “standard engineering” for spacecraft, but Gordon’s example reminds us that it’s a real issue across all engineering domains.
This volume is “about modern views of the structural element in Nature, in technology and in everyday life.” While addressing such modest goals by discussing stress, strain, stiffness, strength and toughness, Gordon also provides context for modern views with literary and historical vignettes that often emphasize the distinction between structural science and practical engineering. If, as the troublesome topic of creep is being introduced, you do not care to learn that “on Mount Olympus the goddess Hebe had the morning chore of fitting the wheels to the chariot of grey-eyed Athene”, this may not be the book for you. But if you are happy to receive a lively analysis of the suitability of materials and construction methods that were applied to Greek chariot wheels, read on. Be assured that praise for Classical construction is not universal, for we are later told that “Greek roofs can only be described as intellectually squalid”. The book is filled with carefully selected examples of structural design rules and “best practices” that have evolved in various human cultures, and in nature itself, along with explanations of the merits and deficiencies of such approaches. A factor of safety is commonly applied to account for uncertainty in material strength and operational load levels, and sometimes to cover “unknown unknowns”. When an engineering design standard calls out a factor of “safety” of 18 for connecting rods (meaning that their operating load should be around 5% of their “strength”), Gordon observes that it is better described as a factor of “ignorance”, because clearly rather a lot is unknown!
Given that Gordon is a professor of materials, it’s unsurprising that he is particularly effective when discussing classes of materials, and their properties, that are suitable for a variety of load types. Tension, compression, shear, bending and cyclic loading all get their due. When does it make sense to subdivide loads among multiple members and when is it better to concentrate them? Gordon explains, for example, that if you’re going to stand upright, two legs are structurally better than four.
When Structures, or Why You Don’t Fall Through The Floor, was first published in 1978, composites were moving beyond novelty, with fiberglass having dominated downhill skis for about a decade, and graphite tennis racquets on the cusp of changing that game. Nevertheless, Gordon observes that creation of composite fibers is enormously energy-intensive, and therefore may end up being limited to niche applications where concentrated loads must be sustained with low mass. On the other hand, nature is good at “extracting energy from diffuse sources and using that energy with uttermost economy”, and it seems to fill the world with a rich variety of structures. If additive manufacturing is the big thing in structures and materials for this decade, and we note that all of biology is additive manufacturing, Gordon’s discussion of structural efficiency is at least timely again, and may indeed by timeless. It deserves your attention now.