Henry Petroski Archives – The Rabbit Hole

Key Takeaways

The colossal disasters that do occur are ultimately failures of design but the lessons learned from those disasters can do more to advance engineering knowledge than all the successful machines and structures in the world. Indeed, failures appear to be inevitable in the wake of prolonged success, which encourages lower margins of safety. Failures in turn lead to greater margins of safety and, hence, new periods of success. To understand what engineering is and what engineers do is to understand how failures can happen and how they can contribute more than successes to advance technology
Engineering has as its principal objective not the given world but the world that engineers themselves create, one which is ever changing. Like our art, we like our structures to be fashionable and since our tastes change, so do our structures
It is the process of design, in which diverse parts of the given world of the scientist and the made world of the engineer are reformed and assembled into something the likes of which Naturr has not dreamed, that divorces engineering from science and marries it to art
We may wonder if human evolution may not be the greatest engineering feat of all time
If ontogeny recapitulates phylogeny, if all that has come to be human races before the fetus floating in its own prehistory, then the child playing relives the evolution of structural engineering in its blocks
One of the most important calculations of the modern engineer is the one that predicts how long it will take before cracks or the simple degradation of its materials threaten the structure’s life
Structural engineers must often deal in probabilities and combinations of probabilities. A safe structure will be one whose weakest link is never overloaded by the greatest force to which the structure is subjected
A scientific hypothesis is tested by comparing its conclusions with the reality of the world as it is
The fundamental feature of all angineering hypotheses is that they state, implicitly if not explicitly, that a designed structure will not fail if it is used as intended
Success is foreseeing failure. Nobody wants to learn by mistakes but we cannot learn enough from successes to go beyond the state of the art. The object of engineering design is to anticipate failure and to design against it. This is done by understanding how much load a structure can carry without letting go or breaking
Designing a bridge or any other large structure is not unlike planning a trip. The end may be clear and simple: to go from here to there. But the means may be limited only by our imaginations. It is this aspect of the writer to engineer analogy that is most helpful in understanding how the celebrated writers and engineers alike learn more from the errors of their predecessors and contemporaries than they do from all the successes in the world. There is the greatest practical benefit in making a few mistakes early in life
Engineering, like poetry, is an attempt to approach perfection. And, engineers, like poets, are seldom completely satisfied with their creations
One of the most comforting means, employed in virtually all engineering designs, hs been the factor of safety. Also known as the factor of ignorance, it is used to provide a margin of error that allows for a considerable number of corollaries to Murphy’s Law to compound without threatening the success of an engineering endeavor. factors of safety are intended to allow for the bridge built of the weakest imaginable batch of steel to stand up under the hackers imaginable truck going over the largest imaginable pothole and bouncing across the roadway in a storm. The factor of safety is calculated by dividing the load require to cause failure by the maximum load expected to act on a structure. The essential idea behind a factor of safety is that a means of failure must be made explicit, and tr load to cause that failure must be calculable or determinable by experiment. This clearly indicates that it ks a failure that the engineer is trying to avoid in his design, and that is why failures of real structures are so interesting to engineers.
Each new structural hypothesis is open to disproof by counterexample, and the rational designer will respond immediately to th credible failure brought to his attention
50-90% of all structural failures are believed to be the result of crack growth. The cracks often grow slowly and only when they reach intolerable proportions for the structure and still go undetected that catastrophic can occur - classic sign of fatigue. Fatigue can theoretically be avoided but overdesigning structures do that peak stresses never exceed the threshold level is not practical
Quality control is supposed to eliminate unacceptably large flaws by minimizing deviations from an acceptable norm and by rejecting inferior workmanship. But, unfortunately, the techniques for the detection of predicting cracks in fabricated structures are wanting. Not only are instruments relatively insensitive, but also their use and interpretation are often more art than science
A second basic design philosophy to obviate structural failure is called the safe-life criterion. Safe-life design, which allows for the inevitability of failure well beyond the service life of the structure, is not so simple to realize. Nuclear reactors have very conservative designs and have proven to have sufficient factors of safety built into the system, and the leak-before-break criterion appears to be a sound concept. A system is only as strong as its weakest link
In engineering, as in nature, bigger is not necessarily even better nor even a good idea
Because Paxton was not steeped in the traditions of either engineering or architecture, he approached design problems without any academically ingrained propensity for a particular structural or aesthetic style. He often solved problems unconventionally in both construction and architectural style
The paradox of engineering design is that successful structural concepts devolve into failures, while the colossal failure contribute to the evolution of innovative and inspiring structures
Computer aided design (CAD) is helpful for making many decisions but one instance it lacks is understanding how a structure might fail. Many worry that we are becoming too dependent on these modeling softwares and that future failures may stem from us not truly understanding what we are doing
The most dramatic failures have occurred in a climate of overconfidence and carelessness, and the least we can learn from those incidents is to be more vigilant
Causes of failure
1. Ignorance
  1. Incompetent men in charge of design, construction or inspection
  2. Supervision and maintenance by men without necessary intelligence
  3. Assumption of vital responsibility by men without necessary intelligence
  4. Competition without supervision
  5. Lack of precedent
  6. Lack of sufficient preliminary information
2. Economy
  1. In first cost
  2. In maintenance
3. Lapses, or carelessness
  1. An engineer or architect, otherwise careful and competent, shows negligence in some certain part of the work
  2. A contractor or superintendent takes a chance, knowing he is taking it
  3. Lack of proper coordination in production of plans
4. Unusual occurrences - earthquakes, extreme storms, fires and the like
5. Limit states
  1. Overload - geophysical, dead, wind
  2. Understrength - structure, materials instability
  3. Movement - foundation settlement, creep, shrinkage
  4. Deterioration - cracking, fatigue, corrosion, erosion
Good judgment is usually the result of experience. And experience is frequently the result of bad judgment. But to learn from the experience of others requires those who have the experience to share the knowledge with those who follow

What I got out of it

A good overview on the basics of engineering