Henry Petroski on Engineering and Factors Of Safety

Henry Petroski, To Engineer Is Human: The Role of Failure in Successful Design (New York: Vintage Books; 1992), p. 98. [American spellings retained.]

‘While engineers can learn from structural mistakes what not to do, they do not necessarily learn from successes how to do anything but repeat the success without change.

And even that can be fraught with danger, for the combination of good luck that might find one bridge built of flawless steel, well-maintained, and never overloaded could be absent in another bridge of identical design but made of inferior steel, poorly maintained or even neglected, and constantly overloaded.

Thus each new engineering project, no matter how similar it might be to a past one, can be a potential failure.

No one can live under conditions of such capriciousness, and the anxiety level of engineers would be high indeed if there were not rational means of dealing with all the uncertainties of design and construction.

One of the most comforting of means, employed in virtually all engineering designs, has been the factor of safety.

‘The factor of safety is a number that has often been referred to as a “factor of ignorance,” for its function is 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 heaviest imaginable truck going over the largest imaginable pothole and bouncing across the road in a storm.’

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