The relationship between success and failure in design constitutes one of the fundamental paradoxes of engineering. The accumulation of successful experience tends to embolden designers to attempt ever more daring and ambitious projects, which seem almost invariably to culminate in a colossal failure that takes everyone by surprise. In the wake of failure, on the other hand, there is generally a renewed conservatism that leads to new and untried design concepts that prove ironically to be eminently successful precisely because the design process proceeds cautiously from fundamentals and takes little for granted. As the new design form evolves and matures, however, the cautions attendant upon its introduction tend to be forgotten, and a new period of optimism and hubris ensues. This cyclic nature of the engineering design climate has been elaborated upon here and elsewhere and is supported by numerous case studies (see Petroski, 1985).

In his thesis on structural accidents and their causes, Sibly (1977) analyzed several large metal bridge failures in a design–historical context and showed that each of them occurred in a design climate characterized by increasing span length, increasing slenderness, increasing confidence in analysis, or decreasing factor of safety (cf. Sibly and Walker, 1977). Generally a combination of such circumstances exists and there are typically warning signs of impending trouble, but they are seldom if ever paid very explicit attention in the evolutionary design process.