Constructions are usually subjected to actions deriving from their normal exploitation, but they can also be subjected to exceptional actions (for example, heavy snowfall, hurricanes, floods, fires, earthquakes and other strong shocks). In such cases, according to the quality of the materials used, to how close to reality the calculation hypotheses taken into account in the design phase come, as well as to any possible execution errors, the constructions may be degraded or even damaged. The damage is the deterioration of a construction or of some of its components, and an accident, in the construction field, is an unpredictable event, usually consisting of collapses or serious damages, with or without human life losses and which involves significant material damages. Usually, construction damages are not brought about by a single cause, but by a combination of multiple causes. Their different succession and intensity makes it more difficult to objectively establish the influence that has led to the damage. There are constructions with serious hidden defects, and no visible defects. In such cases, the damage can be triggered by a minor influence overlapping the unfavourable effect of hidden defects. Numerous investigations have been conducted concerning the structural failures that have occurred over the years and it has been found that, minus a few small exceptions, they are due mainly to human errors and they almost never result from the unfavourable combinations of random events. A structure must be designed and *executed so that, throughout its lifecycle, it should satisfy the following basic requirements: structural resistance, durability, reliability. A different wording of the matter may be criticized in any way: obtaining a high degree of robustness may diminish the minimum cost requirement.. [13] Achieving the 3 objectives (cost, safety, and robustness) may lead to unconventional design solutions: a robust solution may entail an excessive degree of safety for the class of the structure to be executed.
