Choices and Criteria for Seismic Strengthening

Risk assessment is affected by large uncertainties, depending on hazard, structure, damage, and loss analysis. Crucial problems and choices may refer to: (a) hazard parameters, including the definition of appropriate ground motion levels and of their probability to occur; (b) level of knowledge about materials, geometry, detailing; (c) assessed damage and failure modes; and (d) resulting potential for step changes in performances. The cost of attaining a high level of knowledge may significantly reduce the remaining resources, it is therefore important to favor resilient solutions with a creative adoption of appropriate strengthening strategies. In this framework, this article discusses the possible criteria for the mitigation of seismic risk and some of the alternative choices that may be adopted for strengthening, with reference to: (a) the modification of damage and collapse modes strengthening individual elements or locallyincreasing the deformation capacity; (b) the insertion of additional systems resisting to horizontal actions; (c) the introduction of base isolation, with the objective of capacity-protecting the existingstructure; (d) the reduction of displacement demand by added damping or introducing tuned masssystems. Alternative strengthening choices lead to different protection levels and imply different performances that are, in general, represented by non linear or step functions of a cost parameter of the intervention. From these considerations, conceptual “structure driven” strengthening criteria, based on a logical use of resources, are discussed.