Seismic performance of alternative risk-reduction retrofit strategies to support decision making

How can we evaluate the cost-effectiveness of retrofit interventions aiming at reducing the seismic vulnerability of an existing building? What level of shaking intensity should the retrofitted building sustain? These are open questions affecting either the pre-earthquake prevention, the post-earthquake emergency and the reconstruction phases. The (mis)conception that the cost of retrofit interventions would increase linearly with the level of safety required in designing the intervention often discourages stakeholders to consider alternative retrofit options. As a result, a minimum (in some cases prescribed by-law) level of safety is often targeted, leading in some cases to no-action. Furthermore, the performance measure enforcing owners to take action is generally evaluated following a deterministic approach. When the assessment does not directly reflect, even in simplified terms, epistemic uncertainties and aleatory randomness, it can result in misleading confidence on the expected performance. The present study aims at contributing to the delicate decision-making process of retrofitting existing structures, by developing a framework to assist stakeholders with the evaluation of the actual improvement of the performance, expressed in terms of reduction of collapse probability, of alternative strategies of intervention as well as targeted retrofit levels. With reference to a pre-1970 case study building located in New Zealand, alternative retrofit solutions are considered, targeting different levels of performance, and the probability of reaching collapse when considering a set of ground-motions is evaluated, providing a correlation between the deterministic safety index adopted in the design phase (named "Percentage of New Building Standard" adopting New Zealand regulations) and the expected annual probability of collapse.