Seismic amelioration of existing reinforced concrete buildings. Strategy to optimize the amount of reinforcement for joints

Most of the existing Reinforced Concrete (RC) buildings in Italy were built according to obsolete regulations that were not enough aware of issues related to seismic design so that they need to be upgraded by pursuing either amelioration or full seismic rehabilitation. In doing that, the first step is to figure out what is, based on the results of the initial analysis of the structure in its ante-operam version, the best overall dissipative mechanism that could be ob-tained by a number of suitable and economically convenient local interventions. The choice of the overall dissipative mechanism strongly affects the amount of reinforcement to be adopted for the beam-column joints. For new buildings, the current adopted capacity design philosophy pursues an overall beam-sway mechanism in which plastic hinges first form in beams and at last at the base of the columns. On the contrary,for existing ones, often very irregular and gravity-load-dominated, pursuing such overall mechanism may result either uneconomic or even extremely difficult to implement due to the amount of reinforcement to be inserted in the joints. In such cases, an overall dissipative mechanism allowing, at some extent, columns flex-ural plasticizationshould be accepted and clearly identified in advance. Anyway,such ap-proach needs to be addressed properly in order to avoid the formation of column-sways at one story only that would result very dangerous due to the excessive demand of plastic rotations on the resulting hinges. This paper presents two simple models that may help the designer in deal-ing with the operations above. The formeris a model that allows to understand if, given the existing RC building case-study, either the beam-sway or a hybrid beam-column-sway mecha-nism should be conveniently pursued during the design of the retrofitting intervention. The lat-ter isa model that allows to design a hybrid beam-column-sway overall mechanism involving a suitable number of stories such as to guarantee a uniform and reasonable demand of plastic rotations in the involved columns.