Steel bracings in a Rational Strategy for Seismic Retrofitting of RC Existing Buildings

This paper presents the results of the activities carried out by the Research Unit of Salerno University within the framework of the national research project PRIN2005 on “Strategies and Innovative Techniques for Seismic Retrofit of existing RC structures”. The research project is the ideal following of a previous one titled “Innovative Steel Structures for Seismic Protection of Buildings” in which the same authors proposed the bases for the work that will be summarized in the present paper. The focus of the research has been placed on seismic assessment and retrofitting of existing structures with particular attention on RC ones. An innovative and comprehensive procedure has been firstly proposed for seismic vulnerability assessment of those structures. Such a procedure has been conceived within a multi-level performance-Based framework and is based on evaluating two complementary indices which quantify two different aspects of the structural response of the existing structure and its probable deficiencies. The first parameters quantifies how far is the structure (in terms of displacements or forces) from complying with the relevant Limit State requirements, while the second one describes how widespread is the damage in structure as it is struck by the expected seismic event. These two complementary measures needed for a comprehensive assessment of seismic vulnerability of structures are considered as the key points of the “diagnosis” to be carried out on the structure before conceiving the best “therapy” solution for its seismic deficiencies. Consequently, these two parameters are involved in the relationships defining a rational strategy for seismic retrofitting of buildings. As a key feature, that strategy is conceived as a combination of member-level intervention techniques (like confinement aimed to enhance local ductility of members) and structure-level techniques (like shear walls or steel bracings) rather aimed to reduce seismic demand on the existing structure. As a matter of principle, retrofitting can be obtained by using only one of the two mentioned techniques. In particular, member-level techniques would be preferable as few existing members are stressed beyond the corresponding capacity; on the contrary, structure-level techniques could be successfully adopted as seismic demand overcomes the corresponding capacity on a wider number of members. Besides these somehow extreme situations, retrofitting objectives can be achieved by combining the two mentioned techniques; to each one of those combinations a different cost can be associated and a rational strategy is needed to choose the most cost-effective one within the set of technically feasible solutions which can be adopted for the given structure. The formulation of such a strategy is the key subject of the present paper and a possible application with reference to steel bracings will be also shown as a final example on two existing structures.