Seismic Retrofit of Existing Masonry Infill Walls By Means of Sliding Joints

Abstract eng:
The seismic strengthening of existing reinforced concrete structures is a strategic subject in many countries. In Italy, in particular, a large stock of buildings was built from ’50 to ’70 of the nineteen-century without specific earthquake resistance requirements. Most of them have a reinforced concrete moment resisting frame structure infilled with masonry walls. Recent earthquakes showed that the combined in-plane high stiffness and brittleness of the traditional masonry infills, non-engineered for a structural response, represents a threat to those building safety because of the infill frame interaction. Several buildings suffered local failure or total collapse triggered by soft story or shear column mechanisms induced by such interaction, worldwide. Therefore, a retrofit strategy has to consider the actual structural role of the masonry infills and, at the same time, prevent their out of plane failure that can jeopardize the integrity of the building and safety of the occupants. A number of construction details and techniques have been proposed to improve the seismic performance of infilled RC moment resisting frames. Such techniques typically aim at preventing the structure’s collapse during an earthquake by increasing the strength and stiffness of the infill and/or the frame. The increase of the seismic capacity of infilled frames with these techniques can modify substantially their dynamic behavior and eventually lead to brittle failures after their peak resistance is reached. The existing infill significant stiffness complicates the strengthening of the building also when additional bracings systems (steel bracings or structural walls) are attached to the construction because they typically need to be very stiff, more than the existing structure, to limit the building deformation and avoid failure of the infills. As an alternative, in this paper the retrofit approach proposed is based on the reduction of the infills’ stiffness and on the increase of their ductility, in order to limit their interaction with the structural frame. According to recent research results, the masonry infills can be given a ductile and damage free in-plane behaviour thanks to the partitioning of the infill with sliding joints, which are capable to dramatically reduce the detrimental effects of the infill frame interaction. The detailing of such engineered infills for new structures has been presented elsewhere. This paper describes the application of the technique to a prototype existing infill. The prototype intervention was applied to a typical wall of the above recalled existing buildings. A specific detailing was necessary to ensure the desired in and out of plane performance of the infill. Cyclic quasi-static loading was applied both in and out of plane. The results of the test are discussed in terms of deformation capacity, damage levels at different drift and efficiency of the intervention technique. In order to reduce at the minimum the down time of a real structure, the retrofit intervention is designed to be carried out from the outer face of the wall limiting the interference with the ongoing activity inside the building.

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