Seismic Rehabilitation of Substandard R.C. Buildings with Masonry Infills

Seismic deformation demands are localized in areas of stiffness discontinuity, such as in the soft storys of frame structures, where disproportionate damage is often reported in post-earthquake reconnaissance. In many parts of the world, this damage pattern is mitigated using strengthening schemes that include addition of stiffness in the structure so as to limit the magnitude of drift demands. A low-cost retrofitting method is the addition of masonry infills to increase the stiffness of soft storys in low- to mid-rise reinforced concrete (R.C.) structures. This is an easily replaceable remedy in the event of damage that may prove advantageous over R.C. structural systems, owing to the lower forces imparted to the foundation in this retrofit option as compared to more thorough interventions, thereby avoiding extensively invasive retrofit operations in the foundation. Behavioral mechanisms mobilized by masonry infills in successful retrofits are shown to emulate confined masonry behavior. It is also shown that despite their brittleness, well-connected infills can successfully mitigate the occurrence of catastrophic damage by diverting damage localization from the vulnerable regions of the building. The main objective of the current paper is to present a rapid retrofit design methodology, where masonry infills are utilized for strengthening existing substandard constructions in order for their R.C. load-bearing elements to behave elastically in the event of the design earthquake. To facilitate the retrofit design, practical design charts have been derived, to link drift demand to the ratios of infills' area in plan to the total plan area in the critical floor of the structure. Performance criteria, such as target distributions of interstory drift demand, a target estimate of the fundamental period, as required by the designer, and a limit on acceptable displacement ductility in terms of demand for the retrofitted structure, are necessary design decisions that guide the proposed retrofit strategy. Application of the retrofit design through infills is demonstrated through example case studies.