Seismic strengthening of unreinforced masonry buildings with steel elements

This study is concerned with the seismic strengthening of URM walls in which the in-plane lateral resistance is significantly weakened by large openings for doors and windows. The current practice of using steel bracing elements to strengthen these perforated walls was evaluated experimentally for its efficacy. It was found that for the "rocking-critical" masonry wall piers, the overall hysteretic behavior can be significantly improved by installing a steel framing system consisting of vertical and horizontal elements around the wall--without any braces. Vertical elements provide the necessary hold-down forces to stabilize the rocking piers. The piers "rocked" through a number of cycles of large displacements without crumbling or shattering apart, displaying a ductile response. Degradation of their hysteretic response was observed at story drifts in the vicinity of 1% to 1.5%, primarily due to the crushing of the spandrel portion of the wall under increased compression resulting from the hold-down effect of the verticals. Due to the steel frame, the failure mode of rocking wall piers did not change to shear-critical, however, failure under a high compression was observed.; Finite element analyses accurately predicted the envelope response of the test specimens. A simple mechanics based model was developed to predict the load-deflection behavior of a stabilized rocking pier. Since reasonable agreement between the experimental data and the model was found, this model can be used to design the strengthening system in a more rational way.; The strengthened system not only has excellent strength, stiffness and ductility, it also limits the damage to the brittle wall pier, thus providing safety against sudden failure. The post-elastic behavior of the integrated system is more ductile despite the brittleness of the masonry. This strengthening scheme utilizes the considerable load sharing which occurs at almost all load stages between the existing masonry and the added steel elements. The controlled amount of masonry cracking adds to the damping of the system without serious risk to the structure's vertical load carrying capacity. It is also suggested that energy dissipation devices can be used to limit the forces in the verticals and to supplement the system damping for safer and better performance.