Interventions to Structural System of Masonry Buildings and their Effects to their Seismic Response

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Background: Preservation of listed buildings, depending on the importance of each one, requires the conservation of the whole structure or of only the external walls, often called shell of the building, or even only of the façade. In the latter cases, although the new structure is studied to undergo the applied loads according to the codes in force, less research is made to study the response of the remaining structure under seismic loads. Objective: The response of unreinforced masonry (URM) structures with alterations of the original load bearing system to strong ground motions is studied in the present paper. Commonly used radical interventions comprise the addition of a steel or reinforced concrete frame in the interior of the structure after removal of interior load bearing or/and dividing walls. The embedded substructure is designed to support the functional loads of the building and commensurate seismic design forces associated with its mass. In this setting, perimeter walls are relieved of any bearing action apart from resisting the state of stress associated with their self-weight. An important design decision is the extent of contact and interaction that is allowed to occur between the perimeter URM walls and the interior structural system; both options present advantages and disadvantages. Methods: The effect that this design option has on the seismic response of the composite system is studied in this paper using linear elastic finite element analysis. The effect of each intervention is estimated by comparing the principal tensile stresses (pts) developed on the walls before and after each intervention as well as the percentage of the wall areas in elevation where the pts are higher than tensile strength of masonry. Results: It is found that connection of the frame to the masonry walls at several points around the floor and roof perimeters creates a diaphragm action that effectively reduces the out-of-plane bending of the self-standing perimeter URM walls without excessive local stress intensities and increases the shear strength of the building. Lack of contact between the old and new load bearing elements leads to higher intensity stresses due to bending and only the addition of a reinforced concrete tie belt at the top of the walls may mitigate serious damage. Conclusions: The cooperation of the Moment Resisting Frames, irrespective of the material of the frame (reinforced concrete or structural steel) and the walls by connecting the perimeter structural walls with it at floor and roof levels, is more efficient to the stress state of the walls transforming the critical out-of-plane bending of later to shear one, preventing them from out-of-plane collapse.