Seismic retrofitting of URM masonry piers with helical steel reinforcement

Past earthquakes revealed that the brittle nature of unreinforced masonry (URM) structural walls often leads to extensive cacking and shear damage, which can seriously affect the structural integrity and thus compromise the safety of the entire building. Hence, finding an effective seismic retrofitting solution that can increase the safety of existing masonry building stock is of great importance. This paper explores the potential of alternative seismic retrofitting solutions for URM masonry walls - near-surface mounted austenitic stainless-steel helical bars. Being cold rolled from a plain round wire and subsequently tensioned through a free-twisting process, such a reinforcement can not only offer high durability, but also superior mechanical and bond properties, as well as effective redistribution of loads through the retrofitted masonry. In addition, the relatively high flexibility of the bars allows them to be mounted continuously along the joints of the wall, leaving the aesthetic of the retrofitted masonry intact. A total of nine single-leaf clay brick walls were tested under cyclic displacement reversals to examine the seismic performance of the reinforcement in terms of increasing in-plane shear capacity and ductility. Test specimens comprised cantilever walls with various retrofitting patterns, including flexural and shear helical reinforcements installed in the mortar joints or into the vertical slots cut into the masonry. The results showed considerable improvements in the ductility and energy dissipation of the walls after the retrofitting. For most of the retrofitted walls the value of q factors exceeded 4.0, which is greater than the typical q factors for reinforced masonry, thus indicating that large increases in ductility were achieved. The paper highlights the potential of helical stainless-steel bars as a seismic retrofitting reinforcement capable of preserving the structural integrity of masonry structures at increasing displacement demands without affecting the aesthetic of the surface of the walls.