Application of Near Surface Mounted (NSM) technique for Seismic Retrofitting of Heritage Buildings

The high vulnerability of the cultural buildings due to seismic actions urged researchers to retrofit them using reversible and compatible techniques. The application of fiber reinforced polymer (FRP) for enhancing the structural behavior of masonry buildings, under seismic actions, has been studied by many researchers. The near-surface mounting (NSM) technique is a relatively new retrofitting approach that involves bonding FRP bars/strips inserted into grooves cut of mortar joints. This method is an appropriate technique to be used in seismic retrofitting of heritage buildings, because of its low influence on the global mass, high strength-to-weight ratios, minimum durability concern, ease of handling, flexibility and fast installation that improve on-site productivity, low impact of building functions, and low architectural and structural interventions. No.1 Educational heritage building of Tabriz Islamic Art University has flexible diaphragms with unreinforced masonry walls as lateral and gravitational resisting system. The masonry walls have no tie beams or columns and appropriate connection to the floors. Furthermore, the building has irregularity in plane because of length to width ratio more than 3. Another deficiency of this building to resist seismic load is related to the existence of small percentage of masonry wall in transversal direction compared with longitudinal direction, which can be result in torsional irregularity in the structure. The aim of this paper is to investigate the effectiveness of applying NSM technique on seismic performance of this heritage building. For this purpose, a 3D Equivalent frame model was created in 3Muri software to simulate the structural behavior of strengthened masonry walls. The masonry was modeled by macro-modeling approach and FRP bars/strips were modeled as attaching to the masonry walls. A safety index value according to “Guidelines for evaluation and mitigation of seismic risk to cultural heritage”, have been calculated to evaluate the seismic behavior of the building before and after applying retrofitting method. The selected safety index depends on shear strength of the structure, site seismic hazard zone and the desired building performance level. Nonlinear static analysis has been employed by 3Muri software to evaluate the safety index for selected limit states including Ultimate Limit State (SLU) and Damage Limit State (SLD). The distribution of lateral load is done in two different modes; proportional to the shape of the first mode and proportional to story mass. Based on the results of twenty-four pushover analysis, it has been concluded that inserting the FRP strips to the masonry walls and adding new masonry walls reinforced by FRP bars, increase the safety index significantly in both directions, especially in the transversal direction. Also it has been observed that shear strength of the building increased considerably after applying the retrofitting design relative to the existing condition. More uniform distribution of the safety index between different analyses was another positive effect of using this technique. The results of this research demonstrated the advantages of using NSM technique for seismic performance of heritage buildings and are based on a numerical modeling and pushover analysis which uses the performance criteria of safety index, while more experimental and numerical studies are proposed for future studies.