# Out-of-Plane Experimental Study of Strengthening Slender Non-Structural Masonry Walls
## 1 Introduction  
Seismic activity has been a driver of widespread damage and destruction in numerous locations around the globe. In urban areas, the collapse of structural or non-structural elements of buildings (e.g., masonry partition walls, chimneys, parquet walls, etc.) in the event of a strong or even moderate earthquake is often the main cause of a large number of injuries or even fatalities. Many researchers [1, 2] have previously reported that losses caused by the damage of non-structural elements in public buildings can considerably exceed losses caused by the damage of structural elements. From the recent post-earthquake reports following L'Aquila (2009) [3] and Central Italy (2016) [4] earthquakes in Italy, as well as earthquakes in Croatia (Zagreb and Petrinja 2020), it can also be concluded that the collapse of non-structural elements represents a significant share in the sum total of economic losses. Both Italy and Croatia have similar architectural heritage and geotectonic characteristics. A large number of damaged and collapsed non-structural masonry elements have been reported following earthquakes affecting Italian buildings built before 1970 when there were no modern earthquake-safe structural design requirements [4]. Non-structural partition walls built of solid or hollow clay bricks are common in existing buildings. In post-earthquake reports from Petrinja [5, 6] and Zagreb [7], the damage or collapse of such non-structural elements was frequently observed. The main culprits for this kind of collapse are great wall heights, a high specific mass of solid brick masonry, and low out-of-plane seismic resistance. Damage to non-structural elements can already occur at lowerearthquake intensities due to their high vulnerability and the influence of primary structure dynamic floor response. This can result in an increased seismic load on non-structural elements on upper floors, depending on the local resonance impact [8]. Therefore, special attention needs to be given to the non-structural masonry elements where their dynamic properties, such as natural frequencies and damping, should be considered in the process of design. In recent decades, there has been an increasing number of investigations focusing on the out-of-plane behavior (e.g., [9; 10; 11; 12]) or a combination of in-plane and out-of-plane behavior (e.g., [13; 14; 15]) of masonry infills. Despite the fact that masonry partition walls made of horizontal hollow bricks are widely used in Europe's existing buildings, there is only a limited number of experimental studies addressing the behavior of slender non-structural masonry partition walls [16]. Thus, the knowledge of their behavior and design provisions is limited. Recently, a group of researchers [17] conducted in situ out-of-plane experiments on partition walls made of solid and hollow bricks, where the seismic load was simulated with airbags. In general, unreinforced masonry (URM) walls exposed to horizontal loads perpendicular to their plane are the most vulnerable elements in terms of failure mechanisms, as evidenced by post-earthquake damage observations of both public and private buildings [18]. Due to the out-of-plane vulnerability of URM walls and the need to understand their behavior, a few experimental studies have been conducted in the past (e.g., [19; 20; 21; 22; 23; 24]). Nearly all of this research refers to structural masonry walls--particularly to the traditionally built brick or concrete block walls. Only a few out-of-plane experimental studies featuring modern construction walls have been conducted. Initially, a steel reinforcing mesh was used for the out-of-plane anti-seismic strengthening of existing walls using various techniques. High-strength mortar or shotcrete is usually used to attach the steel reinforcing mesh to the existing masonry wall. This type of traditional strengthening is a time-consuming and highly invasive procedure with a further disadvantage of adding mass to the existing structure.