Masonry is a very popular construction material in El Salvador, especially for houses. Normally, these structures have performed adequately during seismic events. However, in masonry structures it exist the interaction of several materials, such as steel and concrete, additionally to the bricks. This fact makes the houses prone to construction errors, which, significantly affect their performance. In the 2001 earthquakes, confined masonry houses due to a loss of confinement in the brick panel, in some cases attributed to a lack of a middle tie beam. In the 1997 technical norm, a middle tie beam was a requirement in confined masonry construction. Nevertheless, in the 2004 revision of the norm, this requirement was dropped without any background research. As such, this study aimed to investigate the effect of the middle tie beam in the shear behavior of confined masonry wall. For this, two sets of wall were used, one tested in the TAISHIN project and the second one tested by T. Goto and T. Azuhata. The methodology consisted in proposing equations to quantify the contribution of the middle tie beam and to analyze the failure modes and damage progression of the specimens. It was found that the middle tie beam acted as horizontal reinforcement, performing in axial tension. When provided with enough main reinforcement it can enhance the strength and ductility of the wall. The addition of the middle tie beam reduced the formation of vertical cracks near the tie columns and limited the expansion of the brick panel.
Keywords: Confined masonry, shear strength, middle tie beam