Out-of-Plane Behavior of Masonry Infill Walls

Past earthquakes have highlighted the vulnerability of masonry infills in the out-of-plane direction. To investigate this vulnerability, it is necessary to test some samples of infills in the out-of-plane direction taking into consideration that the main problem in the simulation of the out-of-plane response is their test setup and calculation of out-of-plane force applied to the infills. One can suggest that multiplying the pressure inside the airbag times its effective area (area of the airbag in full contact with infill) can lead to calculation of the out-of-plane force; but in this paper, it is concluded that the distance between the reaction wall keeping the airbag and the infill affects the effective area of the airbag. When the distance between the reaction walls and the masonry infill wall is smaller, the effective area is closer to the nominal area of the airbag. The effective contact area of the airbag is calculated by dividing the load measured in load cells by the pressure inside the airbag. Based on this result, it is also recommended to use load cells in the test setup to measure the out-of-plane force instead of its calculation by the pressure inside the airbag. After the installation of the out-of-plane test setup, one specimen representing the contemporary construction typology in North of Portugal was tested. In order to investigate its out-of-plane behavior, quasi-static testing was performed on a masonry infill built inside a reinforced concrete frame by means of an airbag system to apply the uniform out-of-plane load to each component of the infill. The main advantage of this testing setup is that the out-of-plane loading can be applied more uniformly in the walls, contrarily to point load configuration. The test was performed under displacement control by selecting the mid-point of the infill as control point. Input and output air in the airbag was controlled by using a software to apply a specific displacement in the control point of the infill wall. Four load cells were attached to the reaction frame to measure the out-of-plane force.
Deformation and crack pattern of the infill confirm the formation of arching mechanism and two-way bending of the masonry infill. Until collapse of the horizontal interface between infill and upper beam in RC frame, the infill bends in two directions. However, the failure of that interface that is known as weakest interface due to difficulties in filling the mortar between bricks of last row and upper beam results in the crack opening trough a well-defined path and the consequent collapse of the infill. It is also investigated that the collapse of the infill was happened suddenly unlike the specimens tested by [1]. This is related to the presence of higher axial force on top of the columns in [1] that resulted in formation of a two-way arching mechanism supported on four sides. Besides, it seems that the presence of higher axial force on top of the columns can compensate the defects of upper interface.