## 1 Introduction
Additionally, the gap between the masonry and frame (especially at the bottom of the beam and top of the masonry wall) might not be properly grouted during construction. Figure 1 shows an example of such low construction precision. The grouting adequacy of the masonry will affect its strength. [PERSON] and [PERSON] [22] conducted an experiment with a masonry wall with a gap of 20 mm between the top of the wall and the bottom of the beam. They reported that the gap decreased the initial stiffness by 60% and the maximum strength by 62%. [PERSON] [23] performed a finite element analysis while changing the size of the gap from 0.01 mm to 1.0 mm. When the size of the gap was small, there was no significant difference in stress distribution as the gap could be filled properly even with a small deformation of the frame. As the size of the gap increases, the change in stress distribution in the infill wall also increases, decreasing the formulation of diagonal struts. Figure 1: Gap between beam and masonry infill wall. In FEMA 306 [24] and ASCE 41 [25], the shear strength of masonry corresponds to BJS strength, estimated as the sum of cohesion and friction, where the friction force is the product of the friction coefficient and the normal force applied to the masonry. ASCE 41 [25] specifies that the normal force shall be calculated from the gravity load transmitted from the upper story and that friction should be ignored when there is a gap between the beam and masonry. However, since the masonry infill is constructed after the frame is finished, a dead load would not produce a normal force to masonry walls and, accordingly, the friction. FEMA 306 [24] suggests that, when the structure deforms laterally, the length of columns decreases due to the effect of curvature shortening, which produces a normal force and, subsequently, a frictional force. However, appropriate lateral displacements must be selected so that normal stresses and friction forces can be calculated. In this study, an experimental program was performed on masonry-infilled frame specimens with varied construction precision and masonry thickness. A total of five specimens, which consist of four masonry-infilled frames and a bare frame, were tested, and the results were analyzed to investigate the effects of construction precision and interaction between the masonry infill and the frame. The test results indicated that the gap in masonry infill decreased the strength by 75% to 80% and stiffness by 55% to 70%. Comparison between the measured and predicted peak strength using the current code shows that the code equations considerably underestimate the strength. This is due to the fact that the contribution of friction in shear resistance of the masonry wall was actually ignored in the current code since no adequate method for estimating the normal force is provided. During the tests, it was observed that the failure mode of the frames changed to shear failure when thick masonry walls were used. In this study, reflecting such an observation, a mechanical model that can explain the shear failure was proposed. The maximum strengths of the specimens calculated using the proposed model were in good agreement with the test results.