Numerical Study of Influence of Infill Walls on the Vertical Irregularity Limit of Turkish Seismic Code

Weak/soft story irregularities are resulted from sudden changes of stiffness, strength and/or mass between adjacent stories. These irregularities introduce a soft zone into a structure where the damage initiates and concentrates, often leading to complete collapse. It is argued that masonry infill walls not constructed at ground floors of some commercial building create such a zone. To examine this phenomenon along stiffness irregularity conditions described in Turkish Seismic Code (2007), four reinforced concrete buildings are produced and tested in this study. The specimens consist of two reference bare frames, one frame with infill wall at the second story and one frame with infill walls at both stories. The frames and bricks used in the infill walls are scaled to 1/3 ratio. The frames are constructed in line with the code requirements. A quasi-static cyclic testing protocol is applied to simulate the earthquake loading while a constant axial loading is utilized to simulate the axial force present in column elements. The results of the experiments show that, at the early stages of time history of loading, the infill walls greatly contribute to the global stiffness of frame system. During this stage, the coefficient of stiffness irregularity indicates that only the frame with infill wall at the second story has a soft story. However, as the frame is further pushed/pulled into nonlinear region, the infill walls become no longer effective and frame behaves as a regular one. It is observed that, during this stage, the critical load carrying components of the frames assume no damage or jeopardized by infill walls. The experimental results of the study are also checked against those of the finite element modeling.