Experimental observations of masonry infilled reinforced concrete frames with openings

The paper summarizes the main experimental findings of a test program at the Universityof Colorado, Boulder on 2/3-scale unreinforced masonry walls bounded by nonductile reinforced concrete frames. The test program was carried out within the frame of a NEESR-SG project headed by Benson Shing at UC San Diego. The cyclic pushover tests on single panels involved two-wythe masonry walls of solid clay bricks with and without window and door openings. The infilled frames were preloaded in the vertical direction by two jacks at 35 kips (156 kN) each to emulate the confinement of a multistory building. Cyclic lateral shear loads of increasing amplitude were applied by the high-performance 220 kip (979 KN) MTS actuator of the Colorado NEES equipment facility. The counterintuitive observations of the test program were the small effects of window and door openings on the overall stiffness and strength properties of the masonry infilled reinforced concrete frames. In fact, the initial elastic stiffness properties exhibited no significant stiffness degradation compared withthe full masonry wall when window and door openings were included in the masonry wall. Moreover, the maximum shear resistance and the residual shear capacity after loss of adhesion did not exhibit significant deterioration when window or door openings were included. In addition, the eccentricity of the window and door openings had little influence on the cyclic performance of the pushover tests when compared with the performance of the masonry wall without opening. In conclusion, the infill masonry walls exhibited surprising resilience up to drift levels of 2% in spite of the brittle nature of unreinforced masonry and nonductile bounding reinforced frames. This apparent ductility of the brittle composite components may be attributed to the overall confinement of the reinforced concrete bounding frame and the vertical preloading.