Seismic Performance of Gravity Load Designed Reinforced Concrete Frames with Unreinforced Masonry Infill Walls

A significant portion of the existing building stock constructed prior to the enactment of modern seismic design provisions consists of Gravity-Load-Designed Reinforced Concrete (GLDRC) frames and unreinforced masonry infill walls which were used as partition walls in those buildings. Common construction practice before modern seismic design codes appeared, allowed the use of columns lap splices above the slab in each floor or above the foundation. The splices were typically 20 to 24 longitudinal bar diameters in length. Shear reinforcement was in the form of stirrups with 90-degree bends and spaced at half the depth of the frame member. As a result, the section at the base of these columns is unconfined and susceptible to shear failure or to a premature failure of the lap splices before yielding of the longitudinal bars, under reversed cyclic loadings in the event of an earthquake. The masonry infill walls used as partitions were often ignored by design engineers since such walls were considered as non resistant architectural elements. However, lessons learned from past earthquakes and from several tests performed have shown that those walls tend to interact with the bounding frame when the structural system is subjected to moderate or severe earthquake ground motions and that such interaction may not be beneficial to the performance of the structure. This paper presents the results of a series of tests, one monotonic and several shake table tests, conducted on two similar 1/2 scale gravity-load-designed reinforced concrete frames containing an unreinforced masonry infill wall built using hollow concrete blocks. One specimen was subjected to a static monotonic lateral loading and the other to prescribed simulated ground motions at different intensities to identify the interaction between the reinforced concrete frame and the masonry infill wall, the degradation in stiffness, and failure mechanism. The tests were conducted at the Earthquake Engineering Research Facility at the University of British Columbia in Vancouver, Canada.