Shake-Table Test of a Partially Grouted Reinforced Masonry Building with Separated Single Grouted Cells

While a number of studies have been carried out to investigate the seismic performance of fully grouted reinforced masonry walls, far less attention has been devoted to partially grouted walls, even though the latter constitutes most of the reinforced masonry construction outside the West Coast. Quasi-static tests on individual wall components have shown that the lateral load resisting mechanism of these walls was vastly different from that of fully grouted reinforced masonry and it resembles the one of masonry-infilled RC frames. It is also recognized that the current masonry design code has deficiencies for the shear strength design of partially grouted components. The behavior of partially grouted masonry is not well understood and information on the system-level performance of these structure is not available.
This experimental study investigated the seismic performance of two full-scale, one-story partially grouted ordinary reinforced masonry wall buildings. The buildings were tested on the outdoor shake-table at the University of California, San Diego (UCSD). Both structures are designed for Seismic Design Category SDS C. Each test structure was symmetric about the center plane along the east-west direction, which was also the direction of the shake-table motion. The clear height of the masonry walls was 152 in. (3.86 m).
The first building (Specimen 1) tested on the shake table (Experiment 1) was a partially grouted masonry wall system designed and constructed according to current code provisions and current practice. The details conform to the current practice in that all the vertically grouted cells were separated by ungrouted masonry. Both the vertical and horizontal reinforcement had Grade 60 No. 4 (129 sq. mm) bars. The positions of the vertical bars satisfied the code requirements in that there was vertical reinforcement of at least 0.2 sq. in (129 sq. mm) in cross-sectional area adjacent to the openings and at wall intersections and that the maximum bar spacing did not exceed 10 ft. (3.05 m). Bond beams with horizontal reinforcement were placed at the first course above the footing and at the top course below the roof slab. The top bond beams were required to connect the roof slab to the walls with ties, but bond beams are normally not used for the first course in practice. The bottom bond beams were introduced here to provide a better performance at the base if the walls were to develop base sliding. While bond beams are required right above and below an opening according to the code, they need not be extended continuously along the whole wall unless they are required to resist shear. For this specimen, the intermediate bond beams were not required to resist shear, but it was deemed beneficial to tie the vertically grouted cells by bond beams at these locations so that adequate frame actions could develop if the walls were to behave like an infill frame as shown in previous studies. The roof system had 8-inch-thick precast hollow-core planks and 4-inch reinforced concrete in order to achieve the desirable seismic mass.
The test structure was instrumented with 178 strain gages on the reinforcing bars, 180 displacement transducers, and 39 accelerometers.
Specimen 1 was subjected to a series of 16 ground motions scaled to different intensity levels. Four different ground motion records were used. They were from the 1940 and 1979 Imperial Valley earthquakes in California, the 1985 Nahanni earthquake in Canada, and the 2011 Mineral earthquake in Virginia. However, most of the test runs had the El Centro 180 record (EC1940) from 1940 Imperial Valley earthquake. The structure was subjected to a white-noise excitation that had a root-mean-square (RMS) amplitude of 0.05g after each earthquake motion to estimate the change of the natural frequency of the specimen.
During testing, significant sliding occurred at the base of the structure even though the sliding shear strength had been calculated during the design. Concrete stoppers were constructed at the base of the structure to prevent sliding and induce damage at the superstructure.
The first shake-table test specimen was able to resist ground motions with effective intensities twice the MCE level without catastrophic failures. However, the structure eventually failed in a very brittle manner.