Experimental cyclic response of RC walls with setback discontinuities

Abstract

After the 2010 Maule, Chile, earthquake (Mw 8.8), several reinforced concrete (RC) buildings presented damage in some walls (crushing of concrete, buckling and fracture of steel reinforcement). These buildings had been designed according to the up-to-date seismic standards in the country and were not expected to suffer damage as severe as what was observed. Common features of the damaged walls were: little or no confinement at the wall ends; relatively high axial load; narrow thicknesses (below 200 mm); and discontinuities in height. Slender RC walls are often used in Chile and commonly, due to architectural constraints, the length of the walls changes between floors designated for parking use and the upper floors, creating a setback in the lower levels. These types of walls are commonly called “flag walls”. The problem of discontinuity has not been investigated experimentally and therefore it is necessary to observe its impact in RC walls. Four structural wall specimens were designed, one with no discontinuity and the other three with different sizes of the setback. They were tested under a nominal constant axial load of 0.1f'cAg and cyclic lateral loads increasing at specific drift levels. Conventional instrumentation and photogrammetry were used to monitor the tests. The experimental results show that the impact of the setback, for the sizes tested, is not significant for the strength but it is relevant for the strength degradation and deformation capacity. Strains in the extreme fibers, curvature and principal strains are studied using photogrammetry. It is found that the plastic hinge length increases with drift, but it remains constrained to the setback region. Another discontinuity is found around the reinforcement of the setback where large concentration of strains can cause premature failure of the wall.