Plastic Hinge Model and Displacement-Based Seismic Design Parameter Quantifications for Reinforced Concrete Block Structural Walls

A practical alternative to the traditional rectangular cross sections of reinforced masonry structural wall systems is to alter the wall ends to allow for smaller compression zone depths, and thus higher curvatures to develop under increased seismic lateral loads. Despite the significantly enhanced seismic performance of flanged and end-confined masonry structural walls compared with their rectangular counterparts, seismic design parameters related to the former two types of walls have not been widely investigated. In addition, prescriptive design requirements for rectangular walls are under continuous development to meet the ongoing research findings in this area. The focus of the current study is to extract specific seismic design parameters of these three types of masonry walls having different end configurations for different aspect ratios when tested under reversed cyclic loads. The parameters investigated include the equivalent plastic hinge lengths, $l_p$, the hysteretic damping levels and the trend of period shift corresponding to stiffness reduction with increased top wall drift. Three approaches were considered to evaluate the equivalent plastic hinge lengths, $l_p$. The analyses showed that, following a widely accepted approach, the $l_p$ values at ultimate loads using the theoretical curvatures and experimental displacement ductility levels were approximately 40, 15, and 20% of the wall length for the rectangular, flanged, and end-confined walls, respectively. A mechanics-based model, accounting for the variation in curvature profile following yielding, strain penetration inside concrete foundation and the effects of inclined flexure-shear cracking, to determine wall top displacements was proposed. At 1% lateral drift, the calculated hysteretic damping ratios were found to be approximately 20% for all walls. At a displacement ductility level of 5.0, corresponding to almost no strength degradation, the period increased by at least 70% compared to that at first yield.