Improvement of seismic performance of ordinary reinforced partially grouted concrete masonry shear walls

Reinforced masonry constitutes about 10% of all low-rise construction in the US. Most of these structures are commercial and school buildings. It may also be used for multi-story hotels, college dormitories, and apartment buildings. The vast majority of reinforced masonry construction in the mid-western and eastern parts of the US is partially grouted (PG) while most of the reinforced masonry construction in the West Coast is fully grouted (FG). The seismic performance of PG reinforced masonry wall systems is not well understood, and there is a critical issue identified in recent research for the design of these structures. The shear strength expression adopted in Masonry Standard Joint Committee Code design (MSJC 2008 and 2011) is likely to be un-conservative for PG walls. The research reported herein aimed at economically competitive design detail to enhance the seismic performance and safety index of PG reinforced masonry shear walls. Hence, this research has especially focused on enhancing the seismic performance of PG walls with different details; the one contained in the MSJC (2013), single grouted cell and bond beam (SS), and another proposed reinforcement detail that include; doubly grouted cells and bond beams (DD), doubly grouted cells and bond beams along with joint reinforcement at every other courses (DDJ), and doubly grouted cells and singly grouted bond beams along with joint reinforcement at every courses (DSJ). The results of 7 full-scale in-plane shear wall tests to investigate the effect of reinforcement detail on the seismic response of planner PG shear walls are presented. In addition to the experimental study, analytical and numerical investigations were also carried out to provide means of predicting and enhancing the seismic response of PG masonry shear walls. Improving the shear-strength expression in the MSJC code for PG walls, based on experimental data generated here and previous studies, is investigated as well.;Experimental studies were conducted at Drexel University and University of Minnesota to achieve the above stated goals. Four full-scale PG reinforced concrete masonry shear walls with different reinforcement details were tested at Drexel University structural laboratory. Three more full scale walls were tested at University of Minnesota structural testing laboratory. Test results demonstrated that the proposed doubly reinforced cells/bond beams detail and doubly reinforced vertical cells/single bond beam with joint reinforcement at every course had a significant effect in increasing wall shear strength and ductility. Using two reinforced and grouted cells/bond beams instead of one cell/bond beam resulted in an enhanced deformation capacity of the walls. It also resulted in an increase of the shear capacity and displacement ductility of the walls by 34% and 47%, respectively. Wall with double reinforced cells/bond beams showed a 60% increase in elastic stiffness compared to the wall with single reinforcing cells.;Because of possible construction difficulties in constructing double reinforced and grouted bond beams, an alternative detail using single reinforced bond beam with joint reinforcement every course is proposed. Tests demonstrate that walls with this configuration had nearly the same elastic stiffness and shear capacity as walls with double bond beams. In addition, wall displacement ductility of increased significantly by 189%. This configuration proved to be the most effective and, therefore, is highly recommended to replace the conventional single reinforcing detail.;A simplified 3D micro model was developed by considering the masonry units, grout and their interfaces. Masonry units and grout are modeled as a solid block and solid concrete, respectively. Cohesive surface-based behavior (interface elements) is used as a discontinuity for hollow and grouted masonry. The mortar joints and concrete masonry units are smeared into one homogeneous material using concrete damage plasticity model (CDP). The traction-separation behavior of the cohesive element is employed to model the units' mortar joints interfaces. Damage initiation is considered based on compressive strength of mortar and grout for the hollow and grouted masonry, respectively. A numerical model was developed based on the experimental results of grout, units, mortar and also different hollow and grouted masonry assemblages constructed and tested before testing the actual walls. The model was able to successfully capture crack pattern and strain distribution of hollow and grouted assemblages. The same model was employed for modeling the walls and for investigating the effect of other parameters controlling the response of the walls.;The results of the numerical modeling were in a good agreement with the experimental results. The numerical model was extended to explore the infilled-frame effect of PG masonry walls and also the effect of various other parameters such as axial compressive stress and boundary conditions on the wall response. As expected, the model showed that by increasing the axial compressive stress and fixing the rotation at the top of the wall (fixed-fixed condition) the shear strength of the wall increased while ductility decreased. The model demonstrates that the infilled-frame action of the wall's grouted masonry parts, through formation of struts, resulted in an increase in wall ductility and shear strength.;Test Results available in the literature for PG masonry walls showed that even implementing a gamma factor in the current MSJC code equation for shear strength of PG walls has not solved the fundamental problem of the over-estimation of the shear strength. The current study investigated a new approach to predict the shear strength mechanism of PG walls through adopting an infilled wall model as opposed to the monolithic wall model used in developing the current code's provisions that was based on test results of FG walls with a correction factor (<1) for PG walls. A new expression has been developed for PG walls based on infilled-frame mechanism. Experimental and analytical results demonstrated that the shear mechanism of PG walls can be predicted using the same concept for infilled-frame. However, struts in PG walls are formed based on the spacing of the vertical grouted cells. The shear strength that the frame (grouted vertical cells and grouted bond beams), can be attained based only on plastic hinges forming in the vertical grouted cells. Available test results from this study and other studies available in the literature demonstrated the accuracy of the proposed strength expression.;The response modification coefficient factor, R, factors calculated for single reinforcing walls were very close to what is specified in ASCE/SEI 7-10. For quantification of the seismic performance factors of such a system, FEMA P695 methodology was used. Results of incremental dynamic analysis revealed that the R factor adopted in ASCE/SEI 7-10 for one story PG buildings do not meet the performance and safety standard of FEMA P695 (ATC 2009). However, the proposed double reinforced cells/bond beams (DD) and double reinforced vertical cells and single reinforcing bond beams with joint reinforcement every courses (DSJ) showed acceptable performance. As a result, MSJC (2013) code needs to restrict the use PG masonry in SDC C and higher to either DD or DJ detail. Alternatively, conventional single reinforcing cells and bond beams can be used if a lower R factor is used.