Behavior of RC Frame with Masonry Infill Panels due to In-plane Monotonic Loading

Abstract

Multistoried masonry infilled reinforced concrete (RC) frame with Open ground floor is a common building construction practice in Bangladesh. Masonry infills in upper floors make the corresponding floors stiffer; resulting stiffness irregularity in RC frames. In contrast, this interaction or masonry panels with frame elements is often neglected in the conventional design analysis such structures. Therefore, an extensive analysis has been performed in the present study to determine the performance of masonry infilled RC buildings with open ground floor due to in-plane monotonic loading. Masonry infill panels were also not recognized as integral structural members in Bangladesh National Building Code (BNBC). It is obvious, the masonry infill panels in a structural frame not only add significant strength and stiffness but also influence the dynamic behavior of the entire building. Three two-storied single-bay laboratory specimens were constructed. The three models represent: a bare frame, 75 mm and 50 mm thick brick frame with open ground floor. These models were fully instrumented and tested under lateral in-plane loading to failure. After the first crack, diagonal strut was visible in the infill which increases the overall shear strength of the infilled frame. It is also evident that, infill increases the strength but reduces ductile behavior of RC frame. Moreover, a software investigation has been performed to evaluate the nature of two-dimensional RC bare, fully infilled and open ground floor infilled frame. The infills are modeled as equivalent diagonal strut. Dead, superimposed and seismic loads were applied in the analysis. The maximum bending moment, shear, deflection and inter story drift are evaluated in both ESA and RSA by applying seismic loading under designed base shear based on BNBC 2006. The study indicates that, such parameters were significantly decreased in case of RSA for open ground infill RC models due to presence of structurally active infill as compared to static analysis. In addition, the presence of infill in upper stories makes those floors stiffer compared to ground floor. As a result, the upper floors behave together in a rigid manner causing excessive sway at ground floor level. In brief, ground floor columns inbuilding with open ground floor based on ESA are over designed for seismic loading. As such the damage of such buildings by soft story mechanism is very high during earthquakes, Finally, highly conservative design is faced in conventional practices when an infill frame is considered as bare and ESA is carried out in place of RSA.