Finite element study of the elastic behaviour of R. C high- rise frame with brick masonry infill

Abstract

In Bangladesh, use of burnt clay brick masonry as infill material for exterior wall and interior partition wall in high-rise building frame is very common. The frame-infill system is used throughout the world only for architectural needs or aesthetic reasons. Present code of practice does not include provision of taking into consideration the effect of infil!. It can be understood that if the effect of infill is taken into account in the analysis and design of frame, the resulting structures may be significantly economic. This study has been undertaken to investigate the elastic behavior of R.C high-rise frame with brick masonry infill subjected to lateral load. In this investigation a 20-storied building with a lift core has been taken as a reference structure. Finite element package (ANSYS 5.4) has been used for two dimensional and three dimensional analysis of this building. Two dimensional beam element has been used in modeling 2-D frame and beam element in space has been used to model the 3-D frame. To model roof Slab and Lift Core, elastic shell element was used. The infill panel in reinforced concrete frame has been modeled in two different ways. In one way, the panel is modeled as a homogeneous continuum considering perfect bond between the frame element and the masonry. Secondly, an equivalent strut is used to simulate the separation of infill panel from the surrounding frame due to bond failure. A number of different cases have been considered for this finite element study, these are (i) Bare frame (ii) Infill as equivalent strut (iii) Infill as homogeneous continuum (iv) Infill as homogeneous continuum with opening (v) Infill as homogeneous continuum with opening & soft story at ground floor. In this study masonry infills are provided along the windward direction and no out-of-plane infills are provided in the analysis. Lateral loads (wind load) at different levels of the building have been calculated according to the method specified in the Bangladesh National Building Code (BNBC,1993). A comparative study has been performed using results of the finite element analysis for different cases. In all cases, deflection, bending moment, shear force and axial force for columns and beams have been studied. From the study it is observed that the stiffness of frames will be significantly increased when masonry infill panels are considered for stiffness computation. From this finite element study, it is found that due to the presence of infill, the moment, shear, and axial force in the columns and beams of the building frame reduces quite significantly. This study indicates that an economy in building design can be achieved if the composition action between building frame and infill panel can be fully utilized.