<p> A moment resisting frame is one of the most commonly used lateral load resisting system in modem structures because it is suitable for low and medium rise buildings and industrial structures. It can be designed to behave in a ductile manner under seismic loads. </p> <p> Masonry infills have traditionally been used in buildings as partitions and for architectural or aesthetic reasons. They are normally considered as non-structural elements, and their effect on the structural system has been ignored in the design. However, even though they are considered non-structural elements, there is mounting evidence that they interact with the frame when the structures are subjected to lateral loads Infill walls have been identified as a contributing factor to catastrophic structural failures during earthquakes. Frame-infill interaction can induce brittle shear failures of reinforced concrete columns by creating a short column. Furthermore, infills can over-strengthen the upper stories of a structure and when they fail a soft first storey is created, which is highly undesirable from the earthquake resistance standpoint. </p> <p> There is a need for an efficient and accurate computational model to simulate the nonlinear hysteretic force-deformation behaviour of masonry infills, which is also suitable for implementation in time-history analysis of large structures. The aim is to develop a simplified advanced and cost-effective model for nonlinear time history analysis and seismic design of masonry infill frame structures. </p> <p> The objective of this research was to develop a practical and economical technique applicable for global analysis of general three-dimensional reinforced concrete infilled frames under lateral loads. Novel finite element model for the infill and the surrounding frame was developed using a special finite element configuration to represent the masonry panel. Some prescribed failure planes in different directions were defined depending on the common failure mode of masonry panels. Moreover, some of contact elements were used on the failure planes to connect among the panel elements, and between the panel elements and the boundary reinforced concrete frame. Different material models were used to represent the behaviour of concrete, reinforcing steel, mortar joints and inclined saw-tooth cracks in the infill panel. Different material models were used to describe the behaviour through and perpendicular to the prescribed failure planes. The proposed model and the used material models were described in details in the first part of this research. </p> <p> The proposed finite element model was verified against experimental and analytical results previously published by others. Different frames configurations, reinforcing details, boundary conditions and material properties were consider in that section to verify the capability of the proposed model to simulate the behaviour of different frames. The overall behaviour "Load-deflection relationship", failure point and failure mode were compared with the experimental and analytical results. Satisfactory agreement with the previously published results was obtained. </p> <p> The study investigates the capability of the proposed model to simulate the behaviour of infilled frames subjected to cyclic loads. Hysteretic loops obtained by using the new model were verified against experimental and analytical results and good correlation were obtained. The failure modes and crack patterns were compared with the experimental results and good agreements were obtained. The proposed model failed to capture some shear cracks in the RC frames as per the experimental results. </p> / Thesis / Doctor of Philosophy (PhD)