A SIMPLIFIED METHOD FOR THE ESTIMATION OF THE SEISMIC RESISTANCE OF RC FRAMES WITH WEAK INFILL PANELS

Reinforced concrete (RC) frames with masonry infill panels are widely used in areas with high seismic activity around the world. Historically, the masonry infills have been considered nonstructural elements and have been ignored in the structural analysis and design. Nonetheless, field evidence and research studies have shown that infills can affect the seismic behavior as they increase the strength and stiffness of a structure, and alter the load-transfer mechanism. The exact influence however, is difficult to evaluate due to the variety of possible failure patterns that may develop. These failure patterns may involve compressive or shear failure of the infill, and flexural or shear failure of the RC members. Hence, the exact failure mechanism is challenging to predict. To address this challenge a number of analysis methods have been proposed in the past but these often require complex computations or have limited application as they were developed based on case-specific data. Moreover, there is a lack of validated analysis guidelines that practicing engineers can use to assess such structures. For these reasons there is still a need for efficient and reliable tools for the analysis of infilled RC frames. This paper will discuss a simplified procedure for the determination of the lateral force-vs.-drift relation of RC frames with masonry infill panels under in-plane loads. The suggested methodology has adopted the shape of the simplified quadri-linear backbone curve used in ASCE 41 for other structural systems. With the proposed method, the force-vs.-drift curve for an infilled RC frame can be estimated considering the frame geometry and basic material properties as the procedure takes into account the different relative strengths and stiffnesses of the infill with respect to the bounding frame. These parameters are of high importance as they are known to influence the failure mechanism of an infilled frame. The framework has been developed with data from experimental and analytical studies from single-bay, single-story specimens. It has also been validated with tests of multi-bay, multi-story structures that are available in the literature. The comparison of the analytical and experimental results indicates that the proposed methodology can successfully capture the main features of the seismic response of infilled frames, including the initial stiffness, peak strength, and residual load capacity. The details of the method and its results when simulating the performance of a number of previously tested complex structures will be discussed in this paper.