Effect of Inclusion of Masonry Infill on Fundamental Time Period

Masonry infill walls in reinforced concrete (RC) frames significantly influence seismic response despite being classified as non-structural elements in design codes. This study investigates the impact of masonry infills on the fundamental time period (FTP) of RC buildings, particularly addressing structures with open ground floors and irregular configurations commonly found in modern high-rise construction. The research methodology employed STAAD Pro software to analyse 168 building frame models with varying parameters, including infill wall thicknesses (150 mm and 250 mm), beam sizes (300-550 mm), column sizes (450-800 mm), and building heights (6-30 stories). Dynamic modal analysis was performed to determine FTP values, which were subsequently compared with existing code-based empirical formulas, including IS 1893:2016, UBC 97, and Eurocode 8. The principal findings reveal that masonry infills reduce FTP by 26-80% compared to bare frame structures, with the reduction percentage directly correlated to the Young's modulus of infill materials. The study demonstrates that current code equations significantly overestimate FTP values, leading to conservative base shear calculations. Through nonlinear regression analysis of the compiled results, a new empirical equation was developed to accurately predict FTP for infilled RC frames. The proposed equation shows superior accuracy compared to existing code formulas when validated against dynamic analysis results. The research contributes to structural engineering by providing a more accurate method for FTP estimation that explicitly considers masonry infill contributions, thereby improving seismic design accuracy. The findings have practical implications for earthquake-resistant design, particularly in regions where infilled RC frames are prevalent. Research limitations include focus on specific infill materials and regular frame configurations, suggesting future work should explore diverse masonry types and irregular structural layouts.