Seismic Analysis of Multi-Storied Buildings with Even and Uneven Story Heights Under Symmetric and Unsymmetric Configurations

Abstract

Seismic behaviour of 15-storey reinforced-concrete (RCC) frame structures was examined for a variety of configuration geometries. The two buildings included in this study were G + 15 RCC frames: one frame had a symmetrical regular floor plan and equal (even) storey heights, and the other frame had an asymmetrical floor plan or irregular storey heights. The buildings were studied in ETABS 2018 using the response spectrum method, as per IS 1893:2016 (Zone IV, biaxial loading). A comparison of key response characteristics, including total base shear, maximum displacement between storeys, maximum relative displacement between storeys, storey shear and stiffness, and the first-mode period in each direction (X and Y), was conducted. The results indicated that the symmetrical, equal-height model exhibited the most beneficial seismic behaviour. The drift distribution was more even, torsional responses were minimal, and the overall stiffness and strength distribution was more uniform. Previous research has noted that “the perfectly symmetrical layout offers better structural stability because they all share similar stiffnesses and therefore similar load distributions” [2], [3]. Comparison of the symmetrical model data with the unsymmetrical/uneven-height model data revealed that the unsymmetrical/uneven-height model showed poorer earthquake performance. The study found that the irregular case had increases in top-floor displacement of approximately 4–5% and increases in peak storey drift of approximately 12% compared with the regular case. Similar findings have been documented in previous studies, which indicate that irregular buildings experience increased lateral displacement and drift when subjected to lateral forces such as those generated by earthquakes. Building irregularities include vertical irregularities due to a soft or uneven storey. Soft-storey frames tend to generate increased displacements and accelerations when subjected to lateral forces during an earthquake. The asymmetrical/uneven-height model had lower storeys that were significantly softer and experienced larger shear force values than the upper storeys, representing a non-linear distribution of stiffness. Finally, the study determined that the response spectrum method can illustrate the difference in seismic response between the two models and confirm its ability to provide comparative seismic assessments of structures.