Combined Effect of Unplanned Storey Addition and Building Orientation on the Seismic Response of Plan-Irregular RC Buildings

Abstract

Rapid urbanization, population growth, rising land values, and limited availability of developable land have encouraged the vertical expansion of existing reinforced concrete (RC) buildings in many developing countries, including Nepal. In seismically active regions, additional storey is often added without detailed structural evaluation, altering the original mass and stiffness distribution of the building and potentially increasing its vulnerability during earthquakes. This concern is particularly significant for plan-irregular structures, where geometric irregularities can lead to torsional effects and uneven seismic force distribution. While previous studies have examined building orientation, structural irregularity, and vertical expansion separately, their combined influence on seismic performance remains insufficiently explored.

This study investigates the effect of unplanned storey addition and seismic excitation orientation on the response of a plan-irregular RC building. A five-storey L-shaped three-dimensional finite element model was developed in ETABS and analyzed using the Equivalent Static Method, Response Spectrum Method, and Linear Time History Analysis following the provisions of IS 1893:2016 and IS 13920:2016. Earthquake motions were applied at orientations of 0°, 30°, 45°, 60°, and 90° to evaluate directional sensitivity. Structural performance was assessed using base shear, natural time period, storey displacement, and inter-storey drift.

The results indicate that the addition of extra storey gradually increased seismic demand on the structure, leading to greater displacement and drift compared to the original building configuration by increasing structural mass and flexibility. The response also varied considerably with excitation orientation producing more critical behavior than others with certain orientations. These findings emphasize the importance of orientation-sensitive seismic assessment before vertical expansion and provide useful guidance for improving the safety and resilience of existing buildings in earthquake-prone regions.