Seismic Fragility Assessment of Flat Slab Structures with and without Drop Panels

Abstract

Flat slab reinforced concrete systems are widely used in building construction due to their architectural efficiency and construction simplicity, but their reduced lateral stiffness and lack of beam elements increase seismic vulnerability, particularly in high seismic regions such as Indonesia. This study evaluates the seismic fragility performance of flat slab structures with and without drop panels subjected to multiple Indonesian seismic hazard regions, including Medan, Bandar Lampung, Yogyakarta, and Nusa Tenggara Timur (NTT), with seismic demand defined based on the Indonesian Seismic Hazard Map developed by the National Center for Earthquake Studies (PUSGEN). A nonlinear static pushover analysis is employed to simulate inelastic structural response, including capacity curves, stiffness degradation, plastic hinge progression, and inter-story drift demand, followed by probabilistic fragility assessment using a lognormal distribution to estimate the probability of exceeding damage states (Slight, Moderate, Extensive, and Collapse). The results indicate that the incorporation of drop panels significantly enhances seismic performance by increasing lateral stiffness, delaying stiffness degradation, and reducing deformation concentration at slab-column connections, resulting in lower inter-story drift ratios and improved ductility compared to systems without drop panels. Fragility curves show a consistent shift toward higher spectral displacement capacity, indicating reduced probability of damage exceedance, while structures without drop panels exhibit earlier nonlinear deterioration and higher collapse vulnerability, particularly in high seismic regions such as Yogyakarta and NTT. The study concludes that drop panels are an effective structural enhancement to improve seismic resilience and reduce collapse risk in flat slab buildings in Indonesian earthquake-prone regions.