Seismic Vulnerability of Waffle Slab Systems: Nonlinear Dynamic Assessment and Development of a Slab Seismic Fragility Index (SSFI)

Abstract

Waffle slabs are widely used in modern construction because they can span large distances while reducing material consumption and overall structural weight. Despite these advantages, their relatively low lateral stiffness can become a major source of seismic vulnerability, especially when simplified assumptions such as the rigid diaphragm model are adopted in structural analysis. This study examines the seismic response of waffle slab systems, with particular attention to the influence of diaphragm flexibility on displacement amplification and damage concentration around slab–column connections. The findings indicate that inadequate in-plane stiffness can generate excessive interstory drift, dynamic amplification effects, and progressive deterioration of slab–column interfaces, ultimately reducing the overall seismic performance of the structure. The research combines previous literature, post-earthquake observations, and advanced numerical simulations to identify the main mechanisms responsible for seismic vulnerability in waffle slab structures. Reported damage patterns include extensive cracking, formation of plastic hinges, bond degradation, and progressive punching shear failure, which in severe cases may result in partial or complete collapse. To better evaluate these effects, an advanced numerical framework is proposed to assess the seismic capacity of waffle slabs and investigate the causes of their reduced lateral stiffness. In addition, a Seismic Fragility Index for Slabs (SSFI) is introduced to quantify vulnerability and classify waffle slab systems according to regional seismic hazard levels. The adopted methodology integrates modal analysis, nonlinear static (pushover) analysis, and nonlinear time-history simulations. A comparison between rigid and flexible diaphragm models highlights the significant influence of slab flexibility on both the overall seismic response and the distribution of structural damage. The findings show that adopting the rigid diaphragm assumption can substantially underestimate displacement demands and overlook critical failure mechanisms, resulting in unconservative seismic evaluations and an increased probability of structural failure.