Dynamics of Flexible Freestanding Structures under Ground Motion Excitation

Freestanding structures are characterized by being unattached at the base, exhibiting rigid or flexible motion under ground motion excitation with various potential responses including rocking (rotation at the base about corner points), pure sliding, slide-rocking, and overturning. While rigid freestanding structures have been extensively studied, the behavior of flexible freestanding structures—particularly the interaction between modes involving sliding—remains underexplored. Existing literature has focused on the deformation and rocking motions of flexible freestanding structures through both analytical and experimental approaches, which have shown indications of sliding at the base. These findings highlight the necessity of incorporating sliding motion into analytical models to improve the accuracy of response predictions and enable comprehensive experimental evaluations without imposing constraints on base motion. To address this knowledge gap, this dissertation develops novel analytical models that integrate three degrees of freedom—deformation, rocking, and sliding—along with updated criteria for mode initiation, transition, and impact mechanisms (the primary source of energy dissipation in freestanding structures). A parametric study is conducted to investigate the influence of key parameters, including flexibility scale (a measure of stiffness), viscous damping, friction, and slenderness ratio. The results indicate that more flexible structures exhibit lower overturning instability when impacted once; however, transitions to slide-rocking and high translational accelerations due to sliding displacements increase overturning likelihood without impact. Increased viscous damping reduces sliding displacement, but high damping ratios result in more overturning with a single impact. Low friction coefficients primarily lead to greater sliding displacements, decreasing overturning by dissipating energy. Unexpectedly, a slight increase in overturning is observed at low frequencies due to transitions from rocking to slide-rocking under high accelerations and very low friction. An experimental program was conducted using a steel tower with stiffeners, attached mass weights, and a concrete pad. The results reveal that sliding is a critical factor contributing to the overturning failure of squat flexible structures, contrary to the expectation of minimal overturning for these configurations. Additionally, a correlation between significant sliding displacements and twisting in tall structures suggests the occurrence of complex three-dimensional motion patterns. Advisor: Christine E. Wittich