The performance of steel braced frames under seismic loading relies heavily on the geometric properties of the bracing members, particularly when using hollow structural sections (HSS). While numerous cross-sectional shapes, width-to-thickness ratios and slenderness ratios can be chosen to optimize system performance, residual damage imposed by large inelastic deformations due to early local buckling in HSS bracing members can create substantial economic losses associated with repair costs and disruption of building function. Although strategies that limit the susceptibility of braces to premature fracture and provide enhanced energy dissipation capacity have been explored, they are typically limited by difficulty in their implementation as a viable retrofit option. The use of a lightweight, pourable and expanding urethane foam within the voids of hollow bracing members circumvents this by providing a unique retrofit opportunity utilizing the inherent void in the member. In addition, experimental testing has shown the foam fill is capable of limiting local buckling in the plastic hinge region, thus prolonging brace fracture life and leading to greater energy dissipation and a more stable structural response. The foam’s lightweight also provides numerous advantages over concrete fill as it has a negligible influence on brace stiffness and yield strength, thus allowing for its use to be seamlessly integrated into the design process without its explicit consideration.
To further explore the utility of the foam fill, an extensive finite element parametric study is conducted using high fidelity models validated with data from experimental testing of foam filled tubular braces under representative seismic loading. The models are able to capture global and local buckling behavior observed in experimental testing as well as the initiation and propagation of brace fracture. Width-to-thickness and slenderness ratios are varied to identify brace geometries that benefit most from the inclusion of foam fill with brace performance assessed with respect to energy dissipation, compressive capacity and ductility.
As structural engineers are faced with constantly evolving challenges, innovative solutions that look to merge disciplines such as seismic engineering and materials science are critical. The cross-disciplinary nature of this research facilitates the development of more structurally resilient infrastructure systems. Furthermore, exposure to the concept of utilizing inherent void space and materials that are not typically used for buildings such as polymer foams will help drive progressive solutions that push the boundaries of our current design standards.