Enhancing Steel Hollow Structural Sections and Braced Frame Performance through Non-Traditional Civil Engineering Materials

Abstract

Steel braced frame structures are commonly used in regions of high seismicity due to their efficiency in resisting lateral forces. Braced frames derive their lateral resistance through a vertical truss system composed of braces that are expected to yield in tension and buckle in compression to dissipate energy induced by severe earthquakes. The braces undergo substantial plastic deformations to allow for surrounding members to respond elastically. However, the performance of braced frames can become severely compromised due to local buckling of the braces and damage associated with inelastic behavior. While hollow structural sections (HSS) are traditionally the preferred type of steel brace, their ability to resist lateral forces quickly degrades after local buckling initiation. This degradation can lead to premature brace fracture and a concentration of structural damage due to large story drifts. To enhance the resilience of steel braced frame systems subject to earthquake loads, non-traditional civil engineering materials are investigated for seismic void fill applications. Transverse plate to HSS column connections are also examined because the inherent void of the HSS column is ideal for the incorporation of fill materials in braced frame systems. An experimental and robust finite element study is undertaken to investigate circular hollow section (CHS) brace performance under large cyclic loads considering a lightweight, expanding, and high-damping polyurethane foam employed in the void of the CHS. The foam fill is shown to reduce the severity and impede the onset of local buckling at the mid-length of the braces, leading to enhanced ductility and energy dissipation capacity. Moreover, the results show that braces with larger diameter-to-thickness ratios can be employed when using the foam fill, leading to an increase in design flexibility and cost savings associated with using less steel. Building on the concept of utilizing the inherent void of HSS, the mechanical behavior of three non-traditional civil engineering materials, a polyester resin compound, carbon foam, and polycarbonate honeycomb, are systematically characterized to assess their viability for seismic void fill applications. Specifically, the materials are tested under monotonic and cyclic loads at quasi-static loading rates and quantities such as energy dissipation capacity, secant stiffness and yield strength are obtained to quantitatively assess their performance. Data necessary for calibrating high-fidelity finite element models of these materials are generated and a better understanding of how these materials will perform under loads that a structure would be expected to experience during an earthquake is obtained. Moreover, valuable insight is gained on how to optimize the use of these materials for seismic void fill applications. Further improvement to braced frame systems is investigated through experimental testing and complementary numerical analyses of transverse plate to rectangular hollow section (RHS) connections, which are typically preferred over longitudinal plate to RHS connections because of their greater stiffness and strength. Findings from this investigation suggest that the design equations’ limits of validity can be extended, leading to greater design flexibility. The results also provide a better understanding of how connection geometry influences the connection behavior. Ultimately, the findings from this work permit a more efficient use of steel and allow for stringent performance-based structural response demands to be met for new and existing (retrofit) braced frame structures. This research also provides a foundation for the incorporation of new materials in the design of resilient structures subject to earthquake loads.