Conventional concentrically braced frames with I-shape braces and bolted brace connections

In low and moderate seismic regions, low-ductility concentrically braced frames (CBFs) are widely used as the seismic force-resisting system for steel structures. Unlike high-ductility CBFs, the capacity-based design principle and additional seismic detailing are not required for such systems, which are referred to as conventional CBFs (CCBFs) in this study. In CCBFs, the brace-to-gusset connections are inherently weaker than the adjoining gusset plates and braces when loaded in tension. This occurs because both the gusset plates and the braces are most often selected based on their respective compressive buckling resistances, and hence, typically have a much greater resistance in tension. As such, brace connections are critical for the seismic behaviour and collapse prevention performance of CCBFs. However, brace connections have received little research attention because they are usually assumed to remain elastic in most capacity-based designs, and as such, their inelastic behaviour is not fully understood at a fundamental level. This is reflected in the different code provisions: in Canada, the seismic design force must be amplified by 1.5 for brace connections in CCBFs unless these connections are proven to be ductile as per CSA S16-19; in New Zealand, for connections in CCBFs, a structural performance factor of 1.0 is required, compared with 0.9 for structural members, which effectively increases the seismic design force demand on connections as per NZS 3404; no analogous requirements exist for CCBFs in the USA as per ANSI/AISC 341-16 or in Europe as per Eurocode 8.The inelastic behaviour of and the seismic deformation demand on CCBF brace connections were studied through a two-level numerical simulation approach, which is presented in this thesis. The bolted flange plate connection of the I-shape brace, which is a common design choice for CCBFs, was selected as the subject of this study.At the connection level, a high-fidelity finite element (FE) simulation procedure was developed for the bolted flange plate connection and validated against laboratory test results. The force transfer mechanism within the branches of the connection was characterized. Subsequently, a parametric study based on the validated numerical simulation procedure was carried out. Three key design parameters, namely, the gusset plate thickness, the flange lap plate thickness, and the web lap plate thickness, were varied to study their effects on both the compressive and tensile behaviour of the brace and the connection assembly. Various deformation mechanisms and failure modes were revealed under both compression and tension. Design recommendations are proposed with regards to attaining better deformation capacity.Based on the knowledge gained from the high-fidelity numerical simulations, a computationally efficient component-based modeling method was developed for the bolted brace connection. The connection was discretized into individual components, and modeled by means of organized springs, which each simulate the behaviour of a component. After validation against experimental test results, the component-based connection model was incorporated into a system-level numerical model for a series of prototype CCBFs. Through nonlinear static and dynamic structural analyses, the seismic behaviour and collapse prevention performance of CCBFs were studied. When loaded in tension, the brace connections deformed much more than the brace, and amplifying the design force by 1.5 was effective in reducing the seismic deformation demand on brace connections. In some cases, a secondary seismic force-resisting mechanism developed and prevented the system from collapse after the primary seismic force-resisting mechanism had failed

Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.