Innovative Steel Braced Frame Systems for Tall Buildings in Seismic Active Regions

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Steel braced frames are widely used to resist wind and earthquake lateral loads for building structures in Canada. The system is easy to design, fabricate and erect. It represents the most cost-effective solution to achieve the lateral strength and stiffness under winds and small earthquakes. However, the system can exhibit poor performance under strong earthquakes such as large inelastic deformations and significant permanent deformations. For tall buildings, it can be prone to concentrations of storey drifts that can lead to global instability and structural collapse. Measures have been implemented in codes to achieve proper seismic response and safety to the public, including higher design seismic loads and stringent height limits. Those have increased construction costs and limited the application of the system. In addition, building owners are likely to face long downtime periods and high repair/reconstruction costs after a major seismic event, which may lead to significant losses for the Canadian economy. The research will focus on the development of two new steel bracing systems that can achieve superior seismic performance, with no or limited damage after an extreme earthquake, such that functionalities can be quickly restored, and repair costs are minimized. The first system is an innovative hybrid bracing system in which an elastic brace and a nonlinear brace are coupled with an elastic beam to generate a positive post-yielding stiffness that ensures stable seismic response and self-centering capabilities. The system can be coupled with an elastic segmental spine system to achieve more uniform storey drift demand over the structure height. The second system builds on the controlled rocking braced frame system that has been developed in the last two decades to eliminate structural damage and permanent structural deformations after a strong earthquake event. The original contribution from the proposed project is a new self-centering storey shear fuse that diminishes the force demand on the structure to reduce its initial cost. This novel shear fuse is inspired from the hybrid braced frame system described in the first project. Both studies will be conducted with the objective of developing braced steel frames suitable for tall buildings located in seismic active regions of eastern and western Canada. The research are expected to lead to effective structural configurations and design methodologies that will have been validated through advanced nonlinear seismic analysis. Shake table experiments and full-scale cyclic tests are also planned to validate the response of the proposed systems. Research will also be conducted to improve design provisions for braced frames using braces with intentional eccentricity and slotted hidden brace connections for HSS members. Testing and numerical simulations will also be performed to assess the plastic rotation capacity of beam-to-column connection in the gravity frame of building structures.