Design Development for Steel Strongback Braced Frames to Mitigate Concentrations of Damage
Steel braced frames are characteristically efficient seismic force-resisting systems. However, multi-story steel braced frames tend to concentrate demands in one or a few stories in response to severe ground shaking. Brace buckling and yielding results in a reduction in story strength and/or stiffness. Unless a mechanism exists to re-distribute the inelastic demands to other stories, demands tend to concentrate in the story where the inelastic response was initiated, indicative of story mechanism behavior.
Research has identified the advantages of using pivoting seismic force-resisting systems, herein termed strongback-braced frames, to mitigate story mechanism behavior. Strongback-braced frames employ an essentially elastic truss, or “strongback”, that provides an explicit mechanism of re-distributing demands to adjacent stories. Yielding and energy dissipation is provided through inelastic actions, or fuses (e.g., through brace yielding/buckling and/or beam plastic hinging). Forces and moments developed in these fuses are transferred vertically to adjacent stories by the flexural stiffness and strength of the strongback. As such, strongback-braced frames are expected to result in more uniform drift distributions, reduced peak inelastic demands, and improved design flexibility compared to conventional seismic force-resisting systems.
Despite being employed successfully in both research and practice, systematic assessment of the strongback’s behavior and practical design methods have not been developed or validated. Since the behavior of strongback systems is not characterized by the formation of story mechanisms, prior studies have found it difficult to proportion the elastic members in the strongback truss and have recognized detailing issues related to large deformation demands induced in the fuses. As such, a series of investigations were aimed at understanding the dynamic behavior and seismic performance of steel strongback-braced frames.
Archetype designs were numerically analyzed to characterize the seismic demands in the strongback elements. A four-story strongback-braced frame was used to benchmark the dynamic behavior observed during nonlinear dynamic analysis. Improved numerical models were calibrated to more realistically simulate the buckling-restrained brace response and to characterize the modeling parameters influencing brace buckling and low-cycle fatigue. The FEMA P695 methodology was used to assess potential design methods based on collapse performance. Extensive parametric studies were carried out on strongback geometries with a range of bracing configurations, ground motion characteristics, and design alternatives.
Higher mode effects were identified as the cause of substantial force amplification in the elastic strongback truss. Unlike typical yielding systems where force demands are limited by the capacity of the fuses in every mode, force demands in the strongback are characterized by a yielding first-mode “pivoting” and elastic higher-mode “bending” force demands. Since the strongback is designed to remain elastic in all modes, it can exhibit significant strength and stiffness in higher mode bending. Under the second and higher modes, the strongback truss remains elastic and continues to accumulate force demands after the fuses have yielded and as the ground shaking intensifies. These force demands in the strongback members can be significantly larger than those estimated per traditional capacity design assuming first mode-only demands.
The addition of a strongback results in improved dynamic response from typical yielding systems, including a more uniform drift profile compared to reference buckling-restrained braced frames. Based on this research, this study proposes recommendations for the design, analysis, and modeling of strongback-braced frames. Simplified static methods to estimate the dynamic demands in the strongback truss are also proposed, including modal pushover and modal enveloping analysis methods.