Withstanding an earthquake places considerable stress on structures. Modern design codes focus on preventing collapse and the safety of occupants, but not necessarily on reuse of the structure after a significant seismic event. Increasing the design-level demands of a structure could make them more resilient. This could be achieved by implementing a spine system which helps reduce the overall structural damage to a single story or set of stories. Research published in the Journal of Structural Engineering focuses on one particular spine frame type, strongback braced frames. SBFs use one half of the frame as the energy-dissipating component and the other half as the spine, but achieving this can be challenging.
In researching SBFs, Peter C. Talley, Mark D. Denavit, and Nicholas E. Wierschem had two objectives: (1) evaluate and compare the performance of frames produced by different design methods, and (2) assess how changes in the strength and stiffness of the strongback affect performance. In their paper, “Evaluation of Methods of Design for Strongback Braced Frames,” they evaluated three methods for SBFs using three buildings of different heights to test for collapse resistance, story drift, and yield in the strongback spine itself. Learn more about how this work advances the development of the SBF system as a viable seismic force-resisting system for highly resilient buildings. Get the full results at https://doi.org/10.1061/JSENDH.STENG-13403. The abstract is below.
Abstract
Strongback braced frames (SBFs) are a relatively new structural system intended to reduce structural damage during seismic events and improve resilience. SBFs combine buckling-restrained braces, which provide the primary lateral resistance and energy dissipation, with a stiff elastic spine to distribute demands across the height of the structure and prevent the formation of weak- and soft-story mechanisms. Designing the spine is challenging, as higher mode effects and partial nonlinear mechanisms have been shown to be significant. These effects, and their interaction, are not fully accounted for by standardized design methods. It is also unclear how stiff and strong the spine must be in order to achieve the desired behaviors. There are proposed procedures for designing SBFs; however, they have not been broadly evaluated, and they have not been compared. This work evaluates two proposed design procedures, the simplified modal pushover analysis (SMPA) and generalized modified modal superposition (GMMS), with a “control” procedure based on current standardized capacity design procedures. A total of nine frames were designed for three buildings using the three procedures. Nonlinear response history analyses were performed to evaluate the differences in behavior resulting from the different design methods. To determine the effect of the strength and stiffness of the strongback, the yield strength and elastic modulus of the strongback members were varied and the analyses repeated. The results of this work show that the GMMS and SMPA design procedures are generally well-calibrated and provide benefits over current standardized procedures in several ways: collapse performance is improved, and yielding in the strongback and residual drifts is reduced. The GMMS procedure results in larger members, but provides similar outcomes to the more-complicated-to-implement SMPA. The insights from this work will assist engineers when implementing these design methods and support the codification of strongback braced frames in design standards.
Get the specifics on how to apply the SBF system to resilient buildings that survive earthquakes in the ASCE Library at https://doi.org/10.1061/JSENDH.STENG-13403.
