Higher‐mode force response in multi‐story strongback‐braced frames

Abstract

Strongback-braced frames employ an essentially elastic steel truss, or strongback, that distributes demands more uniformly to delay or prevent story mechanisms. Because inertial forces are no longer limited by the formation of a story mechanism, strongback-braced frames can exhibit large elastic force demands, particularly in the higher modes. This paper characterizes the higher-mode force response of strongback-braced frames. Four-story archetypes were designed using nonlinear dynamic analyses to incorporate higher-mode force demands into the design process. The response of the archetypes was compared with that of reference buckling-restrained braced frames that were allowed to form story mechanisms. The force demands in the strongback were then described using equivalent-static forces to represent the inertial forces induced by the higher modes. Force demands in the strongback arise from a yielding first-mode 'pivoting' and elastic higher-mode 'bending' response. These higher-mode force demands are elastic, ill-constrained by the strength of the yield mechanism, and depend significantly on the choice of ground motion record used for the analysis. In remaining elastic in the higher modes, the strongback distributes demands more uniformly and mitigates the formation of story mechanisms. Consequently, design and analysis methods for strongback-braced frames need to include estimates for these near-elastic higher-mode force demands.

Modeling uncertainty of specimens employing spines and force‐limiting connections tested at E‐defense shake table

“…With the Northridge record, the addition of spines resulted in increased accelerations above the PGA for the Frame-Spine and Frame-Spine-FLC models compared to the Frame model due to large spectral content near the higher-mode periods and near-elastic higher-mode effects. 46 On average, the maximum floor acceleration was smaller for the Frame-Spine-FLC models than for the Frame-Spine models; using the FLC in the Frame-Spine-FLC models aided in reducing the large floor acceleration at the second floor of the Frame-Spine models, although many models resulted in increased accelerations at the first floor depending on the parameters used to model the FLC. With the JMA Kobe record, floor accelerations tended to be smaller than for the Northridge record and closer to the PGA for all the models, because the JMA Kobe record had less spectral content near the higher-mode periods.…”

“…Models can now be refined post‐test to better represent the upper story response. Unlike story drift ratio, the trends of the peak floor accelerations differed depending on the ground motion record. With the Northridge record, the addition of spines resulted in increased accelerations above the PGA for the Frame–Spine and Frame–Spine–FLC models compared to the Frame model due to large spectral content near the higher‐mode periods and near‐elastic higher‐mode effects 46 . On average, the maximum floor acceleration was smaller for the Frame–Spine–FLC models than for the Frame–Spine models; using the FLC in the Frame–Spine–FLC models aided in reducing the large floor acceleration at the second floor of the Frame–Spine models, although many models resulted in increased accelerations at the first floor depending on the parameters used to model the FLC.…”

“…[38][39][40][41] Additionally, FLC with yielding elements, placed between the spine and MRF, control the magnitude of the forces transferred between the spine and MRF, [42][43][44][45] thereby limiting highermode accelerations and force demands that tend to develop in systems with elastic spines. 46 Due to the presence of the spine and FLC, designs of these types of enhanced lateral-force resisting systems can be highly affected by estimates of story drifts and floor accelerations, which are also affected by modeling uncertainty. 20 In including modeling uncertainty, this study identified the model features of greatest influence to the response of the test specimens and illustrates the variability in drifts and acceleration demands for different sources of modeling uncertainty, particularly for the spine-toframe connection.…”

“…In these systems, vertical elastic spines impose a more uniform drift distribution over the building height to mitigate story mechanisms 38–41 . Additionally, FLC with yielding elements, placed between the spine and MRF, control the magnitude of the forces transferred between the spine and MRF, 42–45 thereby limiting higher‐mode accelerations and force demands that tend to develop in systems with elastic spines 46 . Due to the presence of the spine and FLC, designs of these types of enhanced lateral‐force resisting systems can be highly affected by estimates of story drifts and floor accelerations, which are also affected by modeling uncertainty 20 …”