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Advancements in seismic hazard mitigation and resilience have spurred the exploration and development of lateral-force resisting systems with enhanced performance goals. In this context, brace frames coupled with elastic spines offer a more uniform drift distribution with building height that reduces the concentration of damage in a few stories and therefore reduces the likelihood of a story mechanism. However, properly sizing the spine, along with selecting and placing the energy dissipators, remains challenging and lacks standardized procedures due to 1) the kinematic and force compatibility between the pivoting spine and the energy dissipators and 2) the near-elastic higher-modes demands that are present in this type of system; both closely governed by the selection of the spine strength and stiffness. This study investigates the nonlinear static response of 8-story Strongback Braced Frames (i.e., brace frames where the elastic spine is composed of a pivoting truss called strongback) using a first and second-mode force pattern. The archetypes have different variations in the implementation of the strongback; e.g., keeping the strongback in a separate bay from the main brace frame or embedding the strongback into the frame. In pushing the archetypes with a first-mode force pattern, the strongback imposes more uniform drifts, without a significant increase in the system’s ultimate capacity. In pushing with a second-mode force pattern, the strongback delays the formation of a mechanism and increases the system’s ultimate capacity to values comparable to expected elastic demands under MCE intensities. The distribution of story shears between the main frame and the strongback is assessed, and the implications of higher-mode demands in sizing the brace and ties of the strongback are highlighted. It is expected that results from this paper may begin to inform future code-based provisions in adopting the use of near-elastic spines in enhanced performance systems.
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Preliminary Numerical Analysis of a Strongback Column as a retrofit of a Moment-Resisting Frame
Steel moment-resisting frames (MRFs) are widely used in the United States to resist seismic forces. MRFs have many advantages, including high ductility, architectural versatility, and vetted member and connection detailing requirements. However, MRFs require large members to meet story drift criteria. Moreover, strong-column-weak-beam requirements can result in significant member sizes, and – even in the cases where strong-column-weak-beam requirements are satisfied – MRFs can still be vulnerable to story mechanisms in one or a few stories. Recently, the concept of a strongback has been utilized successfully to delay or prevent story mechanism behavior in braced frames. The strongback is represented by a steel truss or column that is designed to remain essentially elastic, thus allowing the system to transfer inelastic demands across stories. Although systems including strongbacks exhibit more uniform story drift demands with building height and reduced peak drift response, the elastic nature of the strongback can also result in near-elastic higher-mode force demands. This study compares the dynamic response of a baseline MRF to that of a retrofit using a strongback column. Several ground motions are considered to determine which cases produce the largest drift, acceleration, and force demands.
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- PAR ID:
- 10344908
- Publisher / Repository:
- 17th World Conference on Earthquake Engineering (17WCEE)
- Date Published:
- Journal Name:
- 17th World Conference on Earthquake Engineering, 17WCEE
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Structural engineering is moving towards the design of enhanced performing buildings under earthquake events to improve the resiliency of urban communities. Buckling Restrained Braced Frames (BRBF) have been widely adopted to resist lateral loads. However, typical configurations could be subjected to drift concentration, leading to large story drifts and uneven utilization of the BRBs with building height. Studies have suggested that innovative configurations, such as pivoting or rocking frames, can provide a better distribution of the story drift by delaying or preventing story mechanisms and spreading the energy dissipation to adjacent stories across the building height. These types of bracing configurations utilize as essentially elastic spine, or strongback, to induce a global tilting mode. However, since the spine is designed to remain elastic, additional design considerations are needed to size the elements in strongbacks. This study presents a comparative study between traditional chevron BRBF and strongback BRBF systems for a set of buildings with different heights and tributary areas. Results show that the pivoting and rocking strongback result in reduced the peak story drift with more uniform distribution of drift demands. The cost of these alternatives, per frame, was similar to the chevron BRBF.more » « less
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Even with strong-column-weak-beam design requirements, story mechanisms have been observed in Moment Resisting Frames (MRF), resulting in concentrated drift demands that can result in severe structural damage to drift-sensitive components. Frame-Spine systems can redistribute demands with building height, but near-elastic higher-mode effects tend to contribute to floor accelerations, affecting damage to acceleration-sensitive nonstructural components. To mitigate this tradeoff, Force-Limiting Connections (FLCs) have been proposed to reduce accelerations through yielding components between the Frame and Spine, thereby limiting the magnitude of the forces. This study examines the sizing and placement of FLCs in a four-story Frame-Spine system using stochastic simulations. The T-shape yielding element dimensions in the FLC were modeled as random variables at each floor, and Monte Carlo simulations were used to explore their effect on drifts and accelerations. Results show the dominant role of the first-story FLC on balancing drifts and accelerations, while upper-story devices offered limited benefit. Design recommendations are provided to constrain first-story yielding element dimensions within effective bounds that reduce peak accelerations relative to the baseline Frame-Spine configuration.more » « less
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Due to their ability to provide more uniform story drifts with building height, strongback braced frames (SBFs) have been proposed as enhanced-performance structural systems. Conventional design approaches, however, tend to underestimate force demands in strongback elements compared to nonlinear response history analysis (NRHA). To address this gap, alternative methods such as modal pushover analysis (MPA) have been suggested to obtain less computationally intensive estimates of seismic demands. This study presents a statistical assessment of MPA as an estimate of NRHA force demands for an 8-story SBF subjected to 44 far-field ground motion records scaled at the risk-adjusted maximum considered earthquake (MCE𝑟 ) intensity level. Unlike prior studies that compare MPA results to the statistics of the NRHA response, this work treats each ground motion as a separate test to characterize how MPA accounts for record-specific spectral characteristics. Accuracy in estimates of the force demands (i.e., how close the MPA estimates are to the NRHA “truth”) is characterized using root mean square error. Additional comparisons are made across the MPA parameters, such as the number of modes employed, as well as the use of initial versus elongated periods. Results provide a comprehensive statistical assessment of MPA, illustrating that the approach can be sensitive to spectral assumptions and is better suited to aggregated estimates from NRHA.more » « less
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Conventional lateral force-resisting systems can provide a stable, ductile response but also experience significant inelastic demands, rendering repairs impractical or uneconomical. Thus, there is a need for novel structural systems that protect structural and nonstructural components to reduce post-earthquake repairs and downtime. A U.S.-Japan research team – including three U.S. universities, two Japanese universities, and two major experimental research labs – is developing a structural solution to reduce peak drift and acceleration demands, thereby protecting buildings, their contents, and occupants during major earthquakes. The primary components of the system are: (1) steel base moment-resisting frames designed and detailed to behave in the inelastic range and dissipate energy, (2) stiff and strong elastic spines designed to remain essentially elastic to redistribute seismic demands more uniformly over the building height, and (3) force-limiting connections (FLC) that connect the frame to the spines to provide a yielding mechanism that limits acceleration demands. This economical earthquake-resilient system is intended to be used in essential facilities, such as hospitals, where damage to the buildings and contents and occupant injuries must be prevented and where continuity of operation is imperative. The system was recently tested at full scale at the E-Defense shake-table facility in Miki, Japan. This paper provides an overview of pre-test numerical simulations, shake-table test setup and instrumentation, and preliminary test results.more » « less
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