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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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Design and Performance Comparison of Strongback Systems and Typical Chevron BRB Frames
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.
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- Award ID(s):
- 2309829
- PAR ID:
- 10632030
- Publisher / Repository:
- 12th National Conference on Earthquake Engineering
- Date Published:
- Format(s):
- Medium: X
- Location:
- Salt Lake City, UT
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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