An official website of the United States government
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
Proportioning energy-dissipating members in strongback braced frames
Many structural systems are susceptible to soft-story instabilities during earthquakes that are lifethreatening and can lead to damage that is too costly to repair. One way to mitigate damage and reduce the potential for soft-story instability is through the addition of an elastic spine that distributes drifts across the height of a structure. One such system is the strongback braced frame, which replaces one side of a buckling-restrained braced frame with a strongback truss. With the strongback providing vertical continuity, an expanded design space is made available for the arrangement of buckling-restrained braces (BRBs) or other energy-dissipating members. An example of this expanded design space is that a designer could opt to not include BRBs at every story. Methods for proportioning the energy-dissipating resistance in strongback braced frames have been proposed. However, most methods don't allow exploitation of the full design space. The objective of this work is to propose and evaluate a potential method of proportioning energy-dissipating members for arbitrary vertical arrangements within strongback braced frames. For a prototypical building, the BRBs are designed in various configurations using existing methods and with the new method. Nonlinear time history analyses of the resulting designs coupled with a rigid strongback are performed and the results are compared. The impacts of overstrength and P-Δ effects are quantified. The findings support the proposed method of BRB design that enables exploration of the wide design space made available by the strongback.
more »
« less
- Award ID(s):
- 1940197
- PAR ID:
- 10407076
- Date Published:
- Journal Name:
- Proceedings of the Annual Stability Conference Structural Stability Research Council
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Mass timber panels are emerging as an innovative alternative for the design of elastic spines due to their high stiffness- and strength-to-weight ratio, among other factors. Recent research has shown that mass timber panels used in conjunction with steel energy dissipators are promising solutions for enhanced seismic performance. However, the available experimental data at the building scale is still minimal, which limits the understanding, adoption, and development of effective seismic design guidelines for these systems. This research addresses this gap through full-scale quasi-static cyclic testing of a three-story mass timber building. Lateral loads are transferred through Mass Ply Panel (MPP) diaphragms to an MPP spine with vertically-oriented unbonded steel buckling-restrained braces (BRBs) as energy dissipating boundary elements in the first story. The only elements designed to dissipate energy in the inelastic range are the BRBs. The building specimen achieved low-structural damage and enhanced-performance goals, being able to reach a 4% roof drift ratio with little loss of strength and stiffness. The proposed pivoting detail was effective in mitigating compressive damage at the wall toe. To support the experimental campaign and future design procedures, a high-fidelity numerical model of the building was developed using OpenSees.more » « less
-
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.more » « less
-
The Natural Hazards Engineering Research Infrastructure (NHERI) Converging Design project is a collaborative effort between multiple universities and industry entities with the goal of creating a new design paradigm in structural engineering that employs multi-objective optimization to maximize functional recovery while integrating sustainability principles in the design process. The structural design approaches were validated through full-scale shake table testing of a 6-story mass timber structure at the at the Englekirk Structural Engineering Center at University of California, San Diego (NHERI@UCSD) Large High-Performance Outdoor Shake Table (LHPOST6) facility for eventual inclusion in a multi-objective design optimization framework. The shake table testing included three phases. Phase one consisted of a mass timber self-centering rocking wall (SCRW) system with U-shaped flexural plates (UFPs) in both building horizontal directions. Phase two replaced the SCRWs in one principal direction with SCRWs with buckling restrained boundary elements (BRBs) at the first story. Phase three replaced the newer walls from phase two with a resilient steel moment frame and concentric braced-frame (MF/CBF). The data shared includes reports summarizing the testing program, structural drawings, instrumentation setups, and raw data for the series of shake table tests performed during each phase. The data include building responses due to shake table motions simulating scaled historical ground motions and white noise (WN) tests.more » « less
-
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
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- The DOI is not currently available.
