An official website of the United States government
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
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.
more »
« less
- Award ID(s):
- 1926365
- 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
- Location:
- Sendai, Japan
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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
-
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
-
A new friction device using band brake technology, termed the Banded Rotary Friction Damper (BRFD), has been fabricated at the NHERI Lehigh Experimental Facility. The damping mechanism is based on band brake technology and leverages a self-energizing mechanism to produce large damping forces with low input energy. The device is a second-generation BRFD, where the friction mechanism is achieved using two electric actuators. The BRFD generates a damping force as a function of the input force provided by the electric actuators, where the ratio of BRFD force output-to-electric actuator force input is equal to about 112. The paper presents the results of a study using real-time hybrid simulations (RTHS) to investigate the performance of the BRFD’s in mitigating seismic hazards of a two-story reinforced concrete building. The building has two and three special moment resisting frames (SMRFs) in the east-west and north-south directions, respectively. In order to perform the RTHS, the north south SMRF is considered and the BRFD along with a parallel elastic member is used as a base isolation system to mitigate the effects of earthquake hazards by reducing story drift and floor accelerations of the structure. For the RTHS the building and the elastic component of the isolator are part of the analytical substructure while the experimental substructure is comprised of the BRFD. The response of the structure is investigated involving six Maximum Considered Earthquake (MCE) hazard level events that includes three near-field and three far-field ground motions. The explicit, unconditionally stable dissipative Modified KR-α integration algorithm is used to accurately integrate the equations of motion during the RTHS. The model for the reinforced concrete building is created using explicit non-linear force-based fiber elements to discretely model each member of the structure. First, the details of the prototype of the BRFD are presented. Second, the details of the isolator system consisting of a linear spring element and the BRFD are discussed. Finally, the details of the RTHS study and the results are presented. The building’s inter-story peak and residual story drift from base-isolated and fixed-based conditions are compared. Results show that the proposed isolator system produces a significant reduction in both maximum inter-story drift and residual drift, and reduces the damage developed in the structure during the MCE.more » « less
-
null (Ed.)Nonlinear time history analyses were conducted for 5-story and 12-story prototype buildings that used post-tensioned cross-laminated timber rocking walls coupled with U-shaped flexural plates (UFPs) as the lateral force resisting system. The building models were subjected to 22 far-field and 28 near-fault ground motions, with and without directivity effects, scaled to the design earthquake and maximum considered earthquake for Seattle, with ASCE Site Class D. The buildings were designed to performance objectives that limited structural damage to crushing at the wall toes and nonlinear deformation in the UFPs, while ensuring code-based interstory drift requirements were satisfied and the post-tensioned rods remained linear. The walls of the 12-story building had a second rocking joint at midheight to reduce flexural demands in the lower stories and interstory drift in the upper stories. The interstory drift, in-plane wall shear and overturning moment, UFP deformation, and extent of wall toe crushing is summarized for each building. Near-fault ground motions with directivity effects resulted in the largest demands for the 5-story building, while the midheight rocking joint diminished the influence of ground motion directivity effects in the 12-story building. Results for both buildings confirmed that UFPs located higher from the base of the walls dissipated more energy compared to UFPs closer to the base.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- The DOI is not currently available.
