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Phase 2B used the same two-story configuration as 2A, but added weight at both the second floor and roof to increase diaphragm inertial forces. Testing again used scaled Northridge motions (Beverly Hills-14145 Mulhol.) at 50%, 100%, and 125% DE levels, including sign-reversed and repeat motions, with white-noise and impulse tests used to track system properties. In this phase, both the roof and second-floor collectors developed substantial axial forces in addition to flexural demands from story drift, allowing evaluation of collector-to-column connections under combined axial–flexural loading The Phase 2B specimen had the same structural configuration and collector detailing as Phase 2A, but added mass was installed at both the second floor and roof. This produced diaphragm inertial loading at both levels, leading to combined axial and flexural demands in the collectors and their connections. The Phase 2B specimen allowed direct comparison with Phase 2A to study the influence of mass distribution on collector forces.
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Phase 2A: Two-Story Building With Added Mass at the 2nd Floor Only, in Advancing Knowledge on the Performance of Seismic Collectors in Steel Building Structures: Shake Table Tests
Phase 2A used a two-story steel test building with a composite-slab second floor and a bare-steel roof deck. Added weight was applied only at the second floor. Conventional earthquake simulation was used with scaled ground motions from the 1994 Northridge Earthquake (Beverly Hills-14145 Mulhol.) at 50%, 100%, and 200% Design Earthquake (DE) levels, including sign-reversed motions. White-noise and impulse tests were used to identify dynamic properties. In this phase, the second-floor collectors experienced significant axial forces from diaphragm inertial loading, while the roof collectors were mainly subjected to flexural demands due to negligible roof mass. The Phase 2A specimen was a two-story steel building constructed by adding a second story onto the existing Phase 1 test building. It had a composite-slab second floor, a bare-steel roof deck, and perimeter collectors at both levels. Added mass was installed only at the second floor to generate diaphragm inertial forces during the earthquake-simulation tests. This configuration allowed evaluation of collector behavior when significant axial force developed primarily in the second-floor collectors, while the roof collectors experienced mainly flexural demand associated with story drift.
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- Award ID(s):
- 1662816
- PAR ID:
- 10661875
- Publisher / Repository:
- Designsafe-CI
- Date Published:
- Edition / Version:
- 1
- Subject(s) / Keyword(s):
- Collectors seismic collectors collector connections floor diaphragms composite slab steel buildings shake table testing floor acceleration simulation inertial force NHERI@UC San Diego Large High Performance Outdoor Shake Table (LHPOST)
- Format(s):
- Medium: X
- Institution:
- UCSD, University of Arizona
- Sponsoring Org:
- National Science Foundation
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Phase 3 incorporated Buckling-Restrained Braced Frames (BRBFs) into the two-story test building to evaluate collector behavior in the presence of a yielding seismic-force-resisting system. Conventional earthquake simulation was used with scaled ground motions from the 1994 Northridge Earthquake (Beverly Hills-14145 Mulhol.) at 50%, 100%, 150%, and 200% Design Earthquake (DE) levels, including sign-reversed motions. White-noise and impulse tests were used to identify and track dynamic properties. This phase enabled assessment of collector axial force, slab participation, and connection rotation under system-level interaction with brace yielding and load redistribution. For Phase 3, Buckling-Restrained Braced Frames (BRBFs) were added to the same two-story building used in Phases 2. The diaphragm, collector, and connection details remained the same. This specimen was used to evaluate collector behavior in a yielding structural system, including the interaction between diaphragm inertial forces, brace yielding, and load redistribution. Earthquake events consisted of acceleration time histories based on the 1994 Northridge Earthquake record (Beverly Hills-14145 Mulhol.), scaled to different Design Earthquake (DE) intensity levels. Motions were applied in both direct and sign-reversed directions. These events were used to evaluate collector forces, slab participation, inter-story drift, and connection behavior under increasing levels of seismic demand.more » « less
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Phase 1 used a single-story steel test building with a composite slab and perimeter collectors to develop and validate a Floor Acceleration Simulation Testing (FAST) methodology intended to reproduce multistory floor accelerations in a single-story test frame. White-noise and impulse tests were used to identify dynamic properties, followed by earthquake simulation tests at 20%, 50%, and 100% Design Earthquake (DE) levels to observe collector axial force, slab participation, and connection rotation. White-noise tests: White-noise excitation was applied at low amplitude to identify the natural frequencies, damping ratios, and stiffness characteristics of the structure. These tests were typically conducted before and after earthquake events to track changes in dynamic properties as damage accumulated. Impulse tests: Single-pulse excitation was applied through the shake table to evaluate the transient dynamic characteristics of the structure and to supplement the system-identification testing performed using white-noise input. Floor Acceleration Simulation Testing (FAST): In FAST, the objective was to reproduce realistic multistory floor acceleration demands in a single-story test building. Target floor-acceleration histories were obtained from nonlinear response-history analyses of a 12-story BRBF prototype building (SDII). A transfer-function approach in the frequency domain was then used to compute the shake-table input motion required for the single-story specimen to generate these target accelerations. This approach allowed the specimen to respond essentially elastically while reproducing the amplitude and frequency content of multistory floor accelerations. Earthquake simulation tests: Earthquake events consisted of acceleration time histories based on the 1994 Northridge Earthquake record (Beverly Hills-14145 Mulhol.), scaled to different Design Earthquake (DE) intensity levels. Motions were applied in both direct and sign-reversed directions. These events were used to evaluate collector forces, slab partimore » « less
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{"Abstract":["To safely survive an earthquake, and thereby protect its occupants, contents, adjacent property, and passersby, a building structure must transfer the large forces that develop during the earthquake from within the building down to the foundation. Earthquake (lateral) forces are generated by the building weight being accelerated horizontally, and thus most earthquake forces originate in the building's heaviest element, i.e., its floors. A key structural element in the force transfer path to the foundation are collectors, which are either special reinforcement in the floor slab or special beams below the slab, that "collect" the forces in the floor, and transfer them to the primary seismic force-resisting vertical elements (frames, braces, or walls). The loss of collectors or collector connections can be catastrophic, as evidenced by the collapse of the CTV building in the 2011 Christchurch, New Zealand earthquake, which killed 115 people, the largest loss of life in this event, and to some extent the collapse of nine parking garages in the 1994 Northridge, California earthquake. Despite the critical nature of seismic collectors, no research effort, including physical testing, has focused specifically on collectors, and knowledge of their seismic performance is lacking. A challenge in understanding the performance of seismic collectors is the complex nature of the floor system itself, a complicated assemblage of many components of different materials (e.g., steel, metal, and concrete) at different elevations, with multiple purposes and uncertain force paths. Past seismic design methodologies for buildings may have significantly underestimated the collector forces. This lack of knowledge impacts not only new construction but also the assessment and retrofit of existing, especially critical care, facilities in high seismic regions. This condition also applies to older non-seismic compliant steel structures nationwide, where inadequate or non-existent seismic collectors are often a major concern. A better understanding of the performance of steel seismic collectors is needed for safe and economical structures, both in the existing building stock and for new construction. Further, the collector's unique role as the critical link between the floor and the vertical elements provides an opportunity for collectors from trying to "out-strength" the earthquake force to instead serve as an innovative force-limiting element that protects the structure from damage. The goals of this research are to: (1) advance knowledge on the seismic performance, analysis, and design of collectors in steel composite floor systems, and (2) develop new knowledge on the reliable seismic performance and potential benefits of innovative collector concepts that can lead to low-damage structural design. This project will support researchers and graduate students from the University of Arizona, University of California, San Diego, and Lehigh University. The project will benefit from working closely with collaborators who are separately supported, i.e., a researcher and a practitioner in New Zealand and an industry panel of seismic design engineers in the United States. An outreach program will be conducted by the University of Arizona with local K-8 schools identified demographically as possessing student bodies of predominately underrepresented groups. The outreach program will target third, fourth, and eighth grade students to include: (1) slides shows and question and answer sessions on earthquake engineering, (2) career mentoring from graduate and undergraduate students, and (3) hands-on science and math activities.\n\nIn this project, an integrated research program will investigate the performance of seismic collectors for steel composite deck structures using the experimental and computational simulation capabilities afforded by the NSF-supported Natural Hazards Engineering Research Infrastructure (NHERI). The research will involve: (1) large-scale testing of collector elements in a steel composite floor system at the NHERI experimental facility at Lehigh University, (2) shake table testing of a 0.4-scale, single-story, steel composite floor system at the NHERI shake table facility at the University of California, San Diego, and (3) nonlinear analysis of steel structure collector elements, details and surrounding regions under seismic effects, and earthquake simulations of steel buildings under strong earthquakes. The planned experiments on steel collectors, with realistic boundary conditions and inertial forces, will be the first of its kind. New data products and calibrated numerical models will be produced from large-scale physical testing. Analytical models will be developed for the collectors and the collector inertial force paths. Transfer of research results into practice will include: (1) new concepts for low-damage structural design, (2) research-based design recommendations, and (3) assessment and retrofit guidelines."]}more » « less
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ABSTRACT This study integrates analytical and experimental research to develop an innovative shake table testing method called Floor Acceleration Simulation Test (FAST). The primary objective of FAST is to produce an essentially elastic response of a single‐story test specimen to replicate the floor acceleration time history including higher‐mode effects of a target floor in a multistory building experiencing inelastic behavior during an earthquake. The FAST method is well suited for experimental research where the absolute accelerations and the associated inertial forces of the floor diaphragms cannot be simulated by the majority of the conventional test methods. The proposed methodology is based on a transfer function in the frequency domain to compute the required input motion for testing. Considering the physical constraints of a given shake table test facility, guidelines with two response spectra to bracket the natural frequency of the test building are also presented for practical implementation. Experimental validation was carried out on a half‐scale, single‐story steel building featuring a composite floor slab, utilizing the NHERI@UCSD Large High‐Performance Outdoor Shake Table (LHPOST) facility. The results demonstrate the effectiveness of FAST, as both analytical predictions and experimental outcomes confirm its validity. Despite instances of measured floor acceleration amplitude exceeding the target response due to table input motion overshooting in this test program, test results confirmed that the FAST accurately reproduced the intended frequency content, indicative of higher mode effects in the multistory prototype building, in the single‐story test building.more » « less
