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1. Frame-Spine System with Force-Limiting Connections for Low-Damage Seismic-Resilient Buildings

   https://doi.org/10.1007/978-3-031-03811-2_88  

   Fahnestock, L. ( May 2022 , 10th International Conference on Behavior of Steel Structures in Seismic Areas)

   Abstract. A novel structural system is being investigated collaboratively – by an international team including three U.S. universities, two Japanese universities and two major experimental research labs – as a means to protect essential facilities, such as hospitals, where damage to the building and its contents and occupant injuries must be prevented and where continuity of operation is imperative during large earthquakes. The new system employs practical structural components, including (1) flexible steel moment frames, (2) stiff steel elastic spines and (3) force-limiting connections (FLC) that connect the frames to the spines, to economically control building response and prevent damaging levels of displacement and acceleration. The moment frames serve as the economical primary element of the system to resist a significant proportion of the lateral load, dissipate energy through controlled nonlinear response and provide persistent positive lateral stiffness. The spines distribute response evenly over the height of the building and prevent story mechanisms, and the FLCs reduce higher-mode effects and provide supplemental energy dissipation. The Frame- Spine-FLC System development is focusing on new construction, but it also has potential for use in seismic retrofit of deficient existing buildings. This paper provides an overview of the ongoing research project, including selected FLC cyclic test results and a description of the full-scale shake-table testing of a building with the Frame-Spine-FLC System, which represents a hospital facility and includes realistic nonstructural components and medical equipment. 

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2. Modeling uncertainty of specimens employing spines and force‐limiting connections tested at E‐defense shake table

   https://doi.org/10.1002/eqe.3976  

   Astudillo, Bryam ; Rivera, David ; Duke, Jessica ; Simpson, Barbara ; Fahnestock, Larry A. ; Sause, Richard ; Ricles, James ; Kurata, Masahiro ; Okazaki, Taichiro ; Kawamata, Yohsuke ; et al ( July 2023 , Earthquake Engineering & Structural Dynamics)

   In light of the significant damage observed after earthquakes in Japan and New Zealand, enhanced performing seismic force‐resisting systems and energy dissipation devices are increasingly being utilized in buildings. Numerical models are needed to estimate the seismic response of these systems for seismic design or assessment. While there have been studies on modeling uncertainty, selecting the model features most important to response can remain ambiguous, especially if the structure employs less well‐established lateral force‐resisting systems and components. Herein, a global sensitivity analysis was used to address modeling uncertainty in specimens with elastic spines and force‐limiting connections (FLCs) physically tested at full‐scale at the E‐Defense shake table in Japan. Modeling uncertainty was addressed for both model class and model parameter uncertainty by varying primary models to develop several secondary models according to pre‐established uncertainty groups. Numerical estimates of peak story drift ratio and floor acceleration were compared to the results from the experimental testing program using confidence intervals and root‐mean‐square error. Metrics such as the coefficient of variation, variance, linear Pearson correlation coefficient, and Sobol index were used to gain intuition about each model feature's contribution to the dispersion in estimates of the engineering demands. Peak floor acceleration was found to be more sensitive to modeling uncertainty compared to story drift ratio. Assumptions for the spine‐to‐frame connection significantly impacted estimates of peak floor accelerations, which could influence future design methods for spines and FLC in enhanced lateral‐force resisting systems.

    

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3. Design and Assessment of Innovative Dual-Mode Rolling Isolation Systems

   https://doi.org/11244/321054  

   Hadikhan Tehrani, Mohammad ( August 2019 , The University of Oklahoma Libraries)

   null (Ed.)

   Loss of operation or devastating damage to buildings and industrial structures, as well as equipment housed in them, has been observed due to earthquake-induced vibrations. A common source of operational downtime is due to the performance reduction of vital equipment, which are sensitive to the total transmitted acceleration. A well-known method of protecting such equipment is seismic isolation of the equipment itself (or a group of equipment), as opposed to the entire structure due to the lower cost of implementation. The first objective of this dissertation is assessing a rolling isolation system (RIS) based on existing design guidelines for telecommunications equipment. A discrepancy is observed between the required response spectrum (RRS) and the one and only accelerogram recommended in the guideline. Several filters are developed to generate synthetic accelerograms that are compatible with the RRS. The generated accelerograms are used for probabilistic assessment of a RIS that is acceptable per the guideline. This assessment reveals large failure probability due to displacement demands in excess of the displacement capacity of the RIS. When the displacement demands on an isolation system are in excess of its capacity, impacts result in spikes in transmitted acceleration. Therefore, the second objective of this dissertation is to design impact prevention/mitigation mechanisms. A dual-mode system is proposed where the behavior changes when the displacement exceeds a predefined threshold. A new piecewise optimal control approach is developed and applied to find the best possible mechanism for the region beyond the threshold. By utilizing the designed curves obtained from the proposed optimal control procedure, a Kelvin-Voigt device is tuned for illustrative purposes. On the other hand, the preference for protecting equipment decreases as the earthquake intensity increases. In extreme seismic loading, the response mitigation of the primary structure (i.e., life safety and collapse prevention) is of greater concern than protecting isolated equipment. Therefore, the third objective of this dissertation is to develop an innovative dual-mode system that can behave as equipment isolation under low to moderate seismic loading and passively transition to behave as a vibration absorber for the primary structure under extreme seismic loading. To reduce the computational cost of simulating a large linear elastic structure with nonlinear attachments (i.e., equipment isolation with cubic hardening nonlinearity), a reduced order modeling method is introduced that can capture the behavior of such nonlinear coupled systems. The method is applied to study the feasibility of dual-mode vibration isolation/absorber. To this end, nonlinear transmissibility curves for the roof displacement and isolated mass total acceleration are developed from the steady-state responses of dual-mode systems using the harmonic balanced method. The final objective of this dissertation is to extend the reduced order modeling method developed for linear elastic structure with nonlinear attachment to inelastic structures (without attachments). The new inelastic model condensation (IMC) method uses the modal properties of the full structural model (in the elastic range) to construct a linear reduced order model in conjunction with a hysteresis model to capture the hysteretic inter-story restoring forces. The parameters of these hysteretic forces are easily tuned, in order to fit the inelastic behavior of the condensed structure to that of the full model under a variety of simple loading scenarios. The fidelity of structural models condensed in this way is demonstrated via simulation for different ground motion intensities on three different building structures with various heights. The simplicity, accuracy, and efficiency of this approach could significantly alleviate the computational burden of performance-based earthquake engineering. 

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4. Evaluation of Rolling-Type Isolation Systems for Seismic Hazard Mitigation

   https://doi.org/11244/300332  

   Calhoun, Skylar Josef ( August 2018 , The University of Oklahoma Libraries)

   null (Ed.)

   Nonstructural components within mission-critical facilities such as hospitals and telecommunication facilities are vital to a community's resilience when subjected to a seismic event. Building contents like medical and computer equipment are critical for the response and recovery process following an earthquake. A solution to protecting these systems from seismic hazards is base isolation. Base isolation systems are designed to decouple an entire building structure from destructive ground motions. For other buildings not fitted with base isolation, a practical and economical solution to protect vital building contents from earthquake-induced floor motion is to isolate individual equipment using, for example, rolling-type isolation systems (RISs). RISs are a relatively new innovation for protecting equipment. These systems function as a pendulum-like mechanism to convert horizontal motion into vertical motion. An accompanying change in potential energy creates a restoring force related to the slope of the rolling surface. This study seeks to evaluate the seismic hazard mitigation performance of RISs, as well as propose and test a novel double RIS. A physics-based mathematical model was developed for a single RIS via Lagrange's equation adhering to the kinetic constraint of rolling without slipping. The mathematical model for the single RIS was used to predict the response and characteristics of these systems. A physical model was fabricated with additive manufacturing and tested against multiple earthquakes on a shake table. The system featured a single-degree-of-freedom (SDOF) structure to represent a piece of equipment. The results showed that the RIS effectively reduced accelerations felt by the SDOF compared to a fixed-base SDOF system. The single RIS experienced the most substantial accelerations from the Mendocino record, which contains low-frequency content in the range of the RIS's natural period (1-2 seconds). Earthquakes with these long-period components have the potential to cause impacts within the isolation bearing that would degrade its performance. To accommodate large displacements, a double RIS is proposed. The double RIS has twice the displacement capacity of a single RIS without increasing the size of the bearing components. The mathematical model for the single RIS was extended to the double RIS following a similar procedure. Two approaches were used to evaluate the double RIS's performance: stochastic and deterministic. The stochastic response of the double RIS under stationary white noise excitation was evaluated for relevant system parameters, namely mass ratio and tuning frequency. Both broadband and filtered (Kanai-Tajimi) white noise excitation were considered. The response variances of the double RIS were normalized by a baseline single RIS for a comparative study, from which design parameter maps were drawn. A deterministic analysis was conducted to further evaluate the double RIS in the case of nonstationary excitation. The telecommunication equipment qualification waveform, VERTEQ-II, was used for these numerical simulations. Peak transient responses were compared to the single RIS responses, and optimal design regions were determined. General design guidelines based on the stochastic and deterministic analyses are given. The results aim to provide a framework usable in the preliminary design stage of a double RIS to mitigate seismic responses. 

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5. Incremental dynamic analysis and FEMA P695 seismic performance evaluation of a cold‐formed steel–framed building with gravity framing and architectural sheathing

   https://doi.org/10.1002/eqe.3245  

   Leng, Jiazhen ; Buonopane, Stephen G. ; Schafer, Benjamin W. ( January 2020 , Earthquake Engineering & Structural Dynamics)

   The objective of this paper is to present incremental dynamic analysis (IDA) and seismic performance evaluation results for a two‐story cold‐formed steel (CFS)–framed building. The archetype building was designed to current U.S. standards and then subjected to full‐scale shake table tests under the U.S. National Science Foundation Network for Earthquake Engineering Simulation (NEES) program. Test results showed that the building's stiffness and capacity were considerably higher than expected and the building suffered only nonstructural damage even at excitations in excess of Maximum Considered Earthquake levels for a high seismic zone. For the archetype building, three‐dimensional finite element models at different modeling fidelity levels were created using OpenSees. The models are subjected to IDA using the far‐field ground motion records prescribed in Federal Emergency Management Agency (FEMA) P695. Seismic performance quantification following the FEMA P695 procedure shows that if the modeling fidelity only follows the state‐of‐the‐practice, ie, only includes shear walls, unsafe collapse margin ratios are predicted. State‐of‐the‐art models that account for participation from CFS gravity walls and architectural sheathing have overall performance that are consistent with testing, and IDA results indicate acceptable collapse margin ratios, predicated primarily on large system overstrength. Neglecting the lateral force resistance of the gravity system and nonstructural components, as done in current design, renders a safe design in the studied archetype, but largely divorced from actual system behavior. The modeling protocols established here provide a means to analyze a future suite of CFS‐framed archetype buildings for developing further insight on the seismic response modification coefficients for CFS‐framed buildings.

    

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