An SMA cable-based negative stiffness seismic isolator: Development, experimental characterization, and numerical modeling

Shape memory alloy (SMA)-based seismic isolation systems can successfully reduce the peak and residual displacements of bridges during strong earthquake, but they commonly lead to an increased force demands in substructure. This study explores the development of an SMA cable-based negative stiffness isolator to alleviate this problem. The proposed isolator is composed of superelastic SMA cables and a frictional sliding bearing with convex surfaces. The frictional sliding bearing limit the forces transferred to the superstructure and provides energy dissipation, while its built-in negative stiffness mechanism reduces the force demands in substructure. SMA cables provide critical restoring forces, additional energy dissipation, and displacement-limiting capacity. Based on the force balance, the negative stiffness and restoring requirements of the SMA cable-based negative stiffness isolator were analyzed first. Then, a prototype large-scale isolator was designed and fabricated. Next, the experimental testing of the developed isolator was performed under two different vertical load levels. Finally, finite element modeling of the proposed isolator was conducted, and the simulation results and experimental results were compared and discussed. The proposed isolator generates lower forces than the SMA-based zero and positive stiffness isolators and can exhibit stable energy dissipation capabilities with very good displacement-limiting and self-centering capabilities.

Polishing Performance of a Recycled Grinding Wheel Using Grinding Wheel Scraps for the Wet Polishing of Stainless-Steel Sheets

The surfaces of large austenitic stainless-steel sheets, which have side lengths of at least 1 m a sheet thickness of at least 6 mm, used for food tanks and sliding plates in seismic isolation devices, must be finished to a mirror surface. Polishing is performed to improve the surface quality of such sheets and dry machining is typically applied. The problems associated with dry machining are the exhaust heat of machining and treatment of chips. A transition to wet machining is required to solve these problems. In our laboratory, we have developed a wet polishing machine and researched the selection of grinding wheels to develop wet polishing technology for large stainless-steel sheets. In this study, to reduce tool cost and reuse resources, we attempted to manufacture a recycled grinding wheel using snippets of grinding wheel scraps. A polyvinyl alcohol (PVA) aqueous solution was used as the bonding agent for the recycled grinding wheel to reduce environmental load. To overcome the ease of dissolution of PVA in water, we attempted to improve the water resistance of the PVA aqueous solution by incorporating an organic titanium compound. This is one of our efforts to contribute to sustainable development goals. The results are summarized below. (1) A recycled grinding wheel was fabricated by kneading crushed pieces of grinding wheel scrap with a bonding agent. (2) The maintenance of the shape of the recycled grinding wheel was controlled by the concentration of the bonding agent. (3) The recycled grinding wheel with a PVA bonding agent was vulnerable to water. In contrast, the recycled grinding wheel to which the organic titanium compound was added exhibited improved water resistance. (4) The polishing of stainless-steel sheets using the plain PVA recycled wheel was relatively ineffective, but polishing using the recycled wheel with the titanium additive was comparable to polishing with a new grinding wheel.

Modelling lead-rubber seismic isolation bearings using the unified mechanics theory

Lead-rubber seismic isolation bearings (LRB) have been installed in a number of essential and critical structures, like hospitals, universities and bridges, in order to provide them with period lengthening and the capacity of dissipating a considerable amount of energy to mitigate the effects of strong ground motions. Therefore, studying the damage mechanics of this kind of devices is fundamental to understand and accurately describe their thermo-mechanical behavior, so that seismically isolated structures can be designed more safely. Hitherto, the hysteretic behavior of LRB has been modeled using 1) Newtonian mechanics and empirical curve fitting degradation functions, or 2) heat conduction theories and idealized bilinear curves which include degradation effects. The reason for using models that are essentially phenomenological or that contain some adjusted parameters is the fact that Newton’s universal laws of motion lack the term to account for degradation and energy loss of a system. In this paper, the Unified Mechanics Theory – which integrates laws of Thermodynamics and Newtonian mechanics – is used to model the force-displacement response of LRB. Indeed, there is no need for curve fitting techniques to describe their damage behavior because degradation is calculated at every point using entropy generation along the Thermodynamics State Index (TSI) axis. A finite element model of a lead-rubber bearing was constructed in ABAQUS, where a user material subroutine UMAT was implemented to define the Unified Mechanics Theory equations and the viscoplastic constitutive model for lead. Finite element analysis results were compared with experimental test data.

Vertical Seismic Isolation Device for Three-Dimensional Seismic Isolation of Nuclear Power Plant Equipment—Case Study

The purpose of this study was to develop a vertical seismic isolation device essential for the three-dimensional seismic isolation design of nuclear power plant equipment. The vertical seismic isolation device in this study has a concept that can be integrally combined with a conventional laminated rubber bearing, a horizontal seismic isolator with a design vertical load of 10 kN. To develop the vertical seismic isolation device, the vertical spring and the seismic energy dissipation device capable of limiting the vertical displacement of the spring were designed and their performances were verified through actual tests. In this study, the target elevation of the floor is 136 ft, where safety-related nuclear equipment, such as cabinet and remote shutdown console, etc., is installed. The sensitivity studies were carried out to investigate the optimal design vertical isolation frequencies for the target building elevation. Based on the results of the sensitivity study, a disc spring and a helical coil spring were selected for the vertical stiffness design, and the steel damper was selected for the seismic energy dissipation, and their performance characteristics were tested to confirm the design performance. For the steel damper, three types were designed and their energy dissipation characteristics by hysteretic behavior were confirmed by the inelastic finite element analyses and the tests in static fully reversed cyclic conditions. Through the study of the vertical seismic isolation device, it was found that 2.5 Hz~3.0 Hz is appropriate for the optimal design vertical isolation. With results of the vertical seismic isolation performance analysis, the appropriate number of steel dampers are proposed to limit the vertical seismic displacement of the spring within the static displacement range by the design vertical load.

A periodic seismic isolation foundation with an extremely broad low-frequency attenuation zone: Theoretical analysis and experimental verification

The attenuation zones (AZs) of periodic structures can be used for seismic isolation design. To cover the dominant frequencies of more seismic waves, this paper proposes a new type of periodic isolation foundation (PIF) with an extremely wide low-frequency AZ of 3.31 Hz–17.01 Hz composed of optimized unit A with a wide AZ and optimized unit B with a low-frequency AZ. The two kinds of optimized units are obtained by topology optimization on the smallest periodic unit with the coupled finite element-genetic algorithm (GA) methodology. The transmission spectra of shear waves and P-waves through the proposed PIF of finite size are calculated, and the results show that the AZ of the PIF is approximately the superposition of the AZs of the two kinds of optimized units. Additionally, shake tests on a scale PIF specimen are performed to verify the attenuation performance for elastic waves within the designed AZs. Furthermore, numerical simulations show that the acceleration responses of the bridge structure with the proposed PIF are attenuated significantly compared to those with a concrete foundation under the action of different seismic waves. Therefore, the newly proposed PIF is a promising option for the reduction of seismic effects in engineering structures.

Design, Manufacturing, and Performance Evaluation of a Novel Smart Roller Bearing Equipped with Shape Memory Alloy Wires

In this study, a new type of seismic isolation device, called SMA wire-based roller bearing (SMA-RB) is developed and introduced. The SMA-RB has been designed, manufactured, and experimentally tested. This bearing consists of cylindrical roller bearings and SMA wires with straight or cross configurations, as supplementary damping elements. In such a smart bearing, the superelastic SMA wires are passed through the hooks/pulleys attached to the supporting plates of the bearings in different configurations. The rollers provide lateral flexibility, and SMA wires supply energy dissipation and self-centering properties. In the manufacturing stage, a new mechanism for coupling wires (i.e. SMA wire joiner/coupler) is proposed. The results show that SMA wires, made of Nickle Titanium (NiTi), provide a self-centered damping mechanism with almost zero residual deformation which can effectively control the device from over-displacement. While using pulleys and newly designed wire joiners in the SMA-RB, the bearing can experience a stable cyclic behavior. Since the rollers generate a negligible amount of frictional force, the superelastic NiTi wires with a flag-shaped hysteresis mainly contribute to the overall shear hysteretic response of the SMA-RB. A triangular-shaped constitutive model can be used to accurately describe the hysteretic behavior of SMA-RB with different wire configurations.

Analysis of Structure Displacement Formed as a Result of Torsion in Case of Using Rubber-Metal Seismic Isolators

The study of a building with a complex planning solution, rubber-metal laminated seismic isolation supports (hereinafter referred to a RMLSIS) placed at the foundation level and with a reinforced concrete frame-braced system is presented, taking into account the displacement of the structure formed as a result of torsion. The analyses were conducted with the finite element method. The calculation schemes were modeled using the "Lira-SAPR" software. The displacement of the structure formed as a result of torsion of buildings having the same planning solution, different number of floors as well as with and without RMLSIS is estimated. The results obtained show that the displacement values of the structure formed as a result of torsion increased about 40% in buildings without RMLSIS and 25% in buildings with RMLSIS.