C. S. Tsai

Finite element formulations and theoretical study for variable curvature friction pendulum system

The friction pendulum system (FPS), a type of base isolation technology, has been recognized as a very efficient tool for controlling the seismic response of a structure during an earthquake. However, previous studies have focused mainly on the seismic behavior of base-isolated structures far from active earthquake faults. In recent years, there have been significant studies on the efficiency of the base isolator when subjected to near-fault ground motions. It is suggested from these studies that the long-duration pulse of near-fault ground motions results in significant response of a base-isolated structure. In view of this, an advanced base isolator called the variable curvature friction pendulum system (VCFPS) is proposed in this study. The radius of the curvature of VCFPS is lengthened with an increase of the isolator displacement. Therefore, the fundamental period of the base-isolated structure can be shifted further away from the predominant period of near-fault ground motions. Finite element formulations for VCFPS have also been proposed in this study. The numerical results show that the base shear force and story drift of the superstructure during near-fault ground motion can be controlled within a desirable range with the installation of VCFPS. Therefore, the VCFPS can be adopted for upgrading the seismic resistance of the structures adjacent to an active fault.

Interactive Behavior of Structures with Multiple Friction Pendulum Isolation System and Unbounded Foundations

In the last two decades, base isolation has been used for enhanced seismic performance to protect structures against severe earthquakes. The reduction in earthquake forces on a structure is achieved by adding horizontal soft isolation elements between the superstructure and the foundation. The efficiency of the isolator in reducing the seismic energy imparted to a structure is dependent on the flexibility of the supporting soil. The effects of radiation damping and the flexibility of the soil media are the deciding factors for structural design against earthquakes. In order to ensure the safety of isolated structures, the interactive behavior of isolated structures and the unbounded foundation during earthquakes must be considered. In this paper, an advanced analytical model based on the viscoplasticity theory and rigorous finite element derivations for the Multiple Friction Pendulum System (MFPS) is presented. To yield better design accuracy, the consistent infinitesimal finite element-cell method, accounting for the radiation damping of the unbounded soil, was adopted. Significant differences between the system with and without radiation damping were observed. It is implied from this study that to obtain better accuracy, radiation damping should be properly taken into account for structural analysis and design. Quantitative results also reveal that the interaction effects of an isolated building and the unbounded soil medium are important.

Shaking Table Tests of a Full Scale Steel Structure Isolated With MFPS

The base isolation, a kind of passive control technology, has been proved as a very efficient way to ensure the safety of a structure during severe earthquakes both from theoretical study and experimental effort. In general, the base isolation can be classified into two groups, which are sliding type and elastomeric type isolator. In this study, a new base isolator called as Multiple Friction Pendulum System (MFPS) has been proposed. The lubricant material, articulated slider and doubled concave sliding interfaces of MFPS are quite different from that proposed by V. Zayas in 1987. In this study, the MFPS isolator has been equipped beneath each column of a three-story structure at the National Center for Research on Earthquake Engineering to demonstrate its seismic resistance capability. The experimental results from shaking table tests of the 1940 El Centro, 1995 Kobe and 1999 Chi-Chi earthquake show that the proposed isolator can reduce the undesirable seismic response of the structure by lengthening the fundamental period of the structure during earthquakes. The experimental results indicate that the acceleration response of each floor can be lessened significantly as compared with those of the bare structure, and that the stress responses of structural components are limited in the certain range during severe earthquakes. Furthermore, the residual displacements of base isolators are negligible. Therefore, it is shown evidently that the proposed isolator can always bring the base-isolated structure to its initial position after an earthquake. Based on the previous observations, the proposed isolator can be adopted as an effective tool for upgrading the seismic resistibility of a structure. A finite element formulation for the MFPS is also proposed to simulate its mechanical behavior during earthquakes.Copyright

Seismic Behavior of MFPS Isolated Structure Under Near-Fault Sources and Strong Ground Motions With Long Predominant Periods

Base isolation, which has been recognized as a very promising way for upgrading the earthquake-proof capability of existing structures both from theoretical and experimental studies. However, some researchers suspect the efficiency of base isolator under near-fault earthquakes and strong ground motions with long predominant periods. It is suggested from previous studies that earthquakes with long predominant periods always cause severe responses of base-isolated structures. In view of this, a new base isolator called as Multiple-Friction Pendulum System (MFPS) has been proposed in this study to improve the shortcoming of those undesirable phenomenon of base-isolated structures. In order to evaluate the efficiency of MFPS isolators, the shaking table tests of a 3-story steel structure have been performed at NCREE in Taiwan. Experimental results show that the proposed isolator still posses well performance under near source excitations and strong ground motions with long period predominant periods. Therefore, the proposed base isolator can be recognized as a very promising tool for enhancing the seismic-resistance of a structure near seismic faults or on a soft deposit soil.Copyright

The Material Behavior and Isolation Benefits of Ball Pendulum System

Earthquake ground motions often result in significant seismic disasters. Strong ground motions will not only cause damage, but may also cause the collapse of structures. People have to face up the suffering from the earthquake damage, and the indirect loss which may be more serious than the damage itself. For example, the antique breaks in museum, and the equipment damages in hi-tech industries are often in huge loss. Therefore, in addition to promoting the earthquake-resistant capacity of a structure, it is also important to ensure the safety of the ancient valuable objects and the instruments in structures. For this reason, this study is aimed at a new damped rolling type base isolation system named the ball pendulum system (BPS) to be installed under the motion sensitive equipment and proceeding all related studies. The isolation device can isolate earthquake from buildings or equipments in any direction by rolling motions and damping materials. This study has conducted a series of component tests and shaking table tests for examining the behaviors of materials and earthquake proof benefits. From the experiment results, it is found that this device can reduce more than 80% of acceleration response under earthquakes with peak ground acceleration of 450 gal. So, the new rolling isolation system with a damping material can be recognized as a feasible and promising way in mitigating the seismic response of equipment.Copyright

Component Tests and Simulation of Advanced Buckling Restrained Braces

Recently, the earthquake proof technology has been acknowledged to be able to ensure the safety of the structures effectively during earthquakes. In this paper, two advanced buckling restrained braces (BRBs) that include multi-curved reinforced BRB and simplified reinforced BRB are presented. These two braces not only improve the disadvantages of traditional buckling restrained braces but also are more economic than the traditional ones. In order to understand the behaviors of advanced buckling restrained braces, the component tests of the advanced buckling restrained braces were carried out in the Department of Civil Engineering, Feng Chia University, Taichung, Taiwan. The experimental results illustrate that the behaviors of the advanced buckling restrained braces were very stable, as well as the maximum tension forces are close to the maximum compression forces. Furthermore, the Wen’s model in an increment form was utilized to simulate the behaviors of the advanced buckling restrained braces under cyclic loadings. The comparison between the experimental and numerical results shows that the mathematical model could simulate the behaviors of the advanced BRBs well.Copyright

Roles of soil-structure interaction and damping in base-isolated structures built on numerous soil layers overlying a half-space

This study examines the roles of soil-structure interaction (SSI), higher modes, and damping in a base-isolated structure built on multiple layers of soil overlying a half space. Closed-form solutions for the entire system, including a superstructure, seismic isolator, and numerous soil layers overlying a half-space, were obtained. The formulations obtained in this study simply in terms of well-known frequencies and mechanical impedance ratios can explicitly interpret the dynamic behavior of a base-isolated structure interacting with multiple soil layers overlying a half-space. The key factors influencing the performance of the isolation system are the damping ratio of the isolator and the ratio of the natural frequency of the fixed-base structure to that of the isolated structure by assuming that the superstructure moves as a rigid body. This study reveals that higher damping in the base isolator is unfavorable to higher mode responses that usually dominate the responses of the superstructure and that the damping mechanism plays an important role in transmitting energy in addition to absorbing energy. It is also concluded that it is possible to design a soft soil layer as an isolation system for isolating vibration energy.

Application of Direction Optimized-Friction Pendulum System to Seismic Mitigation of Sensitive Equipment

In the recent years, earthquake proof devices have been used to promote the earthquake resistant capabilities of many structures and public constructions. In addition, the high-tech industries are an important key to economic development in some earthquake prone areas, and many historical relics are also located in these areas. Therefore, how to protect the critical equipments from earthquake damage is an important issue. Among many control devices, sliding type isolators such as the FPS, MFPS and TFPS, ect. Isolators are used to lengthen the natural periods of equipment, and to isolate the seismic energy trying to impart to structures. However, the frequency and displacement capacity have been predefined when the radius of curvature of the concave surface or stiffness of base isolator is once determined. In this study, the base isolator with variable frequencies and displacement capacities has been proposed, and several shaking table tests of critical sensitive equipment with the proposed isolators have been carried out in Feng Chia University. The experimental results illustrated that the most responses of tested equipment have been reduced during earthquake.Copyright

Experimental Study of MFPS-Isolated Sensitive Equipment

After observations of many seismic disasters, let is found that many structures were just damaged slightly or even without any damage such as hospitals, high technology factories, computer generator rooms, but huge damage to internal installations was caused by earthquakes. Therefore, in addition to promoting the earthquake-resistant capacity of a structure, it is also important to ensure the safety of ancient objects and instruments in the structure. Structural control has been recognized as an effective and attractive method for preventing structural damage from earthquakes. In this study, shaking table tests of the high-end server equipment equipped with the multiple friction pendulum system (MFPS) were carried out to study the earthquake-proof benefit of the MFPS isolator. The MFPS isolator can not only shift the natural periods of high-end server equipments away from the rich period contents of earthquake motions, but also provide considerable hysteretic friction damping to absorb the input energy of earthquakes to insure the sensitive equipments unharmed during earthquakes.Copyright

RADAS Device Technology for Retrofitting Damaged Structures in 921 Chi-Chi Earthquake

The 921 Chi-Chi earthquake was the most destructive earthquake for Taiwan in the twentieth century. The earthquake caused severe damage or collapse to residential and public structures. In addition to the use of traditional earthquake-resistant technologies for retrofitting damaged structures, new structural control technologies have been also adopted. The RADAS (Reinforced Added Damping and Stiffness) device is a new type of earthquake-proof technology. The RADAS device has been proved as a very reliable energy-absorbing device for seismic hazard mitigation through shaking table tests. In this paper, we will present the application of RADAS devices to damaged structures in the 921 Chi-Chi earthquake. It is also illustrated in this study that new structures equipped with RADAS Devices can enhance seismic resistibility, even if earthquakes exceed ML 7.3 magnitude on the Richter scale. Therefore, it is a sensible choice to use RADAS devices to retrofit damaged structures and to enhance the earthquake-resistant capacity of new structures.Copyright