Nonstructural performance of seismically isolated structuresin the near fault region

This study aims to investigate the influence of soil–structure interaction (SSI) on the seismic response of a nuclear island (NI) building with base isolation. Hence, modal and dynamic time‐history analyses of the NI building with base isolation based on different soil dynamic numerical models that are applicable for homogeneous and layered sites to consider the SSI effect were conducted. After a comparison analysis of natural frequencies, floor response spectra (FRS), and acceleration response of superstructure, the following results are obtained. (i) For homogeneous sites, the degrees of freedom of the horizontal translation and torsion of the NI building with base isolation calculated using different site dynamic numerical models have equivalent natural frequencies. For layered sites, the natural frequencies increase slightly with the improvement of the shear wave velocity of the upper layered soil. (ii) For homogeneous sites, FRS calculated by the massless foundation model is generally larger than those of the dynamic model proposed by the ASCE code and the viscoelastic artificial boundary site model. Both the spectral and peak values of horizontal FRS at a low frequency band calculated by considering the SSI effect are larger than those without the SSI effect. (iii) For layered sites, both the spectral and peak values of horizontal FRS at a low frequency band have an amplification effect compared with that of a homogeneous site. (iv) For layered sites, the calculated horizontal acceleration response does not have evident law that changes with the degree of hardness and softness of the upper soil. Therefore, the SSI effect for particular homogeneous and layered sites should be considered when calculating the seismic response of the NI building with base isolation.

Floor response and design spectra of a museum building before and after isolation are studied; these are used for estimating the response and seismic design of acceleration-sensitive non-structural components. First, two finite-element models of the building before and after seismic isolation are established and verified through ambient vibration tests. Second, seven earthquake motions are chosen to analyse the models; differences between floor response curves and earthquake motions are analysed based on peak acceleration and frequency spectra. Finally, taking the floor response curves as input, the floor acceleration response spectra before and after isolation are calculated, and the design spectra are fitted. Analytical results demonstrate that the dynamic characteristics of the finite-element model agree well with test values, and peak acceleration of the isolated structure is around 75% lower than that of the non-isolated structure. The predominant frequency of the floor response curves is consistent with the natural frequency of the structure. Moreover, the floor acceleration response spectra are influenced by the earthquake motions and the structures, which reflect the characteristics of both. The floor design response spectra are divided into different sections and mathematical formulas are developed. The findings could be applied to the seismic protection design of non-structural components.

The ALSC base isolation system was investigated by testing a church model to the scale 1/3.5 on the two-component shaking table at the IZIIS’ Laboratory, Skopje, Macedonia. The investigation was performed within the frames of the European FP6 project PROHITECH, related to seismic protection of historical buildings by innovative and reversible mixed technology systems. This system was originally developed in 2001 and applied on a model of a steel reservoir. The church model was tested twice: with an active base-isolation system and fixed to the base. The effectiveness of the base-isolation system was investigated by the transfer function/floor response spectra technique. Special software for calculation of the transfer functions and floor response spectra based on experimental data, referred as “SPECTRA”, was developed by the authors of this paper. The main objective of the investigations presented in this paper was evaluation of the effectiveness of the ALSC floating sliding base-isolation system by using the Response Spectra approach. Comparison of the Floor Response Spectra was done for two cases a fixed base model and a base-isolated model. The base response spectrum was calculated from the input time history of the Montenegro earthquake compressed 3.5 times (geometry scale of the model). The Transfer Functions and the Floor response Spectra at a particular level were calculated from the Cross Power Spectra (CPS) and the Auto-Power Spectra. The obtained results showed that the effect of the base-isolation system was considerable, with a low transmissibility of energy from shake table to the model.

Traditional base isolation systems focus on isolating the seismic response of a structure in the horizontal direction. However, in regions where the vertical earthquake excitation is significant (such as near-fault region), a traditional base-isolated building exhibits a significant vertical vibration. To eliminate this shortcoming, a rocking-isolated system named Telescopic Column (TC) is proposed in this paper. Detailed rocking and isolation mechanism of the TC system is presented. The seismic performance of the TC is compared with the traditional elastomeric bearing (EB) and friction pendulum (FP) base-isolated systems. A 4-storey reinforced concrete moment-resisting frame (RC-MRF) is selected as the reference superstructure. The seismic response of the reference superstructure in terms of column axial forces, base shears, floor accelerations, inter-storey drift ratios (IDR) and collapse margin ratios (CMRs) are evaluated using OpenSees. The results of the nonlinear dynamic analysis subjected to multi-directional earthquake excitations show that the superstructure equipped with the newly proposed TC is more resilient and exhibits a superior response with higher margin of safety against collapse when compared with the same superstructure with the traditional base-isolation (BI) system.

A novel adaptive base isolator utilising magnetorheological elastomer

A direct spectra‐to‐spectra method is developed for generating floor response spectra (FRS) for structures under earthquake excitations at multiple supports in terms of ground response spectra (GRS). Only GRS, “t‐response spectra” (tRS), and basic modal information of primary structures, which can be readily obtained from modal analyses, are needed. FRS are separated into dynamic part and quasi‐static part, which are combined by a new combination rule FRSMS‐CQC developed using random vibration theory. FRSMS‐CQC can account for the correlations between various components affecting FRS, that is, the correlation between the responses of oscillators excited by any two vibration modes, the correlation between the response of an oscillator excited by a vibration mode and the response of an oscillator mounted directly on a support, and the correlation between the responses of oscillators mounted on two different supports. In particular, two special cases, that is, excitations in the same direction at two supports being fully correlated and excitations at two supports being uncorrelated, are considered. The direct method can also be applied to generate tertiary response spectra (TRS) from FRS at multiple supports of secondary structures.Numerical example of a piping system mounted on different buildings, which are subjected to tridirectional seismic excitations at the foundation level, is presented to demonstrate the superiority of the proposed method. It is shown that FRS/TRS determined by time‐history (TH) analysis have large variabilities, particularly at FRS/TRS peaks. The proposed direct method, which avoids the deficiencies of time history methods, is of excellent accuracy, efficiency, and simplicity.

Dynamics of structures: theory and applications to earthquake engineering

Rare study is done on floor response spectrum of super-high rise building, but it is an important condition for the seismic response analysis of floor subsidiary structure. Therefore, based on the early calculation model of China Financial Information Mansion, the floor response spectrum is calculated under different input ground motion. The floor and ground response spectrum is compared with each other from the seismic coefficient, dynamic amplification coefficient, characteristic period and the form of response spectrum. The results shows that: the floor seismic coefficient and the magnification coefficient are greater or smaller than the ground ones, the biggest difference of which is nearly 1 times; all the floor character period are greater than the ground ones, the biggest difference of which is over 60%; there are obvious differences between the floor and ground dynamic magnification factor spectra form under some conditions, of which the second peak of the former one is probably very large, even near to the peak of the first one, while the latter has no such phenomenon. Therefore, during the process of calculating the seismic response of floor subsidiary structure, it is necessary to consider the change of floor seismic coefficient, dynamic magnification factor, characteristic period and spectra form based on the main structure.

Effect of second hardening on floor response spectrum of a base-isolated nuclear power plant

Seismic isolation is a seismic protection technique for buildings which has been recently introduced in Peru. More than twenty seismically isolated buildings exist in Peru, at present. Seismic isolators in many of these buildings have been designed using foreign codes developed for foreign seismic conditions in the absence of local design code. These conditions may not accurately represent Peruvian seismicity. The mandatory use of seismic isolators in new major hospital buildings has been established recently in the Peruvian seismic design code. Available studies in Peru indicate that most health centres may be temporarily affected after a rare seismic event. The seismic isolation Peruvian code is being developed taking into account the needs and implications of Peruvian seismicity. This paper presents the design procedure of the seismic isolation system of a representative four storey reinforced concrete hospital block. The requirements of the draft code for seismic isolation and the current seismic code have been used. The design process and verification show reasonable response of the structure in terms of drifts and acceleration even after including maximum and minimum modification factors of properties for the seismic isolation bearings.

Near-fault ground motions generally contain long-period velocity pulses and release huge energy. Severe earthquake damage has been observed in bridges subjected to near-fault ground motions. Moreover, some commonly-used seismic isolation devices cannot balance the relationship between forces and displacements in the structural system. This paper analyzes the typical earthquake damage for bridges subjected to near-fault ground motions by collecting the seismic investigation reports and concludes that deck shifting and unseating are the most typical seismic damage for bridges in the near-fault regions. Constitutive relationship design is the pivotal issue for the seismic isolator design of bridges. Combined with the design method for constitutive relationship of seismic isolation devices, a series of seismic isolation devices for bridges in the near-fault regions are proposed including Buffer cable-sliding friction aseismic bearings (BCSFABs), Steel mesh-reinforced isolation rubber bearings (SMIRBs) and cable-sliding modular expansion joints (CMEJs). Their working principles are introduced, respectively. Easy-repairability, wide-applicability and designability are one of the main developing trends for seismic isolation devices of bridges.

This paper studies the computational seismic response of two damageresistant 20-story tall buildings, located at near-fault regions of strong earthquakes, with a reinforced concrete core-wall to provide the majority of lateral resistance above ground. The first building uses a post-tensioned core-wall designed to rock at the ground level resulting in minimal structural damage in the wall at the maximum considered earthquake (MCE) level of shaking. The second building combines an isolation plane at the base (below ground) of the building with a post-tensioned rocking core wall (dual-isolation system); the dual-isolation system results in minimal structural damage at the MCE level of shaking and reduces significantly demands to non-structural components. The three-dimensional seismic responses of the two buildings are compared to that of a conventionally designed core-wall building designed to develop a flexural plastic hinge in the core-wall. Nonlinear response history analysis is conducted using the two horizontal components of strong historical near fault ground motions. The three building models use a nonlinear beam-truss modeling scheme that explicitly accounts for flexure-shear interaction in RC walls and slabs.

This paper provides a part of series of “Development of an Evaluation Method for Seismic Isolation Systems of Nuclear Power Facilities” [1]–[4]. This part describes the work schedule of this project and the summary of a seismic design for crossover piping system. Since the Southern Hyogo Prefecture Earthquake in 1995, a seismic isolated design has been widely adopted for Japanese typical buildings. The Japanese government accepted utilizing seismic isolation technology for nuclear power facilities with the 2006 revision of the “Regulatory Guide for Reviewing Seismic Design of Nuclear Power Reactor Facilities”. Under these backgrounds, the Japan national project with the participation of all electric power companies and reactor vendors has been started from 2008 to develop seismic isolation systems of nuclear power facilities under the support of the Ministry of Economy, Trade and Industry. In the design of seismic isolated plant, the crossover piping systems, such as Main Steam line and other lines related to the safety system have the important roles for overall plant safety. Therefore, the design of multiply supported piping systems between isolated and non-isolated buildings is one of the major key issues. This paper focuses on the seismic response analysis of Main Steam crossover piping between seismic isolated Reactor Building and non-isolated Turbine Building. Multiple input response spectra and time history analyses of the crossover piping have been performed and the structural integrity of piping and the validity of the multiple input analysis method have been verified based on comparisons with the results obtained by conventional response spectrum analysis using enveloped floor response spectrum.

Damping coefficients for near-fault ground motion response spectra

Seismic isolation is generally considered an effective earthquake protection strategy. As application of seismic isolation increases, decisions on the use of one particular isolator versus another isolator increasingly depend on computed responses with complex analytical models. Accordingly, validation of analytical models to predict primary (structural) and secondary system (non-structural component) response in seismically isolated buildings becomes very important. This paper presents comparisons of experimental and analytical results on the primary and secondary system response of a building model in order to provide information on the accuracy of the predicted response. The tested model was configured as a 6-story building at quarter length scale in a moment-frame configuration, and with the following seismic isolation systems: a) Low damping elastomeric bearings with and without linear or nonlinear viscous dampers, b) Single Friction Pendulum (FP) bearings with and without linear or nonlinear viscous dampers, and c) Lead-rubber bearings. Response quantities compared include story drifts and isolator shear forces and displacements for the primary system, and peak floor total velocities and floor response spectra that relate to secondary system response. This paper presents samples results out of a total of 288 comparisons of experimental and analytical results presented in an MCEER report. It is shown that the primary and secondary system response is computed with sufficient accuracy by the analytical models but some response quantities may be underestimated or overestimated by significant amounts.