Earthquake Input Research Articles

Seismic response analysis and modal identification of a full-scale five-story base-isolated building tested on the NEES@UCSD shake table

This paper presents the analysis of the seismic response and the identification of the dynamic properties of an equivalent linear time-invariant model of a full-scale five-story base-isolated reinforced concrete building tested on the unidirectional NEES@UCSD shake table. A sequence of seven scaled historic earthquake records was applied by the shake table to the building, progressively increasing the seismic demand on the structure and its nonstructural components and systems. The effects of the isolation system on elongating the predominant period of the building, concentrating the displacement in the isolation layer, and augmenting the energy dissipation capacity of the building system are investigated. Low-amplitude white noise (WN) base excitation tests were conducted before and after each seismic test and ambient vibration (AV) data were recorded continuously for a period of approximately sixteen days containing the sequence of seven seismic tests performed. Because of the low intensity of the white noise base and ambient excitations, a quasi-linear response of the base-isolated building is assumed, allowing the modal parameters of an equivalent linear viscously damped time-invariant model to be estimated from vibration data recorded during the WN and AV tests. Different state-of-the-art system identification methods, including output-only and input-output methods, are used to estimate the modal properties of the base-isolated building. Results show that the identified modal parameters obtained from different methods are in good agreement and that the assumption of quasi-linear response for each low-amplitude test is appropriate. The effects of the amplitude of the excitation, earthquake input motions, and environmental conditions are clearly evidenced by the changes induced in the estimated modal parameters of the building.

Development of Retrofitting Technique for Seismically Vulnerable Open Ground Storey Reinforced Concrete Buildings

Retrofitting is a technique wherein the damaged structure is strengthened to original design requirements. During 2001 Bhuj earthquake in Gujarat, India many open ground storey building damaged. So there is an urgent need for retrofitting of damaged buildings. This calls for techniques that are technically sound and economically feasible to upgrade deficient and damaged buildings. Also, Portland cement (PC) production is under critical review due to high carbon dioxide gas released to the atmosphere and Portland cement is also one among the most energy-intensive construction materials. So retrofitting of existing deficient building using eco-friendly material which could promise higher structural performance than the original building is required. The main contributions is development of retrofitting technique of seismically damaged building using Geopolymer concrete and Chevron steel bracing with passive energy dissipation devices. Experimental studies are carried out using the 4 m × 4 m tri-axial shake table test facility to evaluate the seismic performance of bare frame, Open ground storey building, X-plate retrofitted building, Shear wall retrofitted building. Two three storied half scale model buildings, one open ground storey (OGS) building and a bare frame building are tested for different earthquake inputs in the shake table. The acceleration spectrum of IS1893:2002, soft soil is used as seismic input for the above tests. The failure modes and crack pattern of buildings after the seismic test are analysed. The damaged OGS building after the shake table test is retrofitted. The local retrofitting of columns and beam-column joints in the ground storey is carried out using Geopolymer concrete. The global retrofitting of building is formulated with Chevron type steel bracing incorporating X-plate damper in the ground storey. Sweep sine tests on OGS building before and after retrofitting with X-shaped metallic damper is carried out. The natural frequencies and mode shapes of the models are evaluated. The retrofitted OGS building is again subjected to shake table test to the study the seismic performance under varying amplitude as per IS 1893:2002 ground motion. The same model buildings are analysed in SAP 2000 to predict the seismic performance of the retrofitted RC frames.An innovative technique of retrofitting the seismic damaged building using Geopolymer concrete and construction of shear wall is explored. Two Reinforced Concrete shear walls are constructed inside the open ground storey building as global retrofitting technique. Sweep sine test is carried out after retrofitting with shear wall. The fundamental frequency of open ground storey building increased from 3.75 Hz to 7.25 Hz due to the addition of shear wall. The pushover test on building is carried out. The pushover analysis is carried out to study the seismic response of reinforced concrete building with local and global retrofitting using SAP 2000 software. Comparison of experimental and analytical push over curves is plotted. A significant improvement in the strength and stiffness of the open ground storey building after retrofitting with shear wall is observed.

Research on Seismic Performance of Switches with Different Types of Supporting Structure

As an important type of porcelain-type electrical equipment, switches have been seriously damaged in many earthquake disasters. Studies have shown that different types of supporting structure of switches have significant impact on their seismic performance. This paper takes the medium-voltage and the high-voltage switches in a substation in the high-intensity area as the structural prototype, proposes six different types of supporting structure of switches, and explores the influence of the supporting structure on their seismic performance under different seismic intensities. In addition, the single column supporting high-voltage switch was found to have large displacement under 0.4 g earthquake input, and optimization design is needed for this kind of supporting structure. Seven different optimization schemes were proposed in this paper. It was found that single column supporting high-voltage switch with four bracing and single column supporting high-voltage switch with six bracing have excellent seismic performance in high intensity areas, which can be adopted in the optimization design of single column supporting high-voltage switch.

On the influence of earthquakes and soil characteristics on seismic response and performance of isolated bridges

Seismic isolation technology is an effective means of reducing seismic risk and enhancing the structural seismic performance. However, some parameters, such as earthquake inputs and soil characteristics, affect and mitigate the efficiency of this technology. The main purpose of this study is to investigate the simultaneous effects of different records and flexible soils on isolated bridges. To this end, an isolated bridge is assumed to be at different distances from the ruptured fault (Rrup) in order to represent near-fault (NF) and far-field (FF) situations. These records are extracted on different soils, which are categorized based on their shear velocity, to represent different soil behaviours and characteristics. Nonlinear time history analysis (NTHA) is carried out on a typical isolated bridge model using the SAP2000 software. Responses in terms of deck acceleration, base shear, displacement of the isolation system, and the performance of the isolation units are studied. Results demonstrate that for NF zones, the soil effects must absolutely be taken into account. In soft soils, all seismic responses are amplified, leading to higher forces and displacement demands. In such zones, failing to consider this fact during the design process results in largely underestimated seismic displacement and force demands for the isolated bridge system, seriously harming its seismic performance. Likewise, the amplification of responses in soft soils is observed for FF records and it should be considered for these prone areas. However, despite their importance, these records are not as destructive as NF records for isolated bridges on soft soils.

Earthquake input energy prediction considering structural transient response

Statistic and theory based methods are mainly used for predicting earthquake input energy. Statistical energy spectrum methods need abundant time-history simulation and are lack of solid theoretical basis, while theoretical prediction methods are derived from structural dynamics with necessary assumptions, so that these methods have relatively low accuracy resulted from their assumptions. In this work the comparison of simulation and analysis showed that, due to the influence of structural transient response ignored by existing theoretical prediction methods, more than 40% errors exist in prediction of earthquake input energy for structures with low damping ratio and long period. Based on Fourier decomposition, a new theoretical method was proposed for earthquake input energy prediction considering the influence of transient response. The comparative analysis show that the proposed method significantly improves prediction accuracy of earthquake input energy.

Seismic isolation of railway bridges using a self-centering pier

Earthquakes cause severe damages to bridge structures, and rocking isolation of piers has become a superior option for the seismic protection of bridges during earthquakes. A seismic isolation method with free rocking mode is proposed for railway bridge piers with medium height. Experimental and numerical analysis are conducted to evaluate the seismic performance of the rocking-isolated bridge pier. Shaking table test is carried out with a scaled model by using three strong input earthquake records. The measured data includes displacement, acceleration and time history response of the pier-top and the bending moment of the pier-bottom. Test results show that the expected uplift and rocking of the isolated pier occur under strong earthquakes and the rocking-isolated pier has self-centering capacity. Slight damage appears at the collision surface between pier and base due to pier uplift, while there is no damage in the pier body. The bending moment of pier-bottom is less affected by the spectrum of input ground motions. The two-spring model is provided to simulate the isolated pier with free rocking mode under earthquakes. A seismic response analysis model for the rocking-ioslated pier is established with the assistance of OpenSees platform. The simulated results agree well with the measured results by shaking table test. Therefore, the seismic isolation method with a self-centering pier is worthy of promotion for railway bridges in high seismic risk regions.

Seismic Analysis for Cross Transfer Subway Stations in Soft Soil Stratum

This paper investigates the seismic properties of a cross transfer subway station under a horizontal earthquake wave in soft soil stratum. A 3-D simulation analysis model is established considering the dimensions and soil layers of a real structure. The HS-small model is employed to simulate the non-linear stiffness of soft soil. The dynamic responses of a single station case and a cross transfer station case are explored, followed by the direction of the input earthquake wave, the type of soil reinforcement in the overlapping area, had on the dynamic response were also analyzed. The simulation results show that the subway station has better seismic response than the single station, and soil reinforcement can increase the dynamic response of the different components in the overlapping area. As soil reinforcement stiffness increases, the dynamic response increases in both the upper and bottom stations at varying degrees. The findings from this paper provide a reference to the seismic design of cross transfer stations in similar ground.

Study Concept on the Development of an Urban Cyber Physical System for Enhancing the Capability to Respond to Large-Scale Earthquakes

Various technologies for improving earthquake-disaster-mitigation capability in urban communities are being developed and proposed. However, these technologies are sometimes difficult to use in actual applications due to the lack of incentives for owners of buildings or infrastructures, because the owners cannot calculate their cost-effectiveness and thus consider the installation payment for these technologies as a cost to be reduced. To address this problem, we propose the construction of an “Urban cyber physical system (CPS).” This urban CPS evaluates the earthquake-resistant capability of buildings and/or social infrastructures to help owners easily understand the cost-effectiveness of adopting these technologies. The CPS also calculates the effects of newly-developed technologies, thereby allowing owners to accept new technologies based on their effectiveness. The study concept of the urban CPS is as follows: (1) Construction of an “Information platform” by using data aggregation and analysis of existing vibration data of structures, datasets of building (or construction) information modeling and various other available databases; (2) Development of a “Simulation platform” that has a prediction function to calculate the behaviors of urban communities during earthquakes by using data in the Information platform and an identification function to identify structural systems from input earthquake motions and responses of structures; and (3) Establishment of an “Eco-system” to operate the urban CPS in urban community design, based on the perspective of earthquake resilience.

The use of Stick‐IT model for the prediction of direct economic losses

AbstractThe Stick‐IT model (Stick for Infilled frames Typologies), that is a MDOF system consisting of a series of lumped masses connected by nonlinear shear link elements, was introduced to predict the response, in terms of interstory drift ratio (IDR) and peak floor acceleration (PFA), for infilled RC building typologies. Stick model parameters can be defined starting from low level building geometrical features, easily retrievable at the large scale, such as in plan dimensions, number of stories or the percentage of infills openings and considering the infills consistency. This paper investigates the applicability of Stick‐IT to predict the performance and direct economic losses for existing RC infilled frame type buildings. In the framework of a performance‐based assessment, the comparison of seismic‐loss outcomes provided by using either a detailed finite element model (FEM) (L1) or simplified Stick‐IT structural models (L2) are illustrated and discussed. The potential of Stick‐IT model is investigated referring to a real building damaged and repaired after the 2009 L'Aquila earthquake and adopting as earthquake input records registered in the proximity of the building. Story‐level building damage simulated by L1 and L2 models is compared to real damage, showing satisfactory prediction. The comparison of the real repair costs with those evaluated using L1 and L2 models provides encouraging validation. The application suggests that simplified models such as Stick‐IT, despite the low level of information required for its application, can be usefully applied to perform damage and loss scenarios at the large scale. Matlab code to generate the Stick‐IT, as well as the example code for the present study, are openly shared through the repository https://github.com/marcogaetanidaragona/Stick-IT.

Influences on the Seismic Response of the Gravity Dam‐Foundation‐Reservoir System with Different Boundary and Input Models

Dynamic dam‐foundation interaction is great important in the design and safety assessment of the dam structures. Two classic boundary conditions, i.e., the viscous‐spring boundary and the viscous boundary, are employed to consider the radiation damping of the unbounded rock foundation. The input models of seismic excitation of the viscous‐spring boundary and the viscous boundary are derived. The accuracy of the two boundary conditions in the dynamic analysis of the dam foundation is verified through the foundation analysis using an impulsive load. The influences of two boundary conditions and their earthquake input models on the seismic analysis of the Pine Flat and Jin’anqiao gravity dam‐foundation‐reservoir systems are then investigated. The results of displacements, hydrodynamic pressure, and principal stresses show that the agreement between the results of the viscous‐spring boundary and viscous boundary is good. The relative errors of the two models in the Pine Flat and Jin’anqiao gravity dams are both less than 5%. They are both acceptable from an engineering point of view.

Flow-based seismic resilience assessment of urban water transmission networks

In this study, a new framework of seismic resilience estimation for urban water transmission networks was developed. The proposed resilience estimation model consists of three phases: input earthquake generation, hydraulic analysis, and recovery of network facilities. In the earthquake generation phase, the uncertainty of the ground motion is determined using the spatially correlated seismic attenuation law. In the hydraulic analysis phase, a hydraulic simulation is performed in conjunction with EPANET analysis. In the recovery phase, network components are restored, and the performance of the recovered network is evaluated through hydraulic analysis. Then, the seismic resilience curve and recovery costs are calculated. For a numerical simulation, a MATLAB-based computer code was developed for pressure-driven analysis in EPANET simulation. To demonstrate the proposed model, an actual water transmission network in South Korea was reconstructed based on geographic information system data. The performance of the network system was evaluated according to two performance indices: system and nodal serviceability. Finally, the cost of repairing the network facilities and water loss are estimated according to earthquake magnitude and interdependency. Numerical results show that the recovery slope of the resilience curve tends to decrease as the earthquake magnitude and interdependency with the power facilities increase.

Characteristics of Earthquake Input Energy of a Subway Station Structure Based on Probability Density Evolution Method

Understanding seismic energy input and dissipation mechanism is necessary for energy-based seismic design of complex underground structures. Due to the intrinsic uncertainty of ground motion, stochastic methods are usually needed. In this paper, we study the seismic energy input and dissipation mechanism in an underground structure using the probability density evolution method (PDEM). It is found that the cumulative hysteretic energy dissipation ([Formula: see text] of the underground structure is reduced by 55% compared with the above-ground structure due to soil constraint. The columns, walls and slabs at the bottom of the underground structure have a high demand for [Formula: see text], which are vulnerable spots of the structure.

Intensity Measure Based on a Smooth Inelastic Peak Period for a More Effective Incremental Dynamic Analysis

The Incremental Dynamic Analysis (IDA) assesses the global collapse capacity of a structure by plotting its maximum inelastic response, obtained through a non-linear time-history analysis, versus the scaled intensity of different input earthquakes. The seismic intensity is often measured through the spectral acceleration at the fundamental elastic period. However, this can produce highly variable results. An alternative method is presented in this paper that relies on the elongated period, calculated either from the Fourier spectrum of the acceleration at a target building point (inelastic peak period) or from a smooth Fourier spectrum (inelastic smooth peak period). By referring to a reference reinforced concrete building and to a set of 10 spectrum-consistent earthquakes, the paper presents the results of a wide investigation. First, the variation in the elongated period as a function of the seismic intensity is discussed. Then, the effectiveness of the proposed method is assessed by comparing the IDA curves to those obtained through the elastic period or through approximate values of the elongated period given in the literature. The results show that the alternative IDA procedure generates curves with less-dispersed collapse thresholds. A statistical analysis shows significant improvements in the results when the inelastic smooth peak period is adopted.

Identification of joint structural state and earthquake input based on a generalized Kalman filter with unknown input

Accurate and real-time information of structural states and earthquake input is the prerequisite for structural seismic safety assessment and vibration control. When the earthquake input to a structure is not measured, it has been tried to identify structural state and the unknown earthquake input using the measured structural responses as an inverse problem. However, only structural absolute acceleration responses instead of the relative ones can be measured in this case, which limits the real-time performance of the existing Kalman filter with unknown input (KF-UI). In this paper, a generalized Kalman filter with unknown input (GKF-UI) is proposed to identify structural states and unknown earthquake inputs in real-time. Structural motion equations are established in the relative coordinate system and the observation equations are structural absolute acceleration-only measurements. The analytical derivation of the proposed GKF-UI is a direct extension of the classical Kalman filter (KF). It is also proven that the identification by the proposed GKF-UI using the absolute acceleration-only measurements under unknown earthquake excitation does not lead to the so-called drifted results in the previous identifications under unknown external excitations. Moreover, structural modeling errors can be considered in the identification by the proposed method. Then, it is extended to investigate the identification of the structural state of high-rise shear buildings and unknown earthquake inputs by combining the proposed GKF-UI with the modal expansion. Hybrid dynamic motion equations in modal space and observation equations in physical space are established to both reduce the state dimension and keep the one-dimension of unknown earthquake input instead of the increased number of unknown modal inputs. In addition, absolute accelerations are used in the observation equations, which avoids the dilemma that absolute accelerations cannot be converted into relative modal accelerations in the relative coordinate system under unknown earthquake input. Some numerical examples are used to verify the proposed method. Moreover, a shaking table test is adopted to further validate the performances of the proposed method.

Earthquake input and state estimation for buildings using absolute floor accelerations

AbstractAfter earthquakes, structural response such as interstory drift is critical for accurate structural assessment for buildings. Typically, direct integration of absolute floor accelerations does not yield reliable floor displacements due to the long‐period drifts caused by noise, a widely acknowledged challenge. In this case, model‐based estimation strategies can be employed, which often require the ground input for better accuracy. However, in many cases the ground input may not be available for lack of instrumentation or even be unmeasurable due to soil‐structure interaction, hence needs to be estimated. Earthquake input estimation in this case is particularly challenging due to the lack of direct feedthrough term, leading to low observability of system input. As a result, input estimation is sensitive to modeling error, measurement noise, and incomplete measurements. To address this challenge, a hybrid strategy is proposed to estimate earthquake input, states, and acceleration response at unmeasured floors using limited absolute floor acceleration measurements. First, the earthquake input is estimated through a maximum a posteriori (MAP) estimation method, and then the estimated input is combined with Kalman filter to further estimate states and unmeasured responses. A comprehensive assessment was performed through a series of numerical and experimental tests including a comparative study with a popular online model‐based method. While the online method demonstrated certain sensitivity to modeling error and measurement noise due to weak observability, the proposed strategy showed robustness and accuracy under realistic and challenging conditions. Further verification is also performed using a real‐world building structure that experienced earthquake events.

Seismic Response Time-Frequency Analysis of Bedding Rock Slope

According to the potential disaster points of rock bedding slope with weak structural plane along the Sichuan Tibet railway, the shaking table test of earthquake simulation is designed and carried out. The acceleration response and displacement response of slope under different input earthquake conditions are monitored, and the HHT method is mainly used to analyze the seismic response of slope. The results show that the rock bedding slope shows obvious and under the action of seismic waves of different intensities. With the increase of the amplitude value of the input earthquake, the elevation effect and the surface effect gradually weaken. When the amplitude of seismic wave reaches 0.9g, the movement inconsistency of slope on both sides increases, and the slope is gradually separated from the main body of the slope, resulting in slope instability. The characteristics of the seismic signal in time - frequency domain can be better described by the Hilbert-Huang transform. In the time domain, the energy is mainly concentrated in 2-6s and 12-15s, and excellent frequency band is concentrated between 5hz-20hz and 30-80hz. With the increase of elevation, the former increases and the latter decreases. In addition, the change of the peak value of the marginal spectrum can clearly show the development process of the earthquake damage inside the slope. At the height of 100cm, the inflection point can be seen obviously in the slope. It shows that the damage occurs at the height of 100cm, and the degree of earthquake damage near the slope is stronger than that inside the slope. The recognition results of marginal spectrum are in good agreement with the experimental results.

Performance evaluation of curved damper truss moment frames designed using equivalent energy design procedure

Curved damper truss moment frame (CDTMF) system is a novel seismic force-resisting structural system which utilizes the curved dampers integrated into a semi-rigid moment frame to dissipate the earthquake input energy. To ensure the CDTMF has high performance, the state-of the art equivalent energy design procedure (EEDP) is applied to its design. EEDP allows engineers to design CDTMF to achieve different performance objectives at different levels of earthquake shaking intensities. In this study, two different prototype buildings (three and nine-story) were designed using EEDP. The seismic performance of the buildings was assessed using nonlinear time history analysis (NTHA) and incremental dynamic analysis (IDA). The results of the nonlinear dynamic analyses demonstrate that the CDTMFs can achieve the predefined performance targets (selected by the designer) at different earthquake intensity levels. The results of the IDA also show that the EEDP designed CDTMF has a sufficient margin against collapse. The results of this study in general confirm that CDTMF can be used as an effective seismic force-resisting system.

Flow-based seismic risk assessment of a water transmission network employing probabilistic seismic hazard analysis

In this study, a seismic risk assessment model was proposed to evaluate the seismic reliability of a water transmission network. The proposed risk assessment model involves earthquake generation and hydraulic analysis modules. To consider a comprehensive approach, the numerical simulation strategy includes probabilistic seismic hazard analysis, buried pipeline deterioration, numerical modeling of network facilities, and the interdependency between pumping plant and substation. For this purpose, a flow-based MATLAB code has been developed that enables iterative hydraulic analysis using EPANET software. For numerical simulation, the epicenter and earthquake magnitudes were determined based on probabilistic seismic hazard analysis, and a real water transmission network in South Korea was constructed. From the numerical results, two performance indicators (system serviceability and nodal serviceability) and four mean normal status ratios of facilities were adopted to evaluate the network performance. In addition, a component importance measure of facilities in a network system was calculated by introducing a reduction factor. The numerical results using the proposed flow-based model show that the system performance is affected by buried pipeline deterioration and network interdependency, as well as the location and magnitude of the input earthquake. It is thus concluded that the seismic risk assessment of a water transmission network should be performed using a model with a comprehensive approach.

Output-Only Modal Parameter Identification of Systems Subjected to Various Types of Excitation

This study presents a novel modal parameter identification method enabling approximation of the mode shapes of linear systems using white-noise or earthquake inputs. The majority of well-established existing system identification methods perform successfully when the system is excited by broadband white-noise excitation. However, they encounter serious limitations when analyzing the vibrations triggered by nonstationary earthquake inputs. Thus, the presented technique extends the applicability of system identification and modal-based structural health monitoring methods. The method operates in modal space and is based on mode superposition in short windows. The mode shapes are identified using an optimization algorithm minimizing the weighted sum of cross-correlation of frequency response spectra. The technique is validated analytically using simulation results of a simple three-dimensional (3D) structure representing a simplified model of a real bridge pier structure, which enables exact comparison with known properties. The results show the method provides relatively good identification accuracy of modal parameters of systems excited by white-noise and earthquake inputs. The identified modal frequencies showed <1% error, where the mode-shape coefficients were identified within 5% error. The method performs robustly even for high levels of simulated sensor noise and can be readily applied to more complex multiple degree of freedom (MDOF) systems.

Parametric study on the Multangular-Pyramid Concave Friction System (MPCFS) for seismic isolation

A series of comprehensive parametric studies are conducted on a steel-frame structure Finite-Element (FE) model with the Multangular-Pyramid Concave Friction System (MPCFS) installed as isolators. This new introduced MPCFS system has some distinctive features when compared with conventional isolation techniques, such as increased uplift stability, improved self-centering capacity, non-resonance when subjected to near-fault earthquakes, and so on. The FE model of the MPCFS is first established and evaluated by comparison between numerical and theoretical results. The MPCFS FE model is then incorporated in a steel-frame structural model, which is subjected to three chosen earthquakes, to verify its seismic isolation. Further, parametric study with varying controlling parameters, such as isolation foundation, inclination angle, friction coefficient, and earthquake input, is carried out to extract more detailed dynamic response of the MPCFS structure. Finally, limitations of this study are discussed, and conclusions are made. The simulations testify the significant seismic isolation of the MPCFS. This indicates the MPCFS, viewed as the beneficial complementary of the existing well-established and matured isolation techniques, may be a promising tool for seismic isolation of near-fault earthquake prone zones. This verified MPCFS FE model can be incorporated in future FE analysis. The results in this research can also guide future optimal parameter design of the MPCFS.