Showing papers in "Earthquake Engineering & Structural Dynamics in 2000"

Implementation and testing of passive control devices based on shape memory alloys

Abstract: SUMMARY Two families of passive seismic control devices exploiting the peculiar properties of shape memory alloy (SMA) kernel components have been implemented and tested within the MANSIDE project (Memory Alloys for New Seismic Isolation and Energy Dissipation Devices). They are special braces for framed structures and isolation devices for buildings and bridges. Their most important feature is their extreme versatility, i.e. the possibility to obtain a wide range of cyclic behaviour * from supplemental and fully re-centring to highly dissipating * by simply varying the number and/or the characteristics of the SMA components. Other remarkable properties are their extraordinary fatigue resistance under large strain cycles and their great durability and reliability in the long run. In this paper, the working mechanisms of the SMA based devices are outlined and the experimental tests carried out to verify the above-mentioned properties are extensively described. Copyright ( 2000 John Wiley & Sons, Ltd.

Seismically induced racking of tunnel linings

Abstract: An analytical procedure is presented for evaluating the racking deformation of rectangular and circular tunnel linings caused by soil–structure interaction during a seismic event. The procedure, as applied to rectangular linings, is supplementary to that previously published by Penzien and Wu (Earthquake Engineering and Structural Dynamics, 1998; 27:283–300) for circular linings. Copyright © 2000 John Wiley & Sons, Ltd.

Effect of viscous, viscoplastic and friction damping on the response of seismic isolated structures

Abstract: In this paper the efficiency of various dissipative mechanisms to protect structures from pulse-type and near-source ground motions is examined. Physically realizable cycloidal pulses are introduced, and their resemblance to recorded near-source ground motions is illustrated. The study uncovers the coherent component of some near-source acceleration records, and the shaking potential of these records is examined. It is found that the response of structures with relatively low isolation periods is substantially affected by the high-frequency fluctuations that override the long duration pulse. Therefore, the concept of seismic isolation is beneficial even for motions that contain a long duration pulse which generates most of the unusually large recorded displacements and velocities. Dissipation forces of the plastic (friction) type are very efficient in reducing displacement demands although occasionally they are responsible for substantial permanent displacements. It is found that the benefits by hysteretic dissipation are nearly indifferent to the level of the yield displacement of the hysteretic mechanism and that they depend primarily on the level of the plastic (friction) force. The study concludes that a combination of relatively low friction and viscous forces is attractive since base displacements are substantially reduced without appreciably increasing base shears and superstructure accelerations. Copyright © 2000 John Wiley & Sons, Ltd.

Performance of multiple tuned mass dampers for attenuating undesirable oscillations of structures under the ground acceleration

Abstract: Multiple tuned mass dampers (MTMDs) consisting of many tuned mass dampers (TMDs) with a uniform distribution of natural frequencies are considered for attenuating undesirable vibration of a structure. The MTMD is manufactured by keeping the stiffness and damping constant and varying the mass. The structure is represented by its mode-generalized system in the specific vibration mode being controlled using the mode reduced-order method. The optimum parameters of the MTMD are investigated to delineate the influence of the important parameters on the effectiveness and robustness of the MTMD by conducting a numerical searching technique in two directions. The parameters include: the frequency spacing, average damping ratio, mass ratio and total number. The criterion selected for the optimization is the minimization of the maximum value of the dynamic magnification factor (DMF) of the structure with MTMD (i.e. Min.Max.DMF). In this paper, for the sake of comparison, the MTMD(II), which is made by keeping the mass constant and varying the stiffness and damping coefficient, and a single TMD are also taken into account. It is demonstrated that the optimum frequency spacing of the MTMD is the same as that of the MTMD(II) and the optimum average damping ratio of the MTMD is a little larger than that of the MTMD(II). It is also found that the optimum MTMD is more effective than the optimum MTMD(II) and the optimum single TMD with equal mass. Copyright © 2000 John Wiley & Sons, Ltd.

Tuned liquid dampers for controlling earthquake response of structures

Abstract: Numerical simulations of a single-degree-of-freedom (SDOF) structure, rigidly supporting a tuned liquid damper (TLD) and subjected to both real and artificially generated earthquake ground motions, show that a properly designed TLD can significantly reduce the structure's response to these motions. The TLD is a rigid, rectangular tank with shallow water in it. Its fundamental linear sloshing frequency is tuned to the structure's natural frequency. The TLD is more effective in reducing structural response as the ground excitation level increases. This is because it then dissipates more energy due to sloshing and wave breaking. A larger water-depth to tank-length ratio than previous studies suggested, which still falls within the constraint of shallow water theory, is shown to be more suitable for excitation levels expected in strong earthquake motions. A larger water-mass to structure-mass ratio is shown to be required for a TLD to remain equally effective as structural damping increases. Furthermore, the reduction in response is seen to be fairly insensitive to the bandwidth of the ground motion but is dependent on the structure's natural frequency relative to the significant ground frequencies. Finally, a practical approach is suggested for the design of a TLD to control earthquake response. Copyright © 2000 John Wiley & Sons, Ltd.

Parametric investigation of the stability of classical columns under harmonic and earthquake excitations

Abstract: The seismic response of free-standing classical columns is analysed numerically through implementation of the distinct element method. Typical sections of two ancient temples are modelled and studied parametrically, in order to identify the main factors affecting the stability and to improve our understanding of the earthquake behaviour of such structures. The models were first subjected to harmonic base motions. The analysis showed that, for frequencies usually encountered in earthquake motions, intact multi-drum free-standing columns can withstand large amplitude harmonic excitations without collapse. The dynamic resistance decreases rapidly as the period of the harmonic excitation increases. Imperfections, such as initial tilt of the column or loss of contact area due to edge damage, also reduce the stability of the system significantly. The effects of such imperfections could be additive and the cumulative effect of many imperfections may render deteriorating abandoned monuments vulnerable to earthquakes. The response of more complete sections of the temple, such as two columns coupled with an architrave, did not deviate systematically from that of the single multi-drum column or indeed of the equivalent single block. Therefore, a much simpler single block analysis can be used to size-up the seismic threat to the monument. The model of the column of the Temple of Apollo at Bassae was also tested under recorded earthquake motions by scaling-up the acceleration amplitude progressively until collapse of the column. It was found that the columns are particularly vulnerable to long-period impulsive earthquake motions. A comparison of the instability thresholds associated with harmonic excitations and earthquake motions throws more light onto the dynamic response: it appears that around three cycles of monochromatic excitation at the predominant period of the expected earthquake motions lead to a gross prediction of the stability of a classical column during an earthquake. Copyright © 2000 John Wiley & Sons, Ltd.

Estimation of seismic drift demands for frame structures

Abstract: A process is outlined and evaluated for the estimation of seismic roof and storey drift demands for frame structures from the spectral displacement demand at the first mode period of the structure. The spectral displacement demand is related to the roof drift demand for the multi-degree-of-freedom (MDOF) structure using three modification factors, accounting for MDOF effects, inelasticity effects, and P-delta effects. Median values and measures of dispersion for the factors are obtained from elastic and inelastic time history analyses of nine steel moment resisting frame structures subjected to sets of ground motions representative of different hazard levels. The roof drift demand is related to the storey drift demands, with the results being strongly dependent on the number of stories and the ground motion characteristics. The relationships proposed in this paper should prove useful in the conceptual design phase, in estimating deformation demands for performance assessment, and in improving basic understanding of seismic behaviour. Copyright © 2000 John Wiley & Sons, Ltd.

Reduction of pounding effects in elevated bridges during earthquakes

Abstract: Pounding of adjacent superstructure segments in elevated bridges during severe earthquakes can result in significant structural damage. The aim of this paper is to analyse several methods of reduction of the negative effects of collisions induced by the seismic wave propagation effect. The analysis is conducted on a detailed three-dimensional structural component model of an isolated highway bridge. The results show that the influence of pounding on the structural response is significant in the longitudinal direction of the bridge and significantly depends on the gap size between superstructure segments. The smallest response can be obtained for very small gap sizes and for gap sizes large enough to prevent pounding. Further analysis indicates that the bridge behaviour can be effectively improved by placing hard rubber bumpers between segments and by stiff linking the segments one with another. The experimental results show that, for the practical application of such connectors, shock transmission units can be used. Copyright © 2000 John Wiley & Sons, Ltd.

Seismic response control of frame structures using magnetorheological/electrorheological dampers

Abstract: Semi-active control of buildings and structures for earthquake hazard mitigation represents a relatively new research area. Two optimal displacement control strategies for semi-active control of seismic response of frame structures using magnetorheological (MR) dampers or electrorheological (ER) dampers are proposed in this study. The efficacy of these displacement control strategies is compared with the optimal force control strategy. The stiffness of brace system supporting the smart damper is also taken into consideration. An extensive parameter study is carried out to find the optimal parameters of MR or ER fluids, by which the maximum reduction of seismic response may be achieved, and to assess the effects of earthquake intensity and brace stiffness on damper performance. The work on example buildings showed that the installation of the smart dampers with proper parameters and proper control strategy could significantly reduce seismic responses of structures, and the performance of the smart damper is better than that of the common brace or the passive devices. The optimal parameters of the damper and the proper control strategy could be identified through a parameter study. Copyright © 2000 John Wiley & Sons, Ltd.

Linear dynamic modeling of a uni‐axial servo‐hydraulic shaking table system

Abstract: This paper focuses on the development of a linear analytical model (even though servo-hydraulic actuation systems are inherently non-linear, especially for large amplitude simulations — near the performance capacity of the system — linearized models proved experimentally to be quite effective overall in capturing the salient features of shaking table dynamics) of a uni-axial, servo-hydraulic, stroke controlled shaking table system by using jointly structural dynamics and linear control theory. This model incorporates the proportional, integral, derivative, feed-forward, and differential pressure gains of the control system. Furthermore, it accounts for the following physical characteristics of the system: time delay in the servovalve response, compressibility of the actuator fluid, oil leakage through the actuator seals and the dynamic properties of both the actuator reaction mass and test structure or payload. The proposed model, in the form of the total shaking table transfer function (i.e. between commanded and actual table motions), is developed to account for the specific characteristics of the Rice University shaking table. An in-depth sensitivity study is then performed to determine the effects of the table control parameters, payload characteristics, and servovalve time delay upon the total shaking table transfer function. The sensitivity results reveal: (a) a potential strong dynamic interaction between the oil column in the actuator and the payload, and (b) the very important effect of the servovalve time delay upon the total shaking table transfer function. Copyright © 2000 John Wiley & Sons, Ltd.

VFPI: an isolation device for aseismic design

Abstract: Sliding isolators with curved surface are effective base isolation systems incorporating isolation, energy dissipation and restoring mechanism in one unit. However, practical utility of these systems, such as friction pendulum system (FPS) has limitations due to constant isolator period and restoring force characteristics. A new isolator called the variable frequency pendulum isolator (VFPI) that overcomes these limitations while retaining all the advantages has been described in this paper. VFPI has oscillation frequency decreasing with sliding displacement, and the restoring force has an upper bound so that the force transmitted to the structure is limited. The mathematical formulations for the response of a SDOF structure and energy balance are also described. Parametric studies have been carried out to critically examine the behaviour of structures isolated with VFPI, FPS and PF system. From these investigations, it is concluded that the VFPI combines the advantages of both FPS and PF system, without their undesirable properties. The VFPI performance is also found to be stable during low-intensity excitations, and fail-safe during high-intensity excitations. VFPI is found to exhibit robust performance for a wide range of structure, isolator and ground motion characteristics clearly demonstrating its advantages. Copyright © 2000 John Wiley & Sons, Ltd.

Performance‐based design using structural optimization

Abstract: A new methodology for seismic design is proposed based on structural optimization with performance-based constraints. Performance-based criteria are introduced for the seismic design of new buildings. These criteria are derived from the National Guidelines for Seismic Rehabilitation of Buildings (Reference [19], Federal Emergency Management Agency (FEMA), ‘NHERP Guidelines for seismic rehabilitation of buildings’, Report Nos 273 and 274, Washington, DC, 1997) for retrofitting existing structures. The proposed design methodology takes into account the non-linear behaviour of the structure. The goal is to incorporate in the design the actual performance levels of the structure, i.e. how much reserve capacity the structure has in an earthquake of a given magnitude. The optimal design of the structure minimizes the structural cost subjected to performance constraints on plastic rotations of beams and columns, as well as behavioural constraints for reinforced concrete frames. Uncertainties in the structural period and in the earthquake excitation are taken into account using convex models. The optimization routine incorporates a non-linear analysis program and the procedure is automated. The proposed methodology leads to a structural design for which the levels of reliability (performance levels) are assumed to be quantifiable. Furthermore, the entire behaviour of the structure well into the non-linear range is investigated in the design process. Copyright © 2000 John Wiley & Sons, Ltd.

Critical response of structures to multicomponent earthquake excitation

Abstract: This paper aims to develop an improved understanding of the critical response of structures to multicomponent seismic motion characterized by three uncorrelated components that are defined along its principal axes: two horizontal and the vertical component. An explicit formula, convenient for code applications, has been derived to calculate the critical value of structural response to the two principal horizontal components acting along any incident angle with respect to the structural axes, and the vertical component of ground motion. The critical response is defined as the largest value of response for all possible incident angles. The ratio rcr/rsrss between the critical value of response and the SRSS response—corresponding to the principal components of ground acceleration applied along the structure axes—is shown to depend on three dimensionless parameters: the spectrum intensity ratio γ between the two principal components of horizontal ground motion characterized by design spectra A(Tn) and γA(Tn); the correlation coefficient α of responses rx and ry due to design spectrum A(Tn) applied in the x- and y-directions, respectively; and β = ry/rx. It is demonstrated that the ratio rcr/rsrss is bounded by 1 and . Thus the largest value of the ratio is , 1.26, 1.13 and 1.08 for γ = 0, 0.5, 0.75 and 0.85, respectively. This implies that the critical response never exceeds times the result of the SRSS analysis, and this ratio is about 1.13 for typical values of γ, say 0.75. The correlation coefficient α depends on the structural properties but is always bounded between −1 and 1. For a fixed value of γ, the ratio rcr/rsrss is largest if β = 1 and α = ±1. The parametric variations presented for one-storey buildings indicate that this condition can be satisfied by axial forces in columns of symmetric-plan buildings or can be approximated by lateral displacements in resisting elements of unsymmetrical-plan buildings. Copyright © 2000 John Wiley & Sons, Ltd.

Empirical formula for fundamental vibration periods of reinforced concrete buildings in Taiwan

Abstract: More than 30 buildings around Taiwan have been selected to monitor the floor responses under seismic excitation. The structural array monitoring system in each building controls at most 27 channels of accelerometers distributed in several floors. Those buildings were triggered by many events during the past five years of operation. In each building, the records at the basement Can be considered as the ground excitation, and the others at the upper floors are the structural responses. The frequency transfer functions of those buildings can be identified by ARX models, and then the fundamental vibration periods are estimated. The identified fundamental vibration periods using different events are compared in order to ensure the reliability of system identification. An empirical formula in predicting the fundamental vibration period is presented through the regression analysis to the identified fundamental vibration periods of 21 reinforced concrete (RC) moment-resisting frame (MRF) buildings. It is found that the height of a building plays an important role in predicting the fundamental vibration period, compared with the length, width, and time after completion of the building. It is also found that the RC MRF buildings in Taiwan tend to be stiffer than those in the U.S.

Vibration of vertical rectangular plate in contact with water on one side

Abstract: In this paper, the vibratory characteristics of a rectangular plate in contact with water on one side are studied. The elastic plate is considered to be a part of a vertical rectangular rigid wall in contact with water, the edges of which are elastically restrained and parallel to those of the rigid wall. The location and size of the plate on the rigid wall may vary arbitrarily. The water with a free surface is in a rectangular domain infinite in the length direction. The effects of free surface waves, compressibility of the water and the hydrostatic water pressure are neglected in the analysis. An analytical-Ritz method is developed to analyse the interaction of the plate–water system. First of all, by using the method of separation of variables and the method of Fourier series expansion, the exact expression of the motion of water is derived in the form of integral equations including the dynamic deformation of the plate. Then the Rayleigh–Ritz approach is used to derive the eigenfrequency equation of the system via the variational principle of energy. By selecting beam vibrating functions as the admissible functions of the plate, the added virtual mass incremental (AVMI) matrices for plate vibration are obtained. The convergency studies are carried out. The effects of some parameters such as the depth and width of water, the support stiffnesses, location and aspect ratio of the plate and the plate–water size and density ratios on the eigenfrequencies of the plate–water system are investigated. Several numerical examples are given. The validity of AVMI factor approach is also confirmed by comparing the AVMI factor solutions with the analytical-Ritz solutions. The results show that the approach presented here can also be used as excellent approximate solutions for rectangular plates in contact with water of infinite width and/or infinite depth. Copyright © 2000 John Wiley & Sons, Ltd.

Response spectral relationships for rock sites derived from the component attenuation model

Abstract: The seismological model was developed initially from the fundamental relationship between earthquake ground motion properties and the seismic moment generated at the source of the earthquake. Following two decades of continuous seismological research in the United States, seismological models which realistically account for both the source and path effects on the seismic shear waves have been developed and their accuracy rigorously verified (particularly in the long and medium period ranges). An important finding from the seismological research by Atkinson and Boore and their co-investigators is the similarity of the average frequency characteristics of seismic waves generated at the source between the seemingly very different seismic environments of Eastern and Western North America (ENA and WNA, respectively). A generic definition of the average source properties of earthquakes has therefore been postulated, referred to herein as the generic source model. Further, the generic ‘hard rock’ crustal model which is characteristic of ENA and the generic ‘rock’ crustal model characteristic of WNA have been developed to combine with the generic source model, hence enabling simulations to be made of the important path-related modifications to ground motions arising from different types of crustal rock materials. It has been found that the anelastic contribution to whole path attenuation is consistent between the ENA and WNA models, for earthquake ground motions (response spectral velocities and displacements) in the near and medium fields, indicating that differences in the ENA and WNA motions arise principally from the other forms of path-related modifications, namely the mid-crust amplification and the combined effect of the upper-crust amplification and attenuation, both of which are significant only for the generic WNA ‘rock’ earthquake ground motions. This paper aims to demonstrate the effective utilization of the latest seismological model, comprising the generic source and crustal models, to develop a response spectral attenuation model for direct engineering applications. The developed attenuation model also comprises a source factor and several crustal (wave-path modification) component factors, and thus has also been termed herein the component attenuation model (CAM). Generic attenuation relationships in CAM, which embrace both ENA and WNA conditions, have been developed using stochastic simulations. The crustal classification of a region outside North America can be based upon regional seismological and geological information. CAM is particularly useful for areas where local strong motion data are lacking for satisfactory empirical modelling. In the companion paper entitled ‘response spectrum modelling for rock sites in low and moderate seismicity regions combining velocity, displacement and acceleration predictions’, the CAM procedure has been incorporated into a response spectrum model which can be used to effectively define the seismic hazard of bedrock sites in low and moderate seismicity regions. This paper and the companion paper constitute the basis of a long-term objective of the authors, to develop and effectively utilize the seismological model for engineering applications worldwide.

Compensation of time-delay for control of civil engineering structures

Abstract: Time-delay is an important issue in structural control. Applications of unsynchronized control forces due to time-delay may result in a degradation of the control performance and it may even render the controlled structures to be unstable. In this paper, a state-of-the-art review for available methods of time-delay compensation is presented. Then, five methods for the compensation of fixed time-delay are presented and investigated for active control of civil engineering structures. These include the recursive response method, state-augmented compensation method, controllability based stabilization method, the Smith predictor method and the Pade approximation method, all are applicable to any control algorithm to be used for controlled design. Numerical simulations have been conducted for MDOF building models equipped with an active control system to demonstrate the stability and control performance of these time-delay compensation methods. Finally, the stability and performance of the phase shift method, that is well-known in civil engineering applications, have also been critically evaluated through numerical simulations. Copyright © 2000 John Wiley & Sons, Ltd.

Investigation of dynamic cable–deck interaction in a physical model of a cable‐stayed bridge. Part I: modal analysis

Abstract: A description of an experimental investigation involving the study of the dynamic interaction between the cables and the deck/towers system in cable-stayed bridges is presented. The work was carried out on a physical model of a cable-stayed bridge (the Jindo Bridge, in South Korea), whose characteristics of stiffness and mass have been conveniently scaled. Standard modal analysis tests were performed using both an electrodynamic shaker and a shaking table, leading to the creation of a high-quality database, characterizing the dynamic behaviour of the bridge. The study shows the existence of a clear dynamic interaction between the cables and the deck/towers system, associated with the appearance of several groups of mode shapes, at closely spaced frequencies, involving different cable movements, but similar configurations of the girder and towers.

Response spectrum modelling for rock sites in low and moderate seismicity regions combining velocity, displacement and acceleration predictions

Abstract: SUMMARY Seismic hazard modelling for regions of low and moderate seismicity is often hampered by the lack of indigenous strong motion data. Thus, empirical attenuation models developed for analogous d 29:1457}1489) with observations on local isolated earthquake events, to determine representative design response spectra for both force- and displacementbased seismic applications. In CAM, the acceleration and displacement response spectra may be constructed very conveniently, and with reasonable accuracy, using simpli"ed expressions for the spectrum corner periods and the response spectral velocity parameter, for regions of di!erent geological conditions. The comparison of the response spectra determined by CAM with similar response spectra described by numerous well known empirical models (derived in data rich high seismicity regions) provide very useful indications of the credibility and advantages, as well as the limitations, of CAM. The implied increasing stress drop with increasing moment magnitude is a signi"cant observation from the comparative study.

Finite element model updating for structures with parametric constraints

Abstract: This paper presents a finite element (FE) model updating procedure applied to complex structures using an eigenvalue sensitivity-based updating approach. The objective of the model updating is to reduce the difference between the calculated and the measured frequencies. The method is based on the first-order Taylor-series expansion of the eigenvalues with respect to some structural parameters selected to be adjusted. These parameters are assumed to be bounded by some prescribed regions which are determined according to the degrees of uncertainty that exist in the parameters. The changes of these parameters are found iteratively by solving a constrained optimization problem. The improvement of the current study is in the use of an objective function that is the sum of a weighted frequency error norm and a weighted perturbation norm of the parameters. Two weighting matrices are introduced to provide flexibility for individual tuning of frequency errors and parameters' perturbations. The proposed method is applied to a 1/150 scaled suspension bridge model. Using 11 measured frequencies as reference, the FE model is updated by adjusting ten selected structural parameters. The final updated FE model for the suspension bridge model is able to produce natural frequencies in close agreement with the measured ones. Copyright © 2000 John Wiley & Sons, Ltd.

Characteristic of the vertical seismic waves associated with the 1995 Hyogo‐ken Nanbu (Kobe), Japan earthquake estimated from the failure of the Daikai Underground Station

Abstract: The dynamic behaviour of underground structures built by cut-and-cover methods is discussed. A simple model analysis shows that a column supporting the overburden at midspan (central column) can resonate upon incidence of an elastic wave of a specific frequency. The analytical results indicate that not only the size and material properties of the column, but also the static load acting on the column (overburden) is a decisive factor that influences the resonant frequency. Based on the results obtained by the analysis, the mechanism of the failure at the Daikai Underground Station in Kobe caused by the 1995 Hyogo-ken Nanbu, Japan, earthquake is investigated. It is shown that the wave-induced damage to underground structures can concentrate on the sections with specific overburden, and from the induced damage, it is possible to estimate the frequency characteristics of the associated seismic waves. Copyright © 2000 John Wiley & Sons, Ltd.

Natural frequencies measured from ambient vibration response of the arch dam of Mauvoisin

Abstract: The resonance frequencies of the 250-m-high arch dam of Mauvoisin are obtained by way of ambient vibration tests. It is observed that the resonance frequencies initially increase with rising water level and then decrease with a further rise. This is linked to the two competing features of increasing entrained mass of water (reduction of the resonance frequencies) and of dam stiffening due to closing of the vertical construction joints (augmentation of the resonance frequencies). The ambient vibration test results are complemented by those obtained during earthquakes at an array of 12 accelerographs. Copyright © 2000 John Wiley & Sons, Ltd.

Establishing absorbed energy spectra—an attenuation approach

Abstract: For energy-based seismic design, energy demand in the form of absorbed energy spectra was established by an attenuation relationship. The absorbed energy is proposed for evaluating the energy demand in an inelastic system because the absorbed energy is directly related to the pseudo-velocity in the elastic case. Based on a total of 273 ground motion records from 15 significant earthquakes in California, an attenuation relationship of the absorbed energy was established from a two-stage non-linear regression analysis. This relationship was established for a given earthquake magnitude, source-to-site distance, site class, and ductility factor. A similar expression for the normalized absorbed energy was also developed. This study showed that the absorbed energy for near-field ground motions can be significantly larger than that predicted by the attenuation relationship for normal ground motions. Copyright © 2000 John Wiley & Sons, Ltd.

Bayesian probabilistic damage detection of a reinforced‐concrete bridge column

Abstract: SUMMARY A Bayesian probabilistic approach for damage detection has been proposed for the continuous monitoring of civil structures (Sohn H, Law KH. Bayesian probabilistic approach for structure damage detection. Earthquake Engineering and Structural Dynamics 1997; 26: 1259}1281). This paper describes the application of the Bayesian approach to predict the location of plastic hinge deformation using the experimental data obtained from the vibration tests of a reinforced-concrete bridge column. The column was statically pushed incrementally with lateral displacements until a plastic hinge is fully formed at the bottom portion of the column. Vibration tests were performed at di!erent damage stages. The proposed damage detection method was able to locate the damaged region using a simpli"ed analytical model and the modal parameters estimated from the vibration tests, although (1) only the "rst bending and "rst torsional modes were estimated from the experimental test data, (2) the locations where the accelerations were measured did not coincide with the degrees of freedom of the analytical model, and (3) there existed discrepancies between the undamaged test structure and the analytical model. The Bayesian framework was able to systematically update the damage probabilities when new test data became available. Better diagnosis was obtained by employing multiple data sets than just by using each test data set separately. Copyright ( 2000 John Wiley & Sons, Ltd.

Dynamic analysis of structures with Maxwell model

Abstract: A numerical method has been developed for the dynamic analysis of a tall building structure with viscous dampers. Viscous dampers are installed between the top of an inverted V-shaped brace and the upper beam on each storey to reduce vibrations during strong disturbances like earthquakes. Analytically, it is modelled as a multi-degree-of freedom (MDOF) system with the Maxwell models. First, the computational method is formulated in the time domain by introducing a finite element of the Maxwell model into the equation of motion in the discrete-time system, which is based on the direct numerical integration. Next, analyses for numerical stability and accuracy of the proposed method are discussed. The results show its numerical stability. Finally, the proposed method is applied to the numerical analysis of a realistic building structure to demonstrate its practical validity.

Investigation of dynamic cable-deck interaction in a physical model of a cable-stayed bridge. Part II : seismic response

Abstract: The present paper describes an investigation of the effect of dynamic cable interaction with the deck and towers in the seismic response of a cable-stayed bridge. This study involved shaking table tests performed on a physical model of Jindo bridge, in order to validate two alternative numerical models, which differ in terms of consideration of coupled cable/deck and towers modes. The response to artificial accelerograms was calculated and correlated with measured data. Additional numerical simulations are presented in order to clarify the role that cables play in the attenuation or amplification of the structural response. It was found that the cable interference with global oscillations may cause a decrease of the bridge response. However, this ‘system damping’ may not develop in the case where a narrow-band excitation is applied, causing large amplitude of vibrations of some cables, with significant non-linearity, and inducing higher-order modes. Copyright © 2000 John Wiley & Sons, Ltd.

Correlating dynamic characteristics from field measurements and numerical analysis of a high-rise building

Abstract: Using the concept of lumped masses and rigid floor slabs, several mathematical models were built using a popular PC-based finite element program to model a tall building with a frame-core wall structural system. These models were analysed to obtain the first nine mode shapes and their natural frequencies which were compared with those from field measurements, using numerical correlation indicators. The comparison shows several factors that can have a significant effect on the analysis results. Firstly, outriggers connecting the outer framed tube system to the inner core walled tube system have a significant effect on fundamental translational mode behaviour. Secondly, detailed modelling of the core considering major and minor openings as well as internal thin walls has the strongest influence on torsional behaviour, whose measurements were shown to be an important aspect of the dynamic behaviour for the structure studied. Fine tuning of an analytical model requires not just considering variation in values of structural parameters but also attention to fine detail. Copyright © 2000 John Wiley & Sons, Ltd.

Pseudo‐dynamic testing with substructuring at the ELSA Laboratory

Abstract: The paper describes the distinctive features of the pseudo-dynamic test method as implemented at the ELSA reaction-wall facility. Both hardware and software aspects are considered. Particular attention is devoted to the digital control system and to a coupled numerical-experimental substructuring technique allowing realistic earthquake testing of very large structures. Mathematical and implementation details corresponding to this testing technique are given for both synchronous and asynchronous input motion. Selected test results illustrate the advantages of the presented features.

Forced vibration test of a building with semi-active damper system

Abstract: The authors developed a semi-active hydraulic damper (SHD) and installed it in an actual building in 1998. This was the first application of a semi-active structural control system that can control a building's response in a large earthquake by continuously changing the device's damping coefficient. A forced vibration test was carried out by an exciter with a maximum force of 100 kN to investigate the building's vibration characteristics and to determine the system's performance. As a result, the primary resonance frequency and the damping ratio of a building that the SHDs were not jointed to, decreased as the exciting force increased due to the influence of non-linear members such as PC curtain walls. The equivalent damping ratio was estimated by approximating the resonance curves using the steady-state response of the SDOF bilinear hysteretic system. After the eight SHDs were jointed to the building, the system's performance was identified by a response control test for steady-state vibration. The elements that composed the semi-active damper system demonstrated the specified performance and the whole system operated well. Copyright © 2000 John Wiley & Sons, Ltd.

The dynamic response of seabed anchored floating tunnels under seismic excitation

Abstract: In this paper a procedure for analysing the seismic response of seabed anchored floating tunnels is presented. The first step of the research was the development of an ‘ad hoc’ finite element for modelling the behaviour of anchor elements, with particular reference to the problem of transverse oscillations under time varying axial loads. The element was subsequently inserted in a step-by-step procedure for the numerical analysis of non-linear response to multiple-support seismic input; the procedure encompasses simplified modelling of fluid–structure and soil–structure interaction effects. An example of an application is given concerning two 4680 m long floating tunnels with different seabed profiles. Copyright © 2000 John Wiley & Sons, Ltd.