Showing papers in "Earthquake Engineering & Structural Dynamics in 2001"

One-year monitoring of the Z24-Bridge : environmental effects versus damage events

Abstract: When using the analysis of vibration measurements as a tool for health monitoring of bridges, the problem arises of separating abnormal changes from normal changes in the dynamic behaviour Normal changes are caused by varying environmental conditions such as humidity, wind and most important, temperature The temperature may have an impact on the boundary conditions and the material properties Abnormal changes on the other hand are caused by a loss of stiffness somewhere along the bridge It is clear that the normal changes should not raise an alarm in the monitoring system (ie a false positive), whereas the abnormal changes may be critical for the structure's safety In the frame of the European SIMCES-project, the Z24-Bridge in Switzerland was monitored during almost one year before it was artificially damaged Black-box models are determined from the healthy-bridge data These models describe the variations of eigenfrequencies as a function of temperature New data are compared with the models If an eigenfrequency exceeds certain confidence intervals of the model, there is probably another cause than the temperature that drives the eigenfrequency variations, for instance damage Copyright © 2001 John Wiley & Sons, Ltd

Base isolation for near-fault motions

Abstract: Three analytical studies of base-isolated structures are carried out. First, six pairs of near-fault motions oriented in directions parallel and normal to the fault were considered, and the average of the response spectra of these earthquake records was obtained. This study shows that in addition to pulse-type displacements, these motions contain significant energy at high frequencies and that the real and pseudo-velocity spectra are quite different. The second analysis modelled the response of a model of an isolated structure with a flexible superstructure to study the effect of isolation damping on the performance of different isolation systems under near-fault motion. The results show that there exists a value of isolation system damping for which the superstructure acceleration for a given structural system attains a minimum value under near-fault motion. Therefore, although increasing the bearing damping beyond a certain value may decrease the bearing displacement, it may transmit higher accelerations into the superstructure. Finally, the behaviour of four isolation systems subjected to the normal component of each of the near-fault motions were studied, showing that EDF type isolation systems may be the optimum choice for the design of isolated structures in near-fault locations. Copyright © 2001 John Wiley & Sons, Ltd.

Comparing response of SDF systems to near-fault and far-fault earthquake motions in the context of spectral regions

Abstract: In spite of important differences in structural response to near-fault and far-fault ground motions, this paper aims at extending well-known concepts and results, based on elastic and inelastic response spectra for far-fault motions, to near-fault motions. Compared are certain aspects of the response of elastic and inelastic SDF systems to the two types of motions in the context of the acceleration-, velocity-, and displacement-sensitive regions of the response spectrum, leading to the following conclusions. (1) The velocity-sensitive region for near-fault motions is much narrower, and the acceleration-sensitive and displacement-sensitive regions are much wider, compared to far-fault motions; the narrower velocity-sensitive region is shifted to longer periods. (2) Although, for the same ductility factor, near-fault ground motions impose a larger strength demand than far-fault motions—both demands expressed as a fraction of their respective elastic demands—the strength reduction factors Ry for the two types of motions are similar over corresponding spectral regions. (3) Similarly, the ratio um/u0 of deformations of inelastic and elastic systems are similar for the two types of motions over corresponding spectral regions. (4) Design equations for Ry (and for um/u0) should explicitly recognize spectral regions so that the same equations apply to various classes of ground motions as long as the appropriate values of Ta, Tb and Tc are used. (5) The Veletsos–Newmark design equations with Ta=0.04 s, Tb=0.35 s, and Tc=0.79 s are equally valid for the fault-normal component of near-fault ground motions. Copyright © 2001 John Wiley & Sons, Ltd.

Feasibility of using impedance‐based damage assessment for pipeline structures

Abstract: This paper presents the feasibility of using an impedance-based health monitoring technique in monitoring a critical civil facility. The objective of this research is to utilize the capability of the impedance method in identifying structural damage in those areas where a very quick condition monitoring is urgently needed, such as in a post-earthquake analysis of a pipeline system. The basic principle behind this technique is to utilize high-frequency structural excitation (typically greater than 30 kHz) through surface-bonded piezoelectric sensors/actuators to detect changes in structural point impedance due to the presence of damage. Real-time damage detection in pipes connected by bolted joints was investigated, and the capability of the impedance method in tracking and monitoring the integrity of the typical civil facility has been demonstrated. Data collected from the tests illustrates the capability of this technology to detect imminent damage under normal operating conditions and immediately after a natural disaster. Copyright © 2001 John Wiley & Sons, Ltd.

Effect of earthquake ground motions on fragility curves of highway bridge piers based on numerical simulation

Abstract: SUMMARY Fragility curves express the probability of structural damage due to earthquakes as a function of ground motion indices, e.g., PGA, PGV. Based on the actual damage data of highway bridges from the 1995 Hyogoken-Nanbu (Kobe) earthquake, a set of empirical fragility curves was constructed. However, the type of structure, structural performance (static and dynamic) and variation of input ground motion were not considered to construct the empirical fragility curves. In this study, an analytical approach was adopted to construct fragility curves for highway bridge piers of speciCc bridges. A typical bridge structure was considered and its piers were designed according to the seismic design codes in Japan. Using the strong motion records from Japan and the United States, non-linear dynamic response analyses were performed, and the damage indices for the bridge piers were obtained. Using the damage indices and ground motion indices, fragility curves for the bridge piers were constructed assuming a lognormal distribution. The analytical fragility curves were compared with the empirical ones. The proposed approach may be used in constructing the fragility curves for highway bridge structures. Copyright ? 2001 John Wiley & Sons, Ltd.

Evaluation of seismic energy demand

Abstract: In this paper, a method is proposed in order to obtain a simplified representation of hysteretic and input energy spectra. The method is based on the evaluation of the equivalent number of cycles correlated to the earthquake characteristics by the proposed seismic index ID. This procedure allows us to obtain peak values of the hysteretic and input energy that depend on the demanded ductility, on the seismic index ID and on the peak pseudo-velocity. The assessment of the input energy represents a first step towards the definition of a damage potential index capable of taking into account the effect of the duration of the ground motions. Copyright © 2001 John Wiley & Sons, Ltd.

Bayesian spectral density approach for modal updating using ambient data

Abstract: The problem of identification of the modal parameters of a structural model using measured ambient response time histories is addressed. A Bayesian spectral density approach (BSDA) for modal updating is presented which uses the statistical properties of a spectral density estimator to obtain not only the optimal values of the updated modal parameters but also their associated uncertainties by calculating the posterior joint probability distribution of these parameters. Calculation of the uncertainties of the identified modal parameters is very important if one plans to proceed with the updating of a theoretical finite element model based on modal estimates. It is found that the updated PDF of the modal parameters can be well approximated by a Gaussian distribution centred at the optimal parameters at which the posterior PDF is maximized. Examples using simulated data are presented to illustrate the proposed method. Copyright © 2001 John Wiley & Sons, Ltd.

Hysteretic energy spectrum and damage control

Abstract: The inelastic response of single-degree-of-freedom (SDOF) systems subjected to earthquake motions is studied and a method to derive hysteretic energy dissipation spectra is proposed. The amount of energy dissipated through inelastic deformation combined with other response parameters allow the estimation of the required deformation capacity to avoid collapse for a given design earthquake. In the first part of the study, a detailed analysis of correlation between energy and ground motion intensity indices is carried out to identify the indices to be used as scaling parameters and base line of the energy dissipation spectrum. The response of elastoplastic, bilinear, and stiffness degrading systems with 5 per cent damping, subjected to a world-wide ensemble of 52 earthquake records is considered. The statistical analysis of the response data provides the factors for constructing the energy dissipation spectrum as well as the Newmark-Hall inelastic spectra. The combination of these spectra allows the estimation of the ultimate deformation capacity required to survive the design earthquake, capacity that can also be presented in spectral form as an example shows.

On-line identification of non-linear hysteretic structural systems using a variable trace approach

Abstract: In this paper, an adaptive on-line parametric identification algorithm based on the variable trace approach is presented for the identification of non-linear hysteretic structures. At each time step, this recursive least-square-based algorithm upgrades the diagonal elements of the adaptation gain matrix by comparing the values of estimated parameters between two consecutive time steps. Such an approach will enforce a smooth convergence of the parameter values, a fast tracking of the parameter changes and will remain adaptive as time progresses. The effectiveness and efficiency of the proposed algorithm is shown by considering the effects of excitation amplitude, of the measurement units, of larger sampling time interval and of measurement noise. The cases of exact-, under-, over-parameterization of the structural model have been analysed. The proposed algorithm is also quite effective in identifying time-varying structural parameters to simulate cumulative damage in structural systems. Copyright © 2001 John Wiley & Sons, Ltd.

Optimal seismic response control with dampers

Abstract: The usefulness of energy dissipation devices to reduce seismic response of structures is now well established. For a given installation of such devices in a structure, one can easily compute the level of response reduction achieved. However, the solution of an inverse problem of how many devices one would need to achieve a desired level of response reduction in a structure, or to achieve an expected level of performance from a structural system, is not quite as straightforward and well formulated. In this paper, a method is presented to obtain the amount of viscous and visco-elastic damping one would need to obtain a desired level of response reduction. The needed supplemental devices are also optimally distributed in the structure to achieve the best performance. To solve the optimal problem, a gradient-based optimization approach is used. To illustrate the application, numerical results for a 24-storey building structure are presented where the objective is to achieve the maximum reduction in the performance functions expressed in terms of the inter-storey drifts, base shear, or floor accelerations. Other forms of performance functions can also be treated similarly. Copyright © 2001 John Wiley & Sons, Ltd.

Pounding of structures modelled as non‐linear impacts of two oscillators

Abstract: A new formulation is proposed to model pounding between two adjacent structures, with natural periods T1 and T2 and damping ratios ζ1 and ζ2 under harmonic earthquake excitation, as non-linear Hertzian impact between two single-degree-of-freedom oscillators. For the case of rigid impacts, a special case of our analytical solution has been given by Davis (‘Pounding of buildings modelled by an impact oscillator’ Earthquake Engineering and Structural Dynamics, 1992; 21:253–274) for an oscillator pounding on a stationary barrier. Our analytical predictions for rigid impacts agree qualitatively with our numerical simulations for non-rigid impacts. When the difference in natural periods between the two oscillators increases, the impact velocity also increases drastically. The impact velocity spectrum is, however, relatively insensitive to the standoff distance. The maximum relative impact velocity of the coupled system can occur at an excitation period Tn* which is either between those of the two oscillators or less than both of them, depending on the ratios T1/T2 and ζ1/ζ2. Although the pounding force between two oscillators has been primarily modelled by the Hertz contact law, parametric studies show that the maximum relative impact velocity is not very sensitive to changes in the contact parameters. Copyright © 2001 John Wiley & Sons, Ltd.

Improved control algorithm for real‐time substructure testing

Abstract: Real-time substructure testing is a novel method of testing structures under dynamic loading. The complete structure is separated into two substructures, one of which is tested physically at large scale and in real time, so that time-dependent non-linear behaviour of the substructure is realistically represented. The second substructure represents the surrounding structure, which is modelled numerically. In the current formulation this numerical substructure is assumed to remain linear. The two substructures interact in real-time so that the response of the complete structure, incorporating the non-linear behaviour of the physical substructure, is accurately represented. This paper presents several improvements to the linear numerical modelling of substructures for use in explicit time-stepping routines for real-time substructure testing. An extrapolation of a first-order-hold discretization is used which increase the accuracy of the numerical model over more direct explicit methods. Additionally, an integral form of the equation of motion is used in order to reduce the effects of noise and to take into account variations of the input over a time-step. In order to take advantage of this integral form, interpolatuion of the model output is performed in order to smooth the output. The improvements are demonstrated using a series of substructure tests on a simple portal frame. While the testing approach is suitable for cases in which the physical substructure behaves non-linearly, the results presented here are for fully linear systems. This enables comparisons to be made with analytical solutions, as well as with the results of tests based on the central difference method. Copyright \©C 2001 John Wiley and Sons, Ltd

A new proposal for simplified design of buried steel pipes crossing active faults

Abstract: Simplified design methods for obtaining the maximum strain in pipelines crossing active faults proposed by Newmark, Kennedy and Wang have not considered the section deformation of the pipe. In this study, a new simplified method is developed for obtaining the maximum strain in steel pipes crossing faults considering non-linearity of material and geometry of pipe section. It is assumed that the pipe will bend near the fault and the geometry of pipe in the longitudinal direction will change according to a bent deformation. On the other hand, the relation between maximum strain and bent angle has been obtained using a beam–shell hybrid FEM for different pipe-fault conditions. The developed method can be used for calculating the maximum strains for fault-crossing steel pipes with different angles of crossing both in tension and compression, by considering the deformation of the pipe cross-section. Copyright © 2001 John Wiley Sons, Ltd.

Placement of sensors/actuators on civil structures using genetic algorithms

Abstract: The optimal design and placement of controllers at discrete locations is an important problem that will have impact on the control of civil engineering structures. Though algorithms exist for the placement of sensor/actuator systems on continuous structures, the placement of controllers on discrete civil structures is a very difficult problem. Because of the nature of civil structures, it is not possible to place sensors and actuators at any location in the structure. This usually creates a non-linear constrained mixed integer problem that can be very difficult to solve. Using genetic algorithms in conjunction with gradient-based optimization techniques will allow for the simultaneous placement and design of an effective structural control system. The introduction of algorithms based on genetic search procedures should increase the rate of convergence and thus reduce the computational time for solving the difficult control problem. The newly proposed method of simultaneously placing sensors/actuators will be compared to a commonly used method of sensors/actuators placement where sensors/actuators are placed sequentially. The savings in terms of energy requirements and cost will be discussed.

Transverse response of underground cavities and pipes to incident SV waves

Abstract: The transverse response of underground cylindrical cavities to incident SV waves is investigated. Analytical solutions are derived for unlined cavities embedded within an elastic half-space using Fourier–Bessel series and a convex approximation of the half-space free surface. The computed displacements at the half-space free surface and the tangential stresses on the cavity are compared with the results of previous investigations. The analytical solutions are extended to formulate approximate solutions for assessing hoop stresses within cavity liners impinged by low-frequency waves having wavelengths much longer than the cavity diameter. The approximate solutions are compared to existing numerical solutions, and used to evaluate the dynamic response of a flexible buried pipe shaken by the 1994 Northridge earthquake. The proposed approximate model for cavity liners is useful for the seismic analysis of underground pipes and small-diameter tunnels. Copyright © 2001 John Wiley & Sons, Ltd.

Overview of the application of active/semiactive control to building structures in Japan

Abstract: SUMMARY Perhaps one of the most signi:cant technological innovations in the structural engineering :eld is the practical application of active and semiactive control to civil structures. A number of structures integrating active, hybrid, and semiactive response control technologies have been constructed in Japan. Most of them are building structures. This paper provides an overview of those building structures, focusing mainly on the types of buildings that are controlled, and on the types of control devices that are implemented. Future directions of structural engineering are also discussed. Copyright ? 2001 John Wiley & Sons, Ltd.

Relative displacement response spectra with pounding effect

Abstract: To avoid unseating of a deck, an adequate seat width must be provided. The seat width is basically determined from maximum relative displacement between two bridge segments. Under a strong ground excitation, pounding between two decks may occur at a joint. The pounding will affect the response of two bridge segments. This research is conducted to investigate the effect of pounding on the relative displacement between two adjacent bridge segments. A simplified analytical model of two linear single-degree-of-freedom systems is employed. To take into account the pounding, the laws of conservation of momentum and energy are applied. The analytical results are represented in the form of relative displacement response spectra with pounding effect. It is found that due to the pounding the relative displacement can be amplified, resulting in the requirement of a longer seat width to support a deck. The formulation of normalized relative displacement response spectra is presented together with an application example. It is found that the seat width determined from the relative displacement response spectra with pounding effect becomes close to the value specified in the Japanese design specifications for structures with large difference of natural periods.

Evaluation of combination rules for maximum response calculation in multicomponent seismic analysis

Abstract: The existing rules for combining peak response to individual components of ground motion are evaluated. The response values r e to two horizontal components of ground motion estimated by four multicomponent combination rules-SRSS-, 30%-, 40%- and simplified-SRSS-rules-are compared with the critical response, r cr , obtained by the CQC3-rule, which takes into account the direction of the principal ground components with respect to the structural axes and provides the largest response over all possible seismic incident angles, The following results are obtained in the first part of the paper and are valid for any elastic structure and any earthquake design response spectrum: For realistic values of the ratio y of the design spectra for the two principal components of ground motion the SRSS-rule estimate lies between 0.79r cr and 1.00r cr , the Simplified-SRSS-rule estimate lies between 1.00r cr and 1.26r cr , the 40%-rule estimate lies between 0.99r cr and 1.25r cr , and the 30%-rule estimate lies between 0.92r cr and 1.16r cr . None of the multicomponent combination rules account for the increase in response of systems if the vibration periods of the two modes that contribute most to the response to the x- and y-components of ground motion are close to each other. Evaluated in the second part of the paper is the accuracy of the multicomponent combination rules in estimating the response of a range of one-storey systems with (a) symmetrical plan and (b) unsymmetrical plan, and of two multistorey buildings. The SRSS-rule underestimates the response by up to 16% and the other three rules overestimate it by up to 18%. Although these errors appear to be smaller than the many approximations inherent in structural design, they can be eliminated with very little additional computation by using an explicit formula for the critical response based on the CQC3 rule.

Shaking table tests of 1:4 reduced-scale models of masonry infilled reinforced concrete frame buildings

Abstract: Two models of masonry infilled reinforced concrete frame buildings were tested at the shaking table. Models were built in the reduced scale 1:4 using the materials produced in accordance to modelling demands of true replica modelling technique. The first model represented a one-storey box-like building and the second one the two-stories building with plan shaped in the form of a letter H. Models were shaken with the series of horizontal sine dwell motions with gradually increasing amplitude. Masonry infills of tested models were constructed of relatively strong bricks laid in weak mortar. Therefore, typical cracks developed and propagated along mortar beds without cracking of bricks or crushing of infill comers. Data collected from tests will be used in future evaluation, verification and development of computational models for prediction of in-plane and out-of-plane behaviour of masonry infills. The responses of tested models can be well compared with global behaviour of real structures using the modelling rules. The similarity of local behaviour of structural elements, e.g. reinforced concrete joints, is less reliable due to limitations in modelling of steel reinforcement properties. The model responses showed that buildings designed according to Eurocodes are able to sustain relatively high dynamic excitations due to a significant level of structural overstrength.

Dynamic behavior of perimeter lateral‐system structures with flexible diaphragms

Abstract: Building structures are typically designed using the assumption that the floor systems serve as a rigid diaphragm between the vertical elements of the lateral load-resisting system. However, long-floor span structures with perimeter lateral load-resisting systems possess diaphragms which behave quite flexibly. The dynamic behaviour of such structures is dissimilar to the behavior expected of typical structures. This difference can lead to unexpected force and drift patterns. If force levels are sufficiently under-estimated, inelastic diaphragm behaviour can occur, exacerbating the effects of diaphragm flexibility. Such response may lead to a non-ductile diaphragm failure or structural instability due to high drift demands in the gravity system. Analytical models were developed which capture the diaphragm flexibility of structures with long-floor spans and perimeter lateral-systems. Modal examination and time-history analyses were performed to determine the effect of diaphragm flexibility and diaphragm inelastic behaviour on the dynamic behaviour of these structures. Copyright © 2001 John Wiley & Sons, Ltd.

Substructuring of dynamical systems via the adaptive minimal control synthesis algorithm

Abstract: In this paper we consider the concept of modelling dynamical systems using numerical–experimental substructuring. This type of modelling is applicable to large or complex systems, where some part of the system is difficult to model numerically. The substructured model is formed via the adaptive minimal control synthesis (MCS) algorithm. The aim of this paper is to demonstrate that substructuring can be carried out in real time, using the MCS algorithm. Thus, we reformulate the MCS algorithm into a substructuring form. We introduce the concepts of a transfer system, and carry out numerical simulations of the substructuring process using a coupled three mass example. These simulations are compared with direct simulations of a three mass system. In addition we consider the stability of the substructuring algorithm, which we discuss in detail for a class of second-order transfer systems. A numerical–experimental system is considered, using a small-scale experimental system, for which the substructuring algorithm is implemented in real time. Finally we discuss these results, with particular reference to the future application of this method to modelling large-scale structures subject to earthquake excitation. Copyright © 2001 John Wiley & Sons, Ltd.

Seismic isolation of rigid cylindrical tanks using friction pendulum bearings

Abstract: Storage tanks are vulnerable to earthquakes, as numerous major earthquakes have demonstrated. The trend of recent revisions to make seismic design criteria for large-scale industrial storage tanks increasingly stringent has made development of cost-effective earthquake-resistant design and retrofit techniques for industrial tanks imperative. This study assesses the feasibility of seismic base isolation for making liquid-filled storage tanks earthquake resistant. The sliding-type friction pendulum seismic (FPS) bearings are considered rather than the elastomeric bearings because the dynamic characteristics of an FPS-isolated tank remain unchanged regardless of the storage level. This work has devised a hybrid structural-hydrodynamic model and solution algorithm, which would permit simple, accurate and efficient assessment of the seismic response of rigid cylindrical storage tanks in the context of seismic isolation. Extensive numerical simulations confirm the effectiveness of seismic base isolation of rigid cylindrical tanks using FPS bearings. Copyright © 2001 John Wiley & Sons, Ltd.

Inelastic buildings with tuned mass dampers under moderate ground motions from distant earthquakes

Abstract: The effectiveness of tuned mass dampers (TMD) in vibration control of buildings was investigated under moderate ground shaking caused by long-distance earthquakes with frequency contents resembling the 1985 Mexico City (SCT) or the 1995 Bangkok ground motion. The elastic–perfectly plastic material behaviour was assumed for the main structure, with linear TMDs employed by virtue of their simplicity and robustness. The accumulated hysteretic energy dissipation affected by TMD was examined, and the ratio of the hysteretic energy absorption in the structure with TMD to that without it is proposed to be used, in conjunction with the peak displacement ratio, as a supplementary TMD performance index since it gives an indication of the accumulated damage induced in the inelastic structures. For the ground motions considered, TMD would be effective in reducing the hysteretic energy absorption demand in the critical storeys for buildings in the 1.8–2.8 s range. The consequence is reduction in damage of the buildings which would otherwise suffer heavy damage in the absence of TMD, resulting in economical restorability in the damage control limit state. This is of practical significance in view of the current trend toward performance-based design. Copyright © 2001 John Wiley & Sons, Ltd.

Active tendon control of cable – stayed bridges: a large scale demonstration

Abstract: This paper presents a strategy for active damping of cable structures, using active tendons. The first part of the paper summarizes the theoretical background: the control law is briefly presented together with the main results of an approximate linear theory which allows the prediction of closed-loop poles with a root locus technique. The second part of the paper reports on experimental results obtained with two test structures: the first one is a small size mock-up representative of a cable-stayed bridge during the construction phase. The control of the parametric vibration of passive cables due to deck vibration is demonstrated. The second one is a 30 m long mock-up built on the reaction wall of the ELSA test facility at the JRC Ispra (Italy); this test structure is used to demonstrate the practical implementation of the control strategy with hydraulic actuators. Copyright © 2001 John Wiley & Sons, Ltd.

An experimental investigation of water level effects on the dynamic behaviour of a large arch dam

Abstract: The need for full-scale dynamic tests, which are recognized as the most reliable method to evaluate a structure's vibration properties, is increasing as new analysis techniques are developed that take into account the complex interaction phenomenons that occur in dam–reservoir–foundation systems. They are extremely useful to obtain reliable data for the calibration of newly developed numerical methods. The Earthquake Engineering and Structural Dynamics Research Center (CRGP) at the University of Sherbrooke has been developing and applying dynamic testing methods for large structures in the past 10 years. This paper presents the experimental evaluation of the effects of the varying water level on the dynamic response of the 180 m Emosson arch dam in Switzerland. Repeated forced-vibration tests were carried out on the dam during four different periods of the reservoir's filling cycle during a one-year span. Acceleration and hydrodynamic pressure frequency responses were obtained at several locations while the dam was subjected to horizontal harmonic loading. The variation of the resonant frequencies as a function of the reservoir level is investigated. A summary of the ongoing numerical correlation phase with a three-dimensional finite element model for the dam–reservoir–foundation system is also presented. Copyright © 2001 John Wiley & Sons, Ltd.

Active variable stiffness system with non‐resonant control

Abstract: The active variable stiffness (AVS) system is proposed as a seismic response control system. It actively controls structural stiffness of a building to establish a non-resonant state against earthquake excitations, thus suppressing the building's response. It consumes a relatively small amount of energy and maintains the safety of the building in moderate to severe earthquakes. In order to accumulate and analyse practical data, a building was constructed as a trial structure about ten years ago. This paper describes the control algorithm, the applied system, some observed earthquake records, verification of control effectiveness based on simulation analyses, and some issues concerning system maintenance. Through earthquake observations, it was confirmed that the system could select the appropriate stiffness that assures a non-resonant state, which results in a minimum response. Copyright © 2001 John Wiley & Sons, Ltd.

Probabilistic critical excitation for MDOF elastic–plastic structures on compliant ground

Abstract: Earthquake ground motions and their effects on structural responses are very uncertain even with the present knowledge. It is therefore desirable to develop a robust structural design method taking into account these uncertainties. Critical excitation approaches are promising and a new random critical excitation method is proposed for MDOF elastic–plastic shear-building structures on compliant ground. The power (area of power spectral density (PSD) function) and the intensity (magnitude of PSD function) are fixed and the critical excitation is found under these restrictions. In contrast to linear elastic structures, transfer functions and simple expressions for response evaluation cannot be defined in elastic–plastic structures and difficulties arise in describing the peak responses except by laborious elastic–plastic time-history response analysis. Statistical equivalent linearization is used to estimate the elastic–plastic stochastic peak responses approximately. The critical excitation responses are obtained for several examples and compared with those of the corresponding recorded earthquake ground motion. Copyright © 2001 John Wiley & Sons, Ltd.

Use of a shared tuned mass damper (STMD) to reduce vibration and pounding in adjacent structures

Abstract: The dynamic response of tall civil structures due to earthquakes is very important to civil engineers. Structures exposed to earthquakes experience vibrations that are detrimental to their structural components. Structural pounding is an additional problem that occurs when buildings experience earthquake excitation. This phenomena occurs when adjacent structures collide from their out-of-phase vibrations. Many energy dissipation devices are presently being used to reduce the system response. Tuned mass dampers (TMD) are commonly used to improve the response of structures. The stiffness and damping properties of the TMD are designed to be a function of the natural frequency of the building to which it is connected. This research involves attaching adjacent structures with a shared tuned mass damper (STMD) to reduce both the structures vibration and probability of pounding. Because the STMD is connected to both buildings, the problem of tuning the STMD stiffness and damping parameters becomes an issue. A design procedure utilizing a performance function is used to obtain the STMD parameters to result in the best overall system response. Copyright © 2001 John Wiley & Sons, Ltd.

Generating multiple spectrum compatible accelerograms using stochastic neural networks

Abstract: A new neural-network-based methodology for generating artificial earthquake spectrum compatible accelerograms from response spectra was proposed in 1997, in which, the learning capabilities of neural networks were used to develop the knowledge of the inverse mapping from the response spectra to earthquake accelerograms. Recently, this methodology has been further extended and enhanced. This paper presents a new stochastic neural network that is capable of generating multiple earthquake accelerograms from a single-response spectrum. A new stochastic feature to the neural network has been combined with a new scheme for data compression using the replicator neural networks developed in the original method. A benefit of this extended methodology is gaining efficiency in compressing the earthquake accelerograms and extracting their characteristics. The proposed method produces a stochastic ensemble of earthquake accelerograms from any response spectra or design spectra. An example is presented that used 100 recorded accelerograms to train the neural network and several design spectra and response spectra to test this improved methodology. Copyright © 2001 John Wiley & Sons, Ltd.

A new method for non‐stationary random critical excitation

Abstract: Since earthquake ground motions are very uncertain even with the present knowledge, it is desirable to develop a robust structural design method taking into account these uncertainties. Critical excitation approaches are promising and a new non-stationary random critical excitation method is proposed. In contrast to the conventional critical excitation methods, a stochastic response index is treated as the objective function to be maximized. The power (area of power spectral density (PSD) function) and the intensity (magnitude of PSD function) are fixed and the critical excitation is found under these restrictions. It is shown that the original idea for stationary random inputs can be utilized effectively in the procedure for finding a critical excitation for non-stationary random inputs. The key for finding the new non-stationary random critical excitation is the exchange of the order of the double maximization procedures with respect to time and to the power spectral density function. Copyright © 2001 John Wiley & Sons, Ltd.