Supplemental energy-dissipating systems for bride structures have been proposed to mitigate harmful effects of earthquakes. Controllable damping devices are among these systems. They can potentially provide immediate vibration control for structures subjected to destructive seismic motions. In response to these demanding needs, a theoretical and experimental study is performed to evaluate the response of a scaled-model bridge using magneto-rheological fluid (MRF) dampers. The scaled- model bridge is analyzed for its dynamic response to various inputs when MRF dampers are utilized to control the structure. In addition, a graphite/epoxy-concrete column is used in the scaled bridge structure, and its interaction with MRF damper is studied. Procedures and results for the theoretical and experimental testing of the MRF dampers and the scaled-model bridge are presented. MRF dampers proved to significantly improve the response of the structure.

This study illustrates the behavior of a closed-loop vibration control system making use of electrorheological (ER) devices. Material properties of ER suspensions (primarily the yield stress, and visco-elastic properties) increase by several orders of magnitude when subjected to strong electrical fields (kV/mm). Because the electric field controls the yielding behavior of ER materials, ER devices are inherently non- linear. A Lyapunov-based controller is designed to attenuate seismically-induced structural motions. Using this controller, the control decisions are independent of the structural model, and are therefore robust to modeling errors. Structures operated according to the control rule maintain an anti- resonant condition. This is demonstrated in the time-domain and the frequency domain. A numerical study, incorporating an evolutionary model for the ER device illustrates the robustness of this control method to model uncertainties.

A 310 m Nanjing TV transmission tower in China will be installed with an active mass driver on the upper observation deck in order to reduce the acceleration responses under strong winds. The wind-induced structural responses considered in this paper include the coupling effect of lateral and torsional motions. The along-wind and across-wind components of the wind velocity are modeled as random processes defined by the Davenport cross-power spectra. The structural responses, in particular the acceleration, increase due to the coupling effect of lateral and torsional motions. This paper presents the Linear Quadratic Gaussian (LQG) control strategy using accelerations as the feedback quantities to reduce the tower response. Emphasis is placed on the practical applications, such as the limitations on actuator peak force and stroke, limited number of sensors, noise pollution, etc. A state reduced-order system has been established to design the dynamic output feedback controllers. The power spectral density and rms of acceleration responses of the TV transmission tower equipped with an active mass driver have been computed. Simulation results demonstrate that the LQG strategy is remarkable in reducing the lateral and torsional motions of the tower and it is suitable for the full-scale implementation of active mass driver on Nanjing Tower.

This paper presents a new seismic response-control system which a new type of toggle mechanism is installed. The one of authors have indicated that the lever mechanism with auxiliary mass can reduce sufficiently the input effect such as earthquake ground motions under the severe condition that the stiffness of lever arm has very high keeping large arm ratios. This paper develops the toggle system in order to overcome this difficulties and derives theoretical formulas of this system. The theory and control effectiveness of the proposed system are confirmed through a series of shaking table test for the model structure with the toggle system.

This study presents the design, construction, and test results of an electromagnetic stress sensor for monitoring bridge cables and prestressed concrete structures. The sensor uses the reverse magnetostrictive effect found in high elastic limit steels such as those used in cables and in prestressed concrete. This effect is characterized by the variation in the steel's magnetic permeability as a function of its internal stress. Consequently, the internal stresses in this high elastic limit steels can be found by measuring their permeability. The permeability can be measured indirectly by measuring the inductance of a coil placed around or near the cable. We designed a prototype of the sensor with a finite element program. We also used this program to optimize the sensing coil and the measurement frequency and to design the magnetic shielding around the sensor. We built and tested the prototype in our laboratory. We evaluated the sensitivity, precision, linearity, and reliability of the sensor, and also the influence of external thermal and magnetic perturbations on the sensor measurements. The results were very satisfactory. The major advantages of this sensor are its robustness and its ability operate continuously for several decades even in hostile environments. These types of sensors, embedded in or added to the structure, are used to monitor stresses in cables and in prestressed concrete structures used in bridges and nuclear stations.

Strain-sensing concrete is concrete capable of sensing its own strain, as provided by using short carbon fibers (as little as 0.2 vol.%) as an admixture. The sensing ability is related to the change in volume electrical resistivity of the concrete upon straining, as resulted from the change in contact electrical resistivity between fiber and matrix. The gage factor (fractional change in volume resistivity per unit strain) is up to 700. This paper reports that surface treatment of the carbon fibers by using ozone prior to incorporation in concrete improves the strain sensing ability, in addition to improving the mechanical properties. These effects are due to the increase in fiber-matrix bond strength, which is in turn due to the decrease in the contact angle between fiber and water to zero. The contact angle decrease indicates wetting of fiber by water and is due to the increase in fiber surface oxygen concentration and the change from C-O to C = O functional groups on the surface of the fiber. The improvement in strain sensing ability relates to the increase in gage factor and the absence of an irreversible resistivity decrease from cycle to cycle. The improvement in mechanical properties relates to increases in tensile strength, modulus and ductility. The ozone treatment has no effect on the tensile strength, volume electrical resistivity or morphology of the fiber itself.

A simple method to measure metal corrosion using a single- pitch Bragg grating sensor is presented. The prestrained Bragg grating that is only partially coated with a metal can be designed as a corrosion sensor, and the section without coating can be used as a temperature sensor for compensations. After releasing the preloading, the residual stress inside the section with a metal coating can be used to predict the corrosion. Consequently, the two Bragg wavelengths reflecting from the corrosion sensor are separated and have no cross-talk problem in corrosion and temperature signals. The principle of corrosion sensors is that environmental corrosion will result in a thinner thickness on coating, of course, the residual strain will release and cause a Bragg wavelength shifting. Therefore, after the temperature variation is obtained from the free section, the environmental corrosion can be determined according to the released residual strain on the Bragg grating fiber. Two analytical models for corrosion sensors are developed and they are in good agreements. Also, experimental data shows that corrosion Bragg grating sensors are feasible for quantitative analysis.

Normal reinforced concrete lacks the ability to directly respond to the formation of cracking within its own cross section during dynamic loading. A way to introduce this ability is to apply self-healing concepts in the design of reinforced concrete members. Such members could then 'intelligently' react in the event of damaging forces -- by deriving the means of repair from within themselves. Self- healing involves the timed release of adhesive into the member at the time of cracking. Chemically inert encapsulations are filled with adhesive and cast within the cross section of the member. At the onset of cracking, the wall fractures, allowing adhesive to exit and penetrate the developing crack. With this method, adhesives with different characteristics could be applied to different areas of a monolithic, reinforced concrete structural system, in order to accomplish specific results. For example, high strength adhesive could be used in areas where increased stiffness was desired, and more flexible adhesive could be potentially be used to improve energy dissipation or damping. The method would be most appropriate for highly indeterminate structures, where moment redistribution between members tends to 'refocus' stress temporarily. This gives the adhesive time to repair the cracked section and improve local capacity against further damage.

Bridge piers with steel hollow box sections are widely used in Japan for highway bridges. Unlike the steel columns in buildings, these piers are designed with high width-thickness ratios of component plates. This makes them susceptible to damage during severe earthquakes. In order to improve the seismic performance, an energy absorption strategy which localizes damage in structural systems is proposed: concentrating damage in specially designed weak structural elements, with which energy is absorbed during earthquake. Here, Low yield point steel (LYPS) plates attached to steel pier are employed as the weak elements. LYPS is a new kind of material which has low yield stress (around 100 MPa), large ductility and adequate fatigue life. In case of earthquake events, LYPS plates yield to absorb energy and keep the main structure always elastic. In this way, damage is concentrated in LYPS plates which can be easily replaced after earthquake. The paper starts with construction of constitutive model of LYPS with verification by experiment. The energy absorption performance of LYPS plates is studied by 3-dimensional finite element analysis (3DFEM), and optimal design method is proposed.

Most infrastructures are formed by large amount of parts, like members of truss, decks of bridge, and so on. On the connection, relative movement between members causes load redistribution or even failure. Monitoring the integrity of the structural system can assure its performance and safety. TDR monitoring system is proposed here to be a multiple points monitoring system using only one coaxial cable as the sensing and conducting media all the way through the monitored structure. The cable is fixed at both end of each pairs of adjacent members to be the sensing device along itself. Relative movements between each member at the connecting points are monitored simultaneously by sending a fast rise impulse into the cable. When lateral movement applied, the cable will deform and having the effect of adding an equivalent capacitance at that point and causing the signature to jump up. And, the most important ability for TDR system is the location of the relative movement along the cable is very clearly shown in the time domain signature. The pulse of a TDR tester can go a few thousands feet which gives enough monitoring range for most infrastructure.

The Lutrive highway bridges are two twin bridges built in 1972 by the cantilever method with mid-span articulations. The bridge deck consists of a box girder of variable depth. In 1997 this bridge was instrumented with 30 SOFO fiber optic deformation sensors installed inside the box girder. This sensor network is mainly aimed at measuring the short- and long-term spatial displacements of one of the spans. This includes the displacement resulting from the daily variations of temperature and the long-term creep effects. It was found that these same sensors could also be used to capture the quasi-static part of the dynamic deformation of the bridge under traffic load. Although the measurement system can acquire measurements only at intervals a few seconds apart, it was found that these 'snapshots' could give interesting information about the low frequency quasi-static deformations of the bridge. The data from pair of sensors was combined to obtain information about the instantaneous curvature variations of the bridge. This curvature data was then analyzed statistically to extract information about the dynamic traffic loads. This measurement and analysis method was validated in a fatigue test on a concrete slab.

This paper presents an analysis of a bolted lap joint assembly using continuum wave models to examine the scattering of flexural waves across a joint under loss of torque preload. The motivation for this work is derived from monitoring the dynamics of large buildings and bridges to assess the level of damage in bolted or riveted assemblies following a severe loading condition. In the event of such a condition, these joints can loosen or fail completely creating a potentially hazardous environment for the general public while these structures are still in service. It has been suggested that using nondestructive techniques to monitor the health of these bolted connections to determine the extent of damage can minimize such risks to the public. This paper discusses such a health monitoring approach using local mechanical impedance models to characterize the non-linear joint dynamics of a bolted assembly. As the torque on the bolted assembly is varied, the local and global response of a fixed-fixed one- dimensional structure is studied. The loss of torque load is correlated with the changes seen in the local scattering dynamics of the bolted joint. Incoming and outgoing wave dynamics at the joint are separated to compute the frequency dependent local scattering matrix of the joint and determine its affect on the global system response as the torque on the bolted assembly is varied. Analytical results using the continuum wave descriptions of the structural dynamics are compared against experimental data on a beam fixed at both ends with a bolted lap joint located at its center. Using a least squares cost function between experimental and analytical transfer function data, an effective joint stiffness is obtained. Minimizing the cost function leads to an optimal solution for a parameterized model of the joint dynamics and attempts to account for possible non-linearities such as cubic springs, deadband, stiction and hysteresis. Piezoelectric actuators are used to excite the beam in bending. Strain sensor arrays are distributed on both sides of the joint to capture the local scattering properties of the joint as a function of torque load.

Many researchers have proposed damage detection techniques that exploit changes in modal parameters to identify the extent and location of damage in large structures. These analyses, however, generally neglect the effects of environmental changes on modal parameters. Such environmental effects include changes in loads, boundary conditions, temperature, and humidity. Data from real bridge structures indicate that the effects of environmental changes can be significant. In fact, these changes can often mask more subtle structural changes caused by damage. This paper examines a linear adaptive model that may discriminate the changes of modal parameters due to temperature changes from those caused by structural damage or other environmental effects. Data from the Alamosa Canyon Bridge in the state of New Mexico were used to demonstrate the effectiveness of the adaptive filter for this problem. Results indicate that a linear four-input (two time and two spatial dimensions) filter of temperature can reproduce the natural variability of the frequencies with respect to time of day. Using this simple model, we attempt to establish a confidence interval of the fundamental frequency for a new temperature profile in order to discriminate the natural variation due to temperature.

It is well known that the static and dynamic structural response of materials can indirectly indicate the health of structural systems. The changes in natural frequencies, mode shapes, and stiffness matrices due to damage are utilized for determination of occurrence, location and extent of damages. In recent years, many researchers have developed global damage detection algorithms using structural modal response. However most of these methods are off-line techniques based on frequency domain data. In this paper we have proposed real- time damage detection methods based on time domain data. In this method damages in the structure can be detected while the structure is kept on its regular use. The algorithm determines reduction in stiffness and/or damping of the structural elements, while assuming that the mass of the structure does not vary due to damage. This algorithm is based on the state space representation of the structure, which is identified from the time domain data. We have also determined a linear transformation matrix for converting the identified model into a state space representation based on physical coordinates of the structural system. The self-organization and learning capabilities of neural networks can be effectively used for structural damage detection purpose. In this paper a hybrid method for the damage detection has been proposed by combining the features of best achievable eigenvector method and neural network classification techniques for detection of location and extent of damage in the structural systems. The feasibility of the proposed method is verified by using simple three-bar truss structure and a cantilever beam test article.

This paper demonstrates the possibility of incorporating load- dependent Ritz vectors, as an alternative to modal parameters, into a Bayesian probabilistic framework for detecting damages in a structure. Recent research has shown that it is possible to extract load-dependent Ritz vectors from vibration tests. This paper shows that load-dependent Ritz vectors have the following potential advantages for damage detection over modal vectors: (1) In general, load-dependent Ritz vectors are more sensitive to damage than the corresponding modal vectors, and (2) substructures of interest can be made more observable using the load-dependent Ritz vectors generated from particular load patterns. An eight-bay truss structure and a five-story frame example are presented to illustrate the applicability of the proposed approach.

A statistical methodology is presented for optimally locating the sensors in a structure for the purpose of extracting from the measured data the most information about the parameters of the model used to represent structural behavior. The methodology can be used in model updating and in damage detection and localization. It properly handles the unavoidable uncertainties in the measured data as well as the model uncertainties. The optimality criterion for the senor locations is based on information entropy which is a unique measure of the uncertainty in the model parameters. The uncertainty in these parameters is computed by the Bayesian statistical methodology and then the entropy measure is minimized over the set of possible sensor configurations using a genetic algorithm. The information entropy measure is also extended to handel large uncertainties expected in the pre- test nominal model of a structure. In experimental design, the proposed entropy-based methodology provides a rational procedure for comparing and evaluating the benefits of adding more sensors in the structure against the benefits of exciting and observing (measuring) more modes using the existing number of sensors. Simplified models for building and bridge structures are used to illustrate the methodology.

Electrical time domain reflectometry (ETDR) stress/strain sensing technique has been successfully used in geotechnical applications to detect rock deformation and longwall movement. The ETDR sensing method appears to be practical for health monitoring applications of civil concrete structures since durable sensor media can be used. In this paper, feasibility of using an embedded coaxial ETDR cable to detect crack damages in a structure is investigated. Tension and bending tests were performed on edge-notched photoelastic epoxy specimens with embedded coaxial ETDR sensing cables. The test results show that crack lines passing through the embedded sensing cable can be detected. The TDR signal response of the sensing cable reveals not only the location of the crack damage site but also indicates the relative magnitude of the crack opening. The results of the current study strongly suggest that the ETDR sensing technique possesses a great potential for the application of health monitoring of large civil concrete structures.

The superelastic behavior of shape memory alloys (SMAs) provides a stress-strain relation which is characterized by large hysteresis, superelastic effect, large ductility and variation of the material properties at different level of strain. Those properties can be effectively used in a design of SMA dampers for vibration control of bridges. In this paper, a device utilizing bending of SMA bars is experimentally tested. The bars of NiTi are kept on their ends by two rigid elements, moment arms, that are connected to the actuator and reaction wall. The force from the actuator, applied on the moment arms, exerts a uniform bending moment on the NiTi bars. The responses of NiTi alloys with different heat treatment are experimentally studied under cycling loading with various amplitudes. The test results confirmed a large hysteretic response of NiTi damper and its variable response at different displacement level. The experimental results are verified by simple 3D numerical simulations by FEM.

This paper is a case study of a six-story welded moment resisting steel frame (WMRSF) building in the Los Angeles area, which had been shaken during the Northridge earthquake. The objective of study was to assess and compare the effectiveness of seismic retrofit techniques using passive energy dissipation systems. An analytical model of the uncontrolled structure was first defined to perform nonlinear time history analyses under a selection of historic earthquakes. The structure was then retrofitted with rate- dependent and rate-independent supplemental damping devices, and the effectiveness of various damping systems were evaluated.

The design of passive dampers involves the determination of the required capacity of each damper installed at selected locations. Generally, dampers with identical capacities are installed in various story units of a building. However, installing identical dampers in various story units does not achieve the optimal performance for the building and it may result in a conservative and more expensive design. In this paper, a design method, based on the concepts of active control theories, is proposed for the design of the capacities of passive dampers. For most of the passive dampers, the force applied to the structure depends on the drift and velocity across the dampers. From the standpoint of active control, the control force depends only on the local measurements of the displacement (i.e., drift across the damper) and velocity. Controllers of the form described above are designed by decentralized control theories. For this purpose, the method of static output optimal control is modified and applied to the design of passive dampers. Advantages of the proposed method for different types of passive dampers are demonstrated through numerical simulations.

Fiber Bragg grating sensor systems have wide application in the area of civil structures. The advantages of fiber grating strain sensors over electrical strain gauges such as greatly reduced size, EMI resistance, and higher temperature capability make them ideal choices for smart structure applications. Some of these fiber grating sensor systems can measure or detect multiaxis strain, transverse strain, temperature, bridge scouring, ice, and traffic flow.

A novel fiber optic sensors for traffic monitoring applications are presented. They are useful for monitoring the traffic entering and leaving guarded areas, counting traffic on public roads as well as in determining lane occupancy at traffic lights. When installing sensor arrays, the sensors may also be used to provide additional information: measure the speed, wheel base of a vehicle and also to determine the number of axles and vehicle type. The sensors are based on optical fiber or cable, installed inside the asphalt layer of the road, to measure compression or vibrations near the road surface. Two sensor principles were used. One based on the microbending effect and another that utilizes the speckle phenomenon. In both cases the whole length of the fiber acts as a sensor. The microbending sensor requires a special fiber and special set-up, whereas in the speckle sensor a standard cable may be used. Both sensor types were tested in field applications where the harsh environment, especially the heat, sets great demands on installation. In these experiments, speed and vehicle type measurements were carried out with good results. In the paper, we will discuss the advantages and disadvantages of both sensor types and present some field test results. We will also show the benefits of these particular fiber optic sensors over traditional sensors.

In this paper, a new concept for damage detection and long- term health monitoring of structures is presented. The Precursor Transformation Method (PTM) is based on determining the causes (precursors) of change in the measured state of the structure under non-variable loading conditions (e.g. dead loads in bridges). The PTM concept addresses the inability of the current structural monitoring methods to discriminate, in structural behavior terms, the meaning of voluminous measured sensor data on a timely and cost effective basis. This method offers advantages in sensitivity and cost efficiency when compared to conventional vibration-based or parameter estimation methods. PTM was developed as part of a research project sponsored by the Federal Highway Administration on bridge stay cable condition assessment. Measured changes in the state of a structure (displacements, strains, internal forces) can be related to precursors through a transformation matrix. This matrix is formed by determining the patterns of change in the state of structure associated with externally imposed strains (temperatures) or displacements representing possible damage scenarios. A finite element model of the undamaged structure is used to calculate these patterns. The use of an undamaged model of the structure in determining damage patterns simplifies the calculation process significantly, while introducing some approximation in results. Theoretical derivations and special case studies indicate that these approximations are limited to second order effects, and in many cases well within measurement and calculation accuracies. Examples using simulated damages on two truss structures and a cable-stayed bridge are also presented.

The objective of this paper is to make a comparison of the damage detection results obtained from an analysis of the modal parameters extracted from a time domain model and a frequency domain model. In this paper, an autoregressive moving average (ARMA) model and a FRF model will be utilized to extract the modal parameters of a beam. Expressions for the modal parameters using the ARMA model are developed. Next, an established theory of damage localization, which yields information on the location of the damage directly from changes in mode shapes, is selected. Expressions for classification algorithms using the damage indicator functions from the damage localization theory are then generated. Using the classification algorithms, damage localization is attempted for a pinned-pinned beam which contains damage of various degrees. Finally, the modal parameters and damage detection results obtained from the two methods are compared.

The quadrature phase-shifted 3x3 fiber optic Michelson sensors were tried to monitor the health status of a steel truss bridge which was scaled down to 1/15 of the real bridge for the laboratory experiments. The fiber optic sensors and electrical strain gages were bonded on the surface of some frames to sense the strain pattern induced by the abnormal structural behavior. The fiber optic signals were immediately processed by personal computer for the strain determination. In order to confirm the strain sensitivity of the fiber optic sensors, these fiber optic strains were compared with the strains of the strain gages. The static behavior of the bridge was analyzed by finite element analysis with SAP2000. These finite element analysis results were compared with the structural strain pattern obtained by the electrical strain gages and were arranged with the database for the determination of the bridge health condition. It was shown that the breakage of some frames could be detected from the changes in strain pattern.

In multi-hull marine vehicles assembled by FRP sandwich composite materials problems with delamination and skin/core debonding are reported. High frequency vibrations in foam core sandwich materials are investigated to see if it was possible to apply them, together with bending vibrations, in an early damage warning system for delamination detection in marine vessels. This manuscript presents a theory for high frequency vibration in sandwich plates and beams. The core is modeled as a two parameter foundation with shearing interaction effects as well as normal stress effects in the core included. The skins are modeled as ordinary plates or beams on a foundation. Expressions for both anti-symmetric and symmetric modes are given. In addition to the theoretical development, experiments with a simply supported sandwich beam, using a TV-Holography technic, were performed and good accordance between theory and experiments were achieved. The results indicates that disappearance of symmetric modes may be used a parameter for delamination detection. The anti-symmetric modes may be interchangeable with higher bending modes by an early damage warning system. To avoid this, the theory presented may be applied to determine the anti-symmetric frequency values in forehand.

In this paper we consider a few dynamic models of suspension bridges described by partial differential equations with linear and nonlinear couplings. We study analytically the stability properties of these models and the relative effectiveness of aerodynamic and structural damping. Increasing either of these damping coefficients indefinitely does not necessarily increase the decay rate indefinitely. In view of possible disastrous effects of high wind, structural damping is preferable to viscous damping. These results are illustrated by numerical simulation.