Table of contents

Because of environmental and man-made hazards, large civil structures such as highways, buildings and bridges can subject users of such systems to unacceptably large motions, unsafe conditions, and can potentially cause catastrophic damage in the absence of structural control. Structural control has been proposed as a strategy for minimizing these large motions while preserving the integrity of civil infrastructure systems. The closely coupled structural, mechanical, thermal, geological and aerodynamic environment of these structures offers many unique opportunities to apply smart materials and structures technology to control, monitor and assess the state of civil infrastructure systems. The motivation for investigating the application of smart materials and structure technology to large civil infrastructure arises from the desire to mitigate potential risk to the general public and to ensure the viability of civil infrastructure systems. Because of the potential benefit that smart structures technology may offer to mitigate risk to environmental hazards, strong research efforts have been undertaken in the US and Japan to investigate the feasibility of using this technology for structural control.

To address key issues associated with the application of smart structures technology to large civil infrastructure, a workshop was held on 14 November 1996 (Pines and Hiriashi 1996 Proceedings of US-Japan Workshop on Smart Structures Technology: Application to Large Civil Structures) at the University of Maryland to foster discussion on (i) the design and development of smart structures technology for civil infrastructure; (ii) the integration of smart structures technology into large civil infrastructure and (iii) the use of such technology for monitoring and structural control.

The goal of the workshop was to gather Japanese and US experts from academia, industry and government laboratories to assess the state of the art of the technology as applied to the large civil infrastructure. Key objectives of this proposed workshop were:

• to discuss recent applications of smart structures technology to large civil structures

• to identify future research thrusts involving smart structures technology

• to stimulate future cooperative research activities between US and Japanese researchers

• to continue to build and establish long term cooperative research programs between university, laboratory and industry professionals

The specific technical areas of interest for this workshop were:

• active and passive damping

• active vibration control

• smart retrofit/repair

• infrastructure health monitoring/damage assessment

• sensor and actuator technology

The workshop was sponsored by:

• National Science Foundation (NSF)

• NIST

• UJNR TASK Committees C&G

The interest and support of K Chong and S C Liu of the US National Science Foundation is greatly appreciated. We are also indebted to all participants whose attendance and contributions helped to foster important discussions on issues associated with the application of the technology to large civil infrastructure. The workshop served as the basis for this special issue.

D J Pines Guest Editor Department of Aerospace Engineering University of Maryland College Park MD 20742 USA

H Hiraishi Guest Editor Building Research Institute Ministry of Construction Tachihara-1 Tsukuba-shi, Ibaraki 305 Japan

This paper presents a brief introduction to seismic isolation and passive structural response-control buildings in Japan. A total number of 287 projects on seismic-isolated building structures had obtained the required special permission in Japan by the end of September 1996. The effectiveness of seismic isolation buildings has been demonstrated and verified through the 1995 Hyogoken-nanbu (Kobe) earthquake. It has resulted in a remarkable increase of the number of projects on seismic isolation buildings. Passive response-control buildings have been constructed in order to reduce the effects from earthquakes and/or wind induced vibration of buildings. One of the lessons learned from the damage due to the 1995 Hyogoken-nanbu earthquake is the importance of developing seismic design concepts to control the damage of buildings within the repairable level during major earthquakes. The passive response-control technology has a high possibility of achieving this goal. The features and trends in seismic isolation and passive response-control buildings in Japan are introduced.

Three examples of vibration control systems are described. The first is a hybrid mass damper system, which is one type of active vibration control system, as installed on the top floor of a complex triangular building of forty-three stories in order to reduce the response of the building to strong winds and moderate earthquakes. The second is an unbonded brace damper, which is a kind of elasto-plastic damper using low-yield-point steel. It has been installed in a fifteen-story building as an energy absorption member to control severe earthquake motion. The last is a rotational variable damper using an electrorheological fluid. The feasibility of applying this type of damper to a real scale structure as a semi-active control device has been investigated.

This paper presents a qualitative health monitoring technique to be used in real-time damage evaluation of civil infrastructures such as bridge joints. The basic principle of the technique is to monitor the structural mechanical impedance which will be changed by the presence of structural damage. The mechanical impedance variations are monitored by measuring the electrical impedance of a bonded piezoelectric actuator/sensor patch. This mechanical-electrical impedance relation is due to the electromechanical coupling property of piezoelectric materials. This health monitoring technique can be easily adapted to existing structures, since only a small PZT patch is needed, giving the structure the ability to constantly monitor its own structural integrity. This impedance-based method operates at high frequencies (above 50 kHz), which enables it to detect incipient-type damage and is not confused by normal operating conditions, vibrations, changes in the structure or changes in the host external body. This health monitoring technique has been applied successfully to a variety of light structures. However, the usefulness of the technique for massive structures needs to be verified experimentally. For this purpose, a 500 lb quarter-scale deck truss bridge joint was built and used in this experimental investigation. The localized sensing area is still observed, but the impedance variations due to incipient damage are slightly different. Nevertheless, by converting the impedance measurements into a scalar damage index, the real-time implementation of the impedance-based technique has been proven feasible.

A structural dynamic based health monitoring system for large structural systems has been proposed in this article. The proposed method has been verified numerically and experimentally by implementing the scheme on a model of a long span bridge. In the implementation process, the following steps have been identified as being important: (1) finite element modeling for the purpose of establishing the base line, (2) optimal sensor placement to make a scheme economical and (3) damage identification. In the process the authors have identified and discussed the various difficulties that have been encountered and have made suitable recommendations for circumventing the problems.

In this paper, three case studies intending to apply smart materials to civil structures are presented. The first one is a study of response control using piezoelectric actuators. Actuators are inserted into the bottom of a column to produce a bending moment force. A control algorithm using the model matching method is introduced, and this algorithm is checked in shaking table tests of a four story frame. The second one is damage sensing of a structural member, using electric resistance characteristics of shape memory alloys. The relationship between electrical resistance and strain of shape memory alloy wire is studied and the maximum strain of the specimen which is regarded as a structural member is estimated. The third one is an energy dissipation device using super-elastic characteristics of a shape memory alloy. A basic energy dissipation device model using nitinol wire is proposed. The energy dissipation capacity is investigated by device tests, and an analytical model is constructed based on the test results.

Recent advances in smart materials and structures sensor technology offer many unique opportunities to assess the structural integrity of large civil structures. However, the remote operational environment of large civil structures, such as highways, buildings and bridges, makes condition-based health monitoring for damage assessment difficult in the event of a natural disaster. During such disasters, electrical power is lost and cellular phone lines are under heavy usage. This limits the retrieval of very important sensor data. However, recent rulings by the Federal Communication Commission coupled with advances in wireless communication products have now made it possible to circumvent existing wired and cellular infrastructure to retrieve data from smart sensors remotely and more economically. This paper discusses a novel approach using smart sensors and wireless communication technology to monitor the health of large civil structures remotely. Specifically, a remote health monitoring system for large civil structures is developed using spread spectrum wireless modems, data communication software and conventional strain sensors. This system is used to monitor the loads on a laboratory test specimen with a bolted lap joint from as far away as one mile. Commands are issued from a notebook personal computer to instruct the health monitoring system to either excite the structure or acquire data from sensors mounted externally to the structure. Data from measurements made on the structure are then transmitted wirelessly back to a notebook computer for processing and analysis using damage detection algorithms.

This paper reviews an active control algorithm adopted for an active-passive composite tuned mass damper, which is a unique vibration control device equipped into an office building in Tokyo in 1993. The main purpose of this device is to subdue the response motion of tall buildings under random disturbances such as wind pressures and small earthquakes. The main topics in this paper are: (1) the principle of the acceleration feedback algorithm, (2) the expected control performance, (3) the multi-modal control algorithm, (4) the observed performance of the applications using the algorithm.

This paper presents a theoretical study of a hybrid seismic response control system comprising a passive fluid viscous damper and an active control system with a hydraulic actuator and acceleration sensors. The analytical model of this system is derived in the discrete time domain with actuator and damper dynamics considered. Based on digital control theory, a state observer is established for optimal control strategies of the hybrid system. This study shows that the hybrid system gains the advantages of both active and passive control techniques currently in vogue; and that, with the state observer technique, a seismic response control system is more effective and less complicated because full-state feedback seismic control algorithms can be implemented by means of acceleration sensors, and a smaller number of sensors is required. A numerical simulation for seismic response of a building structure with a hybrid control system is presented to demonstrate the effectiveness of this control strategy for the hybrid system.

Electrorheological (ER) materials are suspensions of polarizable particles in a dielectrically strong suspension. When a strong electric field is applied to ER materials, their visco-elastic and, more importantly, yielding properties increase by orders of magnitude. While Newtonian viscous stresses are relatively field independent, dissipative forces in ER dampers are adjusted by varying the field-dependent yield stress. By varying the ratio of the flow-rate-independent yield stresses to Newtonian viscous stresses, ER dampers may be designed to match the requirements of a wide range of applications. In many cases, it is desirable for the ER damper to not only have high force capacities in a compact geometry, but also have a large range of controllable forces.

A class of ER dampers is analysed and illustrated in this paper. The analysis of these device configurations is completed in closed form by virtue of a linear approximation to the non-Newtonian ER Poiseuille flow equation.

These dampers feature multiple concentric cylindrical ducts which can be interconnected in parallel, in series or in combinations thereof. The hydraulic connectivity of the ducts determines, to a great extent, the force-velocity relationship of the device. Within overall size constraints, a tradeoff between the field-controllable force range and force magnitude is controlled by the format of the ducts. Other design variables considered in this paper are the across-flow dimension of the ducts and the number of ducts. Designs are evaluated based on force capacity, range of field-dependent forces, electrical energy requirements and response time. The effects of pre-yield elasticity and particle concentration inhomogeneity are also addressed.

Numerical examples focus on a large-scale damper which requires a very modest amount of external energy (kJ), yet can regulate very large forces (200 kN) and can modify its force by a factor of ten or more within milliseconds.

The US Federal Reserve Board has concluded that the failure of civil infrastructure systems to perform at their expected level may reduce the national gross domestic product (GDP) by as much as 1%. Intelligent-infrastructure systems and components hold promise for improving performance with an excellent cost/benefit ratio. A recent National Science Foundation (NSF) workshop demonstrated the state of the art in research and applications on intelligent materials and structures in Japan and the US. Major investments have been made by the leading Japanese construction companies into developments for enabling intelligent-structure applications for mitigating earthquake and wind damage. A number of innovative concepts and ideas have been developed by American researchers; however, a lack of applications has also become apparent.

This paper reports on the research of a multidisciplinary team working at the University of Cincinnati Infrastructure Institute to conceptualize a wide spectrum of issues in infrastructure condition assessment, health monitoring and intelligent systems by conducting exploratory research on actual operating highway bridges. The writers have made much progress towards developing an intelligent-bridge application. Such an application requires advances in sensors, communication and information technology, global non-destructive evaluation technologies of a rigorous and objective nature and fundamental knowledge on state parameters, loading environments, damage and deterioration mechanisms and performance of actual highway bridges. The writers initiated a strong university-government-industry partnership a decade ago and have explored a global condition assessment methodology based on the structural identification concept. After a decade, they have reached an in-depth realization of the complex multisystem identification and integration which must accompany any `intelligent' systems approach to the infrastructure problem.

The writers are currently maintaining three `typical' steel-stringer highway bridge overpasses as generic test-sites for intelligent structures, condition assessment and health monitoring. One of these bridges, which is forty years old, has been decommissioned. This specimen is being tested to understand damage limit-state behavior. Another ten year old specimen has been tested and monitored for five years to understand operating loading environment and behavior at service limit-states. The third bridge is being monitored during its construction to understand the causes and magnitudes of intrinsic forces. After commissioning, this bridge will have a built-in monitor with 300 embedded/attached sensors, a weigh-in-motion scale and sufficient on-site hardware and software to create an intelligent-bridge prototype. By taking advantage of the linkages provided by research at all three test-sites, it has been possible to understand the complete life-cycle spectrum of loading effects and behavior of the medium length steel-stringer bridge type.

In this paper, the efficacy of magnetorheological (MR) dampers for seismic response reduction is examined. To investigate the performance of the MR damper, a series of experiments was conducted in which the MR damper is used in conjunction with a recently developed clipped-optimal control strategy to control a three-story test structure subjected to a one-dimensional ground excitation. The ability of the MR damper to reduce both peak responses, in a series of earthquake tests, and rms responses, in a series of broadband excitation tests, is shown. Additionally, because semi-active control systems are nonlinear, a variety of disturbance amplitudes are considered to investigate the performance of this control system over a variety of loading conditions. For each case, the results for three clipped-optimal control designs are presented and compared to the performance of two passive systems. The results indicate that the MR damper is quite effective for structural response reduction over a wide class of seismic excitations.

Over the past 30 years detecting damage in a structure from changes in global dynamic parameters has received considerable attention from the civil, aerospace and mechanical engineering communities. The basis for this approach to damage detection is that changes in the structure's physical properties (i.e., boundary conditions, stiffness, mass and/or damping) will, in turn, alter the dynamic characteristics (i.e., resonant frequencies, modal damping and mode shapes) of the structure. Changes in properties such as the flexibility or stiffness matrices derived from measured modal properties and changes in mode shape curvature have shown promise for locating structural damage. However, to date there has not been a study reported in the technical literature that directly compares these various methods. The experimental results reported in this paper and the results of a numerical study reported in an accompanying paper attempt to fill this void in the study of damage detection methods. Five methods for damage assessment that have been reported in the technical literature are summarized and compared using experimental modal data from an undamaged and damaged bridge. For the most severe damage case investigated, all methods can accurately locate the damage. The methods show varying levels of success when applied to less severe damage cases. This paper concludes by summarizing some areas of the damage identification process that require further study.

This paper extends the study of damage identification algorithms summarized in the accompanying paper `Comparative study of damage identification algorithms: I. Experiment' to numerical examples. A finite element model of a continuous three-span portion of the I-40 bridges, which once crossed the Rio Grande in Albuquerque, NM, was constructed. Dynamic properties (resonant frequencies and mode shapes) of the undamaged and damaged bridge that were predicted by the numerical models were then correlated with experimental modal analysis results. Once correlated with the experimental results, eight new damage scenarios were introduced into the numerical model including a multiple damage case. Also, results from two undamaged cases were used to study the possibility that the damage identification methods would produce false-positive readings. In all cases analytical modal parameters were extracted from time-history analyses using signal processing techniques similar to those used in the experimental investigation. This study provides further comparisons of the relative accuracy of these different damage identification methods when they are applied to a set of standard numerical problems.

We develop nonlinear quasi-steady electrorheological (ER) and magnetorheological (MR) damper models using an idealized Bingham plastic shear flow mechanism. Dampers with cylindrical geometry are investigated, where damping forces are developed in an annular bypass via Couette (shear mode), Poiseuille (flow mode) flow, or combined Couette and Poiseiulle flow (mixed mode). Models are based on parallel plate or rectangular duct geometry, and are compared to our prior 1D axisymmetric models. Three nondimensional groups are introduced for damper analysis, namely, the Bingham number, , the nondimensional plug thickness, , and the area coefficient defined as the ratio of the piston head area, , to the cross-sectional area of the annular bypass, . The approximate parallel plate analysis compares well with the 1D axisymmetric analysis when the Bingham number is small, or , or the nondimensional plug thickness is small, . Damper performance is characterized in terms of the damping coefficient, which is the ratio of the equivalent viscous damping constant, , to the Newtonian viscous damping constant, C. In shear mode, the damping coefficient is a linear function of the Bingham number. In flow mode, the damping coefficient is a function of the nondimensional plug thickness only. For the mixed mode damper, the damping coefficient reduces to that for the flow mode case when the area coefficient is large. The quasi-steady damping coefficient versus nondimensional plug thickness diagram is experimentally validated using measured 10 Hz hysteresis cycles for a electrorheological mixed mode damper.