The first phase of the seismically excited base-isolated benchmark building was received well by the structural control community, culminating in the March 2006 journal special issue. The special issue contained contributions from over dozen participants world-wide. While the focus of the Phase I effort was on linear isolation systems, Phase II attempts to galvanize research efforts on control of base-isolated buildings with nonlinear isolation systems. Primarily, friction and hysteretic lead–rubber-bearing (LRB) isolation systems are included in this effort. The superstructure and the control framework remains the same as the Phase I benchmark. The main difference will be in the nonlinear isolation systems used, and consequently the controllers necessary to control such systems. The primary objective of this paper is to present the Phase II benchmark problem definition along with a sample controller for friction isolation system. A sample controller for the LRB isolation system was presented earlier as a part of the Phase I special issue. Included in this paper is a broad set of carefully chosen performance measures, which remain the same as Phase I, so that the participants may evaluate their respective control designs. The control algorithms may be passive, active or semiactive. The benchmark structure considered in the Phase II study is an eight-story base-isolated building that is identical to the one considered for the Phase I study. The base isolation system consists of a combination of linear, nonlinear bearings and control devices. The superstructure is considered to be a linear elastic system with lateral–torsional behavior. The nonlinearities due to the isolators and control devices are limited to the isolation level only. A nonlinear dynamic analysis program and sample controllers are made available to the participants to facilitate direct comparison of results of different control algorithms. Copyright © 2008 John Wiley & Sons, Ltd.
This paper presents an experimental and analytical study of hybrid control of bridges using sliding bearings, with recentering springs, in parallel with servohydraulic actuators. A new control algorithm with absolute acceleration feedback, based on instantaneous optimal control laws, is developed. The developed control algorithm is implemented in a shake‐table study of an actively controlled sliding‐isolated bridge. The objective of implementing the hybrid system is to evaluate its advantages in addition to those due to the passive sliding system. The experimental system used in the shake‐table test is described and the results of the experiments are presented. It is shown that substantial reduction of response acceleration is possible, using hybrid control, while confining the sliding displacement within an acceptable range, and eliminating almost completely postearthquake permanent offsets. Comparisons of the results of hybrid system with results of passive system are presented. The advantages of hybrid control, in addition to those due to passive control, are discussed.
Tuned mass dampers (TMD), active mass dampers (AMD) and hybrid mass dampers (HMD) have been widely applied for vibration control of tall buildings and bridges in the past decade. Recently, the author and his coworkers have developed semiactive or smart tuned mass dampers (STMD) using semiactive variable stiffness systems. STMD's are superior than TMD's in reducing the response of the primary structure. In case the fundamental frequency of the primary structure changes due to damage or deterioration, then the TMD will be off-tune; hence, it will lose its effectiveness significantly, whereas the STMD is robust against such changes as it is always tuned. The author and his coworkers have shown that STMD can provide performance similar to AMD/HMD, but with an order of magnitude less power consumption. In this paper, new adaptive length pendulum STMD's are introduced. The concept of adaptive passive tuned mass dampers (APTMD) is introduced. APTMD is a TMD in which a tuning parameter such as frequency is adjusted passively based on some local mechanical feedback (displacement, velocity, rotation, etc.), but without associated sensing and computer feedback needed in a STMD. Also, the concept of STMD is further developed in this paper and practical STMD's and APTMD's implementation in USA, Japan, and China is presented. Systems with semiactive variable stiffness devices and STMD/APTMD are linear time varying systems (LTV); hence, algorithms are needed for their identification and control. Recently, the author and his coworkers have developed instantaneous frequency tracking control algorithms. In this paper new system identification algorithms based on time frequency methods, such as Empirical Mode Decomposition (EMD), Hilbert Transform (HT), and short time Fourier transform (STFT), are developed. New real time tuning algorithms that identify the instantaneous frequency of the LTV system and tune the STMD are developed based on EMD, HT, and STFT. Systems with STMD subjected to stationary (harmonic, sinsweep, and white-noise) and nonstationary (earthquake) excitations are investigated. The effectiveness of the STMD systems and the new identification and control algorithms is demonstrated by means of numerical simulations and experimental validation. Copyright © 2009 John Wiley & Sons, Ltd.
In this study, negative stiffness brace (NSB) device composed of a series of pre-compressed springs, gap-spring assemblies and links to achieve the desired negative stiffness behavior suitable for application in structural systems is investigated analytically and experimentally. NSB achieves a high force magnification by using the geometry, harnessing the spring force at both the ends and through the usage of multiple pre-compressed springs and links, in series. Series arrangement significantly reduces the stiffness requirements of the pre-compressed springs. NSB has a compact arrangement of components, which reduces space consumption as well as allows easy installation. In this study, the analytical model describing the behavior of the NSB is presented and the effect of various parameters on the behavior is investigated. A scaled NSB device is tested and experimental results of the behavior of the NSB are presented and used to validate the analytical model. Negative stiffness devices and supplemental damping devices have been recently studied extensively for the protection of structural systems subjected to wind and seismic excitations since the combination can significantly reduce accelerations and inter-story drifts, and base shears. NSB with supplemental dampers has significant potential for application in structural systems.
In this study, a negative stiffness brace (NSB) device composed of a series of precompressed springs, gap-spring assemblies, and links to achieve the desired negative stiffness behavior suitable for application in structural systems is investigated analytically and experimentally. The NSB achieves a high force magnification by using the geometry, harnessing the spring force at both the ends, and through the usage of multiple precompressed springs and links in series. The series arrangement significantly reduces the stiffness requirements of the precompressed springs. The NSB has a compact arrangement of components, which reduces space consumption as well as allows easy installation. In this study, the analytical model describing the behavior of the NSB is presented and the effect of various parameters on the behavior is investigated. A scaled NSB device is tested, and experimental results of the behavior of the NSB are presented and used to validate the analytical model. Further, the analytical model of a frame connected to the NSB–gap-spring assembly (GSA) system is developed and validated. The efficacy of the NSB in reducing structural response was studied numerically by considering a single-degree-of-freedom system. Negative stiffness devices and supplemental damping devices have been recently studied extensively for the protection of structural systems subjected to wind and seismic excitations because the combination can significantly reduce accelerations, interstory drifts, and base shears. NSB with supplemental dampers has significant potential for application in structural systems.
A characteristic property of semiconducting single-wall carbon nanotubes (SWCNTs) is distinct near-infrared photoluminescence following excitation by visible light. Theory and experiment show that these optical emission peaks shift predictably in wavelength as nanotubes are compressed or stretched along their axis. We are exploiting this effect in a powerful new method intended for measuring mechanical strain in critical infrastructure components such as airframes, pressurized vessels, pipelines, support beams, etc. The method involves applying a dilute dispersion of nanotubes in a polymeric host onto the surface of the specimen to form a sub-micron thick film in which SWCNTs act as strain sensors. This layer is overcoated with a transparent protective top coat such as a polyurethane varnish. Subsequent strains in the substrate are transmitted to the nanotubes by load transfer. The substrate strain magnitude and direction are then measured by illuminating the surface at any point of interest with a small visible laser beam and spectrally analyzing the resulting near-IR nanotube emission. Single-point measurements currently provide strain magnitude resolution of ca. 50 microstrain, strain angle resolution of ca. 5 degrees, and spatial resolution of ca. 50 mm. Each reading takes less than one second, allowing compilation of strain maps from scanned data. Unlike digital image correlation, which is currently the only commercial non-contact strain technology, the new method can measure strains accumulated when the specimen is not under observation. It thus has the potential for routine use in industrial structural health monitoring as well as in testing and development laboratories. Results will be presented showing strain measurements in metallic, plastic, and concrete specimens. We will also describe recent progress in adapting the technology to camera-based measurements using spectrally resolved fluorescence imaging.
The base-isolated University of Southern California (USC) hospital building experienced strong motion during the 1994 Northridge earthquake. California Strong Motion Instrumentation Program data of the response are available for performance evaluation. The objective of this study is to evaluate the seismic performance of the base-isolated USC hospital building during the 1994 Northridge earthquake. A nonlinear analytical model of the USC hospital building is developed and verified using system identification. The response computed, using the presented analytical modeling techniques, is verified using recorded data. Structural behavior during the Northridge earthquake is evaluated in detail. The base-isolated USC hospital building performed well and reduced the response when compared to a fixed-base structure. The free-field acceleration was 0.49g and peak foundation/ground acceleration was 0.37g. The peak roof acceleration was reduced to 0.21g, nearly 50% of the peak ground acceleration. The peak drift was <30% of the code specification. The bearings yielded and dissipated energy (20%). The superstructure was elastic due to the effectiveness of base isolation. The building is expected to perform well in future earthquakes similar to those used in the original design.
