SUMMARY During severe seismic events, structures designed according to current standards yield and develop inelastic deformations. While the acceleration responses are limited by the yielding strength, these structures develop permanent deformations (and possible damage) due to such yielding. Spectra developed for inelastic structures can help in determining the desired yield levels and the associated inelastic deformations. Some structures made of special materials or equipped with innovative structural systems may yield, but can recover the deformation upon unloading and, thus, may avoid permanent deformations. These structures are known as nonlinear elastic. Often the post yielding excursions are very large and may exceed their toughness (or deformability). By introducing damping in form of supplemental devices, it is possible to control such deformations and keep them within acceptable limits. Spectra for such nonlinear elastic structures and inelastic structures are developed herein, by considering both inherent and supplemental damping. The difference between the two types of damping is addressed both theoretically and numerically. Design examples of several simple structures using the newly developed spectra are presented, which illustrate the importance of lower strength and damping in these nonlinear elastic or inelastic systems. Copyright © 2013 John Wiley & Sons, Ltd.
A newly developed passive device that provides negative stiffness has been implemented within a quarter-scale highway bridge model and subjected to seismic loading via shake table testing. Details of the experimental results and their comparison with numerical simulations under a wide range of ground motions are presented. In addition, performance indices were developed to systematically evaluate the relative performance of different isolation system configurations that employ combinations of positive and negative stiffness as well as various levels of damping. Further, the influence of boundary conditions (rigid versus flexible bridge piers) on the effectiveness of employing negative stiffness devices has been evaluated.
In structural vibration response sensing, mobile sensors offer outstanding benefits as they are not dedicated to a certain structure; they also possess the ability to acquire dense spatial information. Currently, most of the existing literature concerning mobile sensing involves human drivers manually driving through the bridges multiple times. While self-driving automated vehicles could serve for such studies, they might entail substantial costs when applied to structural health monitoring tasks. Therefore, in order to tackle this challenge, we introduce a formation control framework that facilitates automatic multi-agent mobile sensing. Notably, our findings demonstrate that the proposed formation control algorithm can effectively control the behavior of the multi-agent systems for structural response sensing purposes based on user choice. We leverage vibration data collected by these mobile sensors to estimate the full-field vibration response of the structure, utilizing a compressive sensing algorithm in the spatial domain. The task of estimating the full-field response can be represented as a spatiotemporal response matrix completion task, wherein the suite of multi-agent mobile sensors sparsely populates some of the matrix’s elements. Subsequently, we deploy the compressive sensing technique to obtain the dense full-field vibration complete response of the structure and estimate the reconstruction accuracy. Results obtained from two different formations on a simply supported bridge are presented in this paper, and the high level of accuracy in reconstruction underscores the efficacy of our proposed framework. This multi-agent mobile sensing approach showcases the significant potential for automated structural response measurement, directly applicable to health monitoring and resilience assessment objectives.
Floating structures such as spar platforms are typically designed to be compliant or move with environment loadings, rather than resisting them. Hence they are designed so that there is no dynamic amplification in wave frequency response. However, higher order nonlinear effects are produced in low frequency wind excited regions, especially in Spar platforms. It is difficult to separate the nonlinear behavior of the model response from the loading using conventional methods. In this paper, Empirical Mode Decomposition and Hilbert Transform (EMD/HT) is used to identify the nonlinear response of a spar from the model test results. From the measured response the dynamic parameters are estimated as follows: 1) The multi-component response of the floating structure is decomposed into IMF components. 2) Hilbert transform of the input and the IMF signal in the time domain is done to extract the instantaneous dynamic characteristics. 3) Amplitude and frequency dependent frequency response function is used to represent the result of HT identification. The EMD method can identify any changes in system properties in real time and can be effectively used for repair and retrofit.
The seismically excited benchmark problem is based on the newly constructed 91/5 highway over-crossing in Southern California. The goal of this effort is to develop a standardized model of a highway bridge using which competing control strategies, including devices, algorithms and sensors, can be evaluated comparatively. To achieve this goal, a 3-D finite-element model is developed in MATLAB to represent the complex behavior of the full-scale highway crossing. The nonlinear behavior of center columns and isolation bearings are considered in formulating the bilinear force-deformation relationship. The effect of soil-structure interaction is considered by modeling the interaction by equivalent spring and dashpot. The ground motions are considered to be applied simultaneously in two directions. A MATLAB based nonlinear structural analysis tool has been developed and made available for nonlinear dynamic analysis. Passive, semi-active and active control devices are assumed to be installed between the deck and the end-abutments of the bridge. Evaluation criteria and control constraints are specified for the design of controllers.
Structural Health Monitoring (SHM) is an important and growing field in civil engineering. The goal of SHM techniques is to identify, quantify and locate damage in structures. In light of the aging infrastructure and recent failures of important bridges, long-term monitoring techniques are being increasing investigated and adopted. In addition to SHM, structural control (SC) is increasingly adopted in modern structures around the world. In the past two decades a number of SC techniques, including, passive, semi-active, and active control methods have been developed and adopted in civil engineering–particularly, in infrastructure such as important tall buildings, critical facilities, and long span bridges. Both SHM and SC technology face significant challenges due to the size and scale of civil engineering structures. In response of these challenges researchers in the U.S.A and around the world have developed new and innovative techniques.This paper summarizes some of the ongoing research in the U.S.A. in the area of monitoring, damage detection and control in civil engineering structures.
Cables are essential components of the cable-stayed bridges as they serve as the main load-bearing component. Hence, continuous monitoring of such cables becomes necessary as they are vulnerable to the fatigue damage induced by dynamic loads. Sensors are attached to the cables to examine the health of the cables; however, these contact-based sensors can malfunction in harsh weather condition, which makes impossible to estimate the cable health in such unfavorable condition. Therefore, in this paper, we propose a completely noncontact video-based stay-cable tension measurement technique where the video is recorded using a moving handheld camera at a significant distance from the structure itself. Here, the cable tension is determined from vibration-based measurement, but the vibration of the cable recorded in the video includes the true vibration of the cable along with the camera motion. Hence, we amalgamated a series of image processing techniques to nullify the camera movement. First, we detect the camera movement based on the movement of the bridge deck and pylon, which are fixed objects, using Kanade–Lucas–Tomasi (KLT) feature tracking algorithm. Then we nullify the camera movement by using the affine transformation matrix obtained by random sample consensus (RANSAC) algorithm. Subsequently from the steady video, the cable motions are estimated using the phase-based motion estimation technique. From the time history of the cable vibration, real-time frequency variations are estimated using Short-Time Fourier Transform (STFT). Finally, the real-time tension is determined from this dominant frequency variation history using the taut-string theory. This paper shows the significant potential of camera-based sensing techniques in structural health monitoring as the mean estimated tension and the design cable tension are found to be comparable.
Damped outrigger is a viable means for reducing dynamic responses of tall buildings. This study focuses on generalized damped outrigger (GDO) systems. A GDO is composed of a damper for energy dissipation, a negative stiffness device and an inerter for damping enhancement. The GDO system incorporates GDOs at different floors of the tall building optimized to varied structural modes. Frequency equation of a tall building simplified as a cantilever beam with multiple GDOs is first derived by complex modal analysis. A finite different model of such a system is used for verification. Parametric analyses are then performed to compare damping effects of different GDO systems. It is found that a negative stiffness damped outrigger (NSDO) or inerter damped outrigger (IDO) needs to be optimized for maximizing damping of a specific mode. GDOs, respectively, tuned to different modes can largely improve the multimode damping effects. The optimal parameters of the GDOs are slightly different from those in the case when they are installed separately. With both negative stiffness and nonzero inertance, a GDO still needs to be tuned to a specific mode because multimode damping is sensitive to the damper coefficient. The combination of an NSDO optimized to the first mode and an IDO tuned to a higher mode seems the best solution. The IDO additionally improves the first mode damping provided by the NSDO and the two-mode damping is not sensitive to the damper coefficient of the NSDO. The findings are confirmed through seismic response analyses of a tall building with different GDO systems.
Yielding can be emulated in a structural system by adding an adaptive “negative stiffness device” (NSD) and shifting the “yielding” away from the main structural system-leading to the new idea of “apparent weakening” that occurs ensuring structural stability at all displacement amplitudes. This is achieved through an adaptive negative stiffness system (ANSS), a combination of NSD and a viscous damper. By engaging the NSD at an appropriate displacement (apparent yield displacement that is well below the actual yield displacement of the structural system) the composite structure-device assembly behaves like a yielding structure. The combined NSD-structure system presented in this study has a re-centering mechanism thereby avoids permanent deformation in the composite structure-device assembly unless, the main structure itself yields. Essentially, a yielding-structure is “mimicked” without any, or with minimal permanent deformation or yielding in the main structure. As a result, the main structural system suffers less accelerations, less displacements and less base shear, while the ANSS “absorbs” them. This paper presents comprehensive details on development and study of the ANSS/NSD. Through numerical simulations, the effectiveness and the superior performance of the ANSS/NSD as compared to a structural system with supplemental passive dampers is presented. A companion paper presents the NSD and its mechanics in detail.
The implementation of a mechanical negative stiffness device (NSD) within a reduced-scale highway bridge model and its performance under seismic loading conditions is evaluated via shaking table tests. Four different isolation system configurations are considered: isolated bridge (IB), IB with viscous dampers, IB with NSDs, and IB with viscous dampers and NSDs. In addition, two bridge pier configurations were considered: one with flexible piers (mimicking a middle span of a multi-span bridge) and one with braced piers (mimicking a single span bridge supported on abutments). The main feature of the NSD is a large pre-compressed spring, which can push the structure away from its initial undeformed position and thus induce negative stiffness behavior. The experimental results clearly demonstrate the effectiveness of the NSDs in limiting the seismic response of the bridge and provide validation of numerical simulation results wherein numerical models of the bridge model components were calibrated via system identification testing.
