Deck Vibration Research Articles

Performance evaluation of a new ice-resistant jacket platform based on field monitoring

A new ice-resistant jacket platform was designed and constructed in the Bohai Sea, equipped with a series of techniques to mitigate the ice-induced vibrations. The vibration mitigating techniques include the following: 1) Single-leg design with double-ward fixed ice breaking cone at water level, so as to reduce ice load and dynamic response of the structure; 2) the steel-rubber bearing isolators and magnetorheological (MR) dampers, installed between the upper and lower deck of the platform, so as to reduce the vibration of upper deck and to protect the pipeline system transporting natural gas. In order to evaluate the performance of the new platform, different sensors were deployed on the structure to measure the accelerations of the deck levels and the relative displacement between the two decks. Based on the measured data, the ice-resist performance is discussed. It is shown that the conceptual design and related techniques used on this platform can be adopted during the design process of other structures in the future. ► A new ice-resistant jacket platform was designed in the Bohai Sea ► A field monitoring system was built to evaluate the ice-resistant performance ► The vibration of the structure with mitigating facilities gets smaller than that without mitigating facilities ► The new designed platform and accompanying vibration mitigating facilities are effective ► The conceptual design and related techniques can be adopted in the future.

A numerical study on nonlinear vibration of an inclined cable coupled with the deck in cable-stayed bridges

The mechanism and the characteristics of the cable-deck coupled nonlinear vibration in a cable-stayed bridge are studied using numerical methods. A simple three-degrees-of-freedom (d.f.) model, with one independent d.f. for modeling the bridge deck movement, is proposed for describing the nonlinear interactions between the in-plane/out-of-plane vibration of the cable and the oscillation of the bridge deck. The governing equations are discretized with the Galerkin method and then solved with a numerical time integration algorithm. It is pointed out that the periodic variation of cable tension caused by vibration of a bridge deck will lead to the parametric resonance of the stay cable under certain tuning conditions. Numerical results also confirm that energy transfer between different vibration modes and beating phenomenon of the cable-deck vibration may be exhibited in the case of parametric resonance.

Van der Pol-type equation for modeling vortex-induced oscillations of bridge decks

A careful modeling of vortex-induced vibration of a bridge deck allows to avoid fatigue damages and the reduction of travel safety and/or comfort levels for the users. The existing empirical models do not seem to be fully reliable because they are not able to predict the response oscillation amplitude for values of the mass and damping away from those at which their aeroelastic parameters were estimated. A van der Pol-type equation is used in the Ehsan–Scanlan's model, which for practical reasons seems to be the more suitable model for vortex-induced vibration of bridge decks. In this work this model was deeply studied and wind-tunnel tests on a 4:1 rectangular cylinder were performed to apply it to a realistic case study. Numerical integrations of the van der Pol-type equation highlighted its physical coherence to model vortex-induced vibrations of structures prone to the wind action. Moreover, it was verified that the assumptions standing behind the Ehsan–Scanlan's identification procedure of the aeroelastic parameters are justified. Finally, an alternative identification procedure was proposed, consisting in a numerical estimation of the aeroelastic parameters directly from the nonlinear differential equation.

Performance Optimization of the FGX Dry Separator for Cleaning High-Sulfur Coal

The main goal of the present study was not only to deshale (remove pure rock) raw coal extracted from Illinois mines but also to assess the maximum ash separation efficiency and sulfur rejection achievable using the FGX Dry Separator for cleaning raw coals of varying cleaning characteristics. A Model FGX-1 Dry Separator with feed throughput capacity of 10 tph was extensively tested at the Illinois Coal Development Park using multiple coal samples having distinctly different cleaning characteristics. Statistically designed experimental programs were conducted to indentify critical process variables and to optimize FGX Dry Separator performance by systematic adjustments of critical process variable parameters. The coal-cleaning performance of the FGX Dry Separator was evaluated for the particle size range of 4.75–63.5 mm in most cases, although FGX Dry Separator feed consisted of nominal 7minus;63.5 mm run-of-mine coals. Deck vibration frequency, longitudinal deck angle, feeder frequency, and baffle plate height were identified as critical process variables for the FGX Dry Separator. The best cleaning performance obtained from the FGX Dry Separator was described by specific gravity of separation (SG50) and probable error (Ep) values of 1.98 and 0.17, respectively. For a relatively easy-to-clean coal (having a Cleaning Index of 0.72), only about 0.42% of the clean coal (i.e., 1.6 float fraction) present in the feed was lost to the tailings stream. For a relatively difficult-to-clean coal (having a Cleaning Index of 0.53), about 0.98% of the clean coal present in the feed was lost to the tailings stream. The positive impact of having fine materials in the FGX feed stream was also noted in this study. A modified log-logistic partition model was developed using experimental data reported in literature and validated using new experimental data generated in this study. The results showed that this model could be effectively used to predict the FGX Dry Separator coal-cleaning performance.

Study of Verrazano-Narrows Bridge Movements during a New York City Marathon

Series of measurements of the motion of the Verrazano-Narrows Bridge during the November 7, 2004, New York City Marathon were made to evaluate the level of vibrations during the race. This paper presents details of the testing protocols and recorded data analyses as related to the bridge and the runners’ level of vibration perception. From the analysis of the recorded data, the excited local mode of the bridge deck within the range of the runners’ step frequency was identified. In addition, the computed bridge global response using observed runner density and estimated weight showed excellent agreement with the bridge measured response. It was also found that based on the analyses of the recorded data, the bridge deck vibration level during the marathon was within the acceptable range for the runners.

Finite element vibration analysis and measurement of a 255 ft vehicle ferry.

Finite element analysis (FEA) methods have become part of the standard repertoire when designing a ship. This is particularly true for static analyses, although FEA methods are becoming increasingly utilized for analysis of low-frequency vibration as well. This paper discusses the FEA modeling process that was used for analyzing low-frequency (less than 80 Hz) habitability vibration on a 255 ft vehicle ferry. Deck vibration levels were measured after the vessel was built. Measured versus predicted vibration data show good correlation, particularly when the various sources of potential error and uncertainty are considered. The subject ferry was designed to meet the ABS Comfort Class (COMF) vibration criteria. Calculations of deck vibration were performed using NEiNastran to verify compliance with the criteria. Excitations due to propeller hull pressure excitation are highlighted here as these forces created the largest deck responses. However, it is noted that the main propulsion engines were resiliently mounted subsequent to initial analyses using the FE model. This paper discusses pertinent details of the model (element types, properties, etc.), application of hull pressures, damping, analysis methodology, and interpretation of results. Areas of uncertainty and impacts to the results are discussed.

CABLE TENSIONING CONTROL AND MODAL IDENTIFICATION OF A CIRCULAR CABLE-STAYED FOOTBRIDGE

simultaneous effect of several pedestrians depends on their synchronization, in normal circumstances it tends to be a problem of resonance in a vibration mode with significant vertical or horizontal displacements. When the vibration of the deck is perceptible for the pedestrians, synchronization of the human movements and the deck vibration is likely to occur. 2,3 Recently a number of footbridges were built in Portugal within the scope of the urban renovation plans of several cities. This article deals with the modal identification and vibration measurements and finite element (FE) modelling of one of these structures located in Aveiro, Portugal. The circular plan view of the bridge allows the link among the three points located on the banks of the T-shaped junction of two canals (Figs. 1 and 2). Thevibrationmeasurementsarepartoftheon-sitestructural characterization of the bridge that also included load tests, terrestrial photogrammetric survey, and long-term strain measurements using Brag sensors located on the cables and on the mast.

Continuous monitoring of the Øresund Bridge: system and data analysis

The Øresund Bridge opened in July 2000. It is the most striking part of the fixed link across the Øresund connecting Copenhagen (Denmark) and Malmø (Sweden), which further includes a tunnel and an artificial island. The bridge is equipped with a PC-based continuous monitoring system, capable of measuring both static and dynamic quantities such as temperatures, wind characteristics, air humidity, strains and accelerations. The challenges for the design of the monitoring system were the long distances between the monitoring points and the variety of sensors. This paper describes the bridge and the monitoring system components. Some typical measurement data are presented. Finally, the modal parameters of the bridge are extracted from the deck, cable and tower vibrations. This shows that the system does not only give information about sudden events that exceed a certain threshold, but can also be used as a health monitoring system by tracking the evolution of the modal parameters.

Noise Evaluation for Pavement Maintenance in Metropolitan Highway Bridges

Dynamic response of a bridge under traffic load induces acoustic energy at the bridge surface. The acoustic energy change generates an additional coupled noise component caused by vibration of a bridge deck associated with the pavement conditions and moving velocity of the vehicle. This paper presents a three-dimensional finite-element method developed for the dynamic response and noise propagation model, and analyzes the coupled effect induced by traffic loading based on different pavement conditions. Even though vibration-induced noise at the bridge is below the audible frequency range of 20–20,000 Hz , it amplifies the traffic noise source to the highly annoyed level of noise in the metropolitan area. Among several factors that contribute to the traffic noise, interaction between pavement and vehicle is considered according to the different surface roughness and vehicle velocity. The result shows that poor pavement condition contributes to the increase of traffic noise at a high traveling speed of the ...

DYNAMIC STABILITY OF CABLE-STAYED BRIDGE PYLONS

This paper deals with the stability of the pylons of a cable-stayed bridge under the action of time-dependent loads, due to the vibration of the bridge deck. The stability of such problems of cable-stayed bridges is solved by a technique developed in the Laboratory of Metal Structures and Steel Bridges, of National Technical University of Athens (NTUA), as well as Bolotin's technique for the solution of nonlinear problems of dynamic stability. Three cases are studied: pylons with damping, pylons under forced vibration, and pylons subjected to an arbitrary external dynamic load. Useful relations are established by the aforementioned solution method, examples for a variety of pylons are presented, and interesting results regarding the stability of each case are given in diagrams.

Output-only modal identification of a cable-stayed bridge using wireless monitoring systems

The objective of this paper is to present two modal identification methods that extract dynamic characteristics from output-only data sets collected by a low-cost and rapid-to-deploy wireless structural monitoring system installed upon a long-span cable-stayed bridge. Specifically, an extensive program of full-scale ambient vibration testing has been conducted to measure the dynamic response of the 240 m Gi-Lu cable-stayed bridge located in Nantou County, Taiwan. Two different output-only identification methods are used to analyze the set of ambient vibration data: the stochastic subspace identification method (SSI) and the frequency domain decomposition method (FDD). A total of 10 modal frequencies and their associated mode shapes are identified from the dynamic interaction between the bridge’s cables and deck vibrations within the frequency range of 0–7 Hz. The majority of the modal frequencies observed from recording cable vibrations are also found to be associated with the deck vibrations, implying considerable interaction between the deck and cables.

Experimental study on possible vortex shedding in a suspension bridge - Part II - Results when under typhoon Babs and York

Statistical analysis on the measured responses of a suspension bridge deck (Law, et al. 2007) show that vibration response at the first torsional mode of the structure has a significant increase at and beyond the critical wind speed for vortex shedding as noted in the wind tunnel tests on a sectional model. This paper further analyzes the measured responses of the structure when under typhoon conditions for any possible vortex shedding events. Parameters related to the lifting force in such a possible event and the vibration amplitudes are estimated with a single-degree-of-freedom model of the system. The spatial correlation of vortex shedding along the bridge span is also investigated. Possible vortex shedding events are found at both the first torsional and second vertical modes with the root-mean-square amplitudes comparable to those predicted from wind tunnel tests. Small negative stiffness due to wind effects is observed in isolated events that last for a short duration, but the aerodynamic damping exhibits either positive or negative values when the vertical angle of wind incidence is beyond <TEX>${\pm}10^{\circ}$</TEX>. Vibration of the bridge deck is highly correlated in the events at least in the middle one-third of the main span.

Health monitoring data in assessing critical behaviour of bridges

A baseline finite element model has been developed and used for system identification of the Confederation Bridge in Canada. The model was calibrated using the test data collected by the health monitoring system during two field tests. The frequencies and mode shapes obtained from the modal analyses were in good agreement with the values obtained from the tests. The model was used to simulate the bridge responses to the field tests. Continuity of the bridge model beyond the baseline model is essential in analysing its behaviour under moving truck loads. Coupling of the longitudinal movements of the pier tops with vertical movement of the deck and rocking vibration of the drop-in girders were observed in modal analyses of the extended model, which was then used for seismic analyses. The possibility of slippage of the drop-in girder as well as its separation from the bearings during strong ground motions should be considered in designing and future extension of the instrumentation program and the analyses of the recorded data for similar structures.

Nonlinear Models of Cable-Stayed Bridges

Cable-stayed bridges frequently experience vibrations due to a variety of mechanisms, exacerbated by their very low inherent damping. A research group of the University of Bristol has focused lately on the study of cable-stayed bridges, some advances have led to the identification of vortex-induced deck vibrations occurring at the Second Severn Crossing (SSC) and improved methods of analysis of field vibration data. Based on such experience, it aims to study the autoparametric excitation which, due to very great amplitudes, can seriously damage the structure. It has been suggested that this may have been the mechanism of excitation of some large amplitude cable vibrations on real bridges, but the details of the behaviour are not very well understood and several cases of large cable vibrations on full scale bridges have not been fully explained. In this paper we examine a previously established cable-deck model and compare it to a new, more exact model in a different coordinate basis.

Wave impact on elastically supported horizontal deck

Laboratory experiments supported by theoretical investigations were applied to study the wave impact on a horizontal deck, deck vibrations, and phenomena related to wave-induced vibrations. The analysis shows that one can distinguish four characteristic stages of deck vibrations between two consecutive wave impacts. In the first stage, the wave impact induces very high-frequency waves in the deck structure and in the wave flume. There are important components with acoustic frequencies. In the second stage, the deck vibrates in contact with water. The frequencies of vibrations in water are much lower than the corresponding frequencies of the free vibrations in air. The third stage of wave-induced vibrations is a transitional one. At the beginning of the third stage, the deck vibrates in contact with water and at the end of this stage it vibrates in air as a free rigid body. In this process water is shed from the deck. Sometimes the process of shedding is violent, which induces high-frequency vibrations with relatively large amplitudes. The fourth stage of wave-induced vibrations corresponds to the free vibrations of the deck in air. Similar four stages can be distinguished in the pressure records. Additional information regarding wave impact on the deck and the phenomena related to the impact can be obtained by the analysis of mechanical energy. The analysis of mechanical energy is especially helpful in understanding dissipation processes and phenomena in the transitional stage of wave-induced vibrations. In particular, the analysis of energy explains why the amplitudes of acceleration are increasing in the transitional stage of wave-induced vibrations.

A simplified fragility analysis of fan type cable stayed bridges

A simplified fragility analysis of fan type cable stayed bridges using Probabilistic Risk Analysis (PRA) procedure is presented for determining their failure probability under random ground motion. Seismic input to the bridge support is considered to be a risk consistent response spectrum which is obtained from a separate analysis. For the response analysis, the bridge deck is modeles as a beam supported on spring at different points. The stiffnesses of the springs are determined by a separate 2D static analysis of cable-tower-deck system. The analysis provides a coupled stiffness matrix for the spring system. A continuum method of analysis using dynamic stiffness is used to determine the dynamic properties of the bridges. The response of the bridge deck is obtained by the response spectrum method of analysis as applied to multidegree of freedom system which duly takes into account the quasi-static component of bridge deck vibration. The fragility analysis includes uncertainties arising due to the variation in ground motion, material property, modeling, method of analysis, ductility factor and damage concentration effect. Probability of failure of the bridge deck is determined by the First Order Second Moment (FOSM) method of reliability. A three span double plane symmetrical fan type cable stayed bridge of total span 689 m, is used as an illustrative example. The fragility curves for the bridge deck failure are obtained under a number of parametric variations. Some of the important conclusions of the study indicate that (i) not only vertical component but also the horizontal component of ground motion has considerable effect on the probability of failure; (ii) ground motion with no time lag between support excitations provides a smaller probability of failure as compared to ground motion with very large time lag between support excitation; and (iii) probability of failure may considerably increase soft soil condition.

Parametrically excited oscillation of stay cable and its control in cable-stayed bridges

This paper presents a nonlinear dynamic model for simulation and analysis of a kind of parametrically excited vibration of stay cable caused by support motion in cable-stayed bridges. The sag, inclination angle of the stay cable are considered in the model, based on which, the oscillation mechanism and dynamic response characteristics of this kind of vibration are analyzed through numerical calculation. It is noted that parametrically excited oscillation of a stay cable with certain sag, inclination angle and initial static tension force may occur in cable-stayed bridges due to deck vibration under the condition that the natural frequency of a cable approaches to about half of the first model frequency of the bridge deck system. A new vibration control system installed on the cable anchorage is proposed as a possible damping system to suppress the cable parametric oscillation. The numerical calculation results showed that with the use of this damping system, the cable oscillation due to the vibration of the deck and/or towers will be considerably reduced.

Traffic‐induced variability in dynamic properties of cable‐stayed bridge

AbstractThe traffic‐induced variability in the dynamic properties of a cable‐stayed bridge is investigated using ambient vibration measurements. Under a relatively steady wind and temperature environment, the ambient vibration test was conducted on the bridge with normal traffic conditions and totally 24 h acceleration response time histories were recorded. These data are divided into 12 sections with each data section containing 2 h measurements. Thereby the modal variability due to changing traffic loading is investigated through post‐processing of the data in each section in both amplitude and frequency domains. The result indicates that the natural frequencies of the global modes can exhibit as much as 1% variation within a day. The modal amplitudes of each mode as well as the modal deflection at each measurement position vary insignificantly. The damping ratios however are sensitive to the vibration intensity, especially when the deck vibration exceeds a certain level. Copyright © 2002 John Wiley &amp; Sons, Ltd.

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

AbstractThis 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 &amp; Sons, Ltd.

Fully coupled buffeting analysis of Tsing Ma suspension bridge

A fully coupled buffeting analysis of the Tsing Ma suspension bridge in Hong Kong is carried out in this paper. The features of the bridge, its natural frequencies and mode shapes from finite element analysis are first presented. Followed is an introduction to the formulation for fully coupled buffeting analysis of cable-supported bridges, which is featured mainly by a combination of finite element approach and pseudo-excitation method in the frequency domain. The buffeting response of bridge deck is then computed and compared with that from the conventional method. After the comparison, the effects from aeroelastic forces, multimodes and intermodes of vibration on deck response are examined. Finally, the buffeting response of the whole bridge is investigated with aerodynamic forces on not only bridge deck but also bridge towers and main cables. The results show that aeroelastic forces significantly affect the torsional response of the deck. The multimode responses are larger than single-mode responses in the vertical and torsional motions of the deck. The significant intermode responses may exist between the vibration modes of similar mode shapes or similar modal components. The buffeting of towers and main cables affects the lateral vibration of bridge deck and, in turn, the vibration of the bridge deck impacts the buffeting of the bridge towers and main cables.