• Nuclear Engineering and Technology
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• v.33 no.6
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• pp.619-625
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• 2001

An axial-flow-induced vibration model was proposed for a rod supported by two translational springs at both ends. For developing the model, a one-mode approximation was made based on the assumption that the first mode was dominant in vibration behavior of the single span rod. The first natural frequency and mode shape functions for the flow-induced vibration, called the FIV model were derived by using Lagrange＇s method. The vibration displacements at reactor conditions were calculated by the proposed model for the spring-supported rod and by the previous model for the simple-supported(55) rod. As a result, the vibration displacement for the spring-supported rod was larger than that of the 55 rod, and the discrepancy between both displacements became much larger as flow velocity increased. The vibration displacement for the spring-supported rod appeared to decrease with the increase of the spring constant. AS flow velocity increased, the increase rate of vibration displacement was calculated to go linearly up, and that of the rod having the short span length was larger than that of the rod having the long span length although the displacement value itself of the long span rod was larger than that of the short one.

• Steel and Composite Structures
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• v.18 no.3
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• pp.547-563
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• 2015

Deflection in a beam of a composite frame is a serviceability design criterion. This paper presents a methodology for rapid prediction of long-term mid-span deflections of beams in composite frames subjected to service load. Neural networks have been developed to predict the inelastic mid-span deflections in beams of frames (typically for 20 years, considering cracking, and time effects, i.e., creep and shrinkage in concrete) from the elastic moments and elastic mid-span deflections (neglecting cracking, and time effects). These models can be used for frames with any number of bays and stories. The training, validating, and testing data sets for the neural networks are generated using a hybrid analytical-numerical procedure of analysis. Multilayered feed-forward networks have been developed using sigmoid function as an activation function and the back propagation-learning algorithm for training. The proposed neural networks are validated for an example frame of different number of spans and stories and the errors are shown to be small. Sensitivity studies are carried out using the developed neural networks. These studies show the influence of variations of input parameters on the output parameter. The neural networks can be used in every day design as they enable rapid prediction of inelastic mid-span deflections with reasonable accuracy for practical purposes and require computational effort which is a fraction of that required for the available methods.

• Wind and Structures
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• v.35 no.5
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• pp.337-351
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• 2022

As central-slotted box decks usually have excellent flutter performance, studies on this type of deck mostly focus on the vortex-induced vibration (VIV) control. Yet with the increasing span lengths, cable-supported bridges may have critical wind speeds of wind-induced static instability lower than that of the flutter. This is especially likely for bridges with a central-slotted box deck. As a result, the overall aerodynamic performance of such a bridge will depend on its wind-induced static stability. Taking a 1400 m-main-span cable-stayed bridge as an example, this study investigates the influence of a series of deck shape parameters on both static and flutter instabilities. Some crucial shape parameters, like the height ratio of wind fairing and the angle of the inner-lower web, show opposite influences on the two kinds of instabilities. The aerodynamic shape optimization conducted for both static and flutter instabilities on the deck based on parameter-sensitivity studies raises the static critical wind speed by about 10%, and the overall critical wind speed by about 8%. Effective VIV countermeasures for this type of bridge deck have also been proposed.

• Structural Engineering and Mechanics
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• v.63 no.6
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• pp.847-856
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• 2017

Excessive vibrations can occur in long-span structures such as floors or footbridges due to occupant?s daily activity like walking and cause a so-called vibration serviceability issue. Since 1970s, researchers have proposed many human walking load models, and some of them have even been adopted by major design guidelines. Despite their wide applications in structural vibration serviceability problems, differences between these models in predicting structural responses are not clear. This paper collects 19 popular walking load models and compares their effects on structure?s responses when subjected to the human walking loads. Model parameters are first compared among all these models including orders of components, dynamic load factors, phase angles and function forms. The responses of a single-degree-of-freedom system with various natural frequencies to the 19 load models are then calculated and compared in terms of peak values and root mean square values. Case studies on simulated structures and an existing long-span floor are further presented. Comparisons between predicted responses, guideline requirements and field measurements are conducted. All the results demonstrate that the differences among all the models are significant, indicating that in a practical design, choosing a proper walking load model is crucial for the structure?s vibration serviceability assessment.

• Wind and Structures
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• v.10 no.1
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• pp.61-82
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• 2007

This paper presents a novel finite element (FE) model for analyzing coupled flutter of long-span bridges using the commercial FE package ANSYS. This model utilizes a specific user-defined element Matrix27 in ANSYS to model the aeroelastic forces acting on the bridge, wherein the stiffness and damping matrices are expressed in terms of the reduced wind velocity and flutter derivatives. Making use of this FE model, damped complex eigenvalue analysis is carried out to determine the complex eigenvalues, of which the real part is the logarithm decay rate and the imaginary part is the damped vibration frequency. The condition for onset of flutter instability becomes that, at a certain wind velocity, the structural system incorporating fictitious Matrix27 elements has a complex eigenvalue with zero or near-zero real part, with the imaginary part of this eigenvalue being the flutter frequency. Case studies are provided to validate the developed procedure as well as to demonstrate the flutter analysis of cable-supported bridges using ANSYS. The proposed method enables the bridge designers and engineering practitioners to analyze flutter instability by using the commercial FE package ANSYS.

• Wind and Structures
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• v.30 no.5
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• pp.499-509
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• 2020

The aeroelastic stability of a long-span suspension footbridge with a bluff deck (prototype section) was examined through static and dynamic wind tunnel tests using a 1:10 scale sectional model of the main girder, and the corresponding aerodynamic countermeasures were proposed in order to improve the stability. First, dynamic tests of the prototype sectional model in vertical and torsional motions were carried out at three attack angles (α = 3°, 0°, -3°). The results show that the galloping instability of the sectional model occurs at α = 3° and 0°, an observation that has never been made before. Then, the various aerodynamic countermeasures were examined through the dynamic model tests. It was found that the openings set on the vertical web of the prototype section (web-opening section) mitigate the galloping completely for all three attack angles. Finally, static tests of both the prototype and web-opening sectional models were performed to obtain the aerodynamic coefficients, which were further used to investigate the galloping mechanism by applying the Den Hartog criterion. The total damping of the prototype and web-opening models were obtained with consideration of the structural and aerodynamic damping. The total damping of the prototype model was negative for α = 0° to 7°, with the minimum value being -1.07%, suggesting the occurrence of galloping, while that of the web-opening model was positive for all investigated attack angles of α = -12° to 12°.

• Wind and Structures
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• v.2 no.4
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• pp.267-284
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• 1999

Multiple tuned mass dampers are proposed to suppress the vertical and torsional buffeting and to increase the aerodynamic stability of long-span bridges. Each damper has vertical and torsional frequencies, which are tuned to the corresponding frequencies of the structural modes to suppress the resonant effects. These proposed dampers maintain the advantage of traditional multiple mass dampers, but have the added capability of simultaneously controlling vertical and torsional buffeting responses. The aerodynamic coupling is incorporated into the formulations, allowing this model to effectively increase the critical speed of a bridge for either single-degree-of-freedom flutter or coupled flutter. The reduction of dynamic response and the increase of the critical speed through the attachment of the proposed dampers to the bridge are also discussed. Through a parametric analysis, the characteristics of the multiple tuned mass dampers are studied and the design parameters - including mass, damping, frequency bandwidth, and total number of dampers - are proposed. The results indicate that the proposed dampers effectively suppress the vertical and the torsional buffeting and increase the structural stability. Moreover, these tuned mass dampers, designed within the recommended parameters, are not only more effective but also more robust than a single TMD against wind-induced vibration.

• Journal of the Korean Society of Safety
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• v.30 no.5
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• pp.67-73
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• 2015

In this paper, the simple truss model was modified to predict the punching shear strength of long-span prestressed concrete (PSC) deck slabs under wheel load including the effects of transverse prestressing and long span length between girders. The strength of the compressive zone arounding punching cone was evaluated by the stiffness of inclined strut which was modified by considering aging effective modulus. The stiffness of springs which control lateral displacement of the roller supports consists of the steel reinforcement and prestressing which passed through the punching cone. Initial angle of struts was determined by the experimental observation to compensate for uncertainties in the complexities of the punching shear. The validity of computed punching shear strength by modified simple truss model was shown by comparing with experimental results and the experimental results were also compared with existing punching shear equations to determine level of predictability. The modified simple truss model appeared to better predict the punching shear strength of PSC deck slabs than other available equations. The punching shear strength, which was determined by snap-through critical load of modified simple truss model, can be used effectively to examine punching shear strength of long span PSC deck slabs.

• Smart Structures and Systems
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• v.14 no.4
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• pp.679-697
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• 2014

The strain data acquired from structural health monitoring (SHM) systems play an important role in the state monitoring and damage identification of bridges. Due to the environmental complexity of civil structures, a better understanding of the actual strain data will help filling the gap between theoretical/laboratorial results and practical application. In the study, the multi-scale features of strain response are first revealed after abundant investigations on the actual data from two typical long-span bridges. Results show that, strain types at the three typical temporal scales of $10^5$, $10^2$ and $10^0$ sec are caused by temperature change, trains and heavy trucks, and have their respective cut-off frequency in the order of $10^{-2}$, $10^{-1}$ and $10^0$ Hz. Multi-resolution analysis and wavelet shrinkage are applied for separating and extracting these strain types. During the above process, two methods for determining thresholds are introduced. The excellent ability of wavelet transform on simultaneously time-frequency analysis leads to an effective information extraction. After extraction, the strain data will be compressed at an attractive ratio. This research may contribute to a further understanding of actual strain data of long-span bridges; also, the proposed extracting methodology is applicable on actual SHM systems.

• Smart Structures and Systems
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• v.14 no.1
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• pp.55-70
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• 2014

The paper presents a multi-objective optimization strategy for a multi-type sensor placement for Structural Health Monitoring (SHM) of long span bridges. The problem is formulated for simultaneous placement of strain sensors and accelerometers (heterogeneous network) based on application demands for SHM system. Modal Identification (MI) and Accurate Mode Shape Expansion (AMSE) were chosen as the application demands for SHM. The optimization problem is solved through the use of integer Genetic Algorithm (GA) to maximize a common metric to ensure adequate MI and AMSE. The performance of the joint optimization problem solved by GA is compared with other established methods for homogenous sensor placement. The results indicate that the use of a multi-type sensor system can improve the quality of SHM. It has also been demonstrated that use of GA improves the overall quality of the sensor placement compared to other methods for optimization of sensor placement.