Vertical Vortex-induced Vibration Research Articles

Vortex-induced vibration characteristics of two open girders: A comparison of experimental and numerical investigation

Open girder sections are vulnerable to vortex-induced vibrations (VIV) because of their bluff aerodynamic characteristics. A comparative investigation was conducted by wind tunnel tests and numerical simulation. Firstly, based on sectional model wind tunnel tests, the VIV performance of semi-open girder and separated edge-boxes open girder was studied with consideration of the influence of the equivalent mass, wind attack angle and damping ratio. The aerodynamic parameters Scruton number ( S c) and Strouhal number ( S t) of two girders were analyzed contrastively. In addition, VIV amplitudes corresponding to the real bridge girders were calculated by the linear and nonlinear theories. Then, flow fields around two semi-open girder sections were investigated on the basis of computational fluid dynamics. The results show that vertical and torsional VIVs of two open girders are observed, at +3°and +5° wind attack angles. There were two vertical VIV regions and the maximum amplitude in the second vertical VIV region is significantly larger than that in the first one. For the semi-open girder and separated edged-box open girder, the vertical VIV amplitudes at +5° wind attack angle is 115% and 75% larger than at +3° wind attack angle, respectively. The damping ratio could obviously mitigate the VIV of two girders and VIV amplitudes are decreased linearly with the S C number increases. There are vortexes above the girder deck at the opening position of the semi-open girder and the separated edge-boxes open girder. The oblique web and wind faring may break the large vortex into several smaller vortexes at the opening position, thus optimizing the vortex vibration performance of the girder. A thicker shear layer is formed at separated open-girder during the development of airflow towards backward of bridge deck, due to the effect of railings and the blunt boxes on the lower surface.

Experimental study on characteristics of vortex-induced vibration of a twin-box girder and damping effects

To investigate the characteristics of vortex-induced vibration (VIV) of the twin-box girder, we carried out the VIV wind tunnel tests. The VIV of the twin-box girder occurs in the range of 4.680≤Ur≤6.113, with the maximum dimensionless VIV displacement RMS(y)/D=0.03308 at reduced velocity at Ur=5.949. It is interesting that the VIV suddenly disappears at Ur=6.276 at the loading stage, and the large amplitude VIV does not occur at the unloading stage, which indicates that the vibration trajectory with the variation of reduced wind velocity shows significant asymmetry characteristic. In the lock-in region, as the aerodynamic lift force increases, both the phase lag and the VIV displacement show an increasing tendency with the reduced velocity. Flow visualization results show that strong alternately shedding vortex generates around the gap during the VIV. Under the effects of the vortex in the gap, the size of the separation bubble increases with the increase of vibration amplitude, and the wake width becomes wider. As the damping ratio increases, the amplitude of VIV displacement shows a decreasing tendency, the lock-in region becomes narrower and the reduced velocity corresponding to the max VIV displacement decreases. Moreover, the maximum RMS displacement of vertical VIV decays exponentially as the damping ratio increases.

Wind-induced vibrations and suppression measures of the Hong Kong-Zhuhai-Macao Bridge

A series of wind tunnel tests, including 1:50 sectional model tests, 1:50 free-standing bridge tower tests and 1:70 full-bridge aeroelastic model tests were carried out to systematically investigate the aerodynamic performance of the Hong Kong-Zhuhai-Macao Bridge (HZMB). The test result indicates that there are three wind-resistant safety issues the HZMB encounters, including unacceptable low flutter critical wind speed, vertical vortex-induced vibration (VIV) of the main girder and galloping of the bridge tower in across-wind direction. Wind-induced vibration of HZMB can be effectively suppressed by the application of aerodynamic and mechanical measures. Acceptable flutter critical wind speed is achieved by optimizing the main girder form (before: large cantilever steel box girder, after: streamlined steel box girder) and cable type (before: central cable, after: double cable); The installations of wind fairing, guide plates and increasing structural damping are proved to be useful in suppressing the VIV of the HZMB; The galloping can be effectively suppressed by optimizing the interior angle on the windward side of the bridge tower. The present works provide scientific basis and guidance for wind resistance design of the HZMB.

Experimental study on suppression of vortex-induced vibration of bridge deck using vertical stabilizer plates

Despite the wind-resistant design performed during the design stage, an existing bridge may still experience severe vortex-induced vibrations (VIVs) because of the installation of new affiliated facilities during its operation period. Therefore, it is necessary to apply control measures to suppress the VIV induced by such changes in configuration. In this study, the VIV performance of a streamlined box girder bridge during the installation of new auxiliary piping system was investigated through a series of sectional model tests and flow visualization tests. Severe vertical VIVs of the bridge used in this study were observed at lower wind speed ranges during wind tunnel tests. Based on the flow pattern around the sectional model, it was inferred that the flow across the deck bottom surface was entirely separated by the new pipelines, which resulted in the periodic generation of shedding vortices in the wake zone and induced deck vibration. Twin downward vertical stabilizer plates (TDVS) were found to be an effective aerodynamic countermeasure for improving the VIV performance of the bridge and could significantly decrease the amplitude of the vertical VIV motion. Additional flow visualization investigations showed that after the TDVS were installed, the vortex shedding in the wake was mitigated, and the location of the vortex shedding was further away from the deck, and hence, considerably suppressed the vertical VIVs. It was found that the effectiveness of the TDVS in improving the aerodynamic performance was significantly dependent on the TDVS parameters, i.e., the plate height and spacing between plate elements. The optimal values of these parameters were correspondingly identified for the bridge deck used in this study after a comprehensive evaluation.

Effects of leading-edge separation on the vortex-induced vibration of an elongated bluff body

Wind-tunnel experiments were conducted to investigate the effects of leading-edge separation on the vortex-induced vibration (VIV) of an elastically-supported elongated bluff body. Solid wind barriers of various heights were fixed in the leading edge to adjust the flow separation. The vibration signals and the flow field information are acquired simultaneously by a laser-displacement system and particle image velocimetry (PIV) instrument respectively. The result shows that the VIV is excited by consecutive vortices shedding from the leading-edge shear layer. There is a critical height-to-thickness ratio (h/t)critical=0.4, the VIVs are observed when h/t≥0.4, while they are suppressed owing to the insignificant leading-edge separation when h/t<0.4. Three types of VIV are found, two of them are torsional VIV while another one is vertical VIV, which are named T1, T2, and V1 respectively. By placing a wake splitter plate at the trailing edge, V1 is suppressed entirely, T1 is weakened despite the slightly lower amplitude, while T2 transits to limit-cycle oscillations. The flow field information shows that the evolution and shedding pattern of leading-edge vortices vary from mode to mode. The status of the upper-layer leading-edge separated layer significantly affects the formation and the shedding of the trailing-edge vortices. The results reveal that leading-edge separation can play a dominant role in the vortex-induced vibrations of elongated bluff bodies.

Aerodynamic performance of Π-shaped composite deck cable-stayed bridges including VIV mitigation measures

This study systematically investigates the influence of the cross-sectional aspect ratio (B/H) and four different aerodynamic mitigation measures, i.e., the fairing, lower central stability plate, inverted L-shaped edge plate, and lower horizontal edge plate, on the performance of cable-stayed bridges, with π-typed composite deck, i.e., π section, due to vortex-induced vibrations (VIV). The study utilized wind tunnel tests and limited CFD simulations by varying bridge dimensions. First, based on a survey of existing cable-stayed bridges with the π-typed composite deck, three-section models with different aspect ratios were studied and their corresponding vibration characteristics for the vertical and torsional VIVs were analyzed. Then, the vibration suppression effects of mitigation measures with several variants in shape were investigated to determine an effective mitigation measure, i.e., with favorable aerodynamic stability, for the π section deck. The vibration suppression mechanism for three selected cases was further assessed using numerical simulation. The results show that: (a) for the π section model with the aspect ratio in the range of 9.05–11.96, increasing the ratio value will delay the onset wind speed of the vertical and torsional VIVs; (b) In the case of a low ratio of the torsional frequency to vertical bending frequency, there are often special phenomena where the vertical and torsional vibrations coexist. However, a larger aspect ratio would result in more independent vertical and torsional vibrations; (c) Regarding the vibration suppression effect, based on the limited studied cases, a small fairing can be superior to large fairings; (d) Different forms of the lower central stability plate have a good suppression effect on the vertical VIV; (e) The use of inverted L-shaped edge plate with a ​= ​23H and b ​= ​12H is identified to result in the best vibration suppression for both the vertical and torsional vibrations among the considered mitigation measures, where a and b are the size parameters of the inverted L-shaped edge plate; and (f) The lower horizontal edge plate cannot effectively improve the torsional VIV performance. This study offers guidance on the performance of composite cable-stayed bridge with the π section in isolation and equipped with mitigation measures to aid in the engineering design practice.

Experimental and numerical study on vortex-induced vibration of a truss girder with two decks

Vortex-induced vibration (VIV) depends on aerodynamic shapes of bridge girders, which should be treated carefully in the design of long-span bridges. This paper studies the VIV performance of a suspension bridge with the truss girder which contains two separated decks. Although truss girders generally show better VIV performance than box girders, significant vibrations of this type of girders occurred in the wind tunnel tests based on a large-scale sectional model. Several lock-in regions with the same vibration frequency were observed, corresponding to different shedding vortices. Computational fluid dynamics (CFD) simulations were carried out, and monitoring points were set behind different components to study the characteristics of the shed vortices. As the truss girder consists of many members, the results show that various vortices with different dominant frequencies are formed in the wake flow. The vertical VIV of the bridge is probably driven by the vortices behind or above the upper deck, which is related to the guardrails. The torsional VIV of the bridge is probably driven by the vortices behind or below the lower deck, which is related to the service road at lower wind speeds while may be related the vertical stabilizers at higher wind speeds.

Experimental and numerical studies on the vortex-induced vibration of two-box edge girder for cable-stayed bridges

Due to the blunt aerodynamic configuration, the edge-girder bridge deck may sustain severe vortex-induced vibrations (VIVs) that adversely affect bridge usability and security. In the present study, the VIV performance and the related aerodynamic mechanism of the two-box edge girder for a cable-stayed bridge are systematically investigated. Based on the flow pattern and vortex structure around the section simulated by computational fluid dynamics, it is inferred that the aerodynamic interactions of the large-scale vortices under the deck and the shedding vortices in the wake zone periodically change the fluctuating pressures on the section and induce the vibrations of the deck. Moreover, several aerodynamic mitigation countermeasures are designed. Their effects on the vertical VIV are experimentally investigated in detail. It is found that the vertical VIVs are considerably suppressed at different attack angles (−3°, 0°, and +3°) by installing the mini-triangular wind fairings, which can satisfy the bridge wind-resistant design requirements. Further numerical simulations show that after the installation of the effective countermeasures, vortex shedding in the wake zone is mitigated, and the strengths and distortions of large-scale vortices around the upper and lower surfaces of the section also decrease, thereby the vertical VIV is suppressed effectively.

An alternative aerodynamic mitigation measure for improving bridge flutter and vortex induced vibration (VIV) stability: Sealed traffic barrier

This study proposes an alternative aerodynamic mitigation measure, i.e., sealed traffic barrier, for bridges and verifies its efficacy in improving the flutter and vortex induced vibration (VIV) stability of bridges through wind tunnel tests for three distinctive real-world bridge projects. The results show that: (1) The measure of the sealed traffic barrier with regular intervals along the bridge deck has a significant influence on the flutter and VIV performance of the bridge. Thus, a reasonable design of the traffic barrier can simultaneously improve the bridge’s flutter and VIV stability; (2) The sealed traffic barrier with vertical intervals along the bridge can effectively suppress the vertical VIVs of the bridge, while the sealed traffic barrier without vertical intervals, i.e., configured continuously along the bridge, has a better vibration suppression effect on the torsional VIV than that on the vertical VIV; (3) This proposed measure installed on both sides of the bridge deck imposes favorable suppression effect for the flutter vibration, similar to the upward central stabilizer. In case with the sealing form fixed, but a reduced height, for the barrier, the flutter stability at different wind attack angles is slightly decreased; and (4) In certain scenarios, increasing the ventilation rate of the traffic barrier results in an increase for the flutter critical wind speed at positive wind attack angles, but a decrease for the flutter critical wind speed at negative wind attack angles. This aerodynamic mitigation measure has been successfully applied to more than ten other bridge projects based on the wind tunnel tests by the leading authors, offering a reliable approach in improving both flutter and VIV for bridges.

Vortex-induced vibration of bridge decks: Describing function-based model

A describing function (DF)-based model is introduced for the simulation of vortex-induced vibration (VIV) of bridge decks. Similar to the linear frequency response function, the DF is the complex ratio of the first-order component of the nonlinear output to the harmonic input. The DF can be either identified using the forced vibration technique or based on the VIV nonlinear response time history. An iterative procedure is accordingly developed to predict the VIV response with the DF-based model, and an equivalent-damping-ratio-based simplified method is further proposed to efficiently obtain the limit cycle oscillation (LCO) amplitude of VIV. It is demonstrated that the conventional van der Pol-type model is equivalent to a special case of the DF-based model for VIV. Three case studies involving various cross-sections are utilized to validate the simulation accuracy and efficiency of the proposed DF-based model for typical features at VIV lock-in such as LCO and hysteresis phenomena. The vertical VIVs can be well captured by the DF-based model, while its capability of simulating the torsional VIVs requires further improvement. Furthermore, the predictive capability of DF-based model for vertical VIVs of bridge decks within a wide range of mass-damping conditions is highlighted.

Mechanism of suppression of vortex-induced vibrations of a streamlined closed-box girder using additional small-scale components

VIV (vortex-induced vibration) suppression using aerodynamic countermeasures are critical for design of long-span bridges. Strong vertical VIVs were observed in a streamlined closed-box bridge girder subject to wind at an initial attack angle of +3°. CFD simulation, and pressures and displacements measured in wind tunnel tests on a large-scale model (approximately 1:20) were analyzed to reveal the VIV-triggering mechanism for a bridge girder, as well as the VIV-suppression mechanism for bridge girders modified with typical small-scale additional components such as spoilers on crash barriers or guide vanes near maintenance traces. Spoilers can almost eliminate VIVs, while guide vanes can moderately mitigate them. Large-scale vortexes generated from flow separation at leading barriers and leading maintenance trails, referred to as separated vortexes and secondary vortexes, respectively, and called "double vortex mode" together, are responsible for VIVs of an unmodified girder. When spoilers were set on the barriers, the separated vortexes, and then the "double vortex mode" observed on the unmodified bridge girder, could be broken. This greatly reduced the correlation between local aerodynamic forces and general vortex-excited forces (VEFs), and thus the contribution of local aerodynamic forces to general VEFs on the model surface, which caused the VIVs to disappear. As for the unmodified girder, the "double vortex mode" is also responsible for vertical VIVs of a girder modified with guide vanes. This is due to the strong correlation between local aerodynamic forces and general VEFs. However, the phase lags between them have an overall offset of approximately 90° from the approximate synchronous action. Furthermore, a significant decrease in RMS values of pressure coefficients on the model surface dramatically reduce the direct contribution of local aerodynamic forces to general VEFs, especially in the downstream region of the upper surface, thus clearly mitigating VIVs.

A nonlinear numerical scheme to simulate multiple wind effects on twin-box girder suspension bridges

The present study aims to investigate multiple wind effects on long-span twin-box girder bridges by developing a time-dependent nonlinear numerical scheme, which takes into account a range of aerodynamic forces, such as the stationary aerodynamic force, self-excited force, buffeting force and vortex-induced vibration (VIV) force. After calibration and validation, the developed scheme was implemented to investigate wind-induced behaviors of the Xihoumen Suspension Bridge. The results show that the developed scheme has the capability of simulating the nonlinear features of various types of bridge aerodynamic forces and can reproduced the flutter phenomenon from wind tunnel testing and the VIV phenomenon subject to field measurement. In addition, it predicts that, under the critical wind velocity (Ucr) of 95 m/s in smooth flow, the failure mode of the bridge can be defined as the disconnection of hangers at the 1/2L of the main span, while under the Ucr of 85 m/s in turbulence flow, the failure of the bridge can be described as multiple hangers fracture at the 3/4L of the main span with the symmetrical bending-torsional coupled oscillation. Besides, the vertical VIV of the bridge with the maximum amplitude of about 0.2 m and the 7th vertical mode belong to limit cycle oscillation.

Vibration control of vortex-induced vibrations of a bridge deck by a single-side pounding tuned mass damper

This paper proposes a new method to mitigate vortex-induced vibrations (VIVs) of a bridge deck using a single-side pounding tuned mass damper (SS-PTMD). The SS-PTMD is a passive control device and comprises an undamped tuned mass with a pounding boundary covered with viscoelastic (VE) materials layer. A nonlinear force model for describing impact behavior of VE materials is used to simulate the response of a single degree of freedom (SDOF) system controlled by a SS-PTMD. The free pounding experiments are performed to determine the model parameters of impact force and validate the simulation method. The optimal design of SS-PTMD for SDOF system subjected to sinusoidal excitation is carried out by numerical optimization, and the optimized SS-PTMD is applied to control the vertical VIVs of a bridge deck. The control performance is experimentally examined by elastically mounted section model tests in wind tunnel. In addition, the classical wake oscillator model is used to predict the behavior of the coupled fluid-structure system under VIV and explore the control performance of the SS-PTMD. The experimental results show that the maximum response of the bridge deck model was reduced by 94% when a SS-PTMD with mass ratio of 2% was applied. It is also shown that nonlinearity in vortex shedding forces has little influence on control performance of SS-PTMD optimized under sinusoidal excitation.

Effects of attachments on aerodynamic characteristics and vortex-induced vibration of twin-box girder

An experimental investigation was carried out on the effects of attachments on the aerodynamic characteristics and vortex-induced vibration (VIV) of a twin-box girder. In the study, the flow pattern, pressure distribution, vortex shedding frequency, and VIV of five models, namely Model A (twin-box girder without attachments), Model B (twin-box girder with handrails and crash barriers), Model C (twin-box girder with handrails, crash barriers, and wind barriers), Model D (twin-box girder with maintenance rails), and Model E (twin-box girder with full ancillaries), are compared with a bare deck in detail. The results demonstrate that, owing to the large leading flow separation induced by the attachments, the flow characteristics and VIVs of the twin-box girder with attachments differ considerably from the bare twin-box girder. Because of the large porous ratio, the handrails and crash barriers exert relatively weak influences on the surface pressure distribution. However, the leading wind barrier and maintenance rail generate large flow separation at the leading edges, resulting in larger pressure fluctuations, a wider wake, and larger scale vortex with lower shedding frequency than that of the bare deck. Furthermore, owing to the large leading flow separation, the vertical VIV induced by the motion-induced vortex in the gap is suppressed. However, a torsional VIV is generated by the large-scale alternately shedding vortex in the wake of the downstream box girder.

A Simplified Nonlinear Model of Vertical Vortex-Induced Force on Box Decks for Predicting Stable Amplitudes of Vortex-Induced Vibrations

Abstract Wind-tunnel tests of a large-scale sectional model with synchronous measurements of force and vibration responses were carried out to investigate the nonlinear behaviors of vertical vortex-induced forces (VIFs) on three typical box decks (i.e., fully closed box, centrally slotted box, and semi-closed box). The mechanisms of the onset, development, and self-limiting phenomenon of the vertical vortex-induced vibration (VIV) were also explored by analyzing the energy evolution of different vertical VIF components and their contributions to the vertical VIV responses. The results show that the nonlinear components of the vertical VIF often differ from deck to deck; the most important components of the vertical VIF, governing the stable amplitudes of the vertical VIV responses, are the linear and cubic components of velocity contained in the self-excited aerodynamic damping forces. The former provides a constant negative damping ratio to the vibration system and is thus the essential power driving the development of the VIV amplitude, while the latter provides a positive damping ratio proportional to the square of the vibration velocity and is actually the inherent factor making the VIV amplitude self-limiting. On these bases, a universal simplified nonlinear mathematical model of the vertical VIF on box decks of bridges is presented and verified in this paper; it can be used to predict the stable amplitudes of the vertical VIV of long-span bridges with satisfactory accuracy.

Practical countermeasures for the aerodynamic performance of long-span cable-stayed bridges with open decks

Open decks are a widely used deck configuration in long-span cable-stayed bridges; however, incorporating aerodynamic countermeasures are advisable to achieve better aerodynamic performance than a bluff body deck alone. A sectional model of an open deck cable-stayed bridge with a main span of 400 m was selected to conduct a series of wind tunnel tests. The influences of five practical aerodynamic countermeasures on flutter and vortex-induced vibration (VIV) performance were investigated and are presented in this paper. The results show that an aerodynamic shape selection procedure can be used to evaluate the flutter stability of decks with respect to different terrain types and structural parameters. In addition, the VIV performance of Π-shaped girders for driving comfortableness and safety requirements were evaluated. Among these aerodynamic countermeasures, apron boards and wind fairings can improve the aerodynamic performance to some extent, while horizontal guide plates with 5% of the total deck width show a significant influence on the flutter stability and VIV. A wind fairing with an angle of 55° showed the best overall control effect but led to more lock-in regions of VIV. The combination of vertical stabilisers and airflow-depressing boards was found to be superior to other countermeasures and effectively boosted aerodynamic performance; specifically, vertical stabilisers significantly contribute to improving flutter stability and suppressing vertical VIV, while airflow-depressing boards are helpful in reducing torsional VIV.

Mechanism on suppression in vortex-induced vibration of bridge deck with long projecting slab with countermeasures

The wind tunnel test of large-scale sectional model and computational fluid dynamics (CFD) are employed for the purpose of studying the aerodynamic appendices and mechanism on suppression for the vortex-induced vibration (VIV). This paper takes the HongKong-Zhuhai-Macao Bridge as an example to conduct the wind tunnel test of large-scale sectional model. The results of wind tunnel test show that it is the crash barrier that induces the vertical VIV. CFD numerical simulation results show that the distance between the curb and crash barrier is not long enough to accelerate the flow velocity between them, resulting in an approximate stagnation region forming behind those two, where the continuous vortex-shedding occurs, giving rise to the vertical VIV in the end. According to the above, 3 types of wind fairing (trapezoidal, airfoil and smaller airfoil) are proposed to accelerate the flow velocity between the crash barrier and curb in order to avoid the continuous vortex-shedding. Both of the CFD numerical simulation and the velocity field measurement show that the flow velocity of all the measuring points in case of the section with airfoil wind fairing, can be increased greatly compared to the results of original section, and the energy is reduced considerably at the natural frequency, indicating that the wind fairing do accelerate the flow velocity behind the crash barrier. Wind tunnel tests in case of the sections with three different countermeasures mentioned above are conducted and the results compared with the original section show that all the three different countermeasures can be used to control VIV to varying degrees.

Investigation on the effect of vibration frequency on vortex-induced vibrations by section model tests

Higher-mode vertical vortex-induced vibrations (VIV) have been observed on several steel box-girder suspension bridges where different vertical modes are selectively excited in turn with wind velocity in accordance with the Strouhal law. Understanding the relationship of VIV amplitudes for different modes of vibration is very important for wind-resistant design of long-span box-girder suspension bridges. In this study, the basic rectangular cross-section with side ratio of B/D=6 is used to investigate the effect of different modes on VIV amplitudes by section model tests. The section model is flexibly mounted in wind tunnel with a variety of spring constants for simulating different modes of vibration and the non-dimensional vertical amplitudes are determined as a function of reduced velocity U/fD. Two 'lock-in' ranges are observed at the same onset reduced velocities of approximately 4.8 and 9.4 for all cases. The second 'lock-in' range, which is induced by the conventional vortex shedding, consistently gives larger responses than the first one and the Sc-normalized maximum non-dimensional responses are almost the same for different spring constants. The first 'lock-in' range where the vibration frequency is approximately two times the vortex shedding frequency is probably a result of super-harmonic resonance or the "frequency demultiplication". The main conclusion drawn from the section model study, central to the higher-mode VIV of suspension bridges, is that the VIV amplitude for different modes is the same provided that the Sc number for these modes is identical.

Effects of gap width on flow motions around twin-box girders and vortex-induced vibrations

Twin-box girders are employed in long-span suspension bridges for flutter stability. However, the flow around a twin-box girder is notably complicated. The effects of gap width on the flow characteristics at Re=5.58×104 and the vortex-induced vibration (VIV) of a twin-box girder at about Re=1.17×104 were experimentally investigated using a sectional model. In the experiments, the gap ratio (defined as the ratio of the width L between two box girders to the central height of the girder D) varied from 0.855 to 10.260. The flow velocities around the model and the surface pressures of the model were measured during static tests, and the acceleration of sectional model also measured during dynamic tests. In addition, corresponding smoke-wire flow visualization tests were conducted under low wind speeds for the same sectional model. The test results show that all of the pressure characteristics, the aerodynamic force, the Strouhal number, the flow pattern and the VIV are dramatically influenced by the gap width. At a gap ratio of L/D=2.138 (defined as the critical gap ratio), the flow motion around the gap between the upstream and downstream box girders suddenly changes, and an alternate shedding vortex appears. According to vortex shedding pattern of the twin-box girder, the flow can be categorized into three basic patterns: pattern A, vortices alternately shedding behind the downstream box girder when the gap ratio is smaller than the critical gap ratio (L/D<2.138); pattern B, vortices alternately shedding behind the upstream box girder (in the gap) when the gap ratio is in the moderate range (2.138≤L/D≤10.26); and pattern C, vortices freely shedding behind the upstream and downstream box girders when the gap ratio approaches infinity. For the VIVs, the twin-box girder experiences four types of VIV as the gap ratio increases: when L/D<1.710, no VIV occurs; when 1.710≤L/D≤2.138, only vertical VIV occurs; when 2.566≤L/D≤2.933, both vertical and torsional VIVs occur; and when L/D≥3.421, only torsional VIV forms.

Field monitoring and validation of vortex-induced vibrations of a long-span suspension bridge

To investigate full-scale wind-induced vibrations of a long-span suspension bridge with a central span of 1650m, a long-term wind and wind effect monitoring system was created. The basic wind field characteristics along the span-wise direction of the investigated bridge were analyzed. It was found that the wind field along the span-wise direction was inhomogeneous. The full-scale wind pressure distribution around the lower surface of the twin-box girder was also obtained. From the power density functions (PSDs) of the fluctuating pressures, the vortex shedding frequency of the full-scale twin-box girder was determined. A field visualization test was performed, and the flow pattern around the lower surface was obtained. Thirty-seven vortex-induced vibration (VIV) events were observed during the monitoring period. The corresponding wind conditions and vibrations were analyzed in detail. In addition to the wind direction and inflow turbulence, it was found that the inhomogeneity of the wind field along the span-wise direction of the bridge is also a critical factor that affects VIVs of full-scale bridge. The VIVs from a section model test and the full-scale bridge were compared, and it was found that the vertical VIV amplitude of the section model was much smaller than that from the field monitoring results. Moreover, torsional VIVs appeared in the section model test, whereas it was not observed in the full-scale bridge.