Wind-resisted Performance And Control For Transient Structures Of Long-span Suspension Bridges During Construction

The rigidity of the transient structure for the large span suspension bridge is low, the construction time is long, and the wind environment is complicated at the construction stages, so wind effects that cannot take place after construction to be finished may take place. The wind tunnel test whose some characteristics are time–consuming, vigour-squandering and costly is principally used to study the key cases that can affect the construction safety, but is not quite fit to research the cases having the fetures of the little influence on constructon and short construction period which sacrifice time or security and comfort of builders at great cost to some extent. With increasing span of the large span suspension bridge, the construction requirement is becoming higher and higher, time is more precious, and the security and comfort of builders get more and more attention. At the same time, as the numerical technique whose some characteristics are economical, less labor and time is changing quickly, it is possible to settle the all kinds of problems about wind resistance of the large span suspension bridge at the construction stages using the theoretical knowledge and computer. So it has theoretical and practical significance to proceed the theoretical analysis and numerical simulation of wind-resisted performance and control for transient structures of long-span suspension bridges during construction.Based on the review and prospect of wind performance and control for transient structure of large span suspension bridge under the construction stages and the on-site survey, applying the research method by combination of ANSYS, FLUENT, MATLAB and other software and results of wind tunnel tests, the thorough theoretical research of the problems and shortcomings existing in wind-resisted performance and control for transient structures of long-span suspension bridges during construction are carried out in many ways. The main contributions of the dissertation are given as follows:1. To solve the problem of galloping performance of erecting gate type variable section pylon of the large span suspension bridge considering the influence of aerodynamic interference effects, a practical numerical method for galloping analysis of variable section high-rise structure is advanced applying the research method by combination of finite element and CFD and is used to analyzed the galloping performance of key cases for the erecting typon of the large span suspension bridge and research the influence of aerodynamic interference effects on the galloping performance of twin columns. The results show that the numerical method of galloping analysis of variable section high-rise structure and the numerical wind tunnel method are both right. Beacause of aerodynamic interference effects between twin columns, the galloping performance of twin columns is different with single column and it is more disadvantageous for the galloping performance of twin columns to adopt chamfering section than rectangular section in the lateral direction. For the single column in the lateral direction, the galloping coefficients of anterior column with rectangular and chamfering section are greater than zero but those of later column exist the conditon less than zero, and absolute values of the galloping coefficients for the later column of single column are larger than those of twin columns. Beacause the galloping coefficients of twin columns and single column are both greater than zero, the galloping instability can not happen. The galloping critical wind speeds of all cases for the concrete typon in the example are larger than the design wind speed during construction, so the galloping instability of the erecting concrete typon can not happen.2. On the basis of methods of finite element ANSYS model, the wind-induced buffeting behaviour of four cases of concrete pylon at construction stage of long-span suspension bridge was studied. The wind-induced buffeting behaviour and the comfort assessment of four cases of concrete pylon at construction stage of long-span suspension bridge were researched using transient dynamic analysis of finite element ANSYS. Results show that the worst disadvantage state of the pylon at construction stage is not nude pylon ,but case without crossbeam for the comfort of the pylon for the suspension bridge at the construction stages, and so the judging method of the reliability for the pylon of suspension bridge at the construction stages to adopt the RMS and maximum value of the displacement is unscientific and the comfort for the pylon of the suspension bridge should use simultaneously the Dieke mann index.3. On the basis of methods of theoretical derivation and finite element ANSYS modelling, the dynamic behaviour of catwalks without wind-resistant cable of long-span suspension brdige was studied. The vibration frequencies of the first order vertical and side directions, and the first order torsional frequency are derived in theory. The influence of the related parameters was researched using theoretical calculation and model analysis of prestressing cable structures of finite element ANSYS. Results show that the vibration frequencies of the first order symmetry, antisymmetry side directions and antisymmetry vetical direction of the catwalks are independent of the tension of load bearing cables and quality of per-span-length; the influence of pylons on natural frequency of the catwalks without wind-resistant cable is little; the vibration frequencies of the catwalks submit the reduced trend with the increace of the ratio of rise to span; The influences of the positions and population of the transverse overbridges are trivial on low order frequencies and large on the high order frequecies; the parallel cables make almost no difference on the every frequencies and the across cables have no effect on low frequency and have to a certainty advance on the torsional frequencies; the natural frequencies of the catwalks with the bearing ropes made in CFRP and steel are nearly identical,but the catwalks with the bearing ropes made in CFRP reduce the require on the construction machinery such as winding machine.4. Referring to the results of sectional model test and using secondary developed ANSYS finite element software, the finite element modeling for catwalks without wind-resistant cable of a suspension bridge was established. The nonlinear aerostatic stability and the influences of nonlinearity for static wind load, the space between the transverse crossover and rigidity, the initial angle of incidence, the ratio of rise to span for catwalks and the horizontal coupling cables on the aerostatic stability of the catwalks were studied considering the nonlinear effects of catwalks and static wind load and adopting the incremental double iteration method. Results show that the buckling form for catwalk without wind-resistant cable is mainly coupled bending-torsional instability that the torsional deformation has obvious effect on the instability and the lift force and torsional moment are key factors for the instability, the calculation about the stability of the catwalk must be considered simultaneously the effect of the static wind load and the geometry nonlinearity and the aerostatic stability for the catwalk can be overestimated without the influence of structural deformation for the catwalk, the critical wind speed for aerostatic instability can be increased through minishing the spaces between the transverse crossbridges and augumenting the rigidity of those and reduced through augumenting the initial angle of incidence and the ratio of rise to span for catwalks and setting the horizontal coupling cables.5. To solve the problem of galloping instability of the transient main cables.The galloping coefficients of type sections for two construction projects of the transient main cables without wind-resistant measures were investigated for the first time by means of the CFD and finite element soft wares, referring to the erecting some large span suspension bridge, and the two projects were compared. The galloping critical wind speeds of the better construction project are computed. Results show that the galloping performance of the Jian-Ding project is better than that of the Ping-Ding project and the galloping instability can happen for the Ping-Ding project of the long-span suspension bridge during construction.6. On the basis of static, dynamic and transient analysis for the prestressing cable structures, the parametres of controlling the galloping vibration of the transient main cables of some long-span suspension bridge at the construction stages with the assistant cables and detailed scheme for controlling the galloping vibration of those with the assistant cables are investigated adopting the method of the finite element considering the geometry nonlinearity. The review and propect for the galloping vibration and controling vibration with assistant cables are summarized, the finite element modeling of the system for transient main cables and catwalk is established, and the influence of the position of the assistant cables, diposed fashion and the rigidities upon the first vertical bend vibration frequencies of the transient main cables is researched. At the same time, the effect of the damp and the rigidities of the assistant cables on the equivalent damping ratio of the transient main cables inside the plane is studied. In the end,the detailed scheme for controlling the galloping vibration of the transient main cables was studied. Results show that, to increase the 1st vertical vibration frequencies of the transient main cables, the assistant cables should be liad symmetrically and in the vertical direction and the frequency can be augmented through increasing the amount of the assistant cables and setting assistant cables in the middle of the bridge. To increase the diameter and amount of the assistant cables can improve the vibration of the transient main cables inside the plane. In the end, the computation show that the galloping instability for the long-span suspension bridge at construction stages can be controlled when the 11 ssistant cables groups are liad symmetrically and vertically at 12 qual division points of the suspension bridge and the assistant are made up of the reinforcing steel bar of 10mm diameters.7. The deterministic analysis of flutter for stiffening girders of long-span suspension bridges during construction can not meet the needs of actual situations and the probability analysis become the urgent problem to be solved. With a reference to some long-span suspension bridge, adopting the research methods of wind tunnel test and reliability analysis, the flutter reliabilities of stiffening girders for some long-span suspension bridge during construction were computed and meanwhile the effects of some factors on flutter reliability were investigated. Results show that the flutter stability is the worst at the construction early stages of the suspension bridge. There exists nonlinear relationship between the failure probability of flutter during construction and the design wind speeds, and the larger design wind speed is corresponding to the larger failure probability. The coefficients of variation have no effect on the failure probability. The increase of land surface roughness and the angle of attack results in the larger failure probability. It is dangerous for the initial construction to ignore the turbulent characteristics of wind and very conservative for the later construction. The flutter reliability of the non-symmetric construction for stiffening girders is better than that of the symmetric. It is more effective to increase the eccentric masses than to augment the eccentric distances for reducing the failure probability. In the end, the vertical cross suspenders can only increase the flutter reliability during later stages of construction for the symmetric construction of stiffening girders but reduce the flutter reliability during middle stages.