Research On A Structure System Of Super-span Partial Ground-anchored Cable-stayed Bridge With Crossing Stay Cables

The conventional thousand-meter-scale cable-stayed bridges face the challenges of excessive weight dead load,and excessive height pylons,which are becoming the technical bottlenecks for the advance of the long-span cable-stayed bridges.An innovative partial ground-anchored cable-stayed bridge system with crossing stay cables is developed in this study.By anchoring parts of the long stay cables to the ground anchor of the side span and introducing the new concept of crossing stay cable in the middle span,the axial force of the main girder and the height of the pylon can be significantly decreased.To verify the safety,feasibility,and economy of the super span partial ground-anchored cable-stayed bridge with crossing stay cables(the new system),this dissertation compares the mechanical properties of the new system to the conventional cable-stayed bridge system,and performs theory and numerical analyses for the construction scheme.The vibration control of super long cables,auxiliary cables and crossing stay cables are discussed.The main research results are as follows:(1)It is proved that the partial ground-anchored cable-stayed bridge with crossing stay cables can overcome the bottlenecks restricting the span development of conventional cable-stayed bridges.In the new cable-stayed bridge system,long stay cables cross with each other in the mid-span zone of the main span while the other ends of the long cables are anchored to the ground in the side spans.By this design,the long cables result in no additional horizontal pressure to the main girder,and the ratio of pylon height to span length can be reduced.In this paper,the construction scheme of the new cable-stayed bridge is studied.The main girder in the construction stage is controllable and the axial pressure stress of the self-anchored girder will not be increased by setting up traction cable at the cantilever ends.To make preparation for the theoretical research for the following research,finite element models are built for both the new cable-stayed bridge and conventional cable-stayed bridge.(2)The static characteristics of the conventional cable-stayed bridge and the new bridge system with a main span of 1408 m were calculated,including the finish stage,operation stage,and construction stage.The structural responses under vehicle load,temperature and limit static wind were analyzed.The results show that although the parameters of the main girder is determined based on the dead load and moveable loads,the analysis results of the girder can meet the design requirements under temperature actions and limit static wind loads.The static stability safety coefficients of the two cable-stayed bridges are greater than 4,and the stability safety coefficient of the new system is 40% higher than that of the conventional system.The dynamic characteristics of the two cable-stayed bridge systems are compared.It is found that the ground anchor cables of the new system can effectively constraint the longitudinal bending of the pylon and the longitudinal vibration mode of the main girder,but the cross-cable exhibit no significant constraints to the lateral bending of the main girder.(3)The layouts of the auxiliary piers,the tower-to-girder restraint system,the length of cross-cable zone,and the span-to-height ratio of the new cable stayed bridge were also analyzed.Based on the analysis results,the following conclusions can be drawn.(a)The fewer the number of auxiliary piers,the larger the deflection of the main girder under the vehicle loads,and the larger the stress range of the self-anchored cables.Consequently,this may make the tail cables susceptible to fatigue problems.On the other hand,too many auxiliary piers will cause the transverse static wind load response to be too large.Therefore,the number of auxiliary piers is recommended to be 2-3 for the new system.(b)The bending moment to temperatures for the bottom under the rigid junction system and elastic restraint system is huge.Under the action of longitudinal earthquake,the bending moment at the bottom of the tower under the rigid junction system and elastic constraint system is also large.While for the damping constraint system,the bending moment at the branches between the bottom and upper columns is huge.According to the analysis results of the static constraint system and dynamic constraint system,the floating system or semi-floating system is recommended for the constraint system of partially anchored cross-cable cable-stayed bridges.(c)The increase of cross girder segment length has the following effects: the axial pressure of dead load in the main girder decreases;The maximum sagging moment and maximum hogging moment of vehicle load in the main girder both decrease,and the vertical deflection of the middle span increases slightly.The bending moment of ultimate static longitudinal wind in tower bottom increases;The lateral displacement and vertical bending moment of the main girder have no obvious change under the action of static crosswind.The low order of natural vibration frequency increases,and the order of mode changes.(d)The axial pressure of dead load in the main girder increases significantly as the ratio of span to height increases;The mid-span deflection of vehicle load increases;The bending moment of tower bottom and the average force of ground-anchored cable under ultimate static longitudinal wind decrease.The low order natural vibration frequency increases.(4)Three kinds of vibration suppression methods were discussed in order to avoid excessive vibration of the large span partial ground anchor cross cable cable-stayed bridges:(a)The damping effect with semi-active control at the end of cable is greater than that of passive control.However,the dampers installed to the ends of the girder have limited effect to suppress vibration at the mid-span region of the cable.(b)The auxiliary cable network system has an obvious effect to improve the stiffness of the cable fans.It is little difference of vibration control effect for curve auxiliary cable system and straight auxiliary cable system.The auxiliary cable network can not only increase the in-plane vibration frequency of the cables but also increase the frequency of out-of-plane vibration through entangling the auxiliary cables.When the number of the auxiliary cables is less than 5,increasing the auxiliary cables has a significant effect to improve the stiffness of the cable surface.However,when the number of the auxiliary cables is more than 5,the controlling effect does not show significant differences.The increase of auxiliary cable area(tension force)results in an improvement to the rigidity of cable surfaces,especially for high-order vibration modes.(c)the damping ratio of the passive control damper and the semi-active control damper at the cable intersection is significantly higher than that of the non-damper scheme,and the semi-active control scheme is the best;In addition,the damper scheme in partial intersections is better than that in all intersections.(5)Compared with conventional self-anchored cable-stayed bridges and single-span ground anchored suspension bridges,the economy of the new cable-stayed bridge system has the following characteristics:(a)the optimal span-to-height ratio of the new system is about 5.5~6.0;(b)the economic span of the new system is about 900 m ~ 2,100 m,and(c)the optimal ratio between the length of the cross-cable zone and the span for a the new cable-stayed bridge with a span of 1000-2200 m is about 0.1-0.28.