Fabrication Technique And 3D Shape Simulation For Integral Pre-assembly Of Steel Tower Segments In Nanjing 3rd Yangtze River Bridge

Main tower is an important component of the suspension bridge and cable stayed bridge.Generally,it can be as high as 200 to 300 meters.Traditionally,the main tower is made up of steel or concrete.As to the steel tower,normally it is divided into a number of segments in factory and hoisted in bridge sites.Therefore, high vertical degree of the tower is required.It is quite different from the main girder in fabrication.In addition,the shape of the main tower plays an important role in the whole sculpts and influences the visual effect of the bridge,which includs its own shape and how it fits with the surroundings.The modern suspension bridge and cable stayed bridge have been developed for more than 100 years and 50 years respectively.From the Williamsburg bridge (America)to the Akashi-Kaikyo Bridge(Japan,main span:1991m,height of the main tower:285m)and from the Str(?)msund bridge(1955)to the Tataro bridge(main span: 890m,height of the main tower:220m),most of them adopted steel tower.The past 20 years has witnessed the fast growth of the concrete main tower in Europe for economical consideration,such as the Humber bridge in England(suspension bridge, 1981,main span:1410m,height of the main tower:155.5m),the Normandy bridge in France(cable stayed bridge,1995,main span:856m,height of the main tower:203m), and the Great Belt bridge(suspension bridge,1998,main span:1624m,height of the main tower:254m)in Denmark.However,the steel tower is still the first choice in America,Japan and Europe due to its advantages in light deadweight,quick construction and high resistance to earthquake.A Pleth(?)ra of long-span suspension bridges and cable stayed bridges have been built since 1990s in China.All of them adopted the concrete tower because of the lower cost and the tradition of the designer and the builder.Another important reason is the lack of practice on the vertical degree of the steel tower.Of course,the huge concrete towers must undergo the test of the strong earthquake and long durability.Nanjing 3rd Yangtze River Bridge(cable stayed bridge,2005,main span:648m, height of the main tower:215m)is the first cable-stayed bridge adopting steel tower in China.How to borrow overseas experiences in the design,fabrication and construction of the steel tower and consider the condition in China in order to successfully build the first steel tower in China has significant influence to the future development of the steel tower in China.This motivates the research reported in this thesis.Based on his rich experiences in the fabrication of this steel tower,the author reviews the development of the steel tower fabrication technique and summarizes the fabrication technique of Nanjing 3rd Yangtze River Bridge tower.The main work is as follows:(1)The development and characteristic of the steel tower fabrication.During the past century,the fabrication,segment conjunction and erection methods have been developed to drive the drastic technical change from the steel tower from riveted structure to welding structure.In particular,the methods of getting the axis of the segment,the fabrication of the segment,the mechanical cutting of the end face and the inspection method for the segment preassembly have made great progress constantly in order to meet the requirement of the vertical degree.The Information Technology of the computer control was used in the fabrication of the Tatara Bridge,the Akashi-Kaikyo Bridge and Kurushima Bridge at the end of 1990s.(2)Structure analysis of the Nanjing 3rd Yangtze River BridgeThe cable stayed bridge bears a high degree hypostatical structure.The tower is a compressive and flexural component under the loading combinations of the deadweight,vertical and horizontal component of the cable force,deflection of the tower,the wind,the variation in temperature and the earthquake.So the stress analysis, the structure design and the segment joint are crucial to the fabrication and precision control.Based on the original design of the Nanjing 3rd Yangtze River Bridge,the re-analysis was executed.Furthermore,the "metal contact in end face plus tensile long bolts connection" were designed and compared to the "metal contact in end face plus high tensile bolts connection" in order to provide a good reference in the future.(3)Fabrication of the steel tower(welding and assembling)The steel tower supports the cables directly.Considering its structure characteristic,load case and erection method,the segments fabrication standard(the section side length error≤±2mm,the diagonal error≤±3mm,and the torsion error≤3mm)was brought forward referring to the Akashi-Kaikyo Bridge.Based on this standard,the fabrication technological program including the laying off size, assembling precision,controlling of the welding distortion and welding design was made.By using the above technological program,the size of the section,the torsion deformation,and the straightness or curvature were controlled within the range of the precision.(4)Mechanical cutting and precision control of the segments end faceThe steel tower of the Nanjing 3rd Yangtze River Bridge was divided into 22 segments which were connected by the "metal contact in end face plus high tensile bolts connection".So high precision of the mechanical cutting of each segment end face was required.For example,it was specified that planeness≤0.25mm,roughness≤12.5μm,inclination error between the axes and end face≤20"(1/10000). Whereas there are numerous factors influencing cutting precision and if one of them is ignored,the precision standard cannot be reached.Therefore,a precision control system is necessary.Here,the mechanic cutting of the segments and precision control system were introduced in detail.The Information Technology was used to fabricate the traditional steel structure by using the API laser tracker measuring method,and the reactions control system was applied to avoid the distortion of the segment under the gravity and the precision mechanical cutting technique.(5)3D shape simulation analysis of the segments preassemblyAlthough each segment reaches the precision standard,the vertical degree of the erected tower may exceed the standard(1/10000)if the inclination error between the axis and the end face of each segment is in the same direction.The rather that the erection error exists.Because the preassembly is performed outdoors,the influence of the temperature cannot be ignored.How to predict the preassembly shape of the whole tower is a key problem and a pursuing project in other countries.Using 3D coordinate value measured by API,the 3D shape analysis technique simulates the erection of the steel tower to give the error of the axis,the vertical degree and the slab staggering etc.In order to realize the goal,the computer program was developed and verified to achieve satisfying results.(6)Summary of the steel tower erectionThe erection method of the huge steel tower and the measured precision of Nanjing 3rd Yangtze River Bridge were presented.The data indicates that the erection result corresponds to the condition of the preassembly of two segments and the 3D simulation.