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Chongqing Chaotianmen Yangtze River Bridge(Ⅱ)

XIANG Zhongfu

Construction Innovative Technique

Upper steel girders of the bridge were installed in a symmetrical manner from side pivots at both sides to midspan. All members of main structure of the side span were installed, followed by central span's trussed arch and tie rods. After closing trussed arch at midspan, temporary tie rods were installed, which forms stress system of tied arch, and then cranes work on deck were used to install central span's upper and lower girder system and deck slabs. Construction of the bridge was complex with 123 important working conditions.

Make use of temporary piers to erect steel truss of side span. 3 temporary piers that serve as auxiliary support were provided while installing steel girders of side span. Falsework was erected between the 1# temporary pier and side pier. Truss pieces of side span's 1#, 2# were installed on falsework using 1,000t.m tower crane, and the rest truss pieces were assembled with girder erection crane for cantilever. To fulfill the need of adjusting for closing of steel girder's central span and the convenience of transiting between side span and main span, side pivots at both sides were lowered 2.3m in advance when installing steel girders. Side bearers P6, P9 and central bearings P7, P8 were set as fixed bearings respectively during side span installation and central span's cantilever installation. The latter ones maintained longitudinal movement of side bearings.

Cantilever of large arch-based crane of 21,000t.m was used for main arch to install steel trussed arch of main span, which all technical indexes hit international advanced level. (Figure 7).

                                     Figure 7Erection of cable-stayed cantilever of main arch

Use ballast and diagonal tension to realize stability during installation of main arch and pre-control of closing mouth. Anti-toppling stability factor of finished structure gets smaller and smaller with proceeding of cantilever construction. To ensure structural stability during construction, avoid adverse structural stress of steel truss under single cantilever and cut down downward deflection at the end of the cantilever, diagonal tension system was set besides putting ballast on side spans. Using ballast flexibly not only ensured structural stability during construction, but also mended temporary adverse stress of the structure during construction. Construction technique of single strand of diagonal tension steel strands tensioned at one time simplified construction and avoided hidden trouble in safety caused by multiple tensions.

Install the arch first and then the rigid tie rods. Temporary tie rods were set before installation of rigid tie rods, which not only reduced the load of ballast and diagonal tension system that were required to ensure stability of structural installation, but also avoided instability when closing main arch and tie rods.

Pre-bias starting point of installation to make up displacement at closing mouth. Design of mid span's trussed arch needs closing without stress. Sequence of closing is: lower chord - upper chord - diagonal member - horizontal brace. Firstly used temporary closing hinges to realize fast closing of trussed arch's upper and lower chords, and then closed other members after relieving temporary fixing measures pf P8 mobile bearing. For that, that way was used in combination with side and central pivots, relative position of end space of cantilevered steel trussed arch at both sides was adjusted, which fulfilled natural closing free of stress.

Temporary tie rods were installed at E17 node of reinforcing lower chord of the central span after trussed arch was closed. Side pivots were adjusted to design elevation, and then diagonal tension and ballast were removed in an inverted sequence after initial tensioning was done.

After temporary tie rods were set, 900t.m full-rotation deck crane was used to install span after span rigid tie rods at central span in the sequence from bottom to top. After permanent rigid tie rods were closed, remove temporary tie rods, and erect deck slabs of central span.
Effective Monitoring Means

Side spans of the bridge were cantilevered assembled on limited falsework (side span was provided with three temporary piers), while arch truss of main span were cantilevered assembled using diagonal tension. The content of control includes: firstly, properly control dimensions and precision of the members through assurance of design and processing; secondly, control various initial states of construction, such as erecting vertical position and position of central pivot for truss at side pivots, etc; thirdly, process of cantilevered assembly was needed due to influences of errors, over high stresses and insufficient stability etc, though various member stresses, local stability and stability of steel truss have been considered in design; fourthly, monitoring was required for deflection of steel truss during construction of cantilevered assembly, real-time mastering structural spatial conditions and matching degree with theoretical analysis, to ensure smooth closing and steel truss alignment meeting requirements; fifthly, control ballast and diagonal tension for steel truss of side span during cantilevered assembly; sixthly, control closing of main truss, including jacking up pivots; seventhly, control transformation of system, including temporary setting of tie rod, pivot elevation, and adjusting of position; eighthly, carry out construction control on rigid tie rods and bridge way system, as well as monitoring overall structure; ninthly, control deck alignment. Control bridge construction process according to 123 working conditions in reality. Figure 8 shows main working conditions.

                                                         Figure 8: Main Construction of the Bridge

(1) Use real-time target variable weight matrix method with multiple factors and variable parameters to compute target parameter regulation required by construction control, which improves efficiency and optimizes adjusting of construction.

(2) Use tangent assembly computation to substitute traditional computation of fold line assembly, fulfilling spatial simulation analysis for whole process of bridge construction.

(3) Set up monitoring method for super-spanned steel trussed arch where predicted construction conditions combine with parameter correction within condition tolerance.

(4) Steel girder structure's construction alignment is mainly ensured by quality of manufacture at workshop, while site installation of spatial alignment is ensured by controlling matching percentage of bolt holes on the nodes and correcting influence of partial sunshine.

(5) Change elevation of side pivot through lifting force of jacks, fulfilling the target of adjusting alignment. Use a way of putting ballast on side span to ensure anti-toppling stability factor (1.3) of cantilever end. At the same time, improve local stress of the structure during construction relying on flexibility of the ballast (Figure 9).

                                                                       Figure 9: Girder for Ballast

(6) Initial precise positioning (pre-bias compensation) for mobile bearing of main pier (P8) offered guarantee for closing main truss, fulfilling precise control of bridge span(span size of central span in theory of the finished bridge is 552.824m, while the one measured in reality is 552.809m).

(7) Relieving time and mode of three temporary piers used for side span erection have one impressive influence on structural stress. Simulation analysis shows that compulsory relieving through jacking up side pivot is more helpful to structural stress than natural relieving.

(8) Since stayed buckles and suspended tower were dismantled after installation of rigid tie rods, what's more, stayed buckles were tensioned at one time; therefore stability of suspended tower itself faces a great challenge. During control process, reasonably setting pre-bias towards side span for fixing tower makes it gradually drifted towards midspan with erection of steel girders and under influence from temporary tie rods, provided that the fixing tower would not fail.

(9) High precise stress-free closing of main arch and rigid tie rods as required by the design were realized through accurate beforehand analysis and timely process control (error of arch closing: longitudinal 2mm, transverse 9mm, vertical 3mm; error of rigid tie rods' closing: longitudinal 4mm, transverse 7mm, vertical 6mm. Tolerances of the errors are all 30mm).
Finding Temperature Difference of Deck-truss

For full steel structured bridge, specification of each country is not available for temperature effect of different positions within the same structure under the same sunshine condition. Usually extreme temperature of the place is adopted as overall temperature of the system, without considering influence of temperature difference on structural stress at various positions of each member under the same sunshine condition. During construction, it is found that temperature difference exists between steel deck slabs and main trussed girder (arch) of upper chord (15 centigrade for upper layer, and 6 centigrade for lower layer) through dedicated temperature test for deck slab and main truss as well as temperature at various positions. Existence of temperature difference between deck and truss is a totally new technical question we come across during construction of this bridge. The influence of temperature difference on construction can be eliminated through measures such as welding compensation; however, its influence on structural stress of finished bridge must be considered in terms of structure. Based on analysis and research, the original designed deck slabs were changed to partial sectors (6 positions longitudinal to the bridge) through the structural mode of cross girders at main truss's node separated from the deck truss that the main truss linked. Deck-truss composite structure  was newly introduce that transmited shear forces at deck-truss junction under deck-truss temperature difference and various working conditions, which allowed lateral displacement and stress at the end of cross girders of deck slab to be controlled within tolerance. Finding of deck-truss temperature difference of this bridge reveals that the temperature of this type of bridges influences essence, provides theoretical and practical basis for the revision of steel bridge design codes, and is significant to promote construction and development of the same kind bridges throughout the world.

Chongqing is honored as the Bridge City of China, besides large number, multiple types, many innovations and comprehensive influence of bridges, the more important thing is that special geographical environment of Chongqing makes Chongqing people possess strong sense of bridge. They know, identify and love bridges. Bridge culture also develops. One grand holiday-like opening ceremony that lasted for a couple of days was held on April 29th, 2009 for Chongqing Chaotianmen Yangtze River Bridge. Tens of thousands of people gathered there to enjoy the happiness of walking on the bridge (Figure 10). That fully reflected deep affection of people in Bridge City for Chaotianmen Bridge and their concerns upon bridges in Chongqing.

                                         Figure 10
(Author from: Chongqing Jiaotong University)

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