Monitoring calculation of closure change of Extradosed Cable-stayed Bridge

: During the construction of extradosed cable-stayed bridge in Yunnan province, China, the construction unit has made certain changes in the construction process of the closure section due to environmental restrictions: remove the hanging basket after the closure, the sling shall not be provided in closure section, the function of the sling is realized by the hanging basket on the 16th beam. In case of this change, the bridge has been constructed to section 15th. In order to ensure the smooth and orderly progress of each stage in the closure phase, this article is arranged according to the construction plan, appropriate adjustment of related procedures, checking the bridge safety at all stages of construction, the stress and force of the main girder are compared to ensure the safety of the construction after closure changes. Adjust the height of the beam of the 16th and 17th to adapt the new construction plan, and the bridge closure smoothly.


Engineering Overview
The bridge analyzed in this paper is located in Yunnan province in China, main spans of extradosed cable-stayed bridge is 2*85m, bridge width is 27m; the grade of Urban trunk road is Ш, the motor vehicle is two-way and four-lane; the design speed is 30km/h; design load for highway is levelⅡ, the throng is 3.5kN/m 2 ; design useful life of 100 years. The design elevation of the bridge is 454.589-451.389m.
The facade layout of the main bridge is shown in figure1, and the structure of the cable-stayed bridge is adopted. The main beam adopts single box three-compartment large cantilever variable cross-section PC continuous box beam (shown in figure2), the main pier is thin wall type pier, high 28m, and the deck thickness is 5m. The pier beam adopts the consolidation form. The bridge model material is summarized in table 1.   This monitoring calculation adopts the MIDAS Civil8.32 analysis program. The structure of the structure is analyzed by the structure of the space bar system. The structure consists of bridge pier, main beam, main tower and lasso. The structural analysis model is shown in figure 3. The total bridge has 146 units and 172 nodes. The boundary conditions in the analysis are: the bottom end consolidation of the cable tower, the transition pier and the auxiliary pier are simulated by the movable hinge support, and the lower end of the cast-in-place bracket is fixed hinge support.

Construction change requirement
According to the requirements of construction sequence diagram in the original bridge design document, the cantilever of closure strap and its formwork system are 32tons heavy, remove the hanging basket before closure stage, at the end of the closure, the weight is 58tons. But the construction unit proposed: remove the hanging basket after the closure, the sling shall not be provided in closure section, the function of the sling is realized by the hanging basket on the 16th beam, that meansthe former lifting point is located on the 17th beam segment and is 58cm from the starting point, to realize the function of the dragon sling (number of beam segment is shown in Figure 4). The concrete wet weight of all the hedrons is borne by the cantilever end, so the closure weight is increased from 58 to 105tons. When this change is proposed, the bridge has been constructed to 15th beam segment, in order to ensure the successful completion of the sub stages in the closure section, the relevant procedures are adjusted properly in accordance with the construction plan. The hanging basket shall not be removed until it is closed and only moved forward; the weight of the cantilever end is 105tons; after repeated trial calculation, in order to make the bridge state as consistent as possible with the original design, it is necessary to adjust the single tension of 11th cable to 415tons, and make two adjustments after the completion of the bridge, reduce the single tension of 11th cable to 395tons. The closure conditions of the two models are shown in table 2.

Safety checking calculation in construction stage
The stress calculation results of the main girder in each construction stage are shown in figure5 and 6.It can be seen from the picture, the stress change is small compared with the original model and the adjusted model, but they are all within safe limits. The results of pier strength calculation are shown in

Adjustments Calculate of shuttering elevation
Due to the change of construction sequence and plan, the height of the bridge cannot reach the design value under the completed shuttering elevation. Detailed calculations revealed that the maximum difference between the predicted primary bridge and the original design is 17.5mm. Therefore, the beam 16th and 17th sections should be adjusted, and found by the adjusted model, the folding angle of the 17.5mm is generated on the beam 15th-16th section. To mitigate this situation, the following are dealt with: 1) In order to closure, the 17th beam is not changed; 2) Raise the shuttering elevation of the 16th beam segment to 8mm.
At the time of closure,altitude difference is 15mm between west of final closure and side span,altitude difference is 16mm in East side. However, based on 0.78% design data, the theoretical height difference should be 15.6mm,then successfully complete the bridge construction.The table 6 can be show that the calculation of completed bridge after the adjustment of elevation of the formwork. Note：The design elevation is the elevation value of the construction drawing + aftershrinkage creep+ Live load displacement /2

Conclusion
Through the calculation of the stress of the main girder and the cable force at each stage of construction and the calculation results of the elevation of the vertical mold, it can be seen that the bridge is in a safe state after the adjustment of the formwork.
1) The stress of the adjusted formwork is within the safe range, and the bearing capacity meets the requirement during the maximum cantilever stage, and the bridge is in a safe state.
2) The calculation result of cable force satisfies the requirement that the safety factor of stay cable is more than 2.
3) After adjustment, there is a small gap between the cable forces, and the maximum gap is 88ton in cable 10th. Compared with the original model, the difference of 2.4% does not affect the carrying capacity of the bridge. 4) After adjusting the elevation of the formwork of the beam section, the finished bridge shall conform to the design line according to the changed working procedure.