Design And Optimizing Of The Scaffold System For An Overpass Box Girder Bridge Under Complex Condition

For bridge construction using on-site pouring,it is inevitable to adopt some temporary support structures,that is,bracket systems.Although the support system is a temporary auxiliary structure,it is an important guarantee for the safe construction of the bridge and has an important impact on the construction period and cost.The support system has formed many typical application forms in long-term practice.However,with the development of urbanization and the upgrading and expansion of urban road networks,urban elevated bridges,especially interchange projects,often encounter complex site spaces.Restrictions have caused numerous obstacles to the design of the stent system,and it is impossible to directly apply simple and conventional stent systems,which increases the complexity of the stent scheme and the difficulty of safety checking.This paper relies on a large-scale integrated overpass project in a city’s Fourth Ring Elevated Express Project.The construction environment around the proposed overpass site is complicated.Not only six 220KV and 110KV high-voltage lines are erected above,but there is also an existing overpass bridge that crosses the river directly below.Since the proposed crossing is connected with multiple ramps,the beam body width presents a multi-curve gradual form,which needs to be organized and implemented by the cast-in-place process.During the construction of the proposed bridge,it is necessary to ensure the minimum safe operating distance of the overhead high-voltage line above,and after removing part of the horizontal ramp of the existing bridge,it is rebuilt and expanded in-situ into a three-dimensional connecting ramp between the main lines of the upper and lower bridges,while protecting the lower floors.On the premise of the safety of the existing main line bridge,a new overpass cast-in-place bridge over the main line will be built above the vertical projection of the existing main line bridge.Based on the engineering background,research on the selection,design and optimization of the support system was carried out.The main content and results are as follows:(1)Scheme selection and initial design of the support system.In order to meet the schedule requirements of the project plan,the bracket scheme selection was carried out according to the on-site environmental conditions.The existing bridge on the same bridge position was demolished,a temporary foundation was set up in the river channel,and the conventional Bailey beam+steel pipe column less scaffolding system was adopted,and the initial design plan of the scaffolding system was proposed.(2)Simplified calculation and analysis of the force state of the support system after initial design and optimization.In order to ensure the bearing capacity of the support system under the restricted operating environment,the stress state of the main components in the support system was analyzed,including load value,calculation method and internal force distribution.The results show that in the initially designed three-span Bailey beam support system,the bending moment of the Bailey beam is too large,which exceeds its flexural bearing capacity.The original three-span Bailey beam system is optimized to four spans.By reducing the Bailey beam to reduce the bending moment effect of the Bailey beam,and again through the force analysis of the system,it is verified that its bearing capacity meets the requirements.(3)Finite element simulation of the force state and stability of the support system.On the basis of the aforementioned preliminary design calculations,the support system was simulated and analyzed using MIDAS/Civil finite element software,and the force state and overall stability of the various components of the support system were further calculated.The results show that for the four-span Bailey beam bracket system,when four steel pipe columns are used in each row,the force of each member meets the requirements of bearing capacity,and the deformation is within the specification requirements,and the system has good stability.(4)Research on the alternative scheme of steel pipe column in the support system.Taking into account that the complex conditions of the project site caused the support material,especially the steel pipe column,the difficulty of the transportation and installation of the main large load-bearing components has doubled,which has a significant impact on the construction period,cost and safety,so the steel pipe column of the support system has been replaced.Analysis:Replace the φ1000×8mm steel pipe column used in the aforementioned support system with 4φ529×6mm steel pipe columns.The results show that:after the column form is replaced,not only the safety reserve of the support system has been improved,but more importantly,the steel pipe column material model has become smaller,which greatly reduces the construction difficulty and speeds up the construction.The construction convenience and economy have been greatly improved.