Crack Analysis And Control Study Of Separated Twin-box Girder Of Long-span Hybrid Cable-stayed Brige During Construction

In recent years,Separated twin-box concrete girder has been widely used in side span girder of long span hybrid girder cable-stayed bridge at home and abroad.However,this special structural form is prone to crack damage in the construction process.The ‘damage state’will be directly transmitted to the main tower of the bridge through the cable system,then it is transmitted to the steel grider by the stay cables and steel-concrete composite section,resulting in changes in the whole structure operation state.It is very difficult to restore the design standard,which seriously affects the service years of the cable-stayed bridge.Based on the research background of a long-span hybrid girder cable-stayed bridge,this paper analyzes and studies the cracking causes of Separated twin-box concrete girder in the northern span during construction,and puts forward corresponding crack prevention and treatment methods.The main works of this paper are as follows:(1)Referring to the literature at home and abroad,this paper introduces the research status and existing problems of Separated twin-box concrete girder,and summarizes the crack distribution,crack analysis theory and common causes of cracks in concrete bridges.Combined with the investigation results,this paper uses the idea of exclusion method to determine the possible causes of cracks during construction.(2)Based on the analysis theory of hydration heat temperature effect,this paper uses the finite element software MIDAS FEA to establish a three-dimensional solid model.Combined with the stress distribution of the box girder in the hydration heat process,the stress analysis of the possible cracking position is carried out.The effects of ambient temperature,mold temperature and template type on the hydration heat effect have been analyzed.The results show that the hydration heat effect will produce large tensile stress on the inner side of the nozzle,which is the main reason for the cracking at this position.Higher ambient temperature,suitable mold temperature and templates with better thermal insulation properties can reduce the possibility of box girder cracking.In addition,the sudden drop of temperature will cause large tensile stress on the surface of diaphragm and near the manhole,which is one of the main causes of cracks near the manhole.(3)Combined with the engineering practice,this paper selects the appropriate shrinkage prediction model,and uses the method of equivalent temperature load to analyze the shrinkage effect of box girder.It is found that the shrinkage effect will cause the vertical cracks in the middle and lower edge of the diaphragm span,and the shrinkage effect also provides a large tensile stress contribution to the cracks near the chamfer of the tunnel.(4)The whole bridge element model of cable-stayed bridge is established.According to the integral analytical results,the key beam segments under the most unfavorable stress state are selected.Based on the multi-scale modeling theory,the refined model of the beam segment is carried out and its correctness is verified.The spatial stress distribution of the key beam segment is analyzed in detail.The research results show that the key beam section is in a state of compression as a whole,there is a small tensile stress in the local area of the diaphragm,and the maximum compressive stress and the local tensile stress meet the limit values of the specification requirements during the construction period.By comparing the stress analysis results with the crack distribution,the structural load during construction may provide tensile stress for the development of cracks,but it is not the main cause of box girder cracking.At the same time,the parameter sensitivity analysis of prestressing forces and stay cable tension is carried out.The results show that these two parameters have different degrees of influence on the stress and deflection of the main beam under the bridge completion state.(5)In view of the causes of cracks in the construction period of the box girder with separated double edge analyzed in this paper,combined with the structural characteristics of the box girder and the actual construction technology,the crack prevention measures are proposed from the aspects of material selection,optimization of mix proportion,optimization design,maintenance method and construction monitoring.For the existing cracks,according to the characteristics of cracks,this paper proposes appropriate repair measures.